1. business and industrial
2. chemicals industry
3. plastics and polymers

Exploring the Frontiers of Macromonomer
Chemistry
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van de
rector magnificus, prof.dr.ir. C.J. van Duijn, voor een
commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen
op maandag 15 oktober 2012 om 16.00 uur
door
Gemma Claire Sanders
geboren te Colchester, Groot-Brittannië
Dit proefschrift is goedgekeurd door de promotor:
prof.dr. A.M. Van Herk
Copromotoren:
dr.ir. J.P.A. Heuts
en
dr. R. Duchateau
Sanders, G. C.
A catalogue record is available from Eindhoven University of Technology Library
ISBN: 978-90-386-3223-0
Copyright © 2012 by Gemma C. Sanders
The results described in this thesis formed part of the research programme of the
Dutch Polymer Institute (DPI), DPI project # 651
Cover Design: Gemma C. Sanders & Proefschriftmaken.nl || Uitgeverij
BOXPress
Printed by: Proefschriftmaken.nl || Uitgeverij BOXPress
CONTENTS
GLOSSARY
5
SUMMARY
7
SAMENVATTING
9
1 INTRODUCTION
11
Aim
Catalytic Chain Transfer Polymerisation
12
12
Mechanical Aspects
Kinetic Aspects and Determination of CT
13
15
Copolymerisation with Methacrylic Monomers
Copolymerisation with Acrylates/Styrene
16
17
Radical Chemistry with Macromonomers
Post-polymerisation Functionalisation of Macromonomers
Thiol-ene Chemistry
Hydroformylation
16
19
19
20
Outline of Thesis
References
21
23
2 METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE
COMBINATION OF CATALYTIC CHAIN TRANSFER AND
ANIONIC POLYMERISATION
27
Abstract
Introduction
Experimental Section
Results and Discussion
28
29
32
36
Conclusion
References
54
55
Synthesis of Macromonomers
Synthesis of Macroinitiator (Model Studies)
Block Copolymer Synthesis
3 STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
Abstract
Introduction
Experimental Section
Results and Discussion
36
36
39
57
58
59
61
64
Synthesis of ASMA and SMA Copolymers
End Group Determination
Determination of Chain Transfer Constant of COBF for SMA
Copolymers
Post-Polymerisation Reactions of pSMA
Copolymerisation Behaviour
Conclusion
References
64
66
69
72
76
77
79
3
4 THE POLYMERISATION BEHAVIOUR OF EPOXIDISED
MACROMONOMERS DERIVED FROM CATALYTIC CHAIN
TRANSFER POLYMERISATION
Abstract
Introduction
Experimental Section
Results and Discussion
81
82
83
84
88
Synthesis of e-MMA2
Attempted Cationic Ring Opening Homopolymerisation of e-MMA2
Polymerisation of THF in the Presence of e-MMA2
Coupling Reactions of e-MMA2
Conclusion
References
5 STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE
SYNTHESIS OF BLOCK COPOLYMERS
Abstract
Introduction
Experimental Section
Results and Discussion
88
89
95
103
107
108
109
110
111
113
116
Coupling of Macromonomers to Polyurethanes using Thiol-ene
Chemistry
Thiol-ene Reactions of CCT-derived Macromonomers with ThiolFunctionalised Polyethylene
Thiol-functionalised Polyethylene as a Chain Transfer Agent in Free
Radical Polymerisations
Conclusion
References
116
120
125
130
131
6 GRAFT COPOLYMERS AS ANTI-STATIC AND ANTIFOGGING ADDITIVES FOR POLYCARBONATE
SUBSTRATES
Abstract
Introduction
Experimental Section
Results and Discussion
133
134
135
137
139
Synthesis of p(DMAEA-g-(CHMA-co-MMA))
Quarternisation of p(DMAEA-g-(CHMA-co-MMA))
Testing of Coating Properties
Conclusion
References
139
142
143
152
153
ACKNOWLEDGMENTS
155
CURRICULUM VITAE
157
LIST OF PUBLICATIONS
158
4
GLOSSARY
AFCT
AIBN
AMS
BA
BzMA
BzOH
CCG
CCTP
CT
CTA
CHMA
CLD
CM
COBF
DBTDL
DD
DMAEA
DMF
DMPP
DMSO
DPn
DPw
DRI
DSC
FHI
FRP
gHMQC
GPEC
HDI
HFIP
kt
ktr
MA
MALDI-ToF-MS
mCPBA
MMA
Mn
Mp
Mw
NMR
PC
PDI
PE
PEG
Addition-fragmentation chain transfer
2,2’-Azobisisobutyronitrile
α-Methyl styrene
Butyl acrylate
Benzyl methacrylate
Benzyl alcohol
Catalysed chain growth
Catalytic chain transfer polymerisation
Chain transfer constant
Chain transfer agent
Cyclohexyl methacrylate
Chain length distribution
Chain transfer to monomer constant
Bis(difluoroboryl)dimethyl-glyoximato cobalt (II)
Dibutyl tin dilaurate
Danishefsky’s diene
2-(Dimethylamino)ethyl acrylate
N,N-Dimethylformamide
Dimethylphenyl phosphine
Dimethyl sulfoxide
Number-average degree of polymerisation
Weight-average degree of polymerisation
Differential refractive index
Differential scanning calorimetry
Perfluorohexyl iodide
Free radical polymerisation
Gradient-heteronuclear multiple quantum coherence
Gradient polymer elution chromatography
Hexamethylene diisocyanate
1,1,1,3,3,3-Hexafluoroisopropanol
Chain length-averaged termination coefficient
Chain transfer rate coefficient
Maleic anhydride
Matrix assisted laser desorption ionisation time of flight
spectroscopy
3-Chloro-benzenecarboperoxoic acid
Methyl methacrylate
Number-average molecular weight
Peak mass
Weight-average molecular weight
Nuclear magnetic resonance
Polycarbonate
Polydispersity index
Poly(ethylene)
Poly(ethylene glycol)
5
PE-SH
PPG
ppm
RAFT
ROP
S
SEC
t1/2
TCE
Tg
THF
UV
VOC
VAZO-88
λ
Thiol-terminated polyethylene
Poly(propylene glycol) toluene 2,4-diisocyanate
Parts per million
Reversible addition fragmentation chain transfer
polymerisation
Ring opening polymerisation
Styrene
Size exclusion chromatography
Static decay half life
Tetrachloroethylene
Glass transition temperature
Tetrahydrofuran
Ultraviolet
Volatile organic compounds
1,1′-Azobis(cyclohexanecarbonitrile)
Fraction of radicals undergoing termination by
disproportionation
6
SUMMARY
Since the discovery of catalytic chain transfer polymerisation (CCTP) in the 1970s,
the reactive low molecular weight macromonomers characteristic of this technique have
proven to be chemically versatile and easily industrially-scalable. However, despite
detailed investigations into the mechanism and properties of these CCTP-derived
macromonomers, little attention has been paid to the combination of CCTP with other
polymerisation techniques in either industry or academia. Indeed, although CCTP has
been effectively used in combination with radical techniques such as reversible addition
fragmentation chain transfer (RAFT) polymerisation and atom transfer radical
polymerisation (ATRP) and to a certain extent conventional free radical polymerisation
(FRP), non-radical techniques have not been explored. The use of multiple polymerisation
mechanisms in a single polymer eliminates the restrictions of monomer-technique
compatibility, and increases the possible monomer combinations thus polymer properties
obtainable. In this thesis, a toolbox of radical and non-radical polymerisation techniques
as well as post-polymerisation modifications in combination with CCTP have been
explored and developed.
Macromonomers synthesised via CCTP contain a reactive double bond, which can
undergo a Michael addition with lithium ester enolates, resulting in the formation of a
macroinitiator capable of polymerising methacrylic monomers anionically. The resultant
block copolymers contain a predominantly atactic block (derived from the radical CCTP)
and an isotacticoid or syndiotactoid block (derived from the anionic polymerisation). The
nature of the second block depends heavily on the conditions of the polymerisation.
Functional macromonomers based on styrene and maleic anhydride (pSMA) and
styrene, maleic anhydride and α-methyl styrene (pASMA) have been synthesised using
CCTP. Chain transfer constants for these polymerisations have been determined and
detailed 2D NMR analysis was used to reveal that the end group of pSMA is maleic
anhydride-based with a vinylic moiety, and for pASMA the end group is predominantly αmethyl styrene, also with a vinylic moiety. Further post-polymerisation functionalisation
of pSMA via Diels-Alder and thiol-ene reactions has also been explored.
Thiol chemistry has been exploited to synthesise block copolymers. Thiol-ene
reactions have also been used to modify methacrylic macromonomers for use in
polyurethane chemistry to generate triblock copolymers. Synthesis of block copolymers of
poly(ethylene-b-methyl methacrylates) have been attempted using two routes. The
coupling of methyl methacrylate-based macromonomers to thiol-terminated
polyt(ethylene) proved futile under the reaction conditions restrictions set by the
properties of poly(ethylene). The same block copolymers could, however, be realised using
the thiol-terminated poly(ethylene) as a macro-chain transfer agent in a radical
polymerisation of not only methyl methacrylate but also butyl acrylate and styrene.
The vinylic functionality of dimers of methyl methacrylate (MMA) can be
epoxidised using m-chloroperoxybenzoic acid. Model studies of the epoxidised dimer of
MMA (e-MMA2) showed that homopolymerisation of the epoxide results in back-biting of
the epoxide, even under cationic ring opening polymerisation conditions. Polymerisation
of THF in the presence of e-MMA2 (using BF3.OEt2 as the initiator) gave surprising
7
results. e-MMA2 does not copolymerise with THF, but rather catalyses the reaction, then
once the THF maximum conversion has been reached, e-MMA2 end-caps the polymer. eMMA2 has also been used in coupling reactions to amines and (macro)alcohols.
CCTP-derived co-macromonomers of cyclohexyl methacrylate and MMA have
also been synthesised. These co-macromonomers have then been copolymerised radically
with 2-dimethylaminoethyl acrylate (DMAEA) to form graft copolymers to form a range
of polymers containing different grafting densities and graft lengths. The properties of
these polymers were investigated with respect to the suitability of these polymers for use
as anti-static and anti-fogging additive for polycarbonate substrate. As coatings for
polycarbonate, the graft copolymers showed promising results, for both anti-static and
anti-fogging applications.
A toolbox of different chemistries has been developed for use in combination with
CCTP, significantly contributing to both the fields of CCTP and complex architectures.
The potential of these chemistries is large owing to the versatility and range of different
techniques used, providing many options in terms of monomer choice and polymeric
architecture. Although at present there are no direct applications for the majority of the
chemistries developed as part of this thesis, the scope and the variability of the techniques
and monomeric starting materials are redolent with possibilities.
8
SAMENVATTING
Sinds de ontdekking van katalytische ketenoverdracht polymerisatie (Catalytical
Chain Transfer Polymerisation Eng., CCTP) in de jaren 70, heeft deze techniek, met
reactieve macromonomeer karakteristieken, zich bewezen als chemisch veelzijdig en
makkelijk industrieel opschaalbaar.
Ondanks het gedetailleerde onderzoek naar het mechanisme en de eigenschappen
van deze CCTP-macromonomeren is er weinig aandacht besteed aan de combinatie met
andere polymerisatietechnieken door industrie en de academische wereld. Hoewel CCTP
succesvol is gecombineerd met radicaal polymerisatietechnieken, zoals RAFT (Eng.,
reversible addition fragmentation chain transfer) polymerisatie, ATRP (Eng., atom
transfer radical polymerisation) en normale vrije radicaal polymerisatie, zijn niet-radikaal
polymerisatiemechanismes niet onderzocht. Het gebruik van verschillende
polymerisatietechnieken voor één polymeerketen elimineert de restricties van monomeertechniek compatibiliteit en opent de mogelijkheden voor monomeercombinaties en
daarmee de mogelijke eigenschappen van het polymeer. In dit proefschrift is een tool-box
van radicale en niet-radicale polymerisatietechnieken en post-polymerisatie modificatie in
combinatie met CCTP onderzocht en ontwikkeld.
Macromonomeren gesynthetiseerd via CCTP hebben een reactieve dubbele
koolstofverbinding, die een Michael-additie kan ondergaan met een lithium ester enolaat,
resulterend in de formatie van een macroinitiator. De macroinitiator heeft de mogelijkheid
om methacrylaat monomeren anionisch te polymeriseren. Het resulterend blokcopolymeer
heeft voornamelijk een atactisch blok (door de radicale CCTP) en een isotactisch of
syndiotactisch blok (door de anionische polymerisatie). De eigenschappen van het tweede
blok hangen sterk af van de condities van de polymerisatie.
Functionele macromonomeren gebaseerd op styreen en maleïnezuuranhydride
(pSMA) en styreen, maleïnezuuranhydride en α-methylstyreen (pASMA) zijn
gesynthetiseerd door gebruik van CCTP. De ketenoverdracht-constanten voor deze
polymerisaties zijn vastgesteld. Gedetailleerde 2D NMR analyse is gebruikt om te
onthullen dat de eindgroep van pSMA voornamelijk maleïnezuuranhydride met een vinyl
binding is. Voor pASMA is de eindgroep van de keten voornamelijk α-methylstyreen met
een vinyl binding. Verder zijn post-polymerisatie functionalisaties van pSMA door DielsAlder en thiol-ene reacties onderzocht.
Thiol-ene chemie was gebruikt voor de synthese van blokcopolymeren.
Methacrylaat macromonomeren waren modificeerd door thiol-ene reacties voor gebruik in
polyurethaan chemie voor de synthese van triblokpolymerisaties. Twee routes zijn
onderzocht voor het synthetiseren van poly(ethyleen-b-methyl methacrylaat)
blokcopolymeren. Het koppelen van methylmethacrylaat macromonomeren met thiolgetermineerde polyethyleen bleek onmogelijk onder de reactiecondities die door de
eigenschappen van polyethyleen worden beperkt. De blokcopolymeren konden
gerealiseerd worden door het gebruik van het thiol-keteneinde als een macromonomeer
ketenoverdrachtsgroep tijdens de polymerisatie van zowel methylmethacrylaat als
butylacrylaat en styreen.
9
De vinylfunctionaliteit van dimeren van methylmethacrylaat kan geëpoxideerd
worden door m-chloroperoxybenzoëzuur. Modelstudies van het geëpoxideerde dimeer van
MMA (e-MMA2) lieten zien dat de homopolymerisatie van het epoxide resulteerde in
back-bitting van het epoxide, zelfs onder kationische ring-opening polymerisatie condities.
De polymerisatie van THF in de aanwezigheid van e-MMA2 (BF3.OET2 als initiator) gaf
een verrassend resultaat: de e-MMA2 copolymeriseert niet met THF, maar katalyseert de
reactie en end-caps de polyTHF bij maximale conversie. e-MMA2 is ook gebruikt in
combinatie met koppelingreacties met amines en (macro)alcoholen.
CCTP-gebasseerde co-macromonomeren van cyclohexylmethacrylaat en MMA
zijn ook gesynthetiseerd. Deze co-macromonomeren zijn copolymeriseerd met 2dimetylaminoethylacrylaat (DMAEA), resulterend in graftcopolymeren met verschillende
hoeveelheden macromonomeren van verschillende moleculaire gewichten. De
antistatische en anti-condens eigenschappen van deze polymeren waren onderzocht voor
een polycarbonaatsubstraat. Een coating van de graftcopolymeren op polycarbonaat liet
veelbelovende antistatische en anti-condens eigenschappen zien.
Een gereedschapskist van verschillende chemische reacties is ontwikkeld voor
het gebruik in combinatie met CCTP. Dit is een significante bijdrage voor zowel het veld
van CCTP als voor complexe polymeerarchitecturen. Het potentieel van deze chemische
strategieën komt voornamelijk door de veelzijdigheid en de grote mogelijkheden op het
gebied van monomeerkeuze en polymeerarchitectuur. Hoewel momenteel nog geen directe
applicaties voor de chemische combinaties bekend zijn, geven de variaties van technieken
en monomeren veel mogelijkheden.
10
Chapter 1
INTRODUCTION
INTRODUCTION
Chapter 1
AIM
There is a desire, both in academia and in industry, to synthesise polymers with
complex architectures. By using different polymerisation mechanisms, polymer segments
that are not normally found together (i.e. cannot be made via the same polymerisation
technique, or only via very difficult routes) can be combined.1 This is of interest as a route
to developing new materials with novel properties, and can be employed, for example, to
modify the surface of engineering plastics. A common method to achieve this is by using
copolymers with complex polymer architectures, in which one part of the polymer
interacts with substrate providing adhesion, and the other part contains some form of
functionality, such as the reduction of the build-up of static charge or to prevent fogging
of polycarbonate substrates.
Complex polymer architectures, such as block, graft and star copolymers, can be
synthesised via a variety of routes.2 Often, controlled or living polymerisation mechanisms
are used to control the topology of the polymer structure such as atom transfer radical
polymerisation (ATRP),3, 4 reversible addition fragmentation (RAFT) polymerisation,5, 6
nitroxide-mediated radical polymerisation (NMRP),7 anionic8 or cationic9 polymerisation
and coordination polymerisation including chain shuttling chemistry. 10 Using pre-formed
polymers, such as macromonomers, as building blocks allows for the synthesis of
polymers with well-defined segments. The inclusion of macromonomeric building blocks
can be carried out via the same polymerisation mechanism used to synthesise the
macromonomer or via an alternative polymerisation mechanism, removing the restrictions
placed by only using a single polymerisation route for polymer synthesis. 11-13 In recent
literature, living radical polymerisation techniques have been combined with catalytic
olefin polymerisation,1 ring opening polymerisations14 and click chemistry.15
In this thesis, the use of macromonomers derived from catalytic chain transfer
polymerisation (CCTP) will be used as building blocks for the synthesis of block and graft
copolymers. CCTP has been combined with a variety of other polymerisation techniques
and post-polymerisation functionalisation reactions in order to introduce different
monomers and properties into the copolymers.
CATALYTIC CHAIN TRANSFER POLYMERISATION
Catalytic chain transfer polymerisation (CCTP) was first discovered by Smirnov et
al. in the 1980s.16-18 The addition of small quantities of certain low spin Co(II)
compounds, e.g. cobalt porphyrins, were found to dramatically decrease the molecular
weight of polymers made via conventional free radical polymerisation (FRP).
Furthermore, polymer chains with ω-unsaturated end groups were found to be the
predominant, if not the exclusive, product of these polymerisation reactions. Since then,
CCTP found applications in a range of fields such as mould and pavement
manufacturing19, 20 and as low VOC, high solids coatings for the automotive industry. 21, 22
The mechanism of CCTP can be summarised as the abstraction of a hydrogen
radical from a growing polymer chain, and transfer of this radical to a monomer unit. The
resulting species are a terminated polymeric species with an ω-unsaturated end group (a
‘macromonomer’) and a new propagating centre in the form of a monomeric radical
(Scheme 1).16-18
12
Chapter 1
INTRODUCTION
Scheme 1. Catalytic chain transfer in the free radical polymerisation of methyl methacrylate.
The success of these Co(II) species can be attributed to both their catalytic activity
towards chain transfer reactions, as well as their high chain transfer constants (CT)
compared to conventional chain transfer reactions, such as thiols and CBr 4 (Table 1).
Knowledge of CT values enables the prediction of the degree of polymerisation (DPn)
based on the amount of chain transfer agent used.
Table 1. Typical chain transfer constants for the bulk polymerisation of methyl methacrylate at 60
ºC.
Chain Transfer Agent
n-dodecane thiol
CBr4
COBF
CT
Reference
1.2
0.3
(24-40) ×103
23
24
17, 18
A variety of catalytic chain transfer agents based on Co(II) and other transition
metals have been discovered, although the most widely used branch of catalysts are based
on cobaloximes.25 One of the most common chain transfer agents in CCTP, and in fact the
one used in this thesis, is bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF,
Figure 1).26 Only ppm amounts of this catalyst are required to significantly decrease the
molecular weight of the resultant polymers. Note that it is in fact the bis(methanol) adduct
of COBF that is often used in practice.
Figure 1. General structure of COBF
Mechanistic Aspects
The most accepted mechanism for CCTP is via a two-step radical reaction: firstly a
hydrogen radical is abstracted by the Co(II) complex from a growing polymer chain,
resulting in a highly reactive Co(III)H intermediate and a macromonomer. 16-18, 27-38 The
13
INTRODUCTION
Chapter 1
subsequent reduction of Co(III)H back to Co(II) yields a monomeric radical, capable of
propagation (Scheme 2).
Co(III)-Rn
Rn•
Pn(=)
[Co Rn•]
Co(II)
Co(III)H
R1•
M
Scheme 2. Catalytic cycle for COBF-mediated CCTP.
Although CCTP is a highly effective method for producing low molecular weight
polymers based on methacrylic monomers, acrylic monomers and styrene pose a greater
problem. The major difference between the catalytic activity of catalysts, such as COBF,
with methacrylates and acrylates is the degree of formation of Co III-Rn, which is not part
of the catalytic cycle, merely an unwanted side reaction (Scheme 2). The hydrogentransfer is thought to arise via a direct reaction of the cobalt centre with the propagating
radical through a CoHC transition state.16-18, 29 During the radical polymerisation of
methacrylates, most of the cobalt is in the Co(II) state, whereas for acrylates the Co(III)
state is predominant.38, 39 The implication of the increased Co-C interaction is that catalyst
is essentially removed from the catalytic cycle, decreasing the concentration of catalyst
available for chain transfer reactions.16-18, 29 An additional divergence is that CCTP of
acrylates and styrene result in polymers with internal double bonds as the end group
(Scheme 3). Acrylates with an external unsaturation can be synthesised under radical
conditions, but only at high temperatures.40, 41
Scheme 3. Catalytic chain transfer in the free radical polymerisation of styrene.
14
Chapter 1
INTRODUCTION
Kinetic Aspects and Determination of CT
The (instantaneous) number average degree of polymerisation (DPn) for a CCTP
reaction can be described by the Mayo equation (Equation 1), where λ is the fraction of
radicals undergoing termination by disproportionation, 〈 〉 the chain length-averaged
termination rate coefficient, [ ] the overall radical concentration, [ ] the monomer
concentration,
the chain transfer to monomer constant, [ ] the active catalyst
concentration and
the chain transfer constant (defined as ktr/kp, where ktr is the chain
transfer rate coefficient and kp the propagation rate coefficient).42
(
)
〈 〉[ ]
[ ]
[ ]
[ ]
This equation is often simplified to Equation 2, where
polymerisation in the absence of a chain transfer agent.
[ ]
[ ]
( )
is the degree of
( )
Therefore, it can clearly be seen that the higher the [Co] and the greater the CT, the
lower the molecular weight of polymer produced. For very short chains, however, a
modified version of the Mayo equation should be applied (Equation 3). 43
(
[ ]
)
[ ]
( )
The easiest and most widely used method of calculating CT is via the Mayo
method. By determining the Mn or Mw from SEC data, the degree of polymerisation can be
calculated, which is then plotted as 1/DPn or 2/DPw against the ratio of chain transfer
agent concentration to monomer concentration. The slope of this line is the CT, in
accordance with Equation 1. Although using Mn to determine the degree of
polymerisation is theoretically the most accurate way, Mw is often used (as Mw/2×m0) as
it is a more robust experimental parameter, particularly for low molecular weight
polymers. 23, 44, 45 It should also be noted that a polydispersity index of 2 is assumed as this
is the theoretical value found for chain transfer-dominated polymerisations.
However, the Mayo method has one major disadvantage. When dealing with low
molecular weight polymers, it is often difficult to separate the polymer from the solvent
peak in SEC. This means that obtaining an accurate baseline is not always possible, and
thus the determined average molecular weights, particularly Mn, are less reliable giving
an unrealistic CT values. This issue has previously been discussed in detail and a
comparison made between CT values determined using the Mayo method (Mn and Mw
from SEC measurements) and the chain length distribution (CLD) method.23, 46 The CLD
method uses the slope, Λ, of the chain length distribution P(M), plotted as ln(P(M)), vs. M
to determine the chain transfer constant.47, 48 Λ taken in the higher molecular weight
region, ΛH is theoretically the most appropriate, in accordance with Equation 4.
15
INTRODUCTION
(
( ))
(
〈 〉[ ]
[ ]
[ ]
)
[ ]
Chapter 1
( )
More reliable results are often obtained, however, when the slope of the distribution is
taken at the molecular weight of the peak of the original chromatogram, ΛP.49 In this
manner a section of the slope can be selected that is free from “contamination” originating
from solvent peaks or anomalies in the baseline providing a more robust parameter.
Furthermore as Λ is essentially equivalent to 1/DPn, it can be plotted against [Co]/[M]
for a range of [Co] to [M] ratios to determine the chain transfer constant.
RADICAL CHEMISTRY WITH MACROMONOMERS
CCTP has been combined with a range of radical techniques in order to synthesise
block, graft and star copolymers.
Copolymerisation with Methacrylic Monomers
Block copolymers
The copolymerisation of methacrylic macromonomers in the presence of a
methacrylic monomer under radical conditions has been found to result in block
copolymers, or more accurately block macromonomers.49-55 The mechanism by which this
takes place is known as addition fragmentation chain transfer (AFCT, Scheme 4) and is in
fact a precursor to, the more well-known, reversible addition fragmentation chain transfer
polymerisation (RAFT) using dithioester compounds. On addition of a methacrylic radical
to a macromonomer, a relatively unreactive intermediate radical is formed. This
intermediate radical can then undergo β-scission resulting in a new macromonomeric
species and a propagating radical. Propagation can ensue and upon reaction once more
with a macromonomer, the unsaturated ω-end group of the macromonomer is again
transferred terminating the growing polymer chain. In essence, the unsaturated ω-end
group of the original macromonomer is transferred from one chain to another as the
macromonomer act as chain transfer agent.
The CT of CCTP-derived macromonomers, however, is low (around 0.04) especially
when compared to the thio-ester ‘RAFT agents’ used in RAFT. However by using low
concentrations of monomer compared to macromonomer, block macromonomers with low
PDIs and a ‘living’ character can be synthesised.50
16
Chapter 1
INTRODUCTION
Scheme 4. Addition fragmentation chain transfer (AFCT) leading to telechelic (n = 1) or block (n >
1) copolymers in the free radical polymerisation of a methacrylic monomer with a methacrylic
macromonomer.
Telechelic Polymers
The use of dimeric macromonomers (or dimers) of methacrylic monomers in AFCT
reactions results in telechelic macromonomers (Figure 2), i.e. the constituent monomer
units of the dimer are placed at either end of the resulting polymer. 49, 51, 52, 56, 57
Figure 2. Telechelic macromonomer.
As mentioned earlier, CCTP of acrylic monomers and styrene is inefficient and
results in an internal double bond. However, when these monomers are copolymerised
with small amounts (< 10%) of an AMS dimer (AMS 2), polymers with predominantly
AMS-based end groups are observed.57
Copolymerisation with Acrylic Monomers/Styrene
Graft Copolymers
Macromonomers copolymerised in the presence of acrylic monomers or styrene
result in graft copolymers, although the mechanism is not as straight-forward as
originally thought.58-60 The initial presumption was that macromonomers were simply
copolymerised with acrylates to form grafts comprising of the macromonomers. Yamada
and co-workers have since discovered that a certain degree of AFCT occurs before
grafting can occur.54, 55 Regardless of whether a methacrylic or an acrylic monomer
attacks a macromonomer, AFCT is the dominant reaction. However, attack by an acrylic
monomer results in a macromonomer which now contains an acrylic monomer unit in the
penultimate position and the copolymerisation behaviour of macromonomers has been
17
INTRODUCTION
Chapter 1
found to be dependent on the nature of the penultimate unit (Table 2). A methacrylic
penultimate unit results in a labile bond between the penultimate unit and the unsaturated
end group leading to β-scission of the bond, and therefore AFCT. A less labile bond is
formed, on the other hand, when an acrylic monomer, such as methyl acrylate (MA) is
situated in the penultimate position of the macromonomer. Thus, on copolymerisation of
macromonomers with MA or S, AFCT initially occurs, resulting in new macromonomers
which contain MA or S units in positions penultimate to the ω-unsaturated end group.
These new macromonomers, which consist of both the original macromonomer as well as
additional MA or S units form the side chains of the graft copolymers (Scheme 5).
Table 2. Summary of the polymerisation behaviour of macromonomers with different comonomers
at 60ºC.55
Xa
Comonomer
AFCT/(AFCT + COPOLY)b
MMA
MMA
>90%
MMA
MA
>90%
MMA
S
>40%
S or acrylate
S or MA
~0%
X is the penultimate unit in the macromonomer; b AFCT = addition-fragmentation chain transfer,
COPOLY = graft copolymerisation.
a
Scheme 5. Proposed general mechanism for the graft copolymerisation of methacrylic
macromonomers with acrylic monomers.
Star Copolymers
Stars copolymer have also been synthesised via the copolymerisation of pMMA
macromonomers with 1,4-butanediol diacrylate.61
18
Chapter 1
INTRODUCTION
Combination of CCTP with Controlled Radical Polymerisation Techniques
Macromonomers with all the advantages of controlled radical polymerisation
techniques, such as a low PDI and well-controlled molecular weights, have been
synthesised by combining CCTP with ATRP62 or RAFT.63 Polymerisations via ATRP or
RAFT were carried out as normal, however after completion of the reaction, a small
amount of COBF (and if necessary 1 equivalent of monomer and a radical initiator) was
added in order to replace the halogen or RAFT agent end groups, and replace them with
the quintessential CCTP ω-unsaturated end group (Scheme 6).
Scheme 6. Synthesis of polymers via RAFT and ATRP followed by the interchange of end groups
for ω-unsaturated ‘macromonomer’ end groups.
POST-POLYMERISATION FUNCTIONALISATION OF MACROMONOMERS
Thiol-ene Chemistry
The most recent trend in macromonomer chemistry is the post-polymerisation
functionalisation of macromonomers via thiol-ene chemistry.64, 65 The ω-unsaturated end
group CCTP-derived macromonomers is, in general, very reactive towards thiol-ene
chemistry. Although most thiol-ene chemistry is carried out via radical routes,66-68
methacrylic macromonomers can undergo a thia-Michael addition catalysed by phosphinebased catalysts.69, 70 This prevents possible radical-induced fragmentation of the
macromonomers. The proposed mechanistic route for this reaction is shown in Scheme 7,
and is considered to be a nucleophile-mediated hydrothiolation over the terminal double
bond.
19
INTRODUCTION
Chapter 1
Scheme 7. Proposed mechanism for the nucleophile-mediated hydrothiolation of an acrylic carboncarbon bond under phosphine catalysis.
Haddleton and coworkers have used thiol-ene chemistry to attach proteins, such as
salmon calcitonin, to macromonomers by first reducing the disulphide bridge of the
peptide followed an in situ thia-Michael addition.71 Further, thermoresponsive
poly(oligo(ethylene glycol) methyl ether methacrylate)s have been synthesised and the ωunsaturated end groups modified via thiol-ene chemistry to tune the lower critical
solution temperature (LCST) of the polymers.72 CCTP has also been used to synthesise
hyper-branched polymers by using di- and trimethacrylates. These networks can then be
decorated with a wide variety of thiols to tailor the properties of the polymers. 73
Hydroformylation
Smeets et al.74 have successfully converted the ω-unsaturated end groups of
macromonomers into pendant aldehyde moieties (Scheme 8). A ruthenium-based catalyst
was used to carry out a hydroformylation reaction in super-critical carbon dioxide. A high
degree of conversion from the terminal double bond to an aldehyde group was achieved
with high chemoselectivity for methyl methacrylate, styrene and styrene/α-methyl
styrene macromonomers. This opens a pathway to further potential post-polymerisation
modification reactions by conversion of the aldehyde end groups into acids, alcohols or
imines.
20
Chapter 1
INTRODUCTION
Scheme 8. Synthesis of methyl methacrylate, styrene and styrene/α-methyl styrene
macromonomers and the subsequent hydroformylation of the unsaturated end groups into aldehyde
moieties.82
It is clear that a wide range of functional macromonomers can be synthesised via
CCTP, which can subsequently be used as building blocks for more complex polymeric
architectures using both radical and non-radical techniques. In this thesis
macromonomers derived from CCTP have been used in conjunction with various radical
and non-radical polymerisation and functionalisation reactions.
OUTLINE OF THESIS
Chapter 2 explores the use of macromonomers as macroinitiators for the anionic
polymerisation of methacrylic monomers. The resultant polymers consist of an atactic
block (derived from CCTP) and an isotactic or syndiotactic block (derived from the
anionic polymerisation).
In Chapter 3, poly(styrene-co-maleic anhydride) (pSMA) macromonomers have
been synthesised via CCT in the presence and absence of α-methyl styrene. The polymers
were fully characterised and the chain transfer constant of COBF under these conditions
has been determined. Diels-Alder post-polymerisation functionalisation reactions were
successfully carried out on the maleic anhydride-terminated macromonomers.
Chapter 4 investigates the post-polymerisation epoxidation of the unsaturated end
group of a macromonomer. Subsequent ring opening polymerisation reactions have been
proven to be challenging due to a competing cyclisation reaction. These macromonomeric
epoxides, however, have been used in the polymerisation of tetrahydrofuran (THF). It was
found that the epoxide end-caps the polymer chains and controls the molecular weight of
the polymer. The epoxide is also shown to catalyse the homopolymerisation of THF. The
epoxides have also been used as coupling agents with (macro)alcohols and amines.
21
INTRODUCTION
Chapter 1
The use of thiols and thiol-ene chemistry is covered in Chapter 5. Thiol-ene
chemistry has been used to modify methacrylic macromonomers towards isocyanate
reactions with diisocyanate terminated polymers. In addition, the thia-Michael addition
reaction between macromonomers and thiol-functionalised polyethylene (PE-SH) was
investigated as a route to poly(ethylene-b-(methacrylate) block copolymers but proved to
be insurmountable due to concentration limitations and an unfavourable temperature
dependence. Poly(ethylene-b-(meth)acrylate) polymers could be synthesised, however, via
a conventional free radical polymerisation in which PE-SH acts as a chain transfer agent.
Graft copolymers have been synthesised via a radical route in Chapter 6.
Macromonomers based on cyclohexyl methacrylate (CHMA) and methyl methacrylate
(MMA) have been copolymerised with 2-dimethylethylamino acrylate (DMAEA). After
variation of the graft length and grafting density, the resultant polymers have been
quaternised with various iodine-containing compounds. Coatings of these polymers on
polycarbonate have been made and their properties studied to determine their tendency
towards anti-static and anti-fogging behaviours.
22
Chapter 1
INTRODUCTION
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25
INTRODUCTION
26
Chapter 1
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS
VIA THE COMBINATION OF CATALYTIC
CHAIN TRANSFER AND ANIONIC
POLYMERISATION
Catalytic Chain Transfer
Polymerisation (CCTP)
Michael Addition
Macromonomer
Macroinitiator
Anionic
Polymerisation
(Stereo) Block Copolymer
This chapter has been published as: G.C. Sanders, T.J.J. Sciarone, H.M.L. LambermontThijs, R. Duchateau, J.P.A. Heuts, Macromolecules, 2012, 44, 9517-9528
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
ABSTRACT
A novel synthetic pathway toward stereo block copolymers by combining
catalytic chain transfer polymerisation with anionic polymerisation is described.
Catalytic chain transfer polymerisation (CCTP) has been used to synthesise vinylterminated polymers, which, after the Michael addition of α-lithioisopropyl
isobutyrate, were used as macroinitiators for the anionic polymerisation of
methacrylate-type monomers. The resultant polymer consists of a predominantly
atactic block (originating from the free radical polymerisation) and a more
isotactoid or syndiotactoid block (originating from the anionic polymerisation).
28
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
INTRODUCTION
Block copolymers have sparked the interest of academic and industrial chemists
alike for many decades because they provide a facile and versatile route to combining the
properties of two or more homopolymers whilst obtaining a controlled architecture. 1-6
The resultant polymers often show improved properties compared to their homopolymer
counterparts, contributing their greatest assets to the block copolymer. This opens the
door to a broad range of applications areas, such as compatablisers, 7-10 self-assembly,11, 12
drug delivery13, 14 and surfactants,15, 16 which can benefit from the combined properties of
the copolymer. Unfortunately, the use of a sole polymerisation technique to synthesise
the block copolymer frequently restricts the monomers that can be used and the
properties that can be obtained. By combining one of more techniques, a vast array of
block copolymers can be synthesised. Recently, living radical polymerisation techniques
(e.g. ATRP, RAFT and NMRP) have been used in conjunction with catalytic olefin
polymerisation1, ring opening polymerisations,2 ring opening metathesis polymerisation3
and extensively with click chemistry.6 Catalytic chain transfer polymerisation (CCTP),17, 18
on the other hand, has rarely been used in combination with alternative polymerisation
mechanisms2, 15, 19-22 and has never been combined with a non-radical technique.
Stereoblock copolymers of methacrylates were first synthesised by Doherty et al.23
where blocks of syndiotactic and isotactic poly(methyl methacrylate) (pMMA) were made
in the same polymer. This method, which involved the sequential anionic polymerisation
of diphenylmethyl methacrylate and trityl methacrylate followed by hydrolysis and
methylation, is however rather tedious. In 1994, a more direct route was developed
involving the use of trialkyl aluminium species to convert the selectivity of an isotactic
pMMA macroinitiator towards syndiotacticity.24 Group IV metallocenes have also been
used in combination with boron or aluminium species to create stereoblock copolymers of
methacrylates.25 By exchanging a site-controlled polymerisation with a chain-endcontrolled synthesis, the selectivity of the growing polymer chain could be effectively
converted from being isotactic-rich to syndiotactic-rich pMMA. Group IV
metallocene/Lewis acid hybrid catalysts have also been employed to synthesise isotacticsyndiotactic block copolymers via a diastereotopic ion-pairing polymerisation method.26
Although literature surrounding isotactic-syndiotactic pMMA block copolymers is
prevalent, we are not aware of any information regarding stereoblock copolymers of
pMMA in the literature, in which one block is atactic. Atactic-isotactic stereoblock
copolymers of polyacrylamides, on the other hand, have been reported. 27-29 The first block,
made via a RAFT technique, is atactic, the selectivity of which switches to isotacticity
upon addition of a Lewis acid. Carpentier and co-workers30 report, what they claim to be,
the first example of a block copolymer of polystyrene with syndiotactic and atactic blocks
by combining Ziegler-Natta polymerisation with ATRP.
The combination of CCTP with other polymerisation mechanisms has, hitherto,
been underexploited as a technique. In this chapter we demonstrate a novel route to
synthesising block copolymers via the combination of catalytic chain transfer
polymerisation and anionic polymerisation, wherein the blocks have differing tacticities
(Scheme 1). The inclusion of an anionically-derived element allows the possibility of
29
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
introducing “non-radical” properties to the block copolymer, such as tacticity and
potentially crystallinity, via either side- or chain-end control producing iso- or
syndiotactic polymers. This is impossible using CCTP alone. Block copolymers with
differing tacticities are of interest in fields such as coatings. 31 The atactic component can
provide good miscibility with the substrate, whilst the isotactic or syndiotactic blocks tend
to form crystalline domains capable of enhancing the scratch and chemical resistance of
the coatings, protecting the substrate. 32, 33
Scheme 1. Synthetic approach to block copolymers. (a) Catalytic chain transfer polymerisation
(CCTP); (b) Michael addition of α-lithioisopropylisobutyrate; (c) Anionic polymerisation.
The anionic polymerisation of methacrylates enables a degree of stereoregularity to
be introduced, which is almost always impossible using radical methods.34-37 The anionic
polymerisation of methacrylate monomers with lithium ester enolate-based initiators, such
as α-lithioisopropylisobutyrate (1, Scheme 1), proceeds via a Michael addition mechanism.
Unfortunately, the use of this type of initiator means that the polymerisation reactions are
prone to two major problems.38 Firstly, lithium ester enolates, such as 1, are known to
have a tendency to cluster, resulting in inhomogeneous initiation and low initiator
efficiency. In addition, due to the mechanism of this polymerisation technique, a carbanion
is formed, which can attack the ester carbonyl functionality in the methacrylate-based
polymer, resulting in the formation of dead cyclic species. Luckily, there is extensive
literature available with a huge selection of preventative methods for both aggregation
and back-biting termination.34, 38 Care must be taken, however, as the choice of additives
can greatly influence the tacticity of the growing polymer chain.
30
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
In this present study we investigate the use of CCTP-derived macromonomers,
modified with 1, as macroinitiators for the anionic polymerisation of methacrylic
monomers. By combining these two different polymerisation mechanisms, stereoblock
copolymers comprising of an atactic block and a syndio/isotactic block were synthesised.
The versatility of this reaction towards macromonomers based on alternative methacrylic
monomers and different chain lengths was also investigated.
31
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
EXPERIMENTAL SECTION
General Considerations. All syntheses and manipulations of air- and moisture-sensitive
materials were carried out in oven-dried Schlenk-type glassware on a dual manifold
Schlenk line, a vacuum line (typically 1 to 100 mbar), or in a nitrogen-filled glovebox
(typically < 1.0 ppm of oxygen and moisture).
Materials. Methyl methacrylate (MMA, 99%) and benzyl methacrylate (BzMA, 96%)
were purchased from Aldrich, and passed over a column of activated basic alumina to
remove the inhibitor. For anionic polymerisations, MMA and BzMA were purified further
by being dried over CaH2 and distilled, followed by titration against neat trioctyl
aluminium and a second distillation. Triisobutylaluminium (TIBA) was purchased from
Aldrich and used as received. Lithium chloride (98%) was purchased from Aldrich and
dried in a vacuum oven at 100 ºC overnight before introduction to the glovebox.
Azobis(isobutryonitrile) (AIBN) was recrystallised twice from methanol. The
bis(methanol) complex of COBF, COBF(MeOH)2 (2) was prepared as described
previously.39, 40 The chain transfer activity of the complex was determined in methyl
methacrylate (MMA) bulk polymerisation at 60 ºC and found to be equal to 30 × 10 3. For
all experiments, a single batch of catalyst was used. Toluene and tetrahydrofuran (THF)
were purchased from Biosolve and used as received for the CCTP polymerisations. For
use in the macroinitiation experiments, the solvents were dried over molecular sieves
prior to use. α-Lithioisopropylisobutyrate, 1, and MeAl(BHT)2 (3, BHT = 2,6-(t-Bu)2-4Me-C6H2O) were synthesised according to literature methods.41, 42
Synthesis of Macromonomers via CCTP. Complex 2 (see Table 1) and AIBN (42 mg,
0.25 mmol) were placed in a flask equipped with a stirrer bar and underwent three
vacuum-argon cycles. BzMA or MMA (50 mL) and toluene (50 mL) were deoxygenated
and added using a syringe. The mixture was heated to 60 ºC and allowed to react for 16
hours, after which it was quenched by cooling in ice and addition of hydroquinone. For
macromonomers with an Mn < 8,000 g/mol, the residual monomer and solvent were
removed via vacuum evaporation immediately after stopping the reaction. The resulting
product was redissolved in THF, passed over a column of basic alumina and dried at 80 ºC
in a vacuum oven for at least 48 hours to remove all traces of solvent and water. For
macromonomers with an Mn > 8,000 g/mol, the product was further diluted with toluene
and precipitated in a large excess of pentane, then dried in a vacuum oven at 80 ºC for at
least 48 hours to remove all traces of solvent and water.
Synthesis of pMMA2. A 250 mL round-bottom flask was charged with AIBN (213 mg, 1.3
mmol) and 2 (100 mg, 0.26 mmol) inside a glovebox. The flask was closed with a rubber
septum and removed from the glovebox. MMA (100 mL, 936 mmol) was injected via the
septum and the mixture was stirred at 80 ºC for 6 hrs under a nitrogen atmosphere. The
oligomerisation was quenched by addition of hydroquinone and cooling in ice. Residual
monomer was removed under reduced pressure at room temperature. The residual yellow
oil was vacuum distilled (325 mTorr, 38 ºC) to afford the pure dimer in 42% yield. 1H
32
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
NMR (400 MHz, chloroform-d1, 298 K) δ 6.12 (s, 1H, =CH2), 5.43 (s, 1H, =CH2), 3.64 (s,
3H, OMe), 3.55 (s, 3H, OMe), 2.52 (s, 2H, CH2), 1.07 (s, 6H, Me2) ppm. 13C{1H} NMR
(100 MHz, chloroform-d1, 298 K) δ 177.3 (C=Oester), 176.8 (C=Oacryl), 137.3 (C=CH2),
127.8 (=CH2), 51.8 (OMe), 51.6 (OMe), 42.8 (CMe2), 41.0 (CH2), 24.8 (CMe2) ppm.
Synthesis of pBzMA2. A 250 mL round-bottom flask was charged with AIBN (100 mg,
0.61 mmol) and 2 (50 mg, 0.126 µmol) inside a glovebox. The flask was closed with a
rubber septum and removed from the glovebox. BzMA (50 mL) was injected via the
septum and the mixture was stirred at 60 ºC for 16 hrs under a nitrogen atmosphere. The
oligomerisation was quenched by addition of hydroquinone and cooling in ice. Residual
monomer was removed under reduced pressure at elevated temperatures. The residual
yellow oil was vacuum distilled using a Kugelrohr set-up (10-3 mTorr, 200 ºC) to afford
the pure dimer in 60% yield. 1H NMR (400 MHz, chloroform-d1, 298 K) δ 7.38 (m, 10H,
Ph), 6.29 (s, 1H, =CH2), 5.53 (s, 1H, =CH2), 5.20 (s, 3H, OCH2), 5.10 (s, 3H, OCH2), 2.74
(s, 2H, CH2), 1.25 (s, 6H, Me2) ppm. 13C{1H} NMR (100 MHz, chloroform-d1, 298 K) δ
176.7 (C=Oester), 167.2 (C=Oacryl), 137.2 (C=CH2), 136.0 (=CH2), 28.5 (Caromatic), 66.5
(OCH2), 43.0 (CH2), 40.9 (CMe2), 24.9 (CMe2) ppm.
Synthesis of Model ‘Macro’initiator. Compound 1 (10 mg, 72 µmol) was dissolved in 0.4
mL C6D6 and the purity checked via 1H NMR. pMMA2 (14 mg, 72 µmol) was added and
allowed to stir for 1 hour. 1H NMR was used to determine that all of the starting enolate
1 was consumed. The reaction mixture was then quenched with 10% HCl solution, and
the product again analysed using 1H NMR. 1H NMR (400 MHz, THF-d8/toluene-d8, 298
K) δ 4.89 (septet, 1H, OCH(Me)2), 3.40 (s, 6H, OMe), 3.39 (s, 6H, OMe), 2.55 (m, 1H, CH),
1.99-2.10 (m, 2H, CH2), 1.50-1.59 (m, 2H, CH2), 1.06-1.12 (m, 18H, 3×Me2) ppm.
Anionic Polymerisation from Model ‘Macro’initiator. Compound 1 (2.5 mg, 18 µmol)
was dissolved in 0.5 mL dry THF, pBzMA2 (12.5 mg, 36 µmol) was then added and the
mixture allowed stirring for 15 minutes. MMA (90 mg, 900 µmol) was then added and
stirred at 20 ºC for 2 hours. The reaction product was quenched with acidified ethanol and
dried overnight in a vacuum oven. Mn = 31,000 g/mol, PDI = 1.7; Conversion = 90 %.
Reaction of 1 with 2 equivalents of pMMA2. Compound 1 (340 mg, 2.5 mmol) was
suspended in toluene (20 mL) then pMMA2 (1.0 g, 5.0 mmol) was added dropwise. The
clear solution was stirred at room temperature for 18 hrs. Toluene was evaporated under
reduced pressure and the residue was taken up in methanol (20 mL). All volatiles were
evaporated under reduced pressure and the resulting foam was taken up in Et 2O (50 mL).
The organic layer was washed with hydrochloric acid (10%, 30 mL), then separated and
the aqueous layer extracted with Et2O (30 mL). The combined organic fractions were
dried over Na2SO4 and volatiles removed under reduced pressure to give an oil. A small
amount of the monoaddition product Me2C(CO2Me)CH2CH(CO2Me)CH2CMe2CO2iPr was
removed of the oil by vacuum distillation (80 °C, 400 mTorr).
33
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
Typical Procedure for the Isotactic Anionic Polymerisation of MMA. In a glove box,
1 (14 mg, 100 μmol) and LiCl (4 mg, 100 μmol) were dissolved in 0.99 g dry toluene.
Outside the glovebox, 0.01 mL THF was added. The vials were cooled to -78 ºC. MMA
(0.80 g, 8 mmol) was then added under an argon atmosphere and the mixture was stirred
for 30 minutes at -78 ºC. The reaction was quenched with acidified ethanol and
precipitated in pentane. The residual solvent was removed using a vacuum oven at 60 ºC
overnight. In some experiments, as indicated in text, LiCl was omitted and only toluene
used as solvent.
Typical Procedure for the Isotactic Anionic Polymerisation of MMA from a BzMA
Macroinitiator. In a glove box, 1 (14 mg, 100 μmol) and LiCl (4 mg, 100 μmol) were
dissolved in 1 g dry toluene. In a separate vial, the macromonomer (100 μmol) was
dissolved in 0.99 g dry toluene. After dissolution, the two crimp cap vials were removed
from the glove box. To the vial containing 1 and LiCl, 0.01 mL THF was added. Both
vials were cooled to the appropriate temperature. The two solutions where then combined
under an argon atmosphere and allowed to mix for 5 minutes. Cold MMA (0.45 mL, 4.2
mmol) was then added and the mixture was stirred for 4 hours at the desired temperature.
The reaction was quenched with water. The residual solvent was removed using a vacuum
oven at 60 ºC overnight.
Typical Procedure for the Syndiotactic Anionic Polymerisation of MMA from a
BzMA Macroinitiator. Complex 3 (45 mg, 93.4 µmol) and macromonomer (47 µmol)
were dissolved in toluene for 45 minutes. A solution of 1 (47 µmol) in toluene was added
and allowed to stir for 60 minutes. MMA (0.22 g, 2.2 mmol) was added and the reaction
mixture was stirred for 1 hour at room temperature. For longer macromonomers, the
polymerisation time was increased to 2 hours and a solution of TIBA (typically 0.5 wt% of
the macromonomer) was added to the macromonomer 2 hours prior to the addition of 3 in
order to remove any trace of water. Reaction mixtures were quenched with acidified
ethanol and the solvent removed under vacuum. Polymers with an Mn > 8,000 g/mol were
precipitated twice from pentane.
Syndiotactic Anionic Polymerisation of pBzMA. Complex 3 (12 mg, 24 μmol) and a
solution of 1 in toluene (1.6 mg, 12 μmol) were dissolved in 1.5 g toluene to cleanly
generate an initiator/catalyst pair. BzMA (0.21 g, 1.2 mmol) was then added and the
mixture stirred for 2 hours at 20 ºC. The reaction mixture was quenched with acidified
ethanol and precipitated in pentane. 81% conversion; Mn = 15,200 g/mol, PDI = 1.5
(determined against PS standards in THF using SEC).
Syndiotactic Anionic Polymerisation of pMMA. Complex 3 (12 mg, 24 μmol) and a
solution of 1 in toluene (1.6 mg, 12 μmol) were dissolved in 1.5 g toluene to cleanly
generate an initiator/catalyst pair. MMA (0.12 g, 1.2 mmol) was then added and the
mixture stirred for 2 hours at 20 ºC. The reaction mixture was quenched with acidified
34
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
ethanol and precipitated in pentane. 98% conversion; Mn = 9,900 g/mol, PDI = 1.4
(determined against PS standards in THF using SEC).
Measurements. Gel permeation chromatography was carried out using a WATERS 2695
separations module, Model 2487 UV detector (254 nm), and Model 2414 differential
refractive index detector (40 ºC). Injection volume used was 50 µL. Tetrahydrofuran
(Biosolve, stabilised with BHT) was the eluent, flow rate 1.0 ml/min. The column set used
was a PLgel guard (5 m particles) 50 × 7.5 mm precolumn, followed by 2 PLgel columns
in series of 500 Å (5 m particles) and 100 Å (5 m particles), respectively. Calibration
was performed using polystyrene standards (Polymer Laboratories, Mn = 370 up to Mn =
40,000 g/mol). Data acquisition and processing were performed using WATERS
Empower 2 software. 1H NMR spectra were recorded on a Varian Mercury Vx (400 MHz)
spectrometer at 400 MHz chloroform-d1, THF-d8, benzene-d6, toluene-d8 and
tetramethylsilane were used as solvent and internal standard, respectively. MALDI-ToFMS was carried out using a voyager DE-STR spectrometer from Applied Biosystems in
reflector mode. Trans-2-(3-(4-tert-butylphenyl)-methyl-2-propenylidene)-malononitrile
doped with potassium trifluoroacetate was used as the matrix. It was deposited from THF
solution onto a stainless steel sample substrate and the solvent allowed to evaporate. The
polymer was then deposited as a dilute (~1 mg/ml) solution in THF. This resulted in each
polymeric species being observed as its K+ adduct with molecular mass M+31. The
spectrometer was calibrated using poly(ethylene oxide) standards for the lower mass
range and polystyrene standards for the higher mass range. Gradient polymer elution
chromatography (GPEC) was carried out using a Zorbax Eclipse XDB-C8 column, 4.6 x
150 mm, 5 μm on an Agilent 1100 series setup. Initially, the eluent was a 4:1 mix of
methanol/THF for 2 minutes, then gradually changed to pure THF over 25 minutes. The
flow rate was 1 mL/min. The temperature of the column was 50 ºC. Detection was carried
out using a Polymer Laboratories evaporative light scattering detector (ELSD).
Differential scanning calorimetry (DSC) was performed on a TA Q100 DSC.
Approximately 5 mg of dried polymer was weighed in aluminium hermetic pans.
Temperature profiles from 0 to 200 ºC with a heating and cooling rate of 1, 10 and 20
ºC/min were applied. TA Universal Analysis software was used for data acquisition. Glass
transition temperatures were determined from the second heating run.
35
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
RESULTS AND DISCUSSION
Model studies, based on dimers prepared by CCTP, were carried out to establish
the reactivity of these species towards 1, as well as to probe their ability as initiators to
polymerise methacrylic monomers anionically. Once established, macromonomers were
used as macroinitiators. BzMA macromonomers were initially selected as the starting
blocks for the block copolymer due to its UV activity; pBzMA is UV active at 254 nm,
whereas MMA shows no absorption at this wavelength, allowing the simple detection of
pBzMA-containing (block) copolymers via SEC with UV detection.
Synthesis of Macromonomers
Poly(benzyl methacrylate) (pBzMA) and poly(methyl methacrylate) (pMMA)
macromonomers were synthesised by CCTP in toluene at 60 ºC using AIBN as the
initiator. The final properties of these macromonomers are collected in Table 1. The
macromonomers were analysed using 1H NMR to confirm that all had a terminal vinylic
group. For the lower molecular weight macromonomers, this was also confirmed using
MALDI-ToF-MS. The polydisperisty indices of polymers made via CCTP are consistent
with that of chain transfer dominated polymerisation reactions, and are usually around 2.
This can, however, be significantly reduced when CCTP is used in combination with
ATRP43 or RAFT44 but is not applied in this work.
Table 1. The properties of the synthesised pBzMA and pMMA.
SECc
a
b
-1
2 (ppm)
x
Mn/g mol
PDI
pBzMA2
392
pBzMA6
94
0.88
1,150
1.6
pBzMA76
5.5
0.78
13,500
2.2
pMMA2
191
4.3
0.68
1,700
1.6
pMMA15
pMMA40
3.3
0.40
3,800
1.8
pMMA120
2.1
0.84
13,000
2.6
H NMRd
Mn/g mol-1
352
1,050
14,000
200
1,500
4,000
12,000
1
The amount of 2 is defined as moles of 2 per 106 moles of monomer; b Monomer conversions
determined gravimetrically; c Values reported against polystyrene standards; d Calculated based on
the ratio of vinylic protons (5.4 and 6.2 ppm) to ester protons (BzMA: 5.0 ppm, MMA: 3.6 ppm).
a
Synthesis of Macroinitiator (Model Studies)
To investigate the conversion of the macromonomer into a macroinitiator, model
studies were carried out. As a model for the macromonomer, an MMA dimer, pMMA2,
was synthesised using CCTP, followed by purification via vacuum distillation. Equimolar
quantities of the stable enolate Me2C=C(OiPr)OLi (1) and pMMA2 were reacted in
benzene-d6 (Scheme 2).
36
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Scheme 2. Reaction scheme for the synthesis of a macroinitiator and its hydrolysis.
The 1H NMR spectrum of the initially formed product enolate features one broad
signal for all methylene protons, while all other groups give their individual resonances
with resolved coupling patterns. Hydrolysis using 10% aqueous HCl converts the
spectrum into that of the product ester, displaying diastereotopic methylene protons as
well as diastereotopic CMe2 groups and a quintet at 2.55 ppm which corresponds to the
newly formed proton adjacent to the ester functionality, confirming the formation of a
“macro”initiator (Figure 1).
b
f
a
5.0
c
4.5
4.0
3.5
3.0
2.5
d
e
2.0
1.5
1.0
ppm
Figure 1.
and 1.
1H
NMR (400 MHz, THF-d8/toluene-d8) of the “macro”initiator derived from pMMA 2
In a separate experiment to confirm that the resulting coupling product of the
methacrylic dimer with 1 is indeed capable of inducing initiation, the UV-active BzMA
dimer, pBzMA2, was reacted with lithium ester enolate, 1, followed by the addition of
MMA. Polymerisation ensued and the resultant product analysed using SEC equipped
with dual UV/DRI detectors. The molecular weight distribution (Figure 2) shows that a
polymeric material has been synthesised with an Mn of 31,000 g/mol and that the UV and
DRI signals overlay. Since pBzMA2 is UV-active at 254 nm, whereas MMA is inactive,
this result is indicative that pBzMA2 has indeed acted as a macroinitiator and has been
incorporated into the polymer.
37
Chapter 2
w(log M)
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
3.0
3.5
4.0
4.5
5.0
5.5
6.0
log M
Figure 2. Molecular weight distribution (relative to PS, measured in THF) illustrating that
pBzMA2 is capable of initiating the anionic polymerisation of MMA (- - -) DRI signal, (_____) UV
signal.
The addition of two equivalents of pMMA2 to 1, however, resulted in a back-bitten
species (6, Scheme 3). The presence of this species was confirmed via MALDI-ToF-MS
(Figure 3). Back-biting of 4 does not occur as this would result in the formation of an
unstable 4-membered ring. The observed predominance of back-biting in this reaction,
which is in fact a model reaction for propagation of pMMA 2, precludes the lithium enolate
initiated anionic (homo) polymerisation of CCTP-derived macromonomers.
510
520
530
540
550
560
570
m/z
Figure 3. MALDI-ToF-MS of the cyclic back-bitten product of two pMMA2 with 1. Transesterification of the ester groups is also observed.
38
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Scheme 3. Mechanistic reaction scheme of the back-biting reaction of macrointiators.
We can, nevertheless, conclude that (the models for) macromonomers can act as
macroinitiators for the anionic polymerisation of MMA, allowing the synthesis of block
copolymers.
Block Copolymer Synthesis
Atactic-Isotactic Block Copolymers
One of the most common methods to synthesise isotactic pMMA from lithium
ester enolates, such as 1, is by carrying out the polymerisation in toluene. 38 Under these
conditions and at room temperature, isotactic pMMA with an [mm] = 74% can be
synthesised.45 However, as illustrated by the model reactions, the polymerisation of MMA
from a macroinitiator suffers significantly from back-biting reactions, more so than the
simple polymerisation of pMMA from 1; in other words, the (macro)initiator efficiency is
lower than that of 1. The addition of 1% THF and the lowering of the temperature to -78
ºC assists in minimising backbiting termination reactions, 38 although the addition of THF
also affects the tacticity, decreasing the degree of isotacticity (Figure 4). Further addition
of THF (i.e. >1%) results in a significant loss in stereocontrol, 46 justifying the choice of a
99:1 ratio of toluene to THF as a suitable solvent mixture to perform the polymerisations.
39
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
[mm]
Chapter 2
[rr]
[mr]
(a)
(b)
(c)
1.3
1.2
1.1
1.0
0.9
0.8
0.7
ppm
Figure 4. 1H NMR (chloroform-d1, 400 MHz) of pMMA made anionically from 1 in (a) toluene at
20 ºC, [mm] = 74%; (b) toluene/THF (99:1) at -78 ºC, [mm] = 65%; and (c) toluene/THF (99:1) at
-78ºC in the presence of LiCl, [mm] = 61%.
Table 2. Conditions and results for the anionic polymerisation
macroinitiator. pBzMA6 / 1 / MMA = 1:1:10.
Temperature/ºC
Eq. LiCl a
Mnb/g mol-1
1
20
0
2,100
2
0
0
2,400
3
-78
0
1,920
4
20
1
2,020
5
0
1
2,500
6
-78
1
1,800
of MMA using pBzMA6 as the
PDIb
15d
33e
1.9
1.7
1.9
1.6
MMA Conversionc
0.94
1.00
0.26
0.82
1.00
0.50
Molar equivelents of LiCl with respect to 1; b Determined using SEC, measured against PS
standards in THF using DRI detector; c Conversion of monomer determined gravimetrically after 4
hours reaction time; d Bimodal Mp = 2,600 and 83,900 g mol-1; e Bimodal Mp = 2,700 and 122,500 g
mol-1.
a
The in-situ formation of a macroinitiator was carried out by reaction of
pBzMA6 macromonomer and 1, followed by the addition of MMA at a range of
40
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
temperatures and in the absence and presence of LiCl (Table 2). Temperature choice is
paramount in anionic polymerisations, especially when there is an opportunity for
backbiting, as well as other side reactions, to occur. In general, anionic
polymerisations of this type are carried out at -78 ºC.38 The reactions shown in Table
2 also show a strong dependence on temperature. As can be seen in entries 1 -3 and 46, decreasing the temperature decreases the PDI of the resulting polymer. In
comparing entries 1 and 3, this difference is most obvious as the molecular weight
distribution of resultant polymer transforms from bimodal to monomodal. This
suggests a significant reduction in the amount of side reactions that have occurred.
The use of lower temperatures also suppresses the rate of polymerisation, decrea sing
the degree of conversion obtainable after 4 hours.
The use of salts, such as lithium chloride, has been reported to assist in the
deaggregation of lithium ester enolate clusters, allowing homogeneous initiation to
occur.47, 48 From Table 2 it is evident that the presence of lithium chloride does have a
profound effect on the polydispersity of the polymers synthesised. At all temperatures,
a decrease in PDI is observed when the reaction is carried out in the presence of
lithium chloride. The most noticeable being the decrease from a bimodal distribution
with PDI of >15 to a monomodal distribution with PDI of <1.9 when the reaction is
carried out in the presence of LiCl at 0 ºC or even at room temperature. Although the
PDI of the polymer decreases when the temperature is decreased from 0 ºC to -78 ºC
when lithium chloride is used, the difference is not great. It should also be noted here
that the observed PDIs are larger than one would normally expect from an anionic
polymerisation, but one must recall that the starting macroinitiator already has a PDI
 1.6. In addition, the rate of reaction is compromised. Lithium chlor ide does not have
a pronounced effect on the tacticity of the polymer formed (Figure 4).
A more in depth analysis was carried out on the polymer formed in entry 5 (Table
2). A UV detector was used in tandem with a DRI detector for SEC measurements. The
DRI signal produced using SEC for the polymer made in entry 5 clearly shows a shift
towards a higher molecular weight after the reaction has taken place (Figure 5). A shift in
the UV signal is also observed, although the UV and DRI chromatograms of the block
copolymer do not overlap perfectly. This illustrates that while the UV-active
macromonomer does form part of the block copolymer, it may not have been distributed
evenly, suggesting that some pMMA homopolymer has also been produced; pMMA can
be formed as a result of initiation from unreacted 1. The perfect overlap of DRI and UV
signals observed in Figure 2 is not observed here in Figure 5 as a balance exists between
the conversion of macromonomer (n>1) to macroinitiator and backbiting, which does not
exist for the dimers (n=1), as detailed in Scheme 3.
41
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
pBzMA6
p(BzMA6-b-MMAx)
UV
w(log M)
DRI
2.5
DRI
3.0
3.5
4.0
4.5
logM
Figure 5. Molecular weight distribution (relative to PS, measured in THF) of pBzMA6 and
p(BzMA6-b-MMAx) (entry 5, Table 2) normalised to the amount of polymer.
Gradient polymer elution chromatography (GPEC) was carried out to further
confirm the presence of a block copolymer (Figure 6). By using an analytical technique
which separates based on chemical composition as opposed to hydrodynamic volume, it
was possible to distinguish between the two homopolymers (pBzMA 6 and pMMA) and the
block copolymer p(BzMA6-b-MMAx). The observed oligomeric distribution of the
macromonomer has been reported before in literature.49 It is obvious that the resultant
product contains both unreacted pBzMA6 macromonomer and pMMA homopolymer (1328 minutes and 37-38 minutes, respectively), but a broad third peak corresponding to a
copolymer can also be seen, spanning between the two homopolymers. The broadness is
as expected because the polymer consists of a variety of block sizes originating mainly
from the macroinitiator pBzMA6 and also somewhat from the second block.
42
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
(a)
(b)
(c)
Block Copolymer
10
15
20
25
30
35
40
45
Elution Time/Minutes
Figure 6. Gradient polymer elution chromatogram showing (a) pBzMA6; (b) pMMA and (c) the
presence of a p(BzMA6-b-MMAx) (entry 5, Table 2) as well as some residual macromonomer and
pMMA.
The existence of a copolymer was further confirmed using MALDI-ToF-MS
(Figure 7 (a)) in which multiple distributions were observed. As depicted in the spectrum,
a distribution with a repeat unit of 176 Da, corresponding to pBzMA homopolymer is
seen. The end group of this pBzMA has been identified as the result of backbiting (i.e.
after reaction of the macromonomer with 1, the chain end has bitten back onto itself to
form a six-membered ring, as shown in Scheme 3, preventing the initiation of MMA
polymerisation from this polymer chain). Although MALDI-ToF-MS is not quantitative,
minimal unreacted macromonomer is observed in the spectrum, indicating that the first
step of the reaction with the lithium enolate gives almost 100% conversion. A distribution
with a repeat unit of 100 Da is also seen establishing the presence of pMMA, but the end
group could not as yet be determined. Enolates are known to have limited thermal
stability and thus decompose to a wide variety of species. It is assumed that one of these
species is responsible for initiating the polymerisation of MMA, or that this
decomposition reaction has occurred after completion of the reaction. Underlying these
two distributions, which show a much higher intensity, is a third distribution with a
difference of 24 Da between the peaks. This is consistent with the difference between two
43
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
MMA units and one BzMA unit, verifying the presence of a copolymer of MMA and
BzMA. Again, the end group could not be elucidated but it appears to be different to the
end group of pMMA.
(a)
2
2
2
2
2
1a
1a
1a
3
3
3
3
3
2
3
3
3
3
3
3
3
2
(b)4000
42002
4100
4300
3
3
3
4100
3
3
3
4400
3
3
4200
3
3
4300
4500
2
2
m/z
4000
3
3
3
3
3
4400
3
3
3
4500
m/z
Figure 7. MALDI-ToF-MS spectrum of p(BzMA6-b-MMAx) (entry 5, Table 2) (a) unfractionated
and (b) fractionated; showing multiple distributions, where 1a = pBzMA; 2 = pMMA; 3 =
p(BzMA6-b-MMAx).
To unequivocally prove the existence of the block copolymer, isolation of the
copolymer was attempted using GPEC fractionation. Several fractions were taken
between 25 and 35 minutes. This procedure was repeated 15 times in order to obtain
sufficient material to acquire a MALDI-ToF-MS spectrum; the fractions were run a
second time to check the purity. In each case, some residual homopolymer remained in
the spectra. Fortunately, one fraction eliminated all traces of pBzMA giving a much
clearer spectrum containing only pMMA and p(BzMA 6-b-MMAx) (Figure 8). The
MALDI-ToF-MS spectrum of the fractionated species shows two distributions
(Figure 7 (b)). One corresponds to pMMA and the other to p(BzMA-b-MMA), the end
groups of which are consistent with those in the unfractionated spectrum (Figure 7
44
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
(a)). Note that the intensities observed in MALDI-ToF-MS are not representive of
the actual amounts of polymer present as the measurement is not quantitative.
Block Copolymer
pMMA
10
15
20
25
30
35
40
45
Elution Time/Minutes
Figure 8. GPEC chromatogram showing the results of fractionation of p(BzMA 6-b-MMAx)
(entry 5, Table 2).
Free-radical polymerisations are generally highly random and as such produce
atactic chains. However, from literature 45 and model studies we know that 1 is capable
of producing isotactic pMMA, characterised by a high degree of [mm] triads. Under
the conditions used here, pMMA with an isotactoidal tendency is formed, although
the degree of [mm] triads is lower than if only toluene is used as the solvent (F igure
4). 1H NMR was used to determine the extent of the tacticity of p(BzMA 6-b-MMAx)
(entry 5, Table 2) as shown in Figure 9. The CH 3 protons of the pMMA, between 0.6
and 1.2 ppm can be used to indicate the tacticity of the pMMA component of the
polymer. At 1.2 ppm a quinessential [mm] triad can be seen, however both [mr] and
[rr] triads, at 1.0 and 0.8 ppm, respectively, are also present. This suggests that the
polymer is partially isotactic in nature, although there is an element of atacticity
derived from the macromonomer (acting either as the first block or as unreacted
material). It was also difficult to determine the exact degree o f isotacticity due to the
underlying atacticity of the macromonomer.
45
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
(a)
(b)
[mr]
[rr]
[mm]
8
7
6
5
4
3
2
1
0
ppm
Figure 9. 1H NMR (400 MHz, CDCl3) of (a) pBzMA6 and (b) block copolymer p(BzMA6-b-MMAx)
(entry 5, Table 2).
It is clear from the evidence presented above that block copolymers of atactic
benzyl methacrylate and isotactic-rich methyl methacrylate can be made via the
combination of CCTP and anionic polymerisation. However, this route proved to be
rather inefficient due to backbiting termination of the macroinitiator as well as the
formation of pMMA from unreacted 1.
Atactic-Syndiotactic Block Copolymers.
Aluminium compounds have been used previously to control the tacticity of
methacrylic monomers during anionic polymerisations.45, 50 Rodriguez-Delgado et al.45
reported the use of MeAl(BHT)2 (3) in combination with 1 as an effective route to
preventing both clustering of lithium ester enolates and back-biting termination.
Furthermore, the polymerisation of MMA under these conditions can be carried out in
toluene at room temperature to produce syndiotactic pMMA (71.3% [rr], 24.7% [mr],
4.0% [mm]). Further reduction in the temperature to 0 and -40 ºC increases the
stereoselectivity towards higher syndiotacticity, but only by around 8%.
46
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Table 3. Conditions and results for the anionic polymerisation of MMA or BzMA using a
methacrylic macromonomer as the macroinitiator in the presence of 3.
Block Copolymera
Theoretical
Final
Mnb/gmol-1
Mnc/gmol-1
PDIc
Block 1:
Block 2d
Monomer
Conversione
1
p(BzMA6-b-MMA47)
4,500
5,300
2.6
6:30
0.75
2
p(BzMA6-b-MMA95)
10,600
17,400
1.6
6:142
0.94
3
p(BzMA76-b-MMA77)
22,000
19,500
1.9
76:114
0.73
4
p(BzMA76-b-BzMA18)
18,000
16,200
2.2
-
0.24f
5
p(MMA15-b-BzMA43)
9,500
6,000
2.2
15:29
0.97
6
p(MMA40-b-BzMA29)
9,900
10,100
1.7
40:12
0.57
7
p(MMA40-b-MMA50)
8,800
8,000
1.7
-
1.00
8
p(MMA120-b-BzMA26)
22,000
17,600
1.7
120:31
0.50
9
p(MMA120-b-MMA110)
21,300
24,000
1.5
-
0.83
Ratio of monomers determined via SEC; b Corrected for conversion; c Determined using SEC,
measured against PS standards in THF; d Determined using 1H NMR based on CO2CH2Ph (BzMA,
4.8 ppm) and CO2CH3 (MMA, 3.6 ppm) integrals; e Determined gravimetrically after 2 hours
reaction time; f 4.5 hours reaction time
a
To first determine whether this route is feasible, a short BzMA-based
macromonomer (pBzMA6) was used as a macroinitiator. After mixing with 2 equivalents
of 3, 1 equivalent of 1 was added to form a macroinitiator in-situ, followed by 47 or 100
equivalents of MMA (entries 1 and 2 in Table 3, respectively). Figure 10 shows the
molecular weight distributions of the starting macromonomer and the final polymer
synthesised in entry 2 (Table 3). A clear increase in the molecular weight from the
macromonomer to the block copolymer can be observed from 1,150 g/mol to 24,000
g/mol. In addition, the signals recorded from the UV and DRI detectors of the SEC
instrument correlate well with each other, indicating the inclusion of the pBzMA 6
macromonomer in the final polymer. A small amount of unreacted macromonomer
remains, which is more obvious in the UV trace of the product mixture.
47
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
DRI
w(log M)
Block Copolymer
UV
Macromonomer
2.5
3.0
3.5
4.0
4.5
5.0
log M
Figure 10. Molecular weight distribution of p(BzMA6-b-MMA95) normalised based on amount of
polymer.
The molecular weight of the block copolymer obtained in entry 2 is too high for
analysis via MALDI-ToF-MS, however the block copolymer made in entry 1 (Table 3)
has a molecular weight that can be easily analysed using this technique (Figure 11). Three
distinct distributions can be seen. Distribution 3 indicates that a copolymer has been
synthesised; the difference between the peaks is 24 Da (consistent with the difference
between two MMA units and one BzMA unit) and an end group corresponding to two
protons and a connecting group originating from the lithium ester enolate (see overall
scheme, Scheme 1). The most intense distribution (1) corresponds to unreacted
macromonomer, but quantitative conclusions are difficult to draw from the intensity of the
peaks due to the intrinsic nature of the MALDI-ToF-MS technique and the fact that at
lower mass the block copolymer is less prevalent. In addition, a third distribution is seen,
1b, which corresponds to the hydrolysed macroinitiator. This suggests that 3 is highly
efficient at preventing backbiting as the back-bitten product is not observed in this spectra
(contrary to the case where 1% THF in toluene is used to prevent back-biting, vide supra).
The most important result, however, is that no pMMA is observed, which considering the
high propensity and ease of pMMA to ‘fly’ in MALDI-ToF-MS strongly suggests that
reaction of 1 with the macromonomer is more efficient in the presence of 3.
48
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
1
1
1
1b
3
1800
3
1850
3
3
1900
3
1b
3
3
1950
2000
3
3
2050
3
3
2100
3
2150
m/z
Figure 11. MALDI-ToF-MS of p(BzMA6-b-MMA47) where 1 = unreacted pBzMA6, 1b =
hydrolysed pBzMA macroinitiator, 3 = p(BzMA-b-MMA)
Based on the SEC and MALDI-ToF-MS data presented above, it can be concluded
that the use of 3 in the polymerisation of MMA from a pBzMA macromonomer is a
successful route to synthesising (stereo) block copolymers.
The limits of the polymerisation technique have also been investigated by using
longer macromonomers as macroinitiators. In entries 3 and 4 (Table 3), a benzyl
methacrylate macromonomer with an Mn = 13,500 g/mol was used to prepare the
macroinitiator for the polymerisation of both MMA and BzMA. Note that here the use of
a 1:1:2 molar ratio of macromonomer/1/3 (as per entries 1,2 and 5, Table 3) resulted in
the incomplete conversion of the macromonomer into the macroinitiator (as evidenced by
the residual vinylic protons attributed to the macromonomer seen by 1H NMR). It was
thought that this may arise from residual water trapped within the macromonomer due to
the increased viscosity of higher molecular weight macromonomers. Indeed, the addition
of a small amount of TIBA (typically 0.5 wt% compared to the macromonomer) to the
macromonomer solution prior to the reactions alleviated this issue to a certain extent as
TIBA acts as a scavenger for water. Although 3 can also act as a scavenger, the reactivity
is much lower than that of TIBA. Further, TIBA is known to give no adverse effect in
terms of the polymerisation mechanism or tacticity.45 In both cases, where MMA and
BzMA were used as monomers for the second block, an increase in molecular weight is
observed, accompanied by a slight decrease in polydispersity index.
To investigate the versatility of this reaction towards macromonomers based on
different methacrylic monomers, a short MMA-based macromonomer (pMMA15), which
has a similar molecular weight to pBzMA6 of around 1,000 g/mol, was used as the
macroinitiator. A second block of BzMA was polymerised anionically from pMMA 15 as
shown in entry 5 (Table 3). Again an increase in molecular weight was seen (from 1,700
g/mol to 6,000 g/mol, Figure 12) and the DRI and UV detector signals of the block
copolymer overlap well. MALDI-ToF-MS (Figure 13) also confirms the presence of a
49
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
block copolymer. Longer pMMA macromonomers (pMMA40 and pMMA120) can also be
used to produce macroinitiators for both MMA and BzMA polymerisation (entries 6-9,
Table 3).
pMMA15
w(log M)
p(MMA15-b-BzMA43)
UV
DRI
2.5
3.0
3.5
4.0
4.5
5.0
log M
Figure 12. Molecular weight distribution of pMMA15 and p(MMA15-b-BzMA43), normalised based
on amount of polymer.
2
2
2
3
3
3
3
3
1650
1700
1750
3
3
3
1800
3
1850
m/z
Figure 13. MALDI-ToF-MS of p(MMA15-b-BzMA43) where 2 = unreacted pMMA15 and 3 =
p(MMA-b-BzMA).
50
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
The use of 3 in the anionic polymerisation of methacrylates using CCTP-derived
macromonomers as macroinitiators proves to be a highly effective route to synthesising
block copolymers, and has proven its versatility towards different methacrylic monomers
as well as varying chain lengths.
As reported by Rodriguez-Delgado et al.,45 the use of 3 in toluene for the
polymerisation of methacrylic monomers results in syndiotactic polymers. The tacticity of
syndiotactic methacrylic polymers is characterised by an [rr] triad, which can be observed
easily in 1H NMR. At room temperature, pMMA with a tacticity of 71.3% [rr], 24.7%
[mr] and 4.0% [mm] was obtained.45 The polymerisation of BzMA under these same
conditions has not been investigated in the aforementioned article. However for
completeness, pBzMA has been synthesised under the same conditions for the purposes of
this article and using 1H NMR the tacticity determined to be 77.5% [rr], 20.9% [mr] and
1.6% [mm]. Compared to the tacticity of pBzMA made via CCTP (60.6% [rr], 34.5%
[mr] and 4.8% [mm], Figure 14), it is clear that a more syndiotactic pBzMA can be made
under the anionic polymerisation conditions described by Rodriguez-Delgado et al.
[rr]
(a)
[mr]
[mm]
[rr]
(b)
[mr]
[mm]
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
ppm
Figure 14. 1H NMR (400 MHz, CDCl3) of (a) atactic pBzMA macromonomer made via CCTP and
(b) syndiotactic pBzMA made via anionic polymerisation.
Figure 15 shows both the 1H NMR spectra of p(a-BzMA)6 (a) and p(a-BzMA)6-b-(sMMA)95 (b). The block copolymer made in entry 2 (Table 3) was chosen due to its high
degree of polymerisation of MMA compared to BzMA. In this way, the tacticity of the
pMMA block made via anionic polymerisation can be probed more accurately, with
minimal interference of the atactic pBzMA macromonomer. As anticipated, pBzMA 6 is
clearly atactic demonstrated by the methyl protons around 1 ppm (e) in Figure 15 (a). On
the other hand the methyl protons around 1 ppm of p(a-BzMA)6-b-(s-MMA)95 (Figure 15
(b)) show a high concentration of racemic [rr] triads which is indicative of a highly
51
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
syndiotactic material (71% [rr], 27% [mr] and 2% [mm]), which is comparable to the
values obtained for the homopolymer of pMMA under these conditions. 45 The
determination of the tacticity of the remaining polymers synthesised in Table 3 was
investigated using 13C NMR. This was necessary particularly for the BzMA-MMA
copolymers, where the characteristic triads for pMMA and pBzMA were indistinguishable
due to overlapping peaks. In fact, only the C=O region (175 – 179 ppm) in 13C NMR
allowed the adequate separation of the peaks and thus accurate determination of the
tacticity of the copolymers. For the longer macroinitiators, the syndiotacticity calculated
for the second block, regardless of the nature of the starting macromonomer or the
monomer added for the second block, was 79-81% (Figure 16).51 Although the starting
macromonomers synthesised via CCTP (Table 1) generally have a slight tendency
towards syndiotacticity ([rr] = 60-65 %),52 the second blocks show an increase in
syndiotactoid character. The lower molecular weight macroinitiators show a slightly
broader degree of syndiotacticity from [rr] = 74% to [rr] = 82% (Figure 17).53 It is
assumed that even higher syndiotacticities can be achieved when polymerising at lower
temperatures,45 however this is beyond the scope of this chapter.
a
6
e
d
c
b
b
f
e+e'
[rr]
6
95
d+d'
[mr]
a
8
[mm]
c
7
6
5
4
3
2
1
0
ppm
Figure 15. 1H NMR (400 MHz, chloroform-d6) of (a) p(BzMA)6 macromonomer and (b) p(aBzMA)6b-(sMMA)95.
52
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
(a)
(a)
[rr] = 64%
[rr] = 64%
[mr] = 34%
[mr] = 23%
[mm] = 2%
[mm] = 2%
180
179
178
177
176
175
174
180
179
(b)
(b)
178
177
176
175
174
[rr] = 82% (+ [mr]pMMA)
[rr] = 79%
[mr] = 21% (+ [rr]pBzMA)
[mr]pBzMA
180
179
178
177
176
175
[mr] = 18%
[rr]pMMA
[mm]  0%
180
174
ppm
179
[mm]  0%
178
177
176
175
174
ppm
Figure 17. 13C NMR (100 MHz, chloroformd6) showing the tacticity of (a) pBzMA76 and
(b) p(BzMA76-b-MMA77).
Figure 16. 13C NMR (100 MHz, chloroformd6) showing the tacticity of (a) pMMA15 and
(b) p(MMA15-b-BzMA43).
Further examination of the tacticity of the two blocks was attempted using DSC.
Unfortunately, only a marginal difference in Tg between the syndiotactic and atactic
homopolymers was found, spanning the range in which an accurate Tg can be determined
(Figure 17). The relatively short chain lengths as well as the respective degrees of syndioand atacticity are thought to play a role. Applying the Fox-Flory equation, a Tg of 103 ºC
and 117 ºC for atactic and syndiotactic pMMA was found for the homopolymers made via
CCTP and anionic polymerisation, respectively (with similar molecular weights to that of
the block copolymers). In addition, a Tm could not be identified even using a range of
heating rates (1 ºC/minute – 20 ºC/minute), due to the difficulties in achieving thermal
crystallisation of syndiotactic pMMA.54, 55 Regardless, given that the homopolymers of
pMMA and pBzMA made under these anionic conditions give rise to syndiotactic
polymers and the NMR evidence presented in Figures 15-17, it is feasible to assume that
the second blocks made via this route do indeed consist of methacrylic polymers with a
syndiotactoid character.
53
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
Chapter 2
Figure 18. Comparison of DSC of pMMA made anionically ([rr] = 71%) and pMMA made via
CCTP ([rr] = 63%) at 10 ºC/min heating rate. The molecular weights of the polymers are
comparable, Mn ~ 12,000 g/mol.
CONCLUSION
The results of this work show a proof of principle that (stereo) block copolymers
can be synthesised via a combination of catalytic chain transfer polymerisation (CCTP)
and anionic polymerisation techniques. This provides a novel route to synthesising block
copolymers containing blocks of different tacticities. CCTP has been used to synthesise an
atactic block; the second block was then made via anionic polymerisation. The anionic
polymerisation was carried out either in the presence of lithium chloride and a
toluene/THF solvent mixture to give an isotactoid block, or in the presence of
MeAl(BHT)2 (3) to give rise to a second block syndiotactoid in nature. Although block
copolymers could be made using both methods, the use of MeAl(BHT) 2 was much more
efficient, particularly with respect to the conversion of the macromonomer into a
macroinitiator as no evidence of homopolymerisation of the second monomer was
observed. The method also proved to be versatile towards macromonomers of different
methacrylates and chain lengths. Although the synthetic procedures described in this
paper require further optimisation and/or purification steps (depending on the monomers
and method used), we believe this to be the first example of atactic-syndiotactic and
atactic-isotactic block copolymers.
54
Chapter 2
METHACRYLIC STEREOBLOCK COPOLYMERS VIA THE COMBINATION
OF CATALYTIC CHAIN TRANSFER AND ANIONIC POLYMERISATION
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Chromatographia, 2002, 55, 533-540.
H. Schlaad and A. H. E. Müller, Macromol. Symp., 1996, 107, 163-176.
Second block tacticities, for example, p(MMA120-b-BzMA26) = 81:19:0;
p(MMA120-b-MMA110 = 80:19:1; p(BzMA76-b-MMA77) = 79:21:0;
p(BzMA76¬-b-BzMA18) = 81:19:0 ([rr]:[mr]:[mm])
Macromonomer tacticities, for example, pMMA15 = 64:34:2; pMMA120 =
60:39:1; pBzMA6 = 25:40:34; pBzMA76 = 65:33:2 ([rr]:[mr]:[mm])
Second block tacticities, for example p(BzMA6-b-MMA95) = 74:21:1;
p(MMA15-b-BzMA43) = 82:18:0 ([rr]:[mr]:[mm])
Polymer Data Handbook, ed. J. E. Mark, Oxford University Press, 1999, p. 656.
R. Lovell and A. H. Windle, Polymer, 1981, 22, 175-184.
56
Chapter 3
[Type text]
END-FUNCTIONAL STYRENE-MALEIC
ANHYDRIDE COPOLYMERS VIA CATALYTIC
CHAIN TRANSFER POLYMERISATION
This chapter has been published as: G.C. Sanders, R. Duchateau, C.Y. Lin, M.L. Coote,
J.P.A. Heuts, Macromolecules, 2012, DOI: 10.1021/ma301161u
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 3
ABSTRACT
Styrene-maleic anhydride copolymers have been successfully synthesised using
catalytic chain transfer polymerisation employing the low spin bis(difluoroboryl)dimethylglyoximato cobalt (II) (COBF) complex. By partially replacing styrene with α-methyl
styrene (while maintaining the amount of maleic anhydride at 50 mol%) over a range of
ratios, it was shown that the rate of reaction and molecular weight decreases with
increasing α-methyl styrene content. The polymers were characterised using MALDIToF-MS and 1H-13C gHMQC NMR to determine the end groups, which in the presence of
α-methyl styrene was an α-methyl styrene unit with a vinylic functionality. For styrenemaleic anhydride copolymers, the end group was determined to be predominantly maleic
anhydride with a vinylic functionality. Considering the fact that in a styrene-maleic
anhydride copolymerisation the propagating radicals are predominantly of a styrenic
nature, this was a very surprising result, suggesting that the maleic anhydride radicals
undergo a chain transfer reaction which is orders of magnitude faster than that of styrenic
radicals. The chain transfer constant of COBF was determined for the different ratios of
styrene and α-methyl styrene. It was found to increase two orders of magnitude from a
purely styrene-maleic anhydride to a purely α-methyl styrene-maleic anhydride
copolymer. Diels-Alder and thiol-ene reactions were performed on the vinylic end groups
as post-polymerisation modification reactions, as well as graft copolymerisation reactions
of the macromonomers with styrene and butyl acrylate.
58
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INTRODUCTION
Styrene-maleic anhydride copolymers (pSMA) are used extensively in
applications such as engineering plastics, surfactants in the paper-making industry and as
polymer-protein conjugates for drug delivery systems.1 The versatility of these polymers
lies within their properties which include transparency, a high heat resistance, high
dimensional stability and the specific reactivity of the anhydride groups. To fulfil these
varying roles, pSMA copolymers come in a range of molar masses and compositions.
The kinetics of the polymerisation has also been studied at length.2 Maleic
anhydride does not homopolymerise, however in the presence of styrene, a highly reactive
system is created, resulting in an almost perfectly alternating polymer. 3-7 Although most
work relating to pSMA copolymers has been performed via the conventional free radical
route, efforts have been made recently to investigate and evaluate the possibility of
polymerising pSMA using controlled radical routes. Atom transfer radical polymerisation
(ATRP) is incompatible with SMA copolymerisation due to interactions between maleic
anhydride and the catalysts used to mediate these reactions. 2 Nitroxide mediated
polymerisation (NMP) has also been used to copolymerise S and MA but a temperature of
over 80 ºC is required.8-10 This is the upper temperature limit to generate perfectly
alternating copolymers and thus control over the sequence is more difficult. Reversible
addition fragmentation chain transfer polymerisation (RAFT) was more successful at
synthesising pSMA copolymers at lower (and thus optimal) temperatures. 11-15 The
obvious advantage of using a controlled radical polymerisation method over conventional
free radical polymerisation is the ability to control the molecular weight as well as
synthesising highly monodisperse polymers, all with a specific end group. Although these
methods are extremely effective, the experimental requirements and set-up are far from
being industrially applied.
Catalytic chain transfer polymerisation (CCTP),16-19on the other hand, is already
used widely in the coating industry as a route to reducing the volatile organic compound
(VOC) content of coatings.20 CCTP is a very effective route to synthesising polymers with
vinyl functionalities.17-19 The addition of very small quantities of certain low-spin Co(II)
catalysts, such as bis(difluoroboryl)dimethylglyoximato cobalt (II) (COBF), to a
conventional free radical polymerisation results in two major effects. Firstly, the
molecular weight of the polymer chains is controlled via a chain transfer mechanism, and
secondly, due to the mechanism of the chain transfer process, vinylic end groups can be
obtained. The cobalt catalyst abstracts a hydrogen radical from the growing polymer
chain, transferring it to a monomer molecule, thus starting a new chain and reducing the
cobalt back to Co(II). Although this process is highly efficient for methacrylate monomers,
it is approximately two orders of magnitude slower for styrene. Firstly, styrene interacts
with COBF to form strong Co-C bonds, reducing the efficiency of the catalyst and
secondly styrene has no methyl group in the β-position, and thus the proton residing in
this β-position is abstracted preferentially. This results in an “internal” double bond,
which is less reactive in any possible further post-polymerisation reactions. The
drawbacks of an inefficient chain transfer process and the absence of “external” double
bonds in styrene polymerisation can be overcome by the addition of small amounts of α59
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
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Chapter 3
methyl styrene (AMS) as discussed in detail previously.18, 21, 22 Since in a SMA
copolymerisation the propagating radical population is dominated by S radicals, 2 it can be
anticipated that addition of AMS to this copolymerisation will improve the chain transfer
efficiency and lead to the introduction of AMS end groups.
In this chapter, we investigate whether vinyl end-functional pSMA can be
synthesised efficiently using CCTP and whether the addition of AMS indeed makes the
process more efficient. In what follows the chain transfer kinetics using varying amounts
of AMS is discussed and the polymer structures are fully characterised. Finally the scope
of post-polymerisation reactions is discussed.
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EXPERIMENTAL SECTION
General Considerations. All syntheses and manipulations of air- and moisture-sensitive
materials were carried out in oven-dried Schlenk-type glassware on a dual manifold
Schlenk line.
Materials. Styrene (S, 99%), α-methyl styrene (AMS, 99%) and tert-butyl acrylate (BA,
98%) were purchased from Sigma-Aldrich, and passed over a column of activated basic
alumina to remove the inhibitor. Maleic anhydride (MA, 99%) was purchased from SigmaAldrich and used as received. Azobis(isobutryonitrile) (AIBN) and VAZO-88 were
purchased from Sigma-Aldrich and recrystallised twice from methanol. The bis(methanol)
complex of COBF, 1, was prepared as described previously.23, 24 The chain transfer activity
of the complex was determined in methyl methacrylate (MMA) bulk polymerisation and
found to be equal to 46 × 103. For all experiments, a single batch of catalyst was used. 1,4Dioxane (AR, Biosolve) was used as received for conversion measurements but distilled
and stored in the fridge for chain transfer determination experiments. Danishefsky’s diene
was purchased from Sigma-Aldrich and stored under argon. All other materials were
purchased from Sigma-Aldrich and used as received.
Typical Procedure for the Copolymerisation of Styrene, Maleic Anhydride and αMethyl Styrene in the Presence of COBF. COBF (2.7 mg, 6.8 μmol), AIBN (42 mg, 0.25
mmol) and MA (25 g, 0.26 mol) were placed in a flask equipped with a stirrer bar and
underwent three vacuum-argon cycles. S (26 mL, 0.23 mol), AMS (3.3 mL, 0.03 mol) and
1,4-dioxane (50 mL) were degassed and added using a syringe. The mixture was heated to
60ºC and allowed to react for 7 hours. The reaction mixture was quenched by cooling in
ice and addition of hydroquinone. The residual (liquid) monomers and solvent were
removed via vacuum evaporation immediately after stopping the reaction. The resulting
polymer was redissolved in THF and precipitated in a large excess of diethyl ether, then
dried in a vacuum oven at 80 ºC for at least 48 hours.
Chain Transfer Constant Determination. Two stock solutions were made. COBF (30
mg, 75 μmol) was dissolved in 30 mL distilled dioxane. AIBN (20 mg, 122 μmol), AMS (if
applicable) and MA (13.5 g, 1.38 mol) dissolved in 18 mL distilled dioxane. To each finger
Schlenk, the appropriate amount of COBF solution (made up to 1 mL with distilled
dioxane), 3 mL of the AIBN/AMS/MA solution is added and finally the relevant amount
of S is added (S kept separate from AIBN solution, as some evidence of conventional FRP
is seen when they are combined). The finger Schlenks underwent 3 freeze-pump-thaw
cycles and were then placed in an oil bath pre-heated to 60 ºC. After 15 minutes, the
reaction mixture was cooled in ice and hydroquinone added to quench the reaction. For
reactions with AMS, a ratio of 10:9:1 or 10:9.5:0.5 molar ratio of maleic anhydride, styrene
and AMS was used.
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Chapter 3
Diels-Alder Coupling of pSMA with Danishefsky’s Diene. A 2,000 Da pSMA
copolymer (0.1 g, 75 μmol) and Danishefsky’s diene (trans-1-Methoxy-3-trimethylsiloxy1,3-butadiene, 375 μmol) were dissolved in 3 mL xylene in a crimp cap vial. The vial was
then placed in an oil bath and the temperature increased to 180 ºC. The reaction was
stirred at this temperature for 6 days.
Diels-Alder Coupling of pSMA with Cyclopentadiene. A 2,000 Da pSMA copolymer
(0.1g, 75 μmol) and freshly distilled cyclopentadiene (3.75 mmol) were dissolved in 1 mL
acetone. Lithium triflate (LiOTf) or bis(trifluoromethane)sulfonimide lithium salt (LiNTf 2)
was added and reaction stirred at the desired temperature for 2-144 hours.
Thiol-ene Reaction of Citraconic Anhydride with Dodecanethiol. Citraconic
anhydride (0.1 g, 0.9 mmol) and dodecanthiol (0.18 g, 0.9 mmol) were dissolved in 0.5 mL
CDCl3 in an NMR tube equipped with a screw cap. N2 was bubbled through for 10
minutes prior to the reaction. DMPP (10 μL, 0.07 μmol) was then added via syringe and
the reaction allowed to stir at room temperature overnight.
Thiol-ene Reaction of pSMA with Octanethiol. A 2,000 Da pSMA copolymer (0.1 g, 75
μmol) and octanethiol (0.02 g, 150 μmol) were dissolved in 2 mL acetone. N2 was bubbled
through for 10 minutes prior to the reaction. DMPP (10 μL, 75 μmol) was then added via
syringe and the reaction allowed to stir at room temperature overnight.
Graft Copolymerisation of pSMA. A 4,000 Da pSMA copolymer (0.5 g, 125 μmol), S or
BA (4.4 mmol), VAZO-88 (0.05 wt% of monomer and macromonomer) and dioxane (2.7 g)
were weighed into a crimp cap vial and sealed. The solution was stirred until a clear
solution was obtained then argon was bubbled through for 15 minutes. The vial was then
placed in an oil bath at 60ºC for 4 days. The resulting polymer solution was then
quenched by cooling in ice and addition of hydroquinone. Residual monomer and solvent
were removed under vacuum at elevated temperatures.
Graft Copolymerisation of pASMA. A 1,200 Da pASMA copolymer (0.5 g, 417 μmol), S
or BA (9.6 mmol), VAZO-88 (0.05 wt% of monomer and macromonomer) and dioxane (6
g) were weighed into a crimp cap vial and sealed. The solution was stirred until a clear
solution was obtained then argon was bubbled through for 15 minutes. The vial was then
placed in an oil bath at 60ºC for 4 days. The resulting polymer solution was then
quenched by cooling in ice and addition of hydroquinone. Residual monomer and solvent
were removed under vacuum at elevated temperatures.
Measurements. Size exclusion chromatography was carried out using a WATERS 2695
separations module, Model 2487 UV detector (254 nm), and Model 2414 differential
refractive index detector (40°C). The injection volume used was 50 µL. Tetrahydrofuran
(Biosolve, stabilised with BHT) was used as the eluent with a flow rate 1.0 mL/min. The
62
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column set used was a PLgel guard (5 m particles) 50 × 7.5 mm precolumn, followed by
2 PLgel columns in series of 500 Å (5 m particles) and 100 Å (5 m particles)
respectively. Calibration was performed using polystyrene standards (Polymer
Laboratories, Mn = 370 up to Mn = 40,000 g/mol). Reported molecular weight data have
been corrected using the following Mark-Houwink-Kuhn-Sakurada constants: KpS =
1.28·10-4 dL·g-1, apS = 0.712, KpSMA = 5.07·10-5 dL·g-1, apSMA = 0.81. Data acquisition and
processing were performed using WATERS Empower 2 software. 1H, 13C and gHMQC
NMR spectra were recorded on a Varian Mercury Vx (400 MHz) spectrometer at 400
MHz. Acetone-d6 or chloroform-d3 and tetramethylsilane were used as solvents and
internal standard, respectively. MALDI-ToF-MS was carried out using a voyager DESTR spectrometer from Applied Biosystems in reflector mode. Trans-2-(3-(4-tertbutylphenyl)-methyl-2-propenylidene)-malononitrile
doped
with
potassium
trifluoroacetate was used as the matrix. The mixture was deposited from a THF solution
onto a stainless steel sample substrate and the solvent allowed to evaporate. The polymer
was then deposited as a dilute (~1 mg/mL) solution in THF. This resulted in each
polymeric species being observed as its K+ adduct with molecular mass M+31. The
spectrometer was calibrated using poly(ethylene oxide) standards for the lower mass
range and polystyrene standards for the higher mass range.
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Chapter 3
RESULTS AND DISCUSSION
Synthesis of ASMA and SMA Copolymers
Copolymers of α-methyl styrene (AMS), styrene (S) and maleic anhydride (MA)
have been synthesised in the presence of varying amounts of COBF at a reaction
temperature of 60 ºC. In a series of reactions 10% S was replaced with AMS, maintaining
equimolar ratios of styrenic monomers (S + AMS) to maleic anhydride. The molecular
weight evolutions and polymerisation kinetics of these reactions with respect to the
concentration of COBF were investigated and as seen in Figure 1, there was a clear
influence of COBF concentration on the molecular weight of the polymer. As expected,
with increasing [COBF], the molecular weight of the polymers decreased. In addition, the
molecular weight of the polymers remained fairly constant throughout the reaction, which
seems to be a generally observed (but still not understood) phenomenon in CCTP.18, 19, 25
(b)
30,000
0.6
25,000
0.5
20,000
0.4
15,000
0.3
x
Mw/g mol-1
(a)
10,000
0.2
5,000
0.1
0.0
0
0.0
0.1
0.2
0.3
0.4
0.5
x
0
1
2
3
4
5
6
7
8
Time/Hours
Figure 1. The effect of catalyst loading on (a) molecular weight evolutions and (b) conversion vs.
time plots for the COBF-mediated free-radical polymerisation of S/AMS/MA (45/5/50) at 60C in
undistilled dioxane. [COBF] = 456 ppm (■), 91456ppm
(●), 32 ppm (▲), 23 ppm (▼), 9 ppm (♦), 6
ppm
91 ppm
ppm (◄).
32 ppm
23 ppm
9 ppm
From Figure 1b it is also clear that6 ppm
COBF has a negligible effect on the rate of
polymerisation; only at high COBF concentrations is the rate considerably reduced in
accordance with what is generally observed in CCTP.18, 19 It was also observed that at
lower COBF loadings Mw increases (with a corresponding broadening of the distribution)
at higher conversions. This is thought to be a result of the presence of the impurities in
dioxane. Dioxane contains a very small percentage of peroxides (due to decomposition of
dioxane) and these peroxides have a marked effect on the catalytic activity of COBF and
will obviously have a greater effect when using lower amounts of COBF. 17-19 To confirm
this conclusion we measured the chain transfer constants of COBF in an MMA
polymerisation in distilled and undistilled dioxane and found CT values in the region of
44·103 and 9·103 respectively (Figure 2). We therefore decided to measure more exact
values for CT in distilled dioxane (vide infra).
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END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
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Chapter 3
0.08
CT = 44,000
0.07
0.06
2/DPw
0.05
0.04
CT = 46,000
0.03
CT = 9,400
0.02
0.01
0.00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
6
10 [COBF]/[M]
Figure 2. CT values of COBF in bulk MMA (■), MMA in undistilled dioxane (●) and MMA in
distilled dioxane (▲).
We also investigated the effect of replacing some S by AMS on the kinetics and
molecular weight evolution as it is known that AMS affects the rate of reaction and
molecular weight when copolymerised with S in the presence of COBF. 21, 22, 26 By
replacing S with AMS (from 0-10%) in a SMA copolymerisation, whilst maintaining a
constant molar ratio between MA and the styrenic monomers (S+AMS), a similar trend in
behaviour is seen. An increase in the amount of AMS added decreases the molecular
weight (which remains virtually constant with conversion) and decreases the
polymerisation rate (see Figure 3).
(a)
(b)
14,000
0.7
12,000
0.6
10,000
0.5
0.4
6,000
x
Mw/g mol-1
8,000
0.3
4,000
0.2
2,000
0.1
0
0.0
0.0
0.1
0.2
0.3
0.4
0.5
x
0
1
2
3
4
5
6
7
8
Time/Hours
Figure 3. The effect of AMS/S ratio on (a) molecular weight evolutions and (b) conversion vs. time
plots for the COBF-mediated free-radical polymerisation of S/AMS/MA (S+AMS : MA = 50:50) at
60C in undistilled dioxane. [COBF] = 23 ppm. S:AMS = 0:50 (■), 45:5 (●), 48:2 (▲), 50:0 (▼).
65
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
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Chapter 3
In conclusion, it is clear from these results that CCTP leads to efficient molecular
weight control in the copolymerisation of SMA and that, in line with previous results, 21, 22
replacing some of S by AMS leads to a more efficient CCTP process.
End Group Determination
In the previous section we showed that CCTP indeed leads to efficient molecular
weight control, but the polymer microstructure and end groups in these polymerisations
still need to be established. Since an almost perfectly alternating copolymer can be easily
synthesised under conventional free radical conditions by using a 50:50 ratio of styrenic
monomers to maleic anhydride, we expected the polymerisation of S and MA under CCTP
conditions to also result in an (almost perfectly) alternating copolymer. From the
MALDI-ToF-MS spectrum in Figure 4a, which shows a repeat unit of 202 Da (SMA), it is
clear that this is indeed the case. Distributions corresponding to p(SMA)n with 1, 2, 3 and
4 extra S units, p(SMA)n(S)1-4, can also be observed. Besides the potential occurrence of
preferential initiation and transfer reactions, these additional S units originate from the
fact that although MA cannot homopolymerise, S can and thus it is possible that within
the polymer chain there are two S units next to one another. This trend is shown further
in Figure 4b, which shows an alternating topology in the fingerprint of the MALDI-ToFMS contour plot. A slight deviation from a perfectly alternating topology is also evident,
with a higher amount, on average, of S units versus MA units in the polymer chain.
(a)
(b)
18
16
+
p(SMA)9(S)2 K
+
+
p(SMA)8(S)3 K
p(SMA)9(S) K
+
p(SMA)8(S)4 K
Maleic Anhydride
14
12
10
8
6
4
1940
1960
1980
2000
2020
2040
2060
2080
2
2100
2
m/z
4
6
8
10
12
14
16
18
Styrene
Figure 4. MALDI-ToF-MS (THF, K+ salt) (a) spectrum and (b) contour plot of pSMA.
Although there are more styrene units than maleic anhydride units in the
copolymer, close inspection of a 1H-13C gHMQC NMR spectrum (Figure 5) indicates that
maleic anhydride predominantly forms the end group. Figure 5 shows the cross-peak
corresponding to the =CH of an unsaturated MA moiety at 6.5 (1H) and 135 (13C) ppm.
For an internal styrenic double bond, a cross peak at around 5.5 ( 1H) and 114 (13C) ppm
would be expected, which is not observed here.
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4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
ppm
150
140
130
120
110
100
ppm
Figure 5. Partial 1H-13C gHMQC NMR (acetone-d6) spectrum of pSMA.
The result that the polymers formed have predominantly an unsaturated MA end
group is very interesting and was, at first, very surprising for two reasons. First of all,
under the used reaction conditions, the fraction of propagating MA radicals is less than
about 10% of the overall propagating radical population, 2 implying that CT,MA-radicals >>
10  CT,S-radicals. Secondly, at first glance, the MA-radical seems to be similar to an acrylate
radical, for which CT is low. Where this radical differs from an acrylate radical, however, is
the nature of the β-hydrogen atom that is abstracted. Whereas in an acrylate it is
abstracted from a secondary carbon atom, in MA it is abstracted from a tertiary carbon; in
fact, it is a similar carbon as those which lead to the so-called mid-chain radicals in
acrylate polymerisation originating from chain transfer to polymer.27 Hence, in retrospect,
the abstracted hydrogen atom may actually be rather labile. This conclusion is supported
by high-level quantum chemical calculations which show a 40 kJ·mol-1 preference towards
hydrogen transfer from a MA radical over a S radical.33
The addition of AMS to the copolymerisation of S and MA was thought to not only
improve the control over the molecular weight but also produce polymers with an AMS
end group with an “external” double bond. MALDI-ToF-MS data again indicates that an
almost perfectly alternating copolymer of SMA has been synthesised, as evidenced by the
repeating unit of 202 Da, p(SMA) (Figure 6a). Several distributions can be observed. As
with S and MA polymerised without AMS, polymers with a range of units of MA and S
67
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
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Chapter 3
are observed, p(SMA)n(S)x (the labels for distributions where x > 2 have been omitted
from Figure 6a for clarity). In addition to these distributions, a range of AMS units per
p(SMA)n(S)x chain can also be seen, p(SMA)n(S)x(AMS)0-6. Clearly, in these cases, more
than one AMS has been copolymerised with S and MA into the polymer chain. This is
further confirmed by the MALDI-ToF-MS contour plot (Figure 6b), which shows a
distribution of AMS units for a range of SMA units. For simplicity sake we denote all
SMA polymers containing AMS as pASMA in the remainder of this chapter.
(a)
(b)
12
10
+
p(SMA)6(S)(AMS) K
p(SMA)6(AMS)2 K
+
+
p(SMA)6(AMS)3 K
+
p(SMA)7(S)(AMS)2 K
p(SMA)6(AMS)4 K
p(SMA)6(AMS)5 K
p(SMA)6 K
+
+
+
+
-Methyl Styrene
p(SMA)6(AMS) K
8
6
4
+
p(SMA)6(AMS)5 K
+
p(SMA)6(S)2 K
2
1350
1400
1450
1500
2
m/z
4
6
8
10
12
14
16
18
20
Styrene-Maleic Anhydride
Figure 6. MALDI-ToF-MS (THF, K+ salt) (a) spectrum and (b) contour plot of pASMA of
p(ASMA).
In order to establish whether at least one of the AMS units was situated at the
end of the polymer forming the end group, 1H-13C gHMQC NMR was again used. The
cross-peak at 5.5 (1H) and 116 (13C) ppm (Figure 7) indicates the presence of a =CH 2
corresponding to an external double bond of an AMS unit. According to 1H NMR, the
ratio of =CH2 (AMS) to aromatic protons was found to be 1:5, which assuming an almost
equal ratio of S and MA per chain and only one AMS unit gives a number-average
molecular weight of 1,100 g/mol. This is comparable to the value obtained using SEC of
900 g/mol (adjusted using Mark-Houwink parameters), indicating that most chains
contain an AMS end group with an unsaturated “external” double bond.
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4.00
4.50
5.00
5.50
6.00
6.50
ppm
120.0
115.0
110.0
ppm
Figure 7. Partial 1H-13C gHMQC NMR (acetone-d6) spectrum of pASMA.
In conclusion, CCTP can be used to synthesise both PSMA and PASMA
copolymers, in which the end groups have been determined to be an MA and an
unsaturated AMS moiety, respectively (Scheme 1).
Scheme 1. Structures of pSMA and pASMA.
Determination of Chain Transfer Constant of COBF for SMA Copolymers
In order to determine the chain transfer constant (CT) of COBF in S-AMS-MA
copolymersations, polymerisations to low conversion (< 5%) were carried out in freshly
distilled 1,4-dioxane (to minimise deactivation of the catalysts due to the presence of
peroxides) at various COBF concentrations and for four different S:AMS ratios.
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Chapter 3
The easiest and most widely used method of determining the chain transfer
constant (CT) of COBF is the Mayo method. By determining the Mn or Mw from SEC data,
the degree of polymerisation can be calculated, which is then plotted as 1/DPn or 2/DPw
against the ratio of chain transfer agent concentration to monomer concentration. The
slope of this line is the CT, in accordance with Equation 1.
(
)
〈 〉[ ]
⌈ ⌉
[ ]
[ ]
( )
In this equation, λ is the fraction of radicals undergoing termination by
disproportionation, 〈 〉 the chain length averaged termination rate coefficient, [R•] the
overall radical concentration, [M] the monomer concentration, CM the chain transfer to
monomer constant, [Co] the active catalyst concentration and CT the chain transfer
⁄ , where
constant (defined as
is the chain transfer rate coefficient and the
propagation rate coefficient). Although using Mn to determine the degree of
polymerisation is theoretically the most accurate way, Mw is often used (as Mw/2·m0) as it
is a more robust experimental parameter, particularly for low molecular weight polymers.
It should also be noted that a polydispersity index of 2 is assumed, a value which is
commonly found in chain transfer-dominated polymerisations.28-30
The Mayo method has one major disadvantage. When dealing with low molecular
weight polymers, it is often difficult to separate the polymer from the solvent peak in SEC.
This means that obtaining an accurate baseline is not always possible, and thus the
molecular weights, particularly Mn, are less reliable giving an unrealistic CT values. This
issue has previously been discussed in detail and a comparison made between CT values
determined using the Mayo method (Mn and Mw from SEC measurements) and the chain
length distribution (CLD) method.28, 31 The CLD method uses the slope, Λ, of the chain
length distribution P(M), plotted as ln(P(M)), vs. M to determine the chain transfer
constant.32, 33 Λ taken in the higher molecular weight region, ΛH is theoretically the most
accurate, in accordance with equation 2.
(
( ))
(
〈 〉[ ]
[ ]
[ ]
)
[ ]
( )
However, more reliable results are often obtained when the slope of the distribution is
taken at the molecular weight of the peak of the original chromatogram, ΛP.28, 31 Now
plotting ΛP·m0 against [COBF]/[M] gives a plot similar to the conventional Mayo plot
with a slope equal to CT.
In this chapter we used both the Mayo method based on Mw and the CLD
method for determining the chain transfer constants. The obtained Mayo and CLD plots
for the four different S:AMS ratios are shown in Figure 8. An example of the
corresponding molecular weight distributions and lnP(M) plots can be found in Figure 9.
The results for the chain transfer constants are summarised in Table 1.
70
Chapter 3
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
(a)
(b)
107 x [COBF]/[M]
0
1
2
3
107 x [COBF]/[M]
4
5
0.06
6
0
0.009
1
2
3
4
5
6
0.06
0.009
0.008
0.008
0.05
0.05
0.007
0.007
0.006
0.04
0.006
0.02
2/DPw
0.004
0.005
m0
2/DPw
0.005
0.03
0.03
0.004
m0
0.04
0.003
0.02
0.003
0.002
0.002
0.01
0.001
0.01
0.000
4.0
0.00
0.001
0.00
2.0
2.5
3.0
3.5
5
10 x [COBF]/[M]
2.0
2.5
3.0
3.5
0.000
4.5
4.0
107 x [COBF]/[M]
Figure 9. (a) Mayo plots and (b) CLD plots for the determination of the chain transfer constant of
COBF in S, MA and AMS in dioxane at 60 ºC. Molar ratios of S to AMS = 45:5 (black), 48:2 (red),
50:0 (blue) and 0:50 (magenta). Ratio of S/AMS to MA = 1:1. Closed symbols = repeat 1, open
symbols = repeat 2.
(a)
(b)
-10
-12
-14
ln P(M)
-16
-18
-20
-22
-24
3.5
4.0
4.5
5.0
5.5
-26
6.0
0.0
Log(M)
3
5.0x10
4
1.0x10
4
1.5x10
4
2.0x10
4
2.5x10
Figure 9. (a) Molecular weight distributions and (b) ln P(M) vs. M plot used for CT calculations.
71
4
3.0x10
M
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 3
Table 2. Chain transfer constants (CT) in the COBF-mediated free radical polymerisation of S, MA
and AMS at 60C in freshly distilled dioxane.
S:AMSa
Mayo methodb,c
CLD methodc,d
50:0
48:2
45:5
0:50
(0.4 ± 0.1)·103
(1.4 ± 0.3)·103
(1.8 ± 0.3)·103
(11.0 ± 0.9)·103
(0.5 ± 0.1)·103
(1.4 ± 0.4)·103
(1.6 ± 0.4)·103
(11.9 ± 0.7)·103
Molar ratios. Ratio of AMS and S to MA = 1:1; b Mayo method based on Equation 1 using DPn =
Mw/(2·m0); c Average value determined by plotting all points in a single graph and determining the
slope of a best fit line; CLD method based on Equation 2 using ΛP instead of ΛH.
a
First of all it can be seen that there is a good agreement between the two
methods and that with increasing AMS content the chain transfer constant increases (as
expected). What is interesting to note is that, by CCTP standards, the chain transfer
constants are moderately low. Especially when comparing the value obtained for the 50%
AMS / 50% MA system (~ 104) with that for pure AMS (~ 105) then it is an order of
magnitude lower. It is, however, not surprising when one considers the fact that these
chain transfer constants are ratios of the average ktr and the average kp, and in the case of
(A)SMA polymerisation, the average kp is quite high2 (and much higher than the kp for
pure AMS!)22 For a monomer feed fraction fMA = 0.5 in a SMA copolymerisation kp > 2000
dm3·mol-1·s-1,2 which implies that ktr > 106 dm3·mol-1·s-1 in the 50% S / 50% MA
copolymerisation system and ktr > 107 dm3·mol-1·s-1 in the 50% AMS / 50% MA
copolymerisation system (assuming that kp is of the same order of magnitude as that in
SMA). Both values are about an order of magnitude larger than what is observed in an
ordinary CCTP of styrene and in AMS, respectively. This observation suggests that there
may be a strong penultimate unit effect on the transfer reaction in this system, but there
are too many uncertainties in the rate parameters used to draw any firm conclusions.
What seems to be a clear conclusion, however, is that the ktr of MA-radical to COBF is
very high; in fact we expect it be larger 10 7 dm3·mol-1·s-1 (we used here the experimental
result that only MA end groups are observed and the kinetic estimate that only about 10%
of the propagating radical population contains a MA terminal unit). This is in line with
the results from quantum calculations.34
Post-Polymerisation Reactions of pSMA
The CCTP-derived pSMA macromonomers have a predominantly maleic
anhydride end group, which contains a vinylic functionality, making the polymer ripe for a
variety of post-polymerisation modifications. CCTP has been combined with postpolymerisation reactions, such as thiol-ene chemistry, frequently in the last few years.35-42
However, to our knowledge CCTP has never been combined with a Diels-Alder
cycloaddition. In the following section, we describe proof of principle reactions to modify
72
Chapter 3
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
pSMA polymers (obtained using CCTP) via both thiol-ene and Diels-Alder cycloaddition
reactions (Scheme 2).
Scheme 2. Attempted post-polymerisation functionalisations of pSMA.
Diels-Alder Reactions
Diels-Alder reactions require a diene and a dieneophile, usually a single double
bond.43 Although maleic anhydride reacts readily with a range of dienes, the substitution
of a proton with an electron donating group such as the methyl in citraconic anhydride or
in fact the polymer chain in pSMA, significantly decreases the reactivity of the
dienophile.44 Danishefsky’s diene (DD) is known to be one of the most electron-rich dienes
and therefore one of the most effective for electron-deficient dieneophiles.45 Addition of
this diene (in excess) to pSMA at elevated temperatures resulted in a successful DielsAlder reaction (pSMADD), as illustrated by the MALDI-ToF-MS spectrum (Figure 10),
which shows the successful addition of DD to pSMA. The use of 1H NMR as an analytical
tool is problematic due to the broad signals observed.
73
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
pS11MA10
Chapter 3
pS12MA10
pS12MA9
pS14MA7
pS12MA93
pS15MA7
pS20
pS19
2150
2200
2250
2300
2350
m/z
pS18MA-MADD
pS10MA9-MADD
pS11MA9-MADD
pS18MA-MADD
pS9MA10-MADD
2150
2200
2250
2300
2350
m/z
Figure 10. MALDI-ToF-MS spectrum of (a) pSMA and (b) pSMA after reaction with DD.
The Diels-Alder reaction of pSMA with cyclopentadiene (CP) was also attempted.
CP is less reactive compared to DD and so LiOTf or LiNTf2 were employed to catalyse
the reactions.46, 47 However, even though reactions of citraconic anhydride and pSMA ran
to 100% conversion within a few hours (entries 1 and 2, Table 2), no evidence (MALDIToF-MS) of the addition of CP was observed when pSMA was used as the dieneophile,
even at higher catalyst loadings and higher temperatures (entries 3-8, Table 2). It must be
mentioned that the LiOTf and LiNTf2 catalysts are by no means the only catalysts
employed for Diels-Alder reactions44, 48-50 and therefore this reaction may well still be
feasible, simply not using the conditions explored here.
74
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 3
Table 2. Experimental conditions and results for the Diels-Alder coupling between pSMA (or
model compound) and cyclopentadiene.
Reaction
Catalyst
pSMA:CP:catalyst
Temperature/ºC
Conversion
Time/Hours
b
1
LiOTf
1 :2:0.7
2
20
1.0e
2
LiNTf2
1b:2:3.3
2
20
1.0e
3
LiOTf
1c:16:30
16.5
20
0f
4
LiNTf2
1c:16:20
16.5
20
0f
5
LiOTf
1d:50:30
144
20
0f
6
LiNTf2
1d:50:30
144
20
0f
7
LiOTf
1d:50:30
96
40
0f
8
LiNTf2
1d:50:30
96
40
0f
Molar equivalents; b Citraconic anhydride used as model for pSMA; c Mn = 4000 g/mol;
2000 g/mol; e Determined using 1H NMR; f No product observed in MALDI-ToF-MS
a
d
Mn =
Thiol-ene Reactions
The addition of a simple alkyl thiol to pSMA in the presence of dimethylphenyl
phosphine (DMPP) as a catalyst should result in the Michael addition of these two species.
Thiol-ene chemistry can be carried out on the vinylic functionality of MA-type end
groups, as evidenced by model reactions using citraconic anhydride and dodecanthiol
which indicated that the thiol had reacted exclusively with the double bond and not with
the anhydride ring (Figure 11). Although it appears that the thiol predominantly attacks
the more hindered side of the double bond, the complexity of the peak at 2.6 ppm
(corresponding the CH2 adjacent to the S) indicates that some reaction also occurs at the
least hindered position of the vinylic group.
7
6
5
4
3
2
1
0
ppm
Figure 11. 1H NMR (400 MHz, CDCl3) of the reaction product of citraconic anhydride and
dodecanthiol.
75
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 3
An excess of octanethiol was reacted with pSMA in the presence of DMPP to
produce the Michael addition product, as confirmed via MALDI-ToF-MS (Figure 12).
Due to the sheer number of peaks it was difficult to identify whether certain peaks
corresponded to unreacted starting material or the reaction product. In addition the
degree of conversion could not be quantified due to the intrinsic nature of MALDI-ToFMS and the fact that it is the only suitable analytical method. However, the spectrum is
significantly different compared to the starting macromonomer demonstrating that a
successful coupling reaction has taken place.
(a)
pS9MA8
pS9MA7
pS10MA6
pS10MA7
pS11MA6
pS12MA5
pS16
pS12MA4
(b)
pS6MA9OT pS7MA8OT
pS7MA8OT
pS10MA5OT
pS11MA4OT
pS7MA7OT
1660
1680
1700
1720
1740
1760
1780
1800
1820
1840
m/z
Figure 12. MALDI-ToF-MS spectrum of (a) pSMA and (b) pSMA after reaction with octane thiol.
The end groups of maleic-anhydride-terminated polymers have been modified
using both a Diels-Alder cycloaddition and using thiol-ene chemistry.
Copolymerisation Behaviour
Macromonomers made via CCTP have been shown to undergo two different
reactions when in the presence of an additional (methacrylic, acrylic or styrenic) monomer
under free-radical conditions: addition-fragmentation chain transfer (AFCT) and graft
copolymerisation.18, 19 The tendency of a macromonomer to undergo either AFCT of graft
copolymerisation is dependent on the nature of the penultimate monomer unit of the
macromonomer (i.e. the monomer unit next to the unsaturated monomer end group) and
the nature of the secondary monomer.51
Accordingly, we expect pSMA, where the penultimate unit is most likely to be a
styrene unit, and pASMA, where the penultimate unit is most likely a MA or S unit, both
to undergo predominantly graft copolymerisation with either styrene or an acrylate.
76
Chapter 3
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
The copolymerisation of pSMA with 35 equivalents of styrene results in a clear
increase in molecular weight (Figure 13a), and no residual macromonomer is observed.
This indicates that a graft copolymer of p(S-g-SMA) has been obtained. A slightly low
molecular weight shoulder can be observed in the molecular weight distribution, which
could be due to the formation of poly(styrene). The copolymerisation of pSMA with butyl
acrylate (BA) exhibits similar behaviour, and p(BA-g-SMA) is clearly formed (Figure 13a).
PASMA, on the other hand, does not react so readily with styrene and butyl
acrylate and a large amount of macromonomer remains unreacted after completion of the
reaction (Figure 13b). This may simply be due to a slower copolymerisation as the radical
that is formed upon addition to the macromonomer can be stabilized by delocalization of
the electron in the phenyl ring,51 but more extensive copolymerisation studies, beyond the
scope of the current work, would be required to establish this.
(a)
(b)
p(S-g-SMA)
p(BA-co-ASMA)
pSMA
wLog(M)
wLog(M)
p(BA-g-SMA)
p(S-co-ASMA)
pASMA
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
LogM
LogM
Figure 13. Molecular weight distribution of (a) pSMA p(S-g-SMA) and p(BA-b-SMA) and (b)
pASMA p(S-g-ASMA) and p(BA-b-ASMA). Measured against PS standards in THF. Corrected for
conversion.
Although the reason behind the lower reactivity of the pASMA system has not
been completely clarified, it is interesting to note the obvious difference in reactivity
between pSMA and pASMA and that the system with the most efficient CCTP yields the
least reactive macromonomer for further polymerisation.
CONCLUSIONS
In this work, we have demonstrated that low molecular weight, end-functional
polymers of styrene and maleic anhydride can be efficiently synthesised via CCTP and
that replacing part of the styrene with α-methyl styrene makes the process even more
efficient. The polymers prepared without α-methyl styrene are characterised by a reactive
unsaturated maleic anhydride end group that can be post-functionalised by Diels-Alder
and thiol-ene reactions. These polymers are also efficient comonomers in
77
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 3
copolymerisations with styrene and butyl acrylate. The polymers prepared with α-methyl
styrene are characterized by an unsaturated α-methyl styrene end group which showed a
much lower reactivity in further copolymerisation reactions. Hence, if only low-molecular
weight SMA copolymers are required, it is worthwhile considering the addition of a small
amount of α-methyl styrene, which makes the CCTP process more efficient and
necessitates a lower amount of catalyst. However, if the macromonomers are to be used as
comonomers or need to be end-functionalised, then it is probably better to use just styrene
and maleic anhydride.
78
Chapter 3
END-FUNCTIONAL STYRENE-MALEIC ANHYDRIDE COPOLYMERS VIA
CATALYTIC CHAIN TRANSFER POLYMERISATION
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80
Chapter 4
THE POLYMERISATION BEHAVIOUR OF
EPOXIDISED MACROMONOMERS DERIVED
FROM CATALYTIC CHAIN TRANSFER
POLYMERISATION
Electrophilic
Epoxidation
Expected
THF
100
90
80
% ConversionTHF
70
60
50
40
30
Obtained
20
10
0
0
50
100
150
200
250
300
Time/Minutes
Figure 4 Conversion of THF versus time for the copolymerisation of e-MMA2 and THF. Ratio of e-MMA2:THF = 10:100 (■), 4:100 (●) and
1:100 (▲).
This chapter has been published as: G.C. Sanders, B.G.P. van Ravensteijn, R. Duchateau,
J.P.A. Heuts, Polymer Chemistry, 2012, 3, 2200-2208
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
ABSTRACT
In our attempts to combine catalytic chain transfer polymerisation (CCTP) with
ring-opening polymerisation (ROP), we investigated the homopolymerisation and
copolymerisation behaviour of epoxidised CCTP-derived (macro)monomers. Although the
homopolymerisation of these species proved futile, the copolymerisation behaviour of the
epoxides exhibited unexpected behaviour. The cationic ROP of epoxidised CCTP-derived
(macro)monomers was unsuccessful due to the formation of stable 5-membered rings,
incapable of propagation. In a copolymerisation of these epoxides with tetrahydrofuran
(THF), however, we observed that the epoxides did not act as a comonomer but rather as
an end-capper, providing a functional end group. Most surprising, was the observation
that the epoxidised CCTP-derived (macro)monomers significantly catalyse the
homopolymerisation of THF.
82
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
INTRODUCTION
Complex polymer architectures, such as block, graft and star copolymers, can be
synthesised via a variety of routes. 1 Using pre-formed polymers, for example
macromonomers, as building blocks allows for the synthesis of polymers with well-defined
segments. The (co)polymerisation of macromonomeric building blocks can be carried out
via the same polymerisation mechanism used to synthesise the macromonomer or via an
alternative polymerisation mechanism, removing the restrictions placed by only using a
single polymerisation route for polymer synthesis.2-4 In recent literature, living radical
polymerisation techniques have been combined with catalytic olefin polymerisation,5 ringopening polymerisations6 and click chemistry.7
Although the literature is rife with new routes to combining various
polymerisation mechanisms as a method of synthesising new and complex architectures,
there is relatively little mentioned in relation to catalytic chain transfer polymerisation
(CCTP).8-13
In line with previous work carried out in this thesis to combine different
polymerisation mechanisms, we initially set out to combine CCTP with ring-opening
polymerisation (ROP). We considered that as the macromonomers contained a terminal
double bond, this bond could be easily converted into an epoxide, 14, 15 and therefore that
the subsequent ROP of these modified macromonomers would result in a new class of
polymers, with a flexible polyether backbone and functional methacrylic side chains
(Scheme 1). In this chapter these efforts will be described and although our initial aim of
ring-opening of epoxidised methacrylic macromonomers proved futile, some unexpected
and intriguing results in the copolymerisation behaviour of these modified
(macro)monomers have been observed.
Scheme 1. Intended reaction pathway to a new class of polymers.
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THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
EXPERIMENTAL SECTION
General Considerations. All syntheses and manipulations of air- and moisture-sensitive
materials were carried out in oven-dried Schlenk-type glassware on a dual manifold
Schlenk line, a vacuum line (typically 1 to 100 mbar), or in a nitrogen-filled glovebox
(typically < 1.0 ppm of oxygen and moisture).
Materials. Methyl methacrylate (MMA, 99%) and α-methyl styrene (AMS, 99%) were
purchased from Sigma-Aldrich, and passed over a column of activated basic alumina to
remove the inhibitor. Azobis(isobutyronitrile) (AIBN) was recrystallised twice from
methanol. The bis(methanol) complex of bis(boron difluorodimethylglyoximate) cobalt
(II), COBF·(MeOH)2 (1), was prepared as described previously.16, 17 metaChloroperbenzoic acid (mCPBA) (77% pure, Acros), was used without further purification.
Tris(perfluorophenyl)boron (BF15) was synthesized according to literature procedures.18
Solvents (Biosolve) were dried over magnesium sulphate (Sigma-Aldrich) and distilled
before use or taken from the dry solvent system, except for methanol and ethanol, which
were used as received. Benzyl alcohol (BzOH) was purchased from Sigma-Aldrich and
distilled before use. Methyl 2-methylglycidate (e-MMA, 99%) was purchased from SigmaAldrich and stored under an argon atmosphere prior to use. All other chemicals were
purchased from Sigma-Aldrich and used as received.
Synthesis of MMA dimer via CCTP. COBF (34 mg, 8.8·10-5 mol) was premixed in N2purged MMA (200 mL, 1.88 mol) for 1 hour in a degassed flask. AIBN (1.5 g, 9·10-3 mol)
was dissolved in 200 mL N2-purged MMA in a degassed flask while cooling in an ice bath.
The resulting solutions were transferred to the reaction flask (also degassed) with a
cannula. The reaction mixture was heated at 60 °C and stirred for 24 hours, after which it
was cooled in ice and quenched with hydroquinone. The MMA dimer (MMA 2, see Scheme
2) was isolated by a vacuum distillation (bp. 80 °C at 2.9 Torr). Yield = 30%. 1H NMR δ
1.08 (s, 6H, CH3), 2.53 (s, 2H, CH2), 3.56 (s, 3H, OCH3), 3.65 (s, 3H, OCH3), 5.44 (s, 1H,
C=CH2), 6.13 (s, 1H, C=CH2).
Synthesis of AMS2 via CCTP.1 COBF (59 mg, 1.5·10-4 mol) and AIBN (0.837 g, 5·10-3
mol) were introduced to a round bottom flask equipped with a stirrer bar. After
evacuating and re-filling with nitrogen 3 times, 200 mL N2-purged AMS (1.54 mol) was
added. The resulting mixture was heated to 70 °C and stirred. After approximately 20
hours a second shot of COBF (19 mg, 5·10-5 mol) and AIBN (0.334 g, 2·10-3 mol)
dissolved in 10 mL AMS (7.7·10-2 mol) was added. After a further 12 hours the reaction
was cooled in ice and quenched with hydroquinone. The AMS dimer was isolated by a
vacuum distillation (bp. 100 °C at 2.0 Torr). Yield = 31%. 1H NMR δ 1.29 (s, 6H, CH3)),
2.90 (s, 2H, CH2), 4.85 (s, 1H, C=CH2), 5.21 (s, 1H, C=CH2), 7.16-7.37 (m, 10H, Ph).
Epoxidation of MMA2. MMA2 (10 g, 4.6·10-2 mol) was mixed with mCPBA (55g, 0.23
mol) and 55 mL dichloromethane (DCM) and allowed to stir at 20 °C. After 5 days, full
84
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THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
conversion was confirmed using GC. Purification of the MMA 2 epoxide was as follows:
first the solid present in the reaction mixture was removed by vacuum filtration. The solid
was washed with cold DCM. The filtrate was placed in the freezer to enhance
precipitation, after which the solid was again removed by vacuum filtration (again
washing with cold DCM). This procedure was repeated until no precipitation was
observed. If necessary some of the DCM was removed under reduced pressure to prevent
the build-up of large volumes of DCM. After the last vacuum filtration most of the DCM
was removed and a base extraction with a 1:1 mixture of demineralised water and
saturated sodium bicarbonate (NaHCO3) was carried out to remove any remaining acidic
degradation product of mCPBA. The DCM phase was then washed 3 – 5 times. DCM was
removed under reduced pressure before performing a vacuum distillation to obtain pure
MMA epoxide (e-MMA2, see Scheme 2) (bp. 110 °C at 500 mTorr). The obtained yields
were between 50 – 60%. 1H NMR δ 1.21-1.22 (d, 6H, CH3), 1.98 + 2.01 / 2.48 + 2.52 (AB,
d /d, 2H, CH2), 1.72 + 2.74 (d, 1H, OCH2), 2.94 + 2.96 (d, 1H, OCH2), 3.63 (s, 3H, OCH3),
3.71 (s, 3H, OCH3).
Epoxidation of AMS2. mCPBA (10 g, 5·10-2 mol) was dissolved in 60 mL DCM. The
mixture was cooled in an ice bath and AMS2 (10 g, 5·10-2 mol) was added slowly. After 2
hours of reaction in ice, full conversion was confirmed by GC. The AMS 2 epoxide (eAMS2) was isolated via the same purification method as described above (bp. 130 °C at 50
mTorr). Yield = 80%. 1H NMR δ 1.19(s, 3H, CH3), 1.33 (s, 3H, CH3), 2.23 + 2.26 / 2.46 +
2.49 (d, 2H, CH2), 2.59 + 2.60 / 2.65 + 2.66 (AB, d /d, 2H, OCH2), 7.0-7.3 (m, 10H, Ph).
Typical procedure for the attempted cationic ROP of e-MMA2. 2 mL of a BzOH stock
solution (0.3 mL in 100 mL dry DCM) and e-MMA2 (0.5 mL, 2·10-3 mol) were introduced
into the reaction flask followed by the addition of BF 3·OEt2 (20 μL, 1.3·10-4 mol). The
reaction was performed at 20 °C under a nitrogen atmosphere. After the desired reaction
time the reaction mixture was quenched by adding an excess of methanol. When carrying
out the reaction under starved feed conditions, a BzOH stock solution (0.3 mL, 2.8·10-3
mol in 50 mL dry DCM) and a BF3·OEt2 stock solution (1 mL, 0.8 mol in 50 mL dry
DCM) were first prepared. From both stock solutions 2 mL were introduced to a degassed
reaction flask. 1 mL of e-MMA2 was added via a syringe in portions of approx. 0.1 mL
(4·10-4 mol) over 3 hours. The reaction was carried out at 20 °C under N 2 for 24 h.
Sequential feed e-MMA2. A flask containing a stirrer bar was evacuated and refilled 3
times with N2 before use; the reaction flask was closed with a SubaSeal. e-MMA2 (0.35
mL, 1.4·10-3 mol) and BzOH (0.14 mL, 1.4·10-3 mol) were added followed by BF3·OEt2
(0.17 mL, 1.4·10-3 mol). Upon addition of BF3·OEt2 the solution turned orange. After 1
hour (at 20 °C under N2 atmosphere) an NMR sample was prepared by taking 0.1 mL of
the reaction mixture and quenching it with 10 μL of MeOH (BF3·OEt2 : MeOH ≈ 1:1).
This procedure was repeated until 3 more equivalents of e-MMA2 had been added.
85
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
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Chapter 4
Typical procedure for the polymerisations of THF in the presence of e-MMA2. A
flask equipped with a stirrer bar was evacuated and refilled with N 2 three times before use;
the reaction flask was closed with a SubaSeal. e-MMA2 (0.5 mL, 2·10-3 mol) and dry
tetrahydrofuran (THF, 3.25 mL, 5·10-2 mol) were added followed by BF3·OEt2 (50 μL,
4·10-4 mol). Samples were taken throughout the reaction. SEC samples were quenched by
the addition of a large excess of methanol or ethanol. 1H NMR samples were not quenched
but were measured directly. The reference reactions (homopolymerisations of THF) were
performed under identical conditions with reaction times of 24 hours.
NMR tube copolymerisation with THF-d8. THF-d8 (1 mL, 0.0125 mol) and e-MMA2
(0.4 mL, 2∙10-3 mol) were injected in an oven dried NMR tube and closed with a screw cap
with a septum in a glovebox. BF3·OEt2 (15 μL, 1.5∙10-4 mol) was injected via the septum
just prior to the NMR measurements. 1H NMR spectra are recorded every 15 minutes for
4 hours. After 3 hours 1 mL deuterated chloroform was injected to lower the viscosity of
the mixture.
Transesterfication with BzOH. A THF/e-MMA2 polymer (1.5 mL) was dissolved in 3
mL toluene. BzOH (0.43 g, 4∙10-3 mol) was added. Triazobicyclodecene (TBD, 8 mg, 60
μmol) was added as a catalyst and the reaction was allowed to run for 24 hours at 20 °C.
The product was dried in a vacuum oven at 60 °C.
Typical procedure for the investigation of catalytic species. Solid catalytic species
were introduced before degassing and evacuating the reaction flask, liquid catalytic
species were added after 3 degassing/evacuation cycles. e-MMA2 (0.5 mL, 2·10-3 mol) and
dry THF (3.25 mL, 5·10-2 mol) were then added to the flask. The polymerisations were
initiated by the injection of BF3·OEt2 (50 μL, 4·10-4 mol). Samples were taken throughout
the reaction. SEC samples were quenched by the addition of a large excess of methanol. 1H
NMR samples were not quenched but were measured directly.
Measurements. Gel permeation chromatography was carried out using a WATERS 2695
separation module, Model 2487 UV detector (254 nm), and Model 2414 differential
refractive index detector (40 °C). Injection volume used was 50 µL. Tetrahydrofuran
(Biosolve, stabilised with BHT) was the eluent, flow rate 1.0 ml/min. The column set used
was a PLgel guard (5 m particles) 50 × 7.5 mm precolumn, followed by 2 PLgel columns
in series of 500 Å (5 m particles) and 100 Å (5 m particles), respectively. Calibration
was performed using polystyrene standards (Polymer Laboratories, Mn = 370 up to Mn =
40,000 g/mol). Data acquisition and processing were performed using WATERS
Empower 2 software. 1H NMR spectra were recorded on a Varian Mercury Vx (400 MHz)
spectrometer at 400 MHz or Varian Mercury (200 MHz) at 200 MHz. Chloroform-d1 or
THF-d8 and tetramethylsilane were used as solvent and internal standard, respectively.
MALDI-ToF-MS was carried out using a voyager DE-STR spectrometer from Applied
Biosystems in reflector mode. Trans-2-(3-(4-tert-butylphenyl)-methyl-2-propenylidene)86
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
malononitrile doped with potassium trifluoroacetate was used as the matrix. It was
deposited from THF solution onto a stainless steel sample substrate and the solvent was
allowed to evaporate. The polymer was then deposited as a dilute (~1 mg/ml) solution in
THF. This resulted in each polymeric species being observed as its K + adduct with
molecular mass M+31. The spectrometer was calibrated using poly(ethylene oxide)
standards for the lower mass range and polystyrene standards for the higher mass range.
GC analysis was carried out on a Varian 450-GC Gas Chromatograph equipped with a
Varian CP-8400 Auto-Sampler. A temperature profile of 85 – 250 °C with a rate of 5
°C/min was used. The solvent used was toluene or THF with a flow rate of 2.0 mL/min.
87
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
RESULTS AND DISCUSSION
To investigate the potential of the system, with regard to both the epoxidation of
the (macro)monomer as well as the ring-opening polymerisation of the thus obtained
epoxidised (macro)monomers, the dimer of methyl methacrylate, MMA 2, was used as a
model compound in place of a MMA-based macromonomer.
Synthesis of e-MMA2
A dimer of methyl methacrylate, MMA 2, was made under CCTP conditions by
oligomerising MMA in the presence of COBF·(MeOH)2 and employing AIBN as the
radical source (Scheme 2a). The dimer was then isolated in high purity via vacuum
distillation from unreacted monomer and higher oligomers. The dimer contains a terminal
double bond, which is characteristic of CCTP-derived oligomers. Epoxidation of the
double bond was realised by mixing MMA2 with an excess of m-chloroperoxybenzoic acid
(mCPBA) at room temperature (Scheme 2b). Quantitative conversion of MMA2 to the
epoxide of MMA2 (e-MMA2) was achieved after 5 days. After removal of m-chlorobenzoic
acid (the side product of the reaction) and further purification by distillation, a 60% yield
of e-MMA2 was obtained. Figure 1 clearly shows the disappearance of the vinylic protons
of MMA2 and the appearance of a quintessential epoxy AB splitting pattern at 2.7 and 2.9
ppm.
Scheme
2. Synthesis
of 2.e-MMA
Catalytic
chain transfer
polymerisation
Scheme 2 Synthesis
of e-MMA
(a) Catalytic
chain
transfer polymerisation
(CCTP);
(b) Electrophilic (CCTP);
epoxidation (b)
of
2. (a)
MMA2.
Electrophilic
epoxidation of MMA2.
b'
d'
c'
e
d
b
c
a
7
6
5
4
3
ppm
1
Figure 1 H NMR spectra of e-MMA2 and MMA2.
Figure 1 1H NMR spectrum of e-MMA2 and MMA2.
88
2
1
0
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Attempted Cationic Ring Opening Homopolymerisation of e-MMA2
A cationic ring-opening polymerisation method was chosen over an anionic method
for use with monomers containing ester functionalities. In general, cationic ring-opening
polymerisation routes have an increased tolerance towards electrophilic functional groups
reducing the risk of back-biting as the chain end is neutral.19 The addition of an alcohol as
initiator to a cationic ROP has been shown to aid the prevention of small cyclic species
forming by switching the predominant mechanism from an activated chain end mechanism
(ACEM) to an activated monomer mechanism (AMM).20, 21 Indeed, glycidyl methacrylate
has been homopolymerised using poly(ethylene glycol) as an initiator and BF 3·OEt2 as the
catalyst.22
Ring-opening homopolymerisations of e-MMA2 were carried out using either
BF3∙OEt2 or BF15 as the catalyst in combination with benzyl alcohol (BzOH) as the
initiator. Polymerisations were originally carried out under batch conditions, however, a
starved-feed approach was also used in order to minimise back-biting reactions. The
results of the reactions are summarised in Table 1.
Table 1. Overview of cationic ring-opening polymerisation of the e-MMA2. Reaction time = 4 - 8h.
Initiator = benzyl alcohol.
Entry
Catalyst
Solvent
1
2
3
4
BF3∙OEt2
BF15
BF3∙OEt2
BF3∙OEt2
5
BF3∙OEt2
6
7
8
9
10
BF3∙OEt2
BF3∙OEt2
BF3∙OEt2
BF3∙OEt2
BF3∙OEt2
DCM
DCM
DCM
None
n-Butyl
acetate
MEK
DMF
Toluene
DMSO
None
BzOH:Catalyst:eMMA2a
1 : 16 : 45
1 : 16 : 45
1 : 16 : 45
1 : 16 : 45
Feed
Conditionsb
Batch
Batch
Starved
Sequential
Full RO, no pol.
Full RO, no pol.
Full RO, no pol.
Full RO, no pol.
1 : 16 : 45
Starved
Full RO, no pol.
1 : 16 : 45
1 : 16 : 45
1 : 16 : 45
1 : 16 : 45
1:4:1
Starved
Starved
Starved
Starved
Starved
Full RO, no pol.
No RO
Full RO, no pol.
No RO
Full RO, no pol.
Observation
Molar ratio; b Starved-feed conditions: monomer added in portions, where catalyst:monomer =
1:1.5 (molar ratio), excess monomer conditions: all monomer added in one batch at start of reaction.
a
1H
NMR results of the polymerisations show that both BF 3∙OEt2 and B(C6F5)3
(entries 1 and 2, Table 1) are capable of ring-opening e-MMA2. The BF3∙OEt2 / benzyl
alcohol system was investigated further (entry 4, Table 1), and the ring-opening of eMMA2 monitored using 1H NMR. e-MMA2 was added sequentially over a period of 4
hours, adding 1 equivalent of e-MMA2 after taking a sample every hour. The complete
disappearance of the epoxide ring protons combined with the appearance of new signals in
the polyether region of the spectrum (after every sequential addition of e-MMA2)
indicates that complete ring-opening of e-MMA2 has occurred and suggests that
polymerisation of the epoxide had taken place (Figure 2). SEC analyses of the reaction
products, however, revealed that no high molecular weight products had formed and that
89
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
the major product had only a slightly higher molecular weight compared to e-MMA2
(Figure 3). Although a minor amount of higher molecular weight oligomers can also be
seen, this does not correspond to the molecular weights expected. Thus, the 1H NMR and
SEC results appeared to contradict one another, requiring further investigation using 2D
NMR in order to probe the active mechanism.
Increase in signal at 3.4 ppm
Increasing equivalents of e-MMA2 added
Ring-opening of epoxide
e-MMA2
3.5
3.4
2.6
2.5
2.4
2.3
2.2
2.1
2.0
1.9
ppm
wLog(M)
Figure 2. Partial 1H NMRs (200 MHz, CDCl3) showing the increase in intensity of signal at 3.4
ppm and the opening of epoxide rings.
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Log(M)
Figure 3 Molecular
weight distribution of
(
) and the reaction
product
entry 4,the
Table 1 reaction
(-----).
Figure 3. Molecular weight
distribution
ofe-MMA
e-MMA
) ofand
product of entry 4,
2 (______
Table 1 (-----).
2
______
90
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
According to the conditions used, ring-opening and polymerisation should occur
via the activated monomer mechanism (AMM), in which the alcohol initiates the polymer
chain resulting in a neutral chain end which cannot undergo back-biting reactions. This is
clearly not the case in this situation. To probe the nature of the active mechanism, 1H
NMR was used to monitor the ring-opening reaction over time. To a 1:1 molar mixture of
BF3OEt2 and BzOH, e-MMA2 was added sequentially in equimolar batches. After each
addition, 1H NMR was used to confirm complete ring-opening (Figure 2) and SEC to
confirm that propagation was not the major ring-opening mechanism (Figure 4). It should
be noted, however, that an increase in the complexity and number of signals in the
spectrum suggests the possibility of oligomerisation if not polymerisation. It was noticed
that a signal at 3.4 ppm in the 1H NMR (which was previously attributed to splitting of
the methoxy protons) increased as more equivalents of e-MMA2 were added (Figure 2).
1 eq. e-MMA2
2 eq. e-MMA2
3 eq. e-MMA2
4 eq. e-MMA2
5 eq. e-MMA2
RI Signal
6 eq. e-MMA2
17
18
19
20
21
22
Elution Time/Minutes
Figure 4. SEC chromatogram of multiple additions of e-MMA2 (1-5, dashed) to benzyl alcohol in
the presence of BF3.OEt2 compared to e-MMA2 (solid).
1H-13C
gHMBC was used to identify the peak at 3.4 ppm observed in the 1H NMR
spectrum. Strikingly, it seems that only one of the methoxy groups of e-MMA2 is coupled
to a carbonyl (Figure 5a). This indicates that a methoxy group has been eliminated. Ringopening of e-MMA2 with an alcohol (in this case BzOH), results in a species that contains
a hydroxyl group. In a normal cationic ring-opening mechanism, this hydroxyl group
opens the next monomer and propagation continues. However, it is thought that the
alcohol end group may also back-bite onto the ester functionality, forming a ring and
liberating methanol (Scheme 3). The epoxide ring can be opened on both sides (1,2 and
2,1) which results in a 5 or 6 membered ring (2 and 3), respectively. However, the signals
characteristic to a six-membered ring (doublet around 5.4 ppm) are not present, implying
91
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
that only 5-memebered rings are formed. The instability of the 6-membered ring
compared to the 5-membered ring is supported by literature on cyclic esters. 45, 46 The
methanol liberated upon cyclisation of e-MMA2 into 2 plays a further role as it can also
act as an initiator, ring-opening e-MMA2. The ring-opened epoxide can then also backbite to form a 5-memebered ring, again liberating methanol. This hypothesis is supported
by the 1H-13C gHMBC results, which shows cross-peaks at 2.9/3.3 (1H) and 60 (13C) ppm
and 3.0 (1H) and 75 (13C) ppm corresponding to methanol-initiated e-MMA2 (Figure 5b).
(a)
(b)
Figure 5. Partial 1H-13C gHMBC NMR spectra of entry 10, Table 1.
92
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
2
Scheme 3. Cyclic back-biting of e-MMA2 to 2.
Using the information provided by 1H-13C gHMBC in combination with the SEC
results, a mechanism for the reactions has been proposed (Scheme 4). At the start of the
reaction, benzyl alcohol can ring-open e-MMA2 either via a 1,2 or a 2,1 mechanism.
Opening on the 2,1 side results in a positioning of the hydroxyl end group in which backbiting can occur to form a 6-membered ring (3). Compound 3 is not observed in 1H and
13C NMR, however, suggesting that the species is less stable and possibly even formed
reversibly. Ring-opening via a 1,2 mechanism, however, gives rise instead to the more
stable 5-membered ring (2), and is therefore considered a dead chain end. In both cases
methanol is liberated, which can initiate further ring-opening. Consecutive ring-opening
via a 2,1 mechanism may result in propagation and thus some oligomers can be formed (as
observed in SEC), however, as soon as the epoxide is ring-opened via the 1,2 route further
propagation is prevented. It is hypothesised that the major mechanistic pathway is the
ring-opening of e-MMA2 with methanol followed by cyclisation to 2 with further
liberation of methanol. This is consistent with the information provided in Figures 2 and
3.
93
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
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Chapter 4
Scheme 3 Proposed mechanism for the formation of 5-membered rings
Scheme 4. Proposed mechanism for the formation of 5-memebered rings.
Efforts were made to minimise cyclisation by investigating a range of solvents
from non-coordinating to strongly coordinating solvents and in bulk, varying the catalyst
concentration as well as the rate of e-MMA2 addition (Table 1). Similar results were found
for all experiments (excluding reactions carried out in highly coordinating solvents, which
showed no ring-opening at all), with the major product being that of 2. The ring-opening
polymerisation of glycidyl methacrylate, which also contains both epoxide and ester
moieties, under similar conditions does not show any inclination towards back-biting but
instead homopolymerises.22
It is clear that the back-biting cyclisation reaction which occurs when e-MMA2 is
in the presence of an alcohol and a Lewis acid is the predominant mechanism occurring
and also proceeds at a higher rate than propagation. As such, homopolymerisation of eMMA2 to higher molecular weights appears to be difficult.
94
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Polymerisation of THF in the Presence of e-MMA2
It has been made apparent that the homopolymerisation of e-MMA2 poses several
problems. However, when unquenched samples of the homopolymerisation of e-MMA2
were prepared in THF for SEC analysis, a polymer peak, unattributable to p(e-MMA2)
was observed. It was hypothesised that the introduction of a comonomer, such as THF,
may allow the incorporation of e-MMA2 and minimise the chance of back-biting side
reactions.
The homopolymerisation of THF in the presence of a Lewis acid (such as BF3) is a
very slow reaction, reaching a very low conversion even after 24 hours (entry 1, Table 2)
regardless of whether a protic initiator, such as methanol, is used. 20 Surprisingly, when the
polymerisation of THF is carried out in the presence of e-MMA2 a dramatic increase in
the rate of reaction is observed, with the maximum conversion of 75% (for a bulk THF
polymerisation) reached within only 6 hours (entries 2-4, Table 2). Compared to the 2%
conversion achievable after 24 hours in the absence of e-MMA2, the rate enhancement
effect of e-MMA2 is impressive. Indeed, increasing the concentration of e-MMA2 in the
THF polymerisations continues to increase the rate of reaction, attaining maximum
conversion in under 90 minutes (Figure 6a). This is also illustrated by the slopes of firstorder kinetic plots (Figure 6b), however, whether this increase originates from an increase
in propagation rate coefficient, kp, or from an increase in the number of active species,
[P*], cannot be elucidated from these values alone. Note that as the reaction is carried out
in bulk, the maximum achievable conversion is 75 % due to the temperature-dependent
nature of equilibrium polymerisations.19
Table 2. Overview of cationic ring-opening polymerisations of THF and e-MMA2.
Entry
1
2
3
4
BF3 : THF : eMMA2a
1 : 100 : 0
1 : 100 : 10
1 : 100 : 4
1 : 100 : 1
Mn b/gmol-1
PDI
28,000
950
1,900
6,000
1.9
1.6
2.4
3.2
Reaction
time/hrs
24
6
6
6
THF
Conversionc
< 0.02
0.75
0.75
0.75
Calculated
Mnd/gmol-1
760
1,570
5,600
Molar ratio; b Measured by GPC against polystyrene standards; c Conversion of THF determined
using NMR based on the relative integrals of CH2 – CH2 in THF (1.85 ppm) and CH2 – CH2 in
poly(THF) (1.51 ppm) ; d Molecular weight based on the assumption that there is one e-MMA2 per
chain, adjusted for conversion of THF.
a
95
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
(a)
Chapter 4
(b)
kp[P*] = 0.0173
100
kp[P*] = 0.0121
1.4
90
80
1.2
1.0
60
-ln(1-XTHF)
% ConversionTHF
70
50
40
0.8
kp[P*] = 0.0063
0.6
30
0.4
20
0.2
10
0
0.0
0
50
100
150
200
250
300
0
20
40
60
80
100
120
Time /Minutes
Time/Minutes
Figure 6. (a) Conversion of THF versus time and (b) Semilogarithmic kinetic plots for the
copolymerisation of e-MMA2 and THF. Ratio of e-MMA2:THF = 10:100 (■), 4:100 (●) and 1:100
(▲).
Figure 4 Conversion of THF versus time for the copolymerisation of e-MMA2 and THF. Ratio of e-MMA2:THF = 10:100 (■), 4:100 (●) and
1:100 (▲).
A rate increase in a THF polymerisation is not without precedent, 23, 24 but in those
cases the epoxide initiates the chains and copolymerises with THF. However, e-MMA2
does not appear to be consumed until the end of the reaction when THF has reached
maximum conversion (discussed later), indicating that an alternative mechanism is at play
here.
To investigate the origin of this rate enhancement, a THF homopolymerisation
in the absence of e-MMA2 was carried out and after 24 hours a sample was taken for SEC
analysis. The analysed sample showed a high Mn of 28,000 Da, but as expected a low
degree of conversion (2 %), showing that propagation is not the limiting factor. The
initiator efficiency at this stage was calculated to be less than 1 %, implying that initiation
is the problematic step. To the same low-conversion THF homopolymerisation, e-MMA2
was added and samples taken at regular intervals over 30 minutes, and again analysed
using SEC (Figure 7). A new, lower molecular weight distribution began to appear after
15 minutes, and continued to increase with time (and conversion). The original higher
molecular weight distribution remained almost unchanged over the course of the reaction,
clearly showing that e-MMA2 enhanced the initiation step of the polymerisation, rather
than increasing the rate of propagation.
96
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
wLog(M)
0 min (THF homopol.)
15 min
20 min
30 min
3.0
3.5
4.0
4.5
5.0
5.5
Log(M)
Figure 7. Molecular
weight
distributions
of homopolymerisation
THF
and
afterareaddition of the
Figure 5 Molecular
weight distributions
of homopolymerisation
of THF and after addition of theof
MMA
-epoxide.
Distributions
corrected for THF conversion. Measured in THF against PS standards.
MMA2-epoxide.
Distributions are corrected for THF conversion. Measured in THF against PS
standards.
2
(a)
d’’
g
f
Increasing THF Conversion
g
b’
d’
b’
c’
e
d’
e
pTHF
THF
f
h
h
(b)
d’’
MeO-
c’
b’
Figure 8.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
2.00
0.5
1.75
1.50
ppm
Figure 6 1H NMR (200 MHz, THF-d8) of the polymerisation of (a) THF in the presence of e-MMA2 (entry 2, Table 2) and (b) THF-d8 in the
1presence of e-MMA2.
H NMR (200 MHz, THF-d8) of the polymerisation of (a) THF in the presence of
e-
MMA2 (entry 2, Table 2) and (b) THF-d8 in the presence of e-MMA2.
In order to determine the behaviour of e-MMA2 throughout the course of the
reaction, 1H NMR was used to monitor the polymerisation of THF in the presence of eMMA2 with increasing conversion. Firstly, a polymerisation using non-deuterated THF
97
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
and e-MMA2 was performed to determine the conversion of THF into polyTHF over 3
hours (Figure 8b). It was observed that although THF polymerised rapidly, the signals
corresponding to e-MMA2 did not appear to change until the maximum conversion of
THF had been reached. To probe the behaviour of e-MMA2 during the polymerisation
more closely, deuterated THF (THF-d8) was used as the monomer (Figure 8a). In absence
of the broad THF signals it became apparent that not only did the epoxide signals not
alter until maximum conversion of THF had been reached, but also that the final form of
e-MMA2 is that of the 5-membered ring observed in the homopolymerisation of e-MMA2,
2. This suggests that the rate enhancement is not due to the copolymerisation of e-MMA2,
but that e-MMA2 first catalyses the initiation of the THF polymerisation and then
subsequently undergoes a reaction of its own.
From the 1H NMR data presented in Figure 8, it is abundantly clear that e-MMA2
is in the 5-membered ring form, however, to determine whether 2 is attached to the
polyTHF chain, MALDI-ToF-MS was used. Poly(THF-d8) was analysed in order to
distinguish 2 from poly(THF), as 2 with a methoxy initiating group has the same
molecular mass as 3 non-deuterated THF units. Figure 9 shows three distributions,
corresponding to polyTHF with end groups of two 2 units, one 2 and one methoxy, and
two methoxy units.
1980
2000
2020
2040
2060
2080
2100
2120
2140
m/z
Figure 9. MALDI-ToF-MS spectrum of the copolymerisation product of THF-d8 and e-MMA2
(entry 5, Table 2).
Figure 7 MALDI-ToF-MS spectrum of the copolymerisation product of THF-d8 and e-MMA2 (entry 5, Table 2).
The end groups observed when THF is polymerised in the presence of e-MMA2
are shown in Figure 10. We believe that the methoxy end groups originate as a result of
chain scission. The scission of chains via the in-situ formation of BF4- H+, cleaves the ether
bonds, which can then react further with methanol (produced as a result of the formation
98
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
of the cyclic structure, 2) resulting in methoxy-based end groups. As chain scission can
occur at any point in the chain and multiple times within the same chain, polymers with
two methoxy end groups can also be formed.
Figure 10. MALDI-ToF-MS of entry 3 (Table 2), quenched with (a) ethanol and (b) methanol.
To further confirm the presence of 2 as the end group, a transesterification reaction
with benzyl alcohol in the presence of triaxobicyclodecene (TBD) was carried out. Benzyl
alcohol is UV active at 254 nm, whereas polyTHF and e-MMA2 are UV inactive.
Transesterification replaces the methyl groups of the esters in e-MMA2 with benzyl
groups. Measurement of the transesterified polymer using a SEC coupled to a UV
detector, reveals a successful transesterifiaction reaction and also that there are ester
groups present in the polymers proving that 2 is attached to the polymer chains (Figure
11). Although the UV signal does not show the same shape as that of the RI signal, it is
possible that complete transesterification has not occurred, and MALDI-ToF-MS has
already shown that not all chains contain a unit of 2. To quantify the amount of 2 units
attached to the polymer chain, the reaction products (without being worked up) were
heated under vacuum to remove any back-bitten e-MMA2 (similar to end group 2). The
back-bitten e-MMA2 can be distilled at 180 ºC (200 mTorr), however, the reaction
product released no material under the same conditions. Even at higher temperatures, no
volatiles could be removed until 300 ºC (200 mTorr) when a small amount of low
molecular weight polyTHF was removed. This means that all e-MMA2 has been
99
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
consumed corresponding to a theoretical average of one 2 group per chain (entries 2-5,
Table 2), indicating that addition (and cyclisation of e-MMA2) is the main method of
end-capping.
w(Log(M))
UV
DRI
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
Log(M)
Figure 11. Molecular weight distribution of transesterified THF – e-MMA2 copolymer. Measured
in THF against
standards.
FigurePS
8 Molecular
weight distribution of transesterified THF- e-MMA2 copolymer. Measured in THF against PS standards.
Over the course of these experiments it was observed that the evolution of
molecular weight of the polymerisation of THF in the presence of e-MMA2 showed an
unexpected trend. Upon increasing conversion of THF, the molecular weight increases,
however, once conversion starts to reach the maximum obtainable for a bulk THF
polymerisation, and e-MMA2 begins to react, the number-average molecular weight starts
to decrease (Figure 12). We speculate that this could be due to chain scission. The exact
nature and mechanism by which this occurs is outside the scope of this work, however, we
suspect that the presence of trace amounts of water in the system could react with BF 3 to
generate HF. The acidic H+ is capable of cutting chains, reducing the molecular weight. 25
We suspect that this is on-going throughout the reaction, but at the beginning the rate of
chain growth dominates, and as full conversion is reached, chain scission produces a
greater effect on the molecular weight.
100
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
16000
2.8
14000
2.6
2.4
2.2
10000
2.0
PDI
Mw/gmol-1
12000
1.8
8000
1.6
6000
1.4
0
10
20
30
40
50
60
70
80
% Conversion
Figure 12. Evolution of molecular weight of entry 3 (Table 2) with increasing conversion.
To ensure that e-MMA2 is indeed the active catalyst and that the rate enhancement
is not caused by any impurities, mCPBA (and its degradation product) and NaHCO 3 (both
of which were used in the synthesis of e-MMA2 and thus may be present in trace amounts
in the purified e-MMA2) were added to a THF polymerisation. In both cases, no catalytic
effect was observed (Table 3, entry 2 and 3). e-MMA2 has three functional groups (two
esters and an epoxide) to which the observed catalytic effect could be attributed. MMA 2
and dimethyl glutarate both contain two ester groups, which are 3 carbons apart, with and
without the restriction of a double bond, respectively. These two compounds (Table 3,
entries 4 and 5) were added to a THF homopolymerisation but again only a low
conversion was reached, indicating that the epoxide moiety is necessary. Entry 6 (Table
3), however, shows that solely an epoxide is not sufficient, as e-AMS2 also showed no
catalytic effect. Interestingly, e-MMA (entry 7, Table 3) does catalyse the reaction, albeit
more slowly than e-MMA2. This indicates that both epoxide and carbonyl moieties are
required for efficient catalysis of the homopolymerisation of THF.
101
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
Table 3. Potential catalysts for the polymerisation of THF. Reaction time = 8 hours.
1
2
3
(Potential)
Catalyst
None
mCPBA
NaHCO3
4
MMA2
Entry
5
6
7
8
Chemical Structurea
-
‘Catalyst’ :
BF3: THF :
0 : 1 : 100
0.25 : 1 : 100
0.25 : 1 : 100
4 : 1 : 100
Dimethyl
glutarate
AMS2-epoxidee
e-MMA
e-MMA2
% THF Conversionb
< 2c
< 1c
<1c
< 1c
4 : 1 : 100
< 1c
4 : 1 : 100
< 1c
4 : 1 : 100
76c/20d
4 : 1 : 100
75d
Functional group in red; b Conversion of THF determined using 1H NMR; c After 24 hours; d After
2 hours.
a
To place the catalytic effect of e-MMA2 on the polymerisation of THF in
perspective, a comparison can be made between the polymerisation rates for the two most
common THF polymerisation methods and the rate observed upon addition of e-MMA2.
Diphenyliodonium hexafluorophosphate is often used to perform photo-induced living
THF polymerisations. Mah et al.24, 26 investigated the bulk polymerisation of THF in the
presence and absence of an epoxide (epichlorohydrin; ECH) using diphenyliodonium
hexafluorophosphate as the intiator. It should be noted that in this case, the acceleration of
the THF polymerisation is attributed to an increased concentration of 5-membered
oxonium ions, which is clearly not the case for our system as analysis of 1H NMR spectra
measured throughout the reaction reveals that the epoxide is barely consumed in the
beginning stages of the reaction. Mah et al. also found that at high concentrations of
102
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
epoxide (equimolar to THF), the polymerisation of THF was found to be significantly
retarded by incomplete consumption of the epoxide at the start of the reaction. This is in
contrast to our system, which shows a higher rate of polymerisation upon increasing the
e-MMA2 concentration (within the range of e-MMA2 concentrations described here). In
addition, consumption of e-MMA2 only takes place towards the end of the THF
polymerisation. In the presence of ECH in low concentrations (2.2 mol% compared to
THF) 70% conversion was achieved after around 10 hours. In the absence of ECH, a THF
conversion of around 70% was obtained only after 20 – 30 hours (depending on the
temperature). The molecular weights of the polyTHF are much higher (approx.150,000
g∙mol-1) compared to those synthesised using the THF/e-MMA2 system as the initiator
concentration was three times lower than used in the presented experiments. Pchlorophenyldiazonium hexafluorophosphate has also been used as an initiator for cationic
THF polymerisation.27, 28 The conversion of THF reached only 13.8% after 1.5 h (at 20
°C). It must be noted that the obtained molecular weights were again much higher than
reported for the THF/e-MMA2 system, approximately 10,000 g∙mol-1 at a catalyst
concentration of 1.75∙10-2 M (8 times lower than the concentration of e-MMA2 used).
Although these results cannot be directly compared to the polymerisation of THF in the
presence of e-MMA2, it appears that e-MMA2 is a very effective catalyst for the
polymerisation of THF, giving the maximum 70% conversion after 1.5-3 h (depending on
the concentration of e-MMA2 added). In addition, e-MMA2 has the added advantage that
the polymers produced contain a functional end group, which could be potentially used for
further reactions.
Coupling Reactions of e-MMA2
Epoxides are commonly exploited due to the ease at which they undergo
nucleophilic addition reactions. In addition to polymerisation reactions, single reactions
between epoxides and nucleophiles can also be exploited in polymer chemistry, for
example cross-linking and coupling reactions with nucleophiles like amines and alcohols.
Using e-MMA2 as a model for longer epoxidised macromonomers, coupling of the epoxide
to a (macro)alcohol and a diamine were investigated (Scheme 5).
Scheme 5. Coupling reactions of e-MMA2.
103
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
Coupling to hydroxy-terminated poly(ethylene glycol)
Studies into the homopolymerisation of e-MMA2 have shown that the epoxide ring
of e-MMA2 is readily ring-opened by an alcohol. Model reactions using e-MMA2 were
investigated to determine whether a selective reaction between a macro-alcohol and an
epoxidised macromonomer was possible, giving rise to a block copolymer (Scheme 5).
Poly(ethylene glycol) (PEG) can be purchased with hydroxyl end groups, although
on closer inspection via MALDI-ToF-MS actually contains a mixture of hydroxyl and
methoxy end groups (Figure 13a). Reaction of the hydroxyl end groups with e-MMA2 is
expected to follow a similar pathway to that shown in Scheme 3, therefore back-biting to
form end group 2, liberating methanol. Reaction of PEG with e-MMA2, at 20 ºC in DCM
using BF3.OEt2 as the catalyst, did not result in the formation of a block copolymer,
presumably due to the faster reaction of the liberated methanol compared to PEG with eMMA2. However, when the same reaction is carried out in the melt (at 60 ºC) under
vacuum to remove the liberated methanol, the reaction does occur. MALDI-ToF-MS
shows five distributions; three of which correspond to unreacted PEG (Figure 13b). The
remaining two distributions correspond PEG with one end group of 2 attached (the other
end group is either a hydroxyl or methoxy group originating from the starting PEG). As
expected, the PEG with two methoxy end groups does not react and shows a higher
intensity compared to the other unreacted PEG distributions (and in comparison to the
MALDI-ToF-MS of PEG, Figure 13a). The PEG with two hydroxyl units only appears
to react with one e-MMA2 unit, leaving one hydroxyl group unreacted. The reason for
this is unclear, however, it may simply be a result of the relative stoichiometries as the
quantity of hydroxyl groups is unknown and some e-MMA2 may be ring-opened by any
methanol not successfully removed from the system.
However, a proof of principle has been shown for the coupling of e-MMA2 with
hydroxyl-terminated PEG. The application of this proof of principle to longer
macromonomers is likely to require further optimisation in order to achieve good contact
between the polymers in the melt and the removal of methanol.
104
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
(a)
(b)
1420
1440
1460
1480
1500
m/z
Figure 13. MALDI-ToF-MS of (a) hydroxyl-terminated poly(ethylene glycol) and (b) product of
the reaction between poly(ethylene glycol) and e-MMA2.
Coupling to 1,6-hexanediamine
The reaction of epoxides and amines is well-known and particularly important for
crosslinking reactions of resins and coatings. Primary amines react with two epoxides,
therefore primary diamines can react with up to four epoxides (Scheme 5). When a
macroepoxide is used this would result in a 4-armed star polymer. As a proof of principle
reaction, 1,6-hexandiamine was reacted with 4 equivalents of e-MMA2 at 60 ºC in the
absence of any catalyst. After less than 2 hours, the e-MMA2 has begun the react with the
diamine, illustrated by the reduction of the epoxide signals (2.75 and 3 ppm) and
appearance of new signals corresponding to the monoaddition product at 2.8 and 3.2 ppm.
After reaction at 60 ºC for a week, two equivalents of e-MMA2 had reacted, but reaction of
the thus formed secondary amines with e-MMA2 did not appear to have commenced.
Raising of the reaction temperature to 90 ºC for a further 5 days did result in the
consumption of further equivalents of e-MMA2 with a final conversion of 75%. This
corresponds to an average of 3 epoxides per diamine. Unfortunately, even in the absence
of a Lewis acidic catalyst, some e-MMA2 appears to have ring-opened to form the backbitten 5-membered ring structure, as indictaed by the splitting of the methoxy signals at
3.6 - 3.8 ppm and the appearance of a new signal at 2.3 ppm. Presumably this is due to the
presence of trace amounts of water in combination with elevated temperatures.
105
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
Epoxide ring
After 2 hours
After 6 hours
Monoaddition product
After 2 weeks
Diaddition product
4
3
2
1
ppm
Figure 14. 1H NMR (400 MHz, CDCl3) spectra of the coupling of e-MMA2 and 1,6-hexyldiamine.
RI signal
The reaction was also monitored using SEC. The chromatogram obtained at the
end of the reaction shows 4 distinct peaks or shoulders, corresponding to the 4 additions
of e-MMA2 to the diamine, however, the largest peak corresponds to 3 additions,
confirming the values obtained from 1H NMR.
Figure 28: GPC chromatogram of coupling reaction between 1,6-hexanediamine and
ME. Measured against polystyrene___
Normalised to ME.
Figure 15. GPC chromatogram
of e-MMA2 ( standards.
), reaction
product of e-MMA with 1,6___
___
hexyldiamine after 6 hours ( ) and 2 weeks ( ).
This model reaction shows that it is possible to couple epoxidised
(macro)monomers to diamines with around 75% efficiency. Further optimisation of
conditions, and potentially the use of a suitable catalyst, is required before application to
longer epoxidised macromonomers.
106
Chapter 4
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
CONCLUSIONS
Our attempts to synthesise a new class of polymers containing a polyether
backbone and methacrylic side chains via the combination of catalytic chain transfer
polymerisation and ring-opening polymerisation were unsuccessful. However, whilst
investigating the homopolymerisation behaviour of a CCTP-derived epoxide, e-MMA2, it
was discovered that after ring-opening the epoxide forms a stable 5-membered ring
incapable of propagation, expelling methanol. Furthermore, attempts to copolymerise the
epoxide with THF suggest that e-MMA2 is not copolymerised but instead acts as an endcapping agent, again forming the same 5-membered ring observed in the
homopolymerisation of the monomer. In addition, a surprising catalytic effect of e-MMA2
on the polymerisation of THF was seen, significantly increasing the rate of initiation.
Model studies using e-MMA2 have shown that coupling reactions between the (macro)
epoxide and either hydroxy-terminated poly(ethylene glycol) or 1,6-hexyldiamine are
successful opening up new possibilities to synthesise block or star polymers based on
longer epoxidised macromonomers.
107
THE POLYMERISATION BEHAVIOUR OF EPOXIDISED MACROMONOMERS
DERIVED FROM CATALYTIC CHAIN TRANSFER POLYMERISATION
Chapter 4
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-
Chapter 5
STRATEGIES TO EMPLOY THIOL
CHEMISTRY IN THE SYNTHESIS OF BLOCK
COPOLYMERS
This chapter has been published in part as: J. Mazzolini, O. Boyron, V. Monteil, F.
D’Agosto, C. Boisson, G.C. Sanders, J.P.A. Heuts, R. Duchateau, D. Gigmes and D. Bertin,
Polymer Chemistry, 2012, DOI: 10.1039/C2PY20199B
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
Chapter 5
ABSTRACT
In this chapter, three different routes to synthesising block copolymers using thiol
chemistry have been investigated. Nucleophile-mediated thiol-ene chemistry has been
employed to modify CCTP-derived macromonomers which have subsequently been used
to make triblock copolymers with diisocyanates. Thiol-ene chemistry has also been
investigated to be a tool towards coupling thiol-terminated polyethylene and methyl
methacrylate-based macromonomers to form block copolymers. Unfortunately the
conditions required to facilitate this reaction were found to be incompatible with poorlysoluble polymers such as polyethylene. The same block copolymers could however be
made when using the thiol-terminated polyethylene as a chain transfer agent in a radical
polymerisation.
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INTRODUCTION
As discussed in more detail in Chapter 1, the synthesis of polymers with complex
architectures are of great interest to both academics and industrial chemists alike due to
their unique properties.1 In order to include well-defined segments into these copolymers,
pre-formed polymeric building blocks can be used, and by combining these building blocks
via different chemistries or polymerisation techniques, a range of properties can be
obtained that are not possible using a single polymerisation technique. Thiols and their
chemistry have been used extensively in polymer synthesis in order to promote the
combination of multiple methodologies.2 The most predominant reactions, however,
involve either the use of thiols as chain transfer agents (CTA) or in thiol-ene reactions.
Thiol-ene reactions involve the reaction of a thiol over a double bond, and can
proceed via a radical route or a nucleophilic addition. 3-5 Whereas radical thiol-ene
chemistry is applicable to almost any unsaturated group or thiol (although reactivity
strongly depends on the degree of activation of the unsaturation and thiol), the
nucleophilic addition of thiols over a double bond can only occur when the thiol is
conjugated to a carbonyl bond. Conversely, the use of radical thiol-ene chemistry with
CCTP-derived macromonomers runs the risk of fragmentation of the polymer chain due
to the presence of radicals. As discussed in more detail in Chapter 1, the phosphinemediated nucleophilic addition (also known as a thia-Michael addition) of thiols has been
revitalised in recent years, predominantly due to the work of Haddleton and co-workers
applying the technique to CCTP-derived macromonomers.6-9 A range of different thiols,
from alkyl thiols to more functional thiols with hydroxyl, benzyl or even α-keratin groups
have been added to the ω-unsaturated group of a CCTP-derived macromonomer.10-12
Alternatively, the addition of thiol-containing compounds to a radical
polymerisation can result in a decrease in molecular weight due to an increases rate of
chain transfer (to thiol). The resulting polymers contain the CTA as end groups,
providing a route to introducing new functionalities to the ends of the polymer chains.
Further, use of a macro-CTA is a route to synthesising block copolymers based on the
CTA and the polymerised monomer.13
Recently, an interesting macro-thiol has been synthesised by Mazzolini, et al.
consisting of a polyethylene chain terminated with a thiol. This thiol-terminated
polyethylene (PE-SH) gives rise to the possibility of synthesising block copolymers via a
range of routes, and in fact has already been used as a macroinitiator for the ring-opening
polymerisation of D, L-lactide.14 PE-SH has been made via a range of methods but in all
cases well-defined linear polyethylene is first made via a catalysed chain growth (CCG)
route (Scheme 1) using a neodymium-based catalyst in conjunction with n-butyl n-octyl
magnesium.15-18 The resultant polymer chains are attached to a magnesium centre which
can be used to introduce a range of functionalities and end groups to the chain end. 14, 19-22
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Chapter 5
Scheme 1. General mechanism for the catalysed chain growth (CCG) of polyethylene, where M is a
transition metal species and M’ is a main group organometallic species.
Modification of a carbon-magnesium bond to a carbon-thiol bond can be affected
via a variety of routes as described by Mazzolini, et al.21 The most efficient route however,
employs the use of a difulfiram to form PE-dithiocarbonate followed by reduction using
LiAlH4 to PE-SH. Functionalities of around 90% are achievable.
The following chapter is split into several sections. Firstly, small thiols will be
used to modify macromonomers via a thia-Michael addition for use in isocyanate reactions
to form triblock copolymers. Secondly, an investigation into the behaviour of the abovementioned thiol-terminated polyethylene in thiol-ene reactions with CCTP-derived
macromonomers and as a chain transfer agent will be discussed as a route to synthesising
block copolymers of ethylene and (meth)acrylates.
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EXPERIMENTAL SECTION
Materials. Tributyl phosphine (PBu3, Sigma-Aldrich, 97%) and dimethylphenyl
phosphine (DMPP, 99%, Sigma-Aldrich) were purchased and used as received. Methyl
methacrylate (MMA, 99%), tert-butyl acrylate (BA, 99%) and styrene (S, 99%) were
purchased from Sigma-Aldrich, and passed over a column of activated basic alumina to
remove the inhibitor. The bis(methanol) complex of bis[(difluoroboryl)dimethylglyoximato]cobalt(II)bis(methanol), COBF, was prepared as described previously. 23, 24
Azobis(isobutryonitrile) (AIBN, Aldrich) and VAZO-88 (Wacker) were recrystallised
twice from methanol. PE-SH was synthesised by Mazzolini, et al according to conditions
described in the literature using the reduction of in situ generated dithiocarbamate to end
functionalise the PE (Mn= 1440 g.mol-1, PDI = 1.3, functionality = 87%).21 Toluene
(Biosolve) and acetone (Merck, ultra-dry) were used as received. Hexamethylene
diisocyanate (HDI), poly(propylene glycol) toluene 2,4-diisocyanate (PPG, Mn = 2,300
g/mol, ~3.6% isocyanate) and dibutyl tin dilaurate (DBTDL) were purchased from SigmaAldrich and stored under an argon atmosphere. All other reagents were purchased from
Sigma-Aldrich and used without further purification.
Synthesis of MMA Dimer (pMMA2). A 250 mL round-bottom flask was charged with
AIBN (213 mg, 1.3 mmol) and COBF (100 mg, 0.26 mmol) inside a glovebox. The flask
was closed with a rubber septum and removed from the glovebox. MMA (100 mL, 936
mmol) was injected via the septum and the mixture was stirred at 80 ºC for 6 hrs under a
nitrogen atmosphere. The oligomerisation was quenched by addition of hydroquinone and
cooling in ice. Residual monomer was removed under reduced pressure at room
temperature. The residual yellow oil was vacuum distilled (325 mTorr, 38 ºC) to afford
the pure dimer in 42% yield. 1H NMR (400 MHz, chloroform-d1, 298 K) δ 6.12 (s, 1H,
=CH2), 5.43 (s, 1H, =CH2), 3.64 (s, 3H, OMe), 3.55 (s, 3H, OMe), 2.52 (s, 2H, CH 2), 1.07
(s, 6H, Me2) ppm. 13C NMR (100 MHz, chloroform-d1, 298 K) δ 177.3 (C=Oester), 176.8
(C=Oacryl), 137.3 (C=CH2), 127.8 (=CH2), 51.8 (OMe), 51.6 (OMe), 42.8 (CMe2), 41.0
(CH2), 24.8 (CMe2) ppm.
Typical Procedure for the Synthesis of pMMA Macromonomers. COBF (0.83 mg, 2.1
μmol) and AIBN (42 mg, 256 μmol) were placed in a flask equipped with a stirrer bar and
underwent three vacuum-argon cycles. MMA (50 mL) and toluene (50 mL) were
deoxygenated and added using a syringe. The mixture was heated to 60 ºC and allowed to
react for 16 hours, after which it was quenched by cooling in ice and the addition of
hydroquinone. Residual monomer and solvent were removed via vacuum evaporation
immediately after stopping the reaction. The resulting product was redissolved in THF,
passed over a column of basic alumina and dried at 80 ºC in a vacuum oven for at least 48
hours to remove all traces of solvent.
Base-Catalysed Reaction Between pMMA2 and 2-Mercaptoethanol. pMMA2 (0.25 g,
1.25 mmol) and 2-mercaptoethanol (0.09 mL, 1.25 mmol) were charged in a vessel. DMPP
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Chapter 5
(0.18 mL, 1.25 mmol) was added under a N 2 atmosphere. The reaction was allowed to stir
under N2 for 19 hours at RT.
Synthesis of Hydroxyl-Functionalised Macromonomer. MMA macromonomer (1 g,
1500 g/mol, 0.67 mmol) was dissolved in 15 mL chloroform. 2-mercaptoethanol (0.10 mL,
1.3 mmol) and DMPP (0.20 mL, 1.4 mmol) were added under a N 2 atmosphere. The
reaction was allowed to stir under N2 at RT overnight. Solvent was allowed to evaporate,
and residual solvent and unreacted 2-mercaptoethanol were removed using a vacuum
oven.
Coupling of Hydroxyl-Functionalised Macromonomer to HDI. Dry hydroxyfunctionalised macromonomer (0.127 g, ~1500 g/mol, 85 μmol) weighed into a round
bottom flask, and under N2 dissolved in 3 mL dry acetone. Hexamethylene diisocyanate
(10 μL, 60 μmol) and dibutyl tin dilaurate (~2 drops) were added and the reaction mixture
stirred gently at 30 ºC for 2 days. Samples were taken throughout and quenched with
ethanol.
Coupling of Hydroxyl-Functionalised Macromonomer to a PPG. Dry hydroxyfunctionalised macromonomer (0.12 g, ~1550 g/mol, 80 μmol) was weighed into a round
bottom flask, and under N2 dissolved in 0.3 mL dry acetone. Poly(propylene glycol),
toluene 2,4-diisocyanate terminated (0.05 mL, 20 μmol) and dibutyl tin dilaurate (~2
drops) were added and the reaction mixture stirred gently at 40 C for 5 hours. Samples
were taken throughout and quenched with ethanol.
General Procedure for Phosphine Mediated Thia-Michael Addition to pMMA
Macromonomers. A solution of PE-SH (1 eq), pMMA10 (5 eq) in 8 mL of solvent in a
septum-capped vial was first deoxygenated with argon for 15 minutes. Then, PBu 3 was
added through the septum using a syringe and the vial was heated up. Once the reaction
finished, the resulting mixture was cooled down slowly to room temperature, precipitated
in methanol, filtered and the product washed with methanol and dried under vacuum.
Temperature Dependence of Coupling of PMMA2 with Butanethiol. Thiol (0.75
mmol) and PMMA2 (0.25 mmol) were weighed into screw-top vial. DMSO-d6 (1mL) was
added and the vial closed. Argon was bubbled through for 15 minutes. The phosphine
catalyst (PBu3 or DMPP, 0.0125 mmol) was added via a syringe. The vial was placed in a
heated carousel at the desired temperature or on a stirrer plate at room temperature (20
ºC) and stirred for 1 hour. After completion of the reaction, the vial was removed from the
carousel and exposed to air. The solution was placed directly in an NMR tube and
analysed.
Typical Procedure for Polymerisation of BA in the Presence of PE-SH as a Chain
Transfer Agent. PE-SH (15 mg, 11 µmol), BA (0.32 g, 2.5 mmol), VAZO-88 (0.3 wt%,
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from a stock solution of 30 mg VAZO-88 in 40 g toluene) and toluene (1.66 g) were
weighed into a screw-cap vial. Argon was bubbled through the mixture for 15 minutes
then placed in a heated carousel at 80 ºC for 48 hours. The reaction mixture was then
quenched by cooling in ice, followed by the addition of hydroquinone.
Typical Procedure for the Determination of the CT of PE-SH in BA. PE-SH (15 mg,
11 µmol), BA (0.32 g, 2.5 mmol), VAZO-88 (0.3 wt%, from a stock solution of 30 mg
VAZO-88 in 40 g toluene) and toluene (1.66 g) were weighed into a screw-cap vial. Argon
was bubbled through the mixture for 15 minutes then placed in a heated carousel at 80 ºC
for 10 minutes. The reaction mixture was then quenched by cooling in ice, followed by the
addition of hydroquinone. Unreacted PE-SH was removed by drying the sample followed
by redissolution in THF. The solution was then filtered and the filtrate dried in a vacuum
oven for at least 24 hours at 60 ºC to obtain the final polymer used for analysis.
Measurements. 1H and 13C NMR spectroscopy was carried out on either a Bruker DRX
400 spectrometer operating at 400 MHz or a Varian Mercury Vx (400 MHz)
spectrometer at 400 MHz. A mixture of tetrachloroethylene (TCE) and
perdeuterobenzene (C6D6) (2/1 v/v) was used as solvent for the analyses of PE based
materials, chloroform-d1, DMSO-d6 were also used as deuterated solvents. Chemical shifts
() are given in ppm in reference to tetramethylsilane (TMS). High-temperature size
exclusion chromatography (SEC) was performed on a Polymer Laboratories PLXT-20
Rapid GPC Polymer Analysis System (including pump, refractive index detector, and
viscosity detector) at 160 ºC with three PLgel Olexis (300 × 7.5 mm, Polymer
Laboratories) columns in series. 1,2,4-Trichlorobenzene was used as eluent at a flow rate
of 1 mL/min. The molecular weights were calculated with respect to polystyrene
standards (Polymer Laboratories). A Polymer Laboratories PL XT-220 robotic sample
handling system was used as autosampler. Size exclusion chromatography has carried out
using a SEC set-up consisting of a WATERS 2695 separations module, Model 2487 UV
detector (254 nm), and Model 2414 differential refractive index detector (40 ºC). The
injection volume used was 50 µL. Tetrahydrofuran (Biosolve, stabilised with BHT) was
used as eluent with a flow rate 1.0 ml/min. The column set used was a PLgel guard (5 m
particles) 50 × 7.5 mm precolumn, followed by 2 PLgel columns in series of 500 Å (5 m
particles) and 100 Å (5 m particles), respectively. Calibration was performed using
polystyrene standards (Polymer Laboratories, Mn = 370 g·mol-1 up to Mn = 40,000 g·mol1). Data acquisition and processing were performed using WATERS Empower 2 software.
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Chapter 5
RESULTS AND DISCUSSION
Coupling of Macromonomers to Polyurethanes using Thiol-ene Chemistry
In this section the modification of CCTP-derived macromonomers via a thiaMichael addition reaction to hydroxyl-functionalised macromonomers and subsequent
reaction of these modified macromonomers with isocyanates to synthesise triblock
copolymers is described (Scheme 2).
Model studies, based on dimers prepared by CCTP, were carried out to establish
the reactivity of these species towards 2-mercaptoethanol, as well as to probe the ability of
these mono-ω-hydroxy-end functionalised macromonomers to participate in a urethane
reaction with an isocyanate. Once established, macromonomers were employed in the
reaction.
Scheme 2. Synthetic procedure for the synthesis of triblock copolymers, (a) Catalytic chain transfer
polymerisations (CCTP), (b) thia-Michael addition and (c) urethane reaction.
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Synthesis of mono-ω-hydroxy-end functionalised macromonomers
Poly(methyl methacrylate) (pMMAn) macromonomers were synthesised by CCTP
in toluene at 60 ºC using AIBN as the initiator (Scheme 2a). The final properties of these
macromonomers are collected in Table 1. The macromonomers were analysed using 1H
NMR to confirm that all contained a terminal vinylic group. For the lower molecular
weight macromonomers, this was also confirmed using MALDI-ToF-MS.
Table 1. The properties of the synthesised mono-ω-hydroxy-end functionalised pMMAn.
% OH1 (ppm)a
xb
Mnc/g mol-1
PDIc
Mnd/g mol-1
functionalitye
pMMA2
191
200
100
pMMA15
15.5
0.64
1,300
1.2
1,500
94
pMMA10
7.4
0.39
1,300
1.9
1,000
100
pMMA48
3.3
0.40
5,400
1.8
4,800
100
The amount of 1 is defined as moles of 1 per 106 moles of monomer; b Monomer conversions
determined gravimetrically; c Values reported against polystyrene standards; d Calculated based on
the ratio of vinylic protons (5.4 and 6.2 ppm) to ester protons (3.6 ppm); e Calculated based on the
ratio of remaining vinylic protons (5.4 and 6.2 ppm) to ester protons (3.6 ppm).
a
To investigate the conversion of the vinylic moiety of the macromonomer into a
hydroxyl-functionality, model studies were carried out. As a model for the
macromonomer, an MMA dimer, pMMA2, was synthesised using CCTP, followed by
purification via vacuum distillation. An equimolar ratio of pMMA 2, 2-mercaptoethanol
(ME) and DMPP were taken and reacted under an argon atmosphere at room
temperature overnight (Scheme 2b, where n=1; entry 1, Table 1). 1H NMR of the reaction
product revealed that the reaction proceeded to full conversion illustrated by the
disappearance of the vinylic protons at 5.4 and 6.2 ppm. In addition, 13C NMR of the same
reaction product demonstrates that it is exclusively the thiol of ME which reacts over the
double bond, given by the presence of a peak at 35.4 ppm (Figure 1). The addition of the
hydroxyl functionality of over the ω-unsaturated end group of the macromonomer would
be expected to give a signal at around 75 ppm, which in Figure 1 is clearly absent.
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Chapter 5
f
a+a'
d+d'
b+e
Chloroform
c+c'
i g+h
200
180
160
140
120
100
80
60
40
20
0
ppm
Figure 1. 13C NMR (400MHz, CDCl3, RT) of the addition product of pMMA2 and 2mercaptoethanol (Entry 1, Table 1).
Model studies have determined that pMMA2 could be modified to convert the ωunsaturated end group into a hydroxyl-functionality via a thia-Michael addition with
mercaptoethanol. The same methodology was then applied to a range of longer
macromonomers as displayed in Table 1. One equivalent of macromonomer and 2
equivalents of ME were dissolved in chloroform and once purged with argon, 2
equivalents of the catalyst, DMPP, were added and allowed to react overnight at room
temperature. The residual ME was removed via evaporation and the hydroxylfunctionalised polymers analysed with 1H NMR. In all cases almost full conversion was
reached ranging from smaller macromonomers of 1,000 Da to longer macromonomers of
4,800 Da.
Coupling of mono-ω-hydroxy-end functionalised macromonomers to isocyanates
Scheme 2c shows the reaction of a mono-ω-hydroxy-end functionalised
macromonomer with a diisocyanate. The most facile method to determining whether the
reaction between these two compounds (in a ratio of 1:2 macromonomer to diisocyanate)
has occurred is to observe an increase in molecular weight. The resulting polymer
consists of three blocks (ABA) in which the A corresponds to pMMA and B to the
connecting diisocyanate.
Hexamethylene diisocyanate (HDI), was first used as a model for larger
diisocyanates (entry 1, Table 2). HDI was reacted with 2 equivalents of a small
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macromonomer of 1,500 Da (pMMA15) in the presence of a DBTDL. The growth in
molecular weight was monitored via SEC analysis (Figure 2). There is clearly an increase
in molecular weight, as well as the disappearance of the starting material over time,
indicating that the reaction was successful. Due to the interaction behaviour of different
polymers with a SEC column, the Mn and Mw can be significantly different for polymers of
the same molecular weight but different compositions. Mp, however, is less affected by
interaction effects making it a more robust parameter for determining whether coupling of
the macromonomers to the diisocyanate has occurred. The Mp of entry 1 (Table 2)
increased from 1,500 Da to 2,900 Da, which corresponds well to the calculated Mp of 2,600
Da based on 2 macromonomers and the diol. A small shoulder representing the starting
macromonomer can still be seen after 24 hours, but this may be due to stoichiometric
irregularities resulting in the formation of diblocks or a result of trace amounts of water
present in the system, reacting with the isocyanate moieties.
Table 2. Molecular weights of starting macromonomer and starting diisocyanate compared to the
molecular weight of the final polymer (calculated and experimental).
Entry
1
2
3
Macromonomer
Mpa/gmol-1
1,500
1,500
8,500
Diisocyanate
Mpa/g mol-1
~168c
2,300d
2,300d
Calculatedb
Mp/ gmol-1
2,600
6,700
19,300
Final Mpa/g
mol-1
2,900
6,900
17,000
Reaction
Time/hrs
54
72
48
Determined using SEC in THF based on polystyrene standards; b Based on the sum of the Mp of 2 
macromonomer and 1  diisocyanate; c Hexamethylene diisocyante (HDI); d Poly(propylene glycol),
toluene 2,4-diisocyanate.
a
Increasing Time
t = 24 h
w(LogM)
t=0h
t = 0.5 h
1000
10000
LogM
Figure 2. Molecular weight distribution of pMMA10 with HDI. Measured in THF against
polystyrene standards.
Poly(propylene glycol) toluene 2,4-diisocyanate (PPG) is a commercially available
isocyanate-terminated polymer with a molecular weight of around 2,300 Da. In entry 2,
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Chapter 5
Table 2, PPG is reacted with two equivalents of pMMA15, again using DBTDL as the
catalyst. The molecular weight distribution in Figure 3a shows an increase in molecular
weight compared to the starting macromonomer and PPG. The final Mp of 6,900 Da
agrees closely with the calculated Mp based on the Mp of the PPG (3,600 Da) and two
macromonomers (1,550 Da), giving a total of 6,700 Da. Assuming only one
macromonomer had reacted with PPG, an Mp of 5,150 Da would be expected. It should be
noted that PPG was not purified prior to use and thus contains some impurities, one at a
higher and one at a lower molecular weight. Unreacted macromonomer can be observed,
even after 24 hours, which again may be due to the reaction of trace amounts of water
with PPG.
pPG33
pPG33
2.5
p(MMA15-b-PG33-b-MMA15)
3.0
3.5
4.0
w(LogM)
w(LogM)
pMMA15
4.5
p(MMA48-b-PG33-b-MMA48)
pMMA48
3.0
LogM
3.5
4.0
4.5
LogM
Figure 3. Molecular weight distribution of (a) pMMA15 with PPG after 24 hours and (b) entry 3,
Table 3 after 30 hours reaction time. Measured in THF against polystyrene standards.
The coupling of mono-ω-hydroxy-end functionalised macromonomers to
isocyanates can also be applied to longer macromonomers (entry 3, Table 2). An increase
in molecular weight accompanied by a decrease in the concentration of free
macromonomer and diisocyanate is observed (Figure 3b). The increase in Mp agrees well
with the calculated Mp with values of 17,000 and 19,300 Da, respectively.
Thiol-ene Reactions of CCT-derived Macromonomers with Thiol-Functionalised
Polyethylene
As discussed in the introduction, thia-Michael additions of a variety of different
thiol-containing molecules with the vinylic end group of CCTP-derived macromonomers
can be carried out. 9, 25 Amongst those compounds were alkyl thiols, such as 1-dodecane
thiol. It should be noted that, in general, it has been observed that alkyl thiols are less
reactive than other thiols towards thia-Michael addition reactions.10, 11 However, on
reacting a model for a MMA-based macromonomer, pMMA2, with dodecane thiol, almost
complete conversion was achieved within 2 hours when the reaction was carried out in
DMSO (a polar solvent) using DMPP as the catalyst.8 In line with these results, the
following section details our attempts to couple a longer methacrylic macromonomer with
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a thiol-terminated polyethylene via a thia-Michael addition in order to synthesise p(E-bMMA) block copolymers (Scheme 3).
Scheme 3. Expected thia-Michael addition reaction between a pMMA macromonomer and PE-SH.
A methyl methacrylate macromonomer (pMMA10) with an Mn of 1050 g/mol was
synthesised via CCTP. The thiol-terminated polyethylene (PE-SH) was synthesised via
CCG and functionalised via reduction of a dithiocarbamate group (by Mazzolini, et al)21
and shown to have an Mn of 1,440 g/mol and a functionality of 87%.
Table 3. Summary of the reaction conditions of thia-Michael addition reactions between pMMA10
macromonomer and PE-SH and using PBu3 as catalyst.
a
Ene
Solvent
85
Reaction
Time/hrs
Hours
5
pMMA10
Toluene
1:5:0.5
85
20
pMMA10
Toluene
3
1:5:0.5
100
20
pMMA10
Toluene
4
1:5:0.5
100
20
pMMA10
Toluene/DMSOb
5
1:5:1
150
22
pMMA10
DMSO
6
1:5:1
150
22
pMMA10
DMF
7
1:5:1
85
3
MMA
Toluene
8
1:5:1
85
21
MMA
Toluene
9
1:5:1
85
18
MMA
Toluene
10
1:5:1
85
5
MMA
DMF
11
1:5:0.5
85
20
PEG-MA
Toluene
12
1:5:0.5
85
20
PEG-MA
Toluene
Entry
PE-SH:Ene:PBu3a
Temperature/ºC
1
1:5:0.5
2
Molar ratio; b Toluene/DMSO = 3:1 (v/v)
Table 3 gives an overview of the experiments carried out in this study. The success
of the reactions was assessed by precipitation of the reaction product upon cooling, which
after removal of residual solvent was analysed using 1H NMR at 363K in a mixture of
tetrachloroethylene and deuterated benzene (TCE/C 6D6). Initial experiments (entries 1
and 2, Table 3) were carried out in toluene at 85 ºC as polyethylene is insoluble below 80
ºC. PBu3 was employed as the catalyst and the reactions were carried out for 5 and 20
hours respectively. The 1H NMR spectrum of the precipitated polymer indicates that the
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Chapter 5
peak corresponding to the methylene adjacent to the thiol is still present, with an integral
comparable to the starting material. The absence of a peak at 3.5 ppm (methoxy groups of
the pMMA), indicates that no pMMA is present. Further, the mass of the precipitated
polymeric fraction was the same as the starting amount of PE-SH. Even after 20 hours
under the same conditions (Entry 2, Table 3) no pMMA is observed in NMR.
Subsequently, the temperature was increased to 100 ºC in order to aid the reaction (entry
3, Table 3). In addition, as Michael additions are reported to proceed faster in more polar
solvents,8 a mixture of toluene and DMSO (3:1) as reaction solvent was also investigated
(entry 4, Table 3). Again, regardless of the conditions used, the weight of the recovered
polymer after precipitation and analysis of the corresponding 1H NMR spectra showed no
indication that the coupling reaction had worked. Reactions were then performed in more
polar reaction solvents such as pure DMSO or DMF. Under these conditions, it was
necessary to work above the melting temperature of PE-SH (125 ºC) to facilitate the
reaction (entries 5 and 6, Table 3). While no reaction occurred in DMSO, the analysis of
the precipitated polymer obtained in entry 6 (performed in pure DMF) showed the
presence of pMMA (Figure 4).
Figure 4. 1 H NMR (C6D6/TCE 1/2 v/v, 400 MHz, 512 scans, 363 K) of PE-SHc (A), pMMA1o, (B)
and product obtained after entry 6 Table 3 (C).
In addition to the resonances corresponding to unreacted PE-SH, an additional
peak at 3.4 ppm can be observed, corresponding to the methoxy protons of pMMA. The
122
Chapter 5
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
absence of vinylic protons at 5.5 and 6.2 ppm indicates the pMMA present is not the
starting material. Furthermore, the quadruplet at 2.3 ppm (corresponding to the
methylene adjacent to the thiol in PE-SH) has lost its definition. This suggests the
presence of a second underlying triplet, corresponding to the methylene adjacent to the
expected thioether link in p(E-b-MMA). Considering the harsh conditions required to
obtain this result, further improvement and optimisation of the conditions may prove
difficult.
Additional experiments were also performed with other methacrylate containing
molecules. MMA monomer (entries 7 to 10, Table 3) was used as a model molecule and
PEG-methacrylate (entries 11 and 12, Table 3) as an alternative macromonomer to
pMMA10. In all the cases, the reactions were unsuccessful, emphasising the difficulty in
reacting methacrylate-type functionalities with PE-SH.
In order to better comprehend the low degree of reactivity of pMMA 10 with PESH, it was hypothesised that reaction of the trialkyl phosphine catalyst with the double
bond of the methacrylate, generating a phosphonium ion and the pMMA-based anion
(Chapter 1, Scheme 7), may be a reversible reaction, and thus the kinetics may be
influenced by the reaction temperature. In order to test this hypothesis a model system,
similar to the PE-SH/pMMA system in terms of chemical reactivity, but without
solubility problems at lower temperatures, was studied. A methyl methacrylate dimer
(pMMA2) was chosen as a model for the macromonomer and butanethiol as a model for
PE-SH. The experiments were carried out in closed vessels at three different
temperatures: 20 oC, 60 ºC and 85 ºC, using reaction conditions from a recent publication
by Haddleton et al.8, who observed full conversion after one hour with a similar catalyst
(DMPP) and thiol (propane thiol). Figure 5 clearly shows the difference in the peak height
of the vinylic protons (at 5.5 and 6.1 ppm) at the different temperatures. For a more
quantitative result, the conversion of the reactions was calculated based on the
(remaining) integral of the vinylic protons to the (total) methoxy protons (between 3.5
and 3.7 ppm). The methoxy protons at 3.7 ppm shift to 3.5 ppm as the reaction proceeds.
The reaction at room temperature shows 85% conversion after one hour, which is
comparable to the results obtained by Haddleton et al.,8 who obtained 100% conversion
after one hour with a shorter thiol. On raising the temperature to 60 ºC, an increase in the
conversion to 95% is seen. However, on further increasing the temperature to 85 ºC, the
conversion drops to 0%. Although not as drastic, a similar trend was found when using
PBu3 (i.e., the catalyst used in our previous attempts to couple the two polymers) (Figure
6). To our knowledge, such a dependency of the efficiency of thia-Michael addition upon
increasing temperature has not been reported to date. For polymers soluble only at high
temperatures, this effect can indeed be problematic.
123
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
=CH2
Chapter 5
OCH3
(a)
*
(b)
*
(c)
*
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
ppm
Figure 5. 1H NMR (400 MHz, DMSO-d6) spectra of the coupling product of butanethiol and
PMMA2 using DMPP as the catalyst at (a) 20 ºC, (b) 60 ºC and (c) 85 ºC (starred peak is water).
(a)
=CH2
OCH3
*
(b)
*
(c)
*
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
ppm
Figure 6. 1H NMR (400 MHz, DMSO-d6) spectra of the coupling product of butane thiol and
PMMA2 using PBu3 as the catalyst at (a) 20 ºC, (b) 50 ºC and (c) 150 ºC.
Despite this unexpected unreactivity between PE-SH and methacrylate-type
groups, a series of selective reactions to acrylaye-methacrylate-type molecules could be
124
Chapter 5
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
carried out, as the reactivity of PE-SH towards acrylates is much higher than towards
methacrylates.8 Using this route, macromonomers of PE can be synthesised with a
methacrylate-type end group via a thia-Michal addition route, which has been used
previously to synthesise macromonomers based on alternative polymers.26, 27 PE-SH was
reacted with the commercially available 3-(acryloyloxy)-2-hydroxypropyl methacrylate
(AHPMA) and acryloyloxy ethyl methacrylate (AEMA), both of which contain an acrylate
and methacrylate moiety. As an alternative coupling reaction, 2-isocyanatoethyl
methacrylate (ICEM) was reacted with PE-SH via a thiol-isocyanate reaction to again
yield a methacryate-functional polyethylene. Block copolymers could also be made directly
by the thia-Michael addition reaction of a commercially available PEG functionalised with
an acrylate end group (PEG-A) with PE-SH. More details of these reactions can be found
in reference 28.
It has been shown that the thia-Michael addition of alkyl thiols with methacrylatetype groups is efficient only under very specific conditions (solvent, catalyst, temperature)
and that the use of PE-SH as a polymeric alkyl thiol requires solubility conditions that can
hardly meet the narrow window of reaction conditions to be truly practical. Although it is
clear from this study that the addition of PE-SH to methacrylate functionalities is
problematic, this low reactivity has been used advantageously to prepare methacrylatetype macromonomers from PE-SH.
Thiol-functionalised Polyethylene as a Chain Transfer Agent in Free Radical
Polymerisations
It is not trivial to synthesise block copolymers of ethylene and methyl
methacrylate via the thiol-ene coupling of a thiol-terminated polyethylene and an MMAbased macromonomer. These same polymers can, in principle, be made by using the thiolterminated polyethylene as a chain transfer agent in a radical polymerisation of monomers
such as MMA.
The same PE-SH of Mn = 1,440 g/mol and with a functionality of 87% was
employed in a series of radical polymerisations as a chain transfer agent. The reactions,
detailed in Table 4, were carried out at 80 ºC in toluene, using VAZO-88 as the radical
source.
125
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
Chapter 5
Table 4. Synthesis of polymers using PE-SH as a chain transfer agent.
Entry
1
2
3
1
2
3
4
5
6
Monomer
(M)
BA
BA
BA
MMA
MMA
MMA
S
S
S
PE:Ma
xb
1:100
1:50
No PE
1:100
1:50
No PE
1:100
1:50
No PE
1.00
1.00
1.00
0.69
0.65
0.78
0.63
0.69
0.69
Mwc,d/
g mol-1
9,500f
7,800f
15,600f
6,800
7,000
11,400
16,900g
14,100g
54,900g
PDId
2.6f
2.6f
3.8f
1.6
1.5
6.8
2.1g
1.3g
1.9g
Mwd,e/
g mol-1
-h
-h
-h
8,590
7,110
13,540
18,760
8,670
55,750
PDI
PE:Mf
Mn MMAe
-h
-h
-h
1.9
1.8
2.9
2.9
2.3
1.9
1:72
1:34
1:103
1:32
1:56
1:33
-
9,200
4,400
10,300
3,200
5,800
3,400
-
Molar ratio; b Conversion of monomer, determined gravimetrically; c Crude polymer; d Measured using hightemperature SEC in trichlorobenzene based on polystyrene standards; e Polymer filtered from THF; e
Determined using 1H NMR; f Measured in THF based on polystyrene standards; g Polystyrene has refractive
index greater than SEC solvent whereas polyethylene has a refractive index lower than the solvent; h Polymer
soluble in toluene.
a
PE-SH was added in different concentrations to the polymerisation of butyl
acrylate (BA) and compared to a polymerisation of BA in the absence of the chain transfer
agent (entries 1-3, Table 4). Surprisingly, the resulting polymers were soluble in toluene
at room temperature, contrary to PE-SH alone which is insoluble. This indicates that all
PE-SH chains are attached to enough BA to render the copolymers soluble in the reaction
medium at room temperature. Due to this evidential solubility, the polymers could be
measured using SEC in THF. A distinctly lower molecular weight was observed for the
polymers synthesised in the presence of PE-SH (entries 1 and 2, Table 4) compared to
entry 3 (Table 4) where no PE-SH was present. This trend coupled to the presence of a
monomodal distribution and a PDI close to 2 proves that chain transfer is the dominant
polymerisation mechanism; PE-SH acts as a chain transfer agent in the polymerisation of
BA. The presence of BA in the block copolymer was confirmed using 1H NMR and the
appearance of a strong signal at 1.4 ppm corresponding to the tert-butyl group of BA
(Figure 7c). The ratio of PE-SH to monomer was also determined using 1H NMR and
found to be comparable to theoretical targets, although some discrepancies presumably
due to evaporation of monomer during experiment preparation do arise.
126
Chapter 5
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
(d)
(c)
(b)
(a)
9
8
7
6
5
4
3
2
1
0
ppm
Figure 7. 1H NMR (400 MHz, TCE/C6D6, 353 K) spectra of (a) PE-SH, (b) p(E-b-MMA), (c) p(E-bBA) and (d) p(E-b-S).
MMA was also polymerised using PE-SH as a chain transfer agent (entries 4-6,
Table 4), and again a lower molecular weight polymer was obtained when PE-SH was
present (Figure 8a). Unlike the p(E-b-BA) copolymers, some unreacted insoluble PE-SH
was observed and could be detected using high-temperature SEC measurements. Isolation
of p(E-b-MMA) was achieved by filtering the polymer solution from THF, whereby the
insoluble PE-SH fraction was removed, illustrated by SEC measurements (Table 4).
Confirmation of the presence of pMMA in the block copolymer was shown by the
presence if a signal at 3.5 ppm corresponding to the methoxy protons of MMA (Figure
7b). The polymerisation of styrene in the presence of PE-SH (entries 7-9) showed similar
results (Figure 8b) and the appearance of aromatic signals around 6.5-7 ppm indicates the
presence of a PS block (Figure 7d).
127
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
(a)
Chapter 5
(b)
PE-SH
100eq S
50eq S
PS (no CTA)
w(Log(M))
w(Log(M))
PE-SH
100eq MMA
50eq MMA
PMMA (no CTA)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Log(M)
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Log(M)
Figure 8. Molecular weight distributions of (a) entries 1-3 (Table 4) and (b) entries 4-6 (Table 4)
compared to PE-SH. Measured in trichlorobenzene at 160 ºC using polystyrene standards.
The difference in behaviour of BA compared to S and MMA could be explained
using the relative chain transfer constants (CT) as in general acrylates have a higher CT
than methacrylates and styrene when thiols are used as CTAs.2
In order to characterise the activity of PE-SH as a chain transfer agent, the chain
transfer constant (CT) was determined. Firstly, a series of polymerisations with varying
concentrations of PE-SH compared to BA were carried out and stopped at low
conversions. After filtering from THF to remove unreacted PE-SH, the residual polymers
were analysed using SEC and 1H NMR. From SEC the Mw was determined and plotted as
2/DPw in a Mayo plot (Figure 9a); the DPw was based on the pBA block only. The chain
transfer constant was determined to be 1.9. The CT was also determined using an Mn
based on the relative integrals of BA and PE-SH in 1H NMR; this time plotted as a 1/DPn
in a Mayo plot (Figure 9), giving a CT of 6.3. In principle, NMR should give a more
accurate and absolute Mn, although certain assumptions are made, for example, that the
length of the PE block is the same as the starting PE-SH. Determination of an absolute
molecular weight cannot be measured using the SEC set-up used in this work. Firstly, the
calibration standards used are based on polystyrene which has a different hydrodynamic
volume to pBA. Secondly, even using pBA standards or conversion using Mark-Houwink
parameters, an absolute molecular weight cannot be determined as the PE block will affect
the hydrodynamic volume. Regardless of this slight difference in CT and the respective
shortcomings of the different methods, the values are of the same order of magnitude as
reported values for alkyl thiols in BA polymerisations (Table 5)2, 29 and consolidated by
Tung et al who found that the CT of macrothiols based on butadiene is comparable to their
smaller counterparts.30 The influence of reaction solvent should not be disregarded when
comparing the values to the literature.
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STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
Chapter 5
(a)
(b)
0.09
0.028
0.12
0.030
0.026
0.07
0.025
0.022
0.10
0.06
0.08
0.05
0.015
0.04
0.016
1/DPn
0.018
1/DPn
0.09
2/DPw
0.020
0.020
2/DPw
0.08
0.11
0.024
0.03
0.07
0.010
0.014
0.02
0.06
0.012
0.005
0.010
0.01
0.05
0.004
0.005
0.006
0.007
0.008
0.009
0.004
0.010
0.006
0.008
0.010
0.012
0.014
[PE-SH]/[MMA]
[PE-SH]/[BA]
Figure 9. Mayo plots based on Mw obtained using SEC measured in THF against PS standards (■)
and Mn obtained using 1H NMR (□) for (a) BA and (b) MMA
The chain transfer constant for PE-SH in MMA was also determined using both
Mw from SEC and Mn from 1H NMR (Figure 9b) and the chain transfer constants were
calculated to be 2.4 and 6.3, respectively (Table 5), again in the same order of magnitude
as those found in literature.2, 29 Again, the degree of polymerisation was based solely on
the pMMA block length.
Table 5. Chain transfer constants (CT) for BA and MMA using PE-SH as a CTA compared to
literature results using dodecane thiol.29
PE-SH (this work)a
BA
MMA
a
SEC
1.9
2.4
NMR
6.3
6.3
Dodecane thiol
(Reference 29)b
1.5
0.7
Conditions: VAZO-88, toluene, 80 ºC; b AIBN, benzene, 60 ºC.
It is surprising to note, however, that a higher chain transfer constant is obtained
(based on SEC results) for pMMA compared to pBA, against the general observation that
acrylates tend to have a higher CT than methacrylates with thiol-based CTAs. However,
this could be due to differences in hydrodynamic volume compared to the polystyrene
standards used for SEC. Application of Mark-Houwink parameters to convert the
molecular weights does not significantly affect the CT value. The closeness of the values
obtained using NMR may be more representative.
Block copolymers of p(E-b-MMA), p(E-b-S) and p(E-b-BA) have been synthesised
by using PE-SH as a chain transfer agent in a radical polymerisation. The chain transfer
constants of these systems have been determined and although these do not fully agree
with literature values they are in the same order of magnitude. Although this is by no
means the first time polymers with a similar composition have been synthesised,31-35 this
129
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
Chapter 5
method does provide a facile and versatile route towards block copolymers of olefins and
(meth)acrylic and styrenic monomers.
CONCLUSIONS
Three different routes to synthesising block copolymers using thiol chemistry have
been investigated. Nucleophilic thiol-ene reactions have been used to modify CCTPderived macromonomers into hydroxyl-functionalised polymers that can be reacted with
diisocyanate-terminated polymers to make triblock copolymers. This technique has the
potential to be applicable for coupling macromonomers to almost any isocyanateterminated polymer. Thiol-ene chemistry was also investigated as a method to couple
macromonomers to thiol-terminated polyethylene. However, a strong dependence on the
rate of reaction on the reaction temperature was found to severely hinder the reaction
restricting the use of poorly-soluble polymers like PE in these types of thiol-ene reactions.
Similar block copolymers could, however, be synthesised by using PE-SH as a chain
transfer agent in a radical polymerisation of methyl methacrylate, styrene and butyl
acrylate. Taken as a whole, simple chemistry has been exploited to synthesise block
copolymers which cannot be made using a sole polymerisation technique. Furthermore,
these techniques promise to be versatile and applicable to a wide range of monomers.
130
Chapter 5
STRATEGIES TO EMPLOY THIOL CHEMISTRY IN THE SYNTHESIS OF
BLOCK COPOLYMERS
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132
Chapter 6
GRAFT COPOLYMERS AS ANTI-STATIC AND
ANTI-FOGGING ADDITIVES FOR
POLYCARBONATE SUBSTRATES
Anti-Static/Anti-Dust
Anti-Fog
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
ABSTRACT
A range of graft copolymers of cyclohexyl methacrylate, methyl methacrylate and
2-(dimethylamino)ethyl acrylate have been synthesised via catalytic chain transfer
polymerisation and subsequently been used as macromonomers in a conventional free
radical polymerisation. After quarternisation of the amine-containing backbone, these
graft copolymers were solution cast onto polycarbonate substrates and the surface
properties investigated. The coatings were tested for their viability as anti-static and antifogging additives, providing promising results.
134
Chapter 6
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
INTRODUCTION
Industrially, there is a desire to improve certain properties of engineering
plastics, either in order to improve the existing properties or to introduce novel
characteristics to the plastic. Engineering plastics have excellent bulk properties, but
often have inferior surface properties.1 For example aromatic polycarbonates (PC) have
excellent bulk properties, such as their thermal stability, excellent impact resistance and
optical clarity making them suitable for a wide range of applications. 2, 3 However, PC
suffers from a build-up of static charge, making their usage hazardous in certain
application areas.4 Similarly, PC also has a tendency to fog, reducing the transparency of
the material which is undesirable in applications such as greenhouses, 5 sports equipment
such as diving goggles and ski masks,6 analytical equipment, for example microscopes,7
spectacle lenses, and supermarket freezer doors.
In order to improve the properties of PC, additives such as anti-static and antifogging additives, can be added. Anti-static and anti-fogging additives work via similar
mechanisms (Figure 1). The additives are added to the polycarbonate during the extrusion
process after which, due to their amphiphilic nature, the additives migrate to the surface of
the polymer. The hydrophobicity of the polycarbonate ensures the organisation of the
additives on the polymer surface, increasing the polarity of the surface. For anti-static
applications this newly hydrophilic surface attracts water, allowing any build-up of charge
to be dissipated via the water molecules. For anti-fogging applications, the newly
hydrophilic surface means that there is better spreading of water across the surface. On
exposure to water, the contact angle of the water droplet on the surface now tends to 0º,
spreading out over the surface to form a continuous layer rather than discreet droplets
which reflect the light, giving the appearance of fog.8, 9
(a)
(b)
(c)
air
H2O
H2O
air
H2O
H2O
Figure 1. Simple representation of anti-fog/anti-static mechanism. (a) Additive is randomly
distributed in the polymer matrix following extrusion processing, (b) additive migrates to polymerair interface and (c) presence of additive at surface attracts water molecules from the air, allowing
charge to be dissipated through the water molecules.
135
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
There are several requirements that need to be fulfilled by anti-static and antifogging agents to be effective. Firstly, the additive must increase the hydrophilicity of the
surface of a substrate. In order for this to be achievable, the additive must be suitably
composed to migrate to the surface (as they are conventionally introduced during the
compounding process). Secondly, once they reach the surface they must provide good
coverage ensuring a homogeneous layer of the additive. Currently, most anti-static and
anti-fog additives for polycarbonate are based on small molecules consisting of ammonium
or phosphonium salts.10-13 However these additives have a limited lifetime as they are
easily removed from the polymer surface. Therefore there is a drive in industry to develop
new additives, which are less susceptible to removal from the surface of the polycarbonate.
One of the most obvious avenues to explore is the use of polymeric anti-static and anti-fog
additives. For example, Howarter, et al have grafted fluorinated oligomers to silica
surfaces using isocyanate chemistry.9, 14 These modified surfaces showed promise in antifogging tests. Alternatively, Tajitsu has used highly conductive butadiene rubber particles
doped with Li+ or K+ dispersed in a poly(methyl methacrylate) matrix as anti-static
polymer films.15, 16
In order for a polymeric additive to be successful, it must be both compatible with
the substrate as well as carrying out its function as an anti-static or anti-fogging agent.
Copolymers with complex polymer architectures, in which one part of the polymer
interacts with substrate providing adhesion and the other part containing the
functionality, fulfil these requirements. In this chapter, PC-compatible macromonomers
derived from catalytic chain transfer polymerisation (CCTP) are copolymerised with a
quarternisable acrylate to form graft copolymers (discussed in more detail in Chapter 1). 1720 Although graft copolymers made via this method do not have as well-defined grafts as
expected due to a concomitant addition-fragmentation chain transfer (AFCT) reaction,
they are anticipated to have a comb-like structure merely the structure of the grafts will
have a more complex architecture. These polymers have then been tested with respect to
their anti-static and anti-fogging properties as coatings for PC. Testing was carried out
using the polymers as coatings to determine the potential of the system with the view to
use these polymers in the future as additives for PC.
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
EXPERIMENTAL SECTION
General considerations. All syntheses and manipulations of air- and moisture-sensitive
materials were carried out in oven-dried Schlenk-type glassware on a dual manifold
Schlenk line.
Materials. Methyl metharylate (MMA, 99%, Sigma-Aldrich), cyclohexyl methacryate
(CHMA, 97%, Alfa Aesar) and 2-(dimethylamino)ethyl acrylate (DMAEA, 98%, Alfa
Aesar) were passed over a column of activated basic alumina to remove the inhibitor.
Azobis(isobutryonitrile) (AIBN) was purchased from Sigma-Aldrich and recrystallised
twice from methanol. The bis(methanol) complex of bis[(difluoroboryl)dimethylglyoximate]cobalt II (COBF), was prepared as described previously.21, 22 The
chain transfer activity of the complex was determined in methyl methacrylate (MMA)
bulk polymerisation at 60 ºC and found to be equal to 30 × 103. For all experiments, a
single batch of catalyst was used. Toluene, pentane, tetrahydrofuran (THF),
Hexafluoroisopropanol (HFIP) and ethanol (AR, Biosolve) were used as received. Lexan
GLX143 polycarbonate (PC) was provided by Sabic IP.
Synthesis of p(CHMA-co-MMA) via CCTP. COBF (1.0 mg, 2.5 µmol) and AIBN (42
mg, 254 μmol) were weighed into a round bottom flask equipped with a magnetic stirrer
bar. The flask was then evacuated and re-filled 3 times. Argon was bubbled through
toluene and a stock solution of cyclohexyl methacrylate (CHMA, 25 mol%) and methyl
methacrylate (MMA, 75 mol%) for 30 minutes prior to use. Toluene (50 mL) and
MMA/CHMA stock solution (50 mL) were added to the flask. The reaction was carried
out at 60 ºC for 18 hours with continuous stirring, after which it was quenched by cooling
in ice and the addition of hydroquinone. The residual solvent and MMA was removed via
rotary evaporation. The polymer was then redissolved in toluene and precipitated in
pentane to remove residual catalyst and monomer and dried under vacuum for 24 hours.
Synthesis of p(DMAEA-g-(CHMA-co-MMA)) via free radical polymerisation.
Macromonomer (35g, 16 mmol), AIBN (0.5 wt % based on DMAEA and macromonomer),
DMAEA (23 g, 0.16 mol) and 100 g toluene were placed in a vial. N 2 was bubbled through
for 30 minutes and then the reaction mixture heated to 60 ºC and stirred for 72 hours.
Additional shots of AIBN (0.5 wt %) were added after each 24 hours. Quenching of the
reaction was achieved by cooling in ice followed by the addition of hydroquinone. The
polymer mixture was then precipitated into pentane, re-dissolved in THF and then reprecipitated into pentane and dried overnight in a vacuum oven.
Quarternisation of polymers with methyl iodide. p(DMAEA-g-(CHMA-co-MMA)) (20
g, 67 wt% DMAEA) was dissolved in 3 L THF. Methyl iodide (MeI, 11 mL, 0.18mol) was
then added and the reaction mixture stirred at room temperature. After a few minutes the
polymer began to precipitate out of solution however the reaction was left to stir
137
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
overnight in order to reach completion. Residual MeI and THF were evaporated from the
precipitated polymer. The polymer was then dried in a vacuum oven at 40 ºC overnight.
Quarternisation of polymers with perfluorohexyl iodide. p(DMAEA-g-(CHMA-coMMA)) (6.4 g, 67% DMAEA) was dissolved in 150 mL DMF and heated to 85 ºC.
Perfluorohexyl iodide (FHI, 6.5 mL, 30 mmol) was then added and the reaction mixture
stirred at 85 ºC for 7 days. The polymer was then dialysed (1000 Da) against water to
remove residual FHI and DMF. The polymer was then isolated by removing the water
using a freeze-drier.
Coatings. The quarternised polymers were dissolved in water/ethanol mixtures and
applied to a polycarbonate substrate using a coil applicator. Coating thicknesses were
varied by using different applicators and different solution concentrations. Once applied,
the coatings were left to dry then placed in a vacuum oven at 20 ºC overnight.
Measurements. Size exclusion chromatography (SEC) was measured on a system
equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index
detector (40 ºC), a Waters 2707 autosampler and a PSS PFG guard column followed by 2
PFG-linear-XL (7 µm, 8 × 300 mm) columns in series at 40 ºC. HFIP with potassium
trifluoroacetate (3 g/L) was used as eluent at a flow rate of 0.8 mL/min. The molecular
weights were calculated against poly(methyl methacrylate) standards (Polymer
Laboratories, Mp = 580 Da up to Mp = 7.1 × 106 Da). Data acquisition and processing
were performed using WATERS Empower 2 software. 1H, 13C and gHMQC NMR spectra
were recorded on a Varian Mercury Vx (400 MHz) spectrometer at 400 MHz. D 2O,
DMSO-d6 or chloroform-d3 and tetramethylsilane were used as solvents and internal
standard, respectively. Surface resistivity measurements were carried out on a Keithley
8009 Resistivity Test Fixture and measured using a 6517A electrometer/high resistance
meter. The measuring time was 600 seconds with an alternative voltage of 500 V. The
measurements were carried out according to the 6517 HiR Test. Static decay
measurements were carried out on a Static Honestmeter Type S-5109 (Shishido
Electrostatic Ltd.) using a voltage of 10 kV. Measurements carried out at 23ºC and 50%
relative humidity. Static contact angles were measured using a Kruss Drop Shape Analysis
System DSA 10 MK2 instrument at room temperature. 1 µL deionized water was used
and contact angles recorded for up to 160 seconds. Dynamic advancing and receding
contact angles were recorded using a Dataphysics OCA30 instrument at room
temperature. A drop of deionized water was swollen with a further 2 µL at 0.1 µL/s and
then retracting the same amount at the same speed. There was a 1s delay period between
injection and retraction. All data given in this chapter is the average of at least 3
measurements.
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Chapter 6
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
RESULTS AND DISCUSSION
Synthesis of p(DMAEA-g-(CHMA-co-MMA))
Graft copolymers have been synthesised using a two-step procedure. Firstly,
CCTP was used to synthesise well-defined macromonomers of cyclohexyl methacrylate
(CHMA) and methyl methacrylate (MMA), then these macromonomers were
copolymerised with 2-(dimethylamino)ethyl acrylate (DMAEA) via a conventional freeradical technique (FRP) (Scheme 1).
Scheme 1. Synthesis of quarternised p(DMAEA-g-(CHMA-co-MMA)).
Copolymers of cyclohexyl methacrylate (CHMA) and methyl methacrylate (MMA)
containing 15-25 wt% CHMA have been found to be miscible with polycarbonate (PC) 23
and therefore have been deemed a suitable monomer combination for use as the
macromonomer-derived grafts of the copolymer. Macromonomers of CHMA and MMA
were synthesised via CCTP (Table 1) using ppm amounts of COBF as the chain transfer
catalyst and AIBN as the radical source. The reactions were carried out at 60ºC in toluene
and the resultant polymers purified by precipitation in pentane. Macromonomers of two
different lengths were synthesised and found to have a CHMA content of 17-20 wt%,
determined using via 1H NMR (Figure 2b).
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
Table 1. Properties of p(CHMA-co-MMA) copolymers made via CCTP.
Entry
MM1
MM2
COBF/ppm
6.1
2.6
Conversion
0.53
0.67
Mna/g mol-1
1,580
2,900
DPnb
PDIa
1.4
1.7
FMMAb
83
80
20
41
Determined using SEC based on polystyrene standards in THF; b Determined using 1H NMR
based on the integral of the vinylic protons (6.2 and 5.5 ppm), CH (CHMA, 4.7 ppm) and OCH 3
(MMA, 3.6 ppm).
a
OCH3 (MMA)
CH2/CH3
//
CH (CHMA)
CH2N (DMAEA)
(a)
(b)
CH (CHMA)
=CH2
6
OCH3 (MMA)
5
CH2/CH3
4
3
2
1
0
ppm
Figure 2. 1H NMR (400 MHz, CDCl3) spectra of (a) p(DMAEA-g-(CHMA-co-MMA)) and (b)
p(CHMA-co-MMA).
The CHMA-MMA macromonomers were then copolymerised with 2(dimethylamino)ethyl acrylate (DMAEA) in the presence of AIBN as the radical initiator,
again using toluene as the solvent and a reaction temperature of 60 ºC (Scheme 1). The
consumption of the vinylic bonds of both the monomer and macromonomer was followed
using 1H NMR, and additional initiator was added until full conversion of both the
monomer and macromonomer had been reached. The resultant graft copolymers have
been analysed using 1H NMR and SEC (Table 2). The ratio of macromonomer to
monomer (DMAEA) was shown to be similar when determined experimentally from
NMR (Figure 2a) and when calculated based on monomer feed and DMAEA conversion.
A clear shift in the molecular weight from the macromonomer to the graft copolymers can
also be seen, for example in Figure 3, for all four graft copolymers. The four graft
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
copolymers consist of two different starting macromonomers (MM 1 and MM2) and two
different grafting densities for each macromonomer.
Table 2. Properties of p(DMAEA-g-(CHMA-co-MMA)). G1a and G1b are synthesised using MM1,
and G2a and G2b from MM2.
Entry
Target ratio of
MM:Ma
Ratio
MM:Ma,b
Mnc/gmol-1
PDIc
Grafts per
chaind
G1a
G1b
G2a
G2b
1a:35
1a:10
1b:35
1b:15
1:33
1:10
1:32
1:16
19,200
8,900
13,200
10,700
1.8
1.6
1.5
1.4
3
2.5
1.5
1.5
a Molar
ratio; b Determined using 1H NMR based on the ratio of integrals of CH (CHMA, 4.7 ppm),
OCH3 (MMA, 3.6 ppm) and CH2N (DMAEA, 4.1 pm); c Determined via SEC, measured against
poly(methyl methacrylate) standards in HFIP; d Approximate, calculated based on: grafts per chain
= Mn, graft copolymer (SEC)/Mn, macromonomer (NMR) + (DMAEA units per MM × molar mass of
DMAEA).
(b)
w(LogM)
w(LogM)
(a)
1000
10000
1000
100000
10000
100000
M (g/mol)
M (g/mol)
Figure 3. Molecular weight distributions of (a) GP1 and (b) GP3 ( _____) compared to respective
starting macromonomers (……). Measured in HFIP against poly(methyl methacrylate) standards.
GPEC measurements confirm the presence of a copolymer (Figure 4). MM 1 has an
elution time of 20 – 28 minutes (Figure 4c), which is not observed for G1b (Figure 4b)
indicating that all of the macromonomer has been consumed during the copolymerisation
process. pDMAEA elutes at 2 – 5 minutes (Figure 4a), which can also be observed in the
copolymer chromatogram (Figure 4b), overlapping with the graft copolymer peak.
141
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
(a)
(b)
Graft copolymer
(c)
0
10
20
30
40
Elution Time/Minutes
Figure 4. Gradient polymer elution chromatograms of (a) pDMAEA, (b) G 1b and (c) MM1
Hence it can be concluded that graft copolymers consisting of a DMAEA
backbone and CHMA-MMA-based macromonomer-derived grafts can be synthesised via
CCTP and FRP. In order to ensure that the copolymer consists of a highly hydrophilic
section, the backbone must then be quarternised.
Quarternisation of p(DMAEA-g-(CHMA-co-MMA))
Increasing the hydrophilicity of the polymer backbone by quarternising the amine
groups of the DMAEA serves two purposes. Due to the larger differences in polarity
between the backbone and grafts, not only does the presence of a hydrophilic backbone
encourage migration of the polymer to the surface (if compounded into the PC) but also
instigates organisation of the polymer on the surface thus increasing the hydrophilicity of
the surface.
The amine groups of the graft copolymers were quarternised using two different
quarternising agents: methyl iodide (MeI) and perfluorohexyl iodide (FHI) as shown in
Table 3. The quarternisations using MeI were carried out in THF and as the reaction
proceeded, the quarternised polymer precipitated out of solution. Quarternisation with
FHI was carried out in DMF and allowed to react for 1 week at 85 ºC before purification
using dialysis and freeze-drying. FHI was chosen for its fluorinated groups which, it was
142
Chapter 6
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
hoped, would improve the migrational behaviour of the polymer (when extruded), while
maintaining similar surface modification properties. It is also important to note that
complete removal of any free amines is required as these can have undesired effects on the
PC especially at higher temperatures.
It was immediately clear from 1H NMR data that the quarternisation of these
polymers was a success due to the presence of a large peak corresponding to water, drawn
in from the atmosphere (or work-up procedure) due to the hygroscopic nature of the
quarternised amines. The degree of quarternisation was determined by comparing the
relative integrals of the protons of the quarternised and unquarternised amines.19, 24-26
Table 3. Overview of synthesised polymers
Macromonomer
Graft
Copolymer
Quarternised
Graft
Copolymer
MM1
G1a
G1a-m97
G1a-m60
G1a-m40
G1a-f10
G1a-f100
G1b
G1b-m99
G1b-f26
MM2
G2a
G2a-m94
G2a-f42
G2b
G2b-m100
G2b-f21
Descriptiona
Fhydrophilic monomerb
p[(DMAEA33-g-(MMA18-co-CHMA2))]3
quarternised with MeI (97%)
p[(DMAEA33-g-(MMA18-co-CHMA2))]3
quarternised with MeI (60%)
p[(DMAEA33-g-(MMA18-co-CHMA2))]3
quarternised with MeI (40%)
p[(DMAEA33-g-(MMA18-co-CHMA2))]3
quarternised with HFI (10%)
p[(DMAEA33-g-(MMA18-co-CHMA2))]3
quarternised with HFI (100%)
p[(DMAEA10-g-(MMA18-co-CHMA2))]2.5
quarternised with MeI (99%)
p[(DMAEA10-g-(MMA18-co-CHMA2))]2.5
quarternised with HFI (26%)
p[(DMAEA32-g-(MMA35-co-CHMA6))]1.5
quarternised with MeI (94%)
p[(DMAEA32-g-(MMA35-co-CHMA6))]1.5
quarternised with HFI (42%)
p[(DMAEA16-g-(MMA35-co-CHMA6))]1.5
quarternised with MeI (100%)
p[(DMAEA16-g-(MMA35-co-CHMA6))]1.5
quarternised with HFI (21%)
0.68
0.39
0.26
0.07
0.68
0.39
0.10
0.52
0.23
0.38
0.08
Degree of quarternisation determined from the ratio of unquarternised (2.65 ppm) and
quarternised (3.40 ppm) α-amino methyl proton integrals (MeI) in D2O19, 26 or integral of
unquarternised (2.13 ppm) and quarternised (3.04 ppm) α-amino methyl proton integrals (FHI) in
DMSO-d6;24, 25 b Based on mole fraction of DMAEA in graft copolymer and degree of
quarternisation.
a
As can be seen in Table 3, a range of graft copolymers of CHMA, MMA and
DMAEA have been quarternised to varying degrees using both MeI and FHI. These
polymers were subsequently tested for their anti-static and anti-fogging properties.
Testing of Coating Properties
Ultimately, the polymers made in the previous section need to be extruded into a
PC matrix and various factors investigated, however in order to establish the viability of
143
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
using these polymers as anti-static and anti-fogging agents, coatings of these polymers
were applied directly to PC substrates.
The polymers synthesised in Table 3 were applied using a bar applicator to a
standard PC substrate at a range of different thicknesses from a water/ethanol solvent
mixture. These coatings were then dried overnight at 20 ºC in a vacuum oven. The
properties of these coatings were investigated in terms of their anti-static and antifogging properties.
Anti-static and Anti-dust Properties. In order for a material to be deemed ‘anti-static’ or
‘anti-dust’, it must be able to dissipate charge. This can be measured using surface
resistivity or static decay measurements; these two techniques show a roughly linear
correlation with one another. The surface resistivity of a material is defined as the
resistance to the flow of electrical current across the surface of the material. Insulators,
such as PC, have surface resistivities in the realm of 10 16-1018 Ohms/sq, whereas a
conducting material has a lower resistivity of 104-106 Ohmssq2. Materials with anti-static
or anti-dust properties tend to have surface resistivities between 10 9-1012 and 1012-1014
Ohms/sq respectively.
The surface resistivity of a range of coated PCs (Table 4) were tested for selected
coatings as a technique to determine the viability of these coatings to act as anti-static or
anti-dust agents. As a reference point, uncoated PC has a surface resistivity of 10 18
Ohms/sq. The measured coatings all have a much lower surface resistivity than that of
uncoated PC (Table 4), suggesting an improvement of the anti-static and anti-dust
properties. The suitability of the coatings for use as anti-static and anti-dust additives can
also be measured by determining the static decay half-life of the material, i.e. the time it
takes for half an applied static charge to be dissipated over the surface of the material. PC
is an insulator, and therefore untreated PC has a very long static decay half-life (t1/2).
Measurement of t1/2 after application of a coating from a polymer synthesised in Table 3,
clearly shows that the surface of the PC has been modified (Table 4). For a surface of a
material to be considered an ‘anti-dust’, a t1/2 < 60 seconds is suggested. Almost all of the
coatings fall in a region considered suitable for anti-dust applications, which is in
agreement with surface resistivity measurements.
144
Chapter 6
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Table 4. Anti-static properties of quarternised polymers.
PC
G1a-m97b
G1a-m60b
G1a-m40b
G1b-m99b
G2a-m94c
G2b-m100c
G1a-f10d
G1a-f100d
G1b-f26d
G2a-f42e
G2b-f21e
Dry film
thickness/nma
10
100
680
2000
7000
10000
100
100
100
2000
7000f
10000f
100
2000
7000
10000
100
2000
7000
10000
1800
6300g
9000g
100
1800f
6300f
9000f
1800
6300
9000
1800h
6300h
9000h
Surface
Resistivity/Ωm-2/sq
1.83x1018
1.60x1011
9.72x109
8.79x1010
5.24x1010
5.80x1010
5.47x1010
1.60x1011
4.26x1011
1.43x1010
9.89x1010
1.42x1010
1.29x1011
1.61x1010
3.11x1011
8.01x1012
1.30x1013
6.64x1012
6.79x1013
8.23x1013
3.80x1013
2.03x1014
3.63x1015
-
Static Decay
(t1/2)/sec
∞
11
<1
<1
<1
<1
<1
<1
<1
<1
<1
3.0
2.0
<1
<1
<1
<1
<1
<1
1.6
2.0
11.5
9.1
9.8
18
35.2
28
20
19
24
22
117
2893
1419
AntiStatic
























AntiDust


































Theoretical film thickness based on applicator size and solid content; b1 g polymer dissolved in 6 g
water and 3 g ethanol; c 1 g polymer dissolved in 3 g ethanol and 6 g water; d 1g polymer dissolved
in 5 g water and 5 g ethanol; e 1 g polymer dissolved in 2.5 g water and 7.5 g ethanol; f Minor
cracking of film; g patchy coverage of coating; h hazy film
a
Firstly, the effect of the coating thickness was studied and it was observed that
there is very little influence of dry film thickness on the surface resistivity, indicating that
a homogeneous layer of the polymers has been applied to the PC substrate (Figure 5),
even at film thicknesses of 10 nm. This indicates that only a very thin layer is required for
total coverage of the surface. Total coverage is required for efficient transfer of charge
over the substrate surface. A lower surface resistivity is observed for film thickness of 100
nm compared to thicker films. This is seen consistently for all coatings measured and may
145
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
be related to the fact that the age of the plaques was different, giving rise to the possibility
of polymeric rearrangement. Further, the measurements were carried out on a different
day to the other film thicknesses. Static decay measurements also confirm that film
thickness does not play a large role in the properties of the coatings (Table 4). All values
fall below 1 second, bar the 10 nm film which was found to have a slightly longer half-life,
but still well within the range considered acceptable for anti-static applications.
Surface Resistivity/sq
1012
1011
1010
109
1
10
100
1000
10000
Dry Film Thickness/nm
Figure 5. The effect of the film thickness of G1a-m97 on the surface resistivity. Open symbol =
coating aged longer and measured on different day.
The degree and type of quarternisation was also investigated. In order for a
material to be considered suitable for anti-dust applications, a surface resistivity of 10 12 –
1014 Ohms/sq is required. The polymers (partially) quarternised with FHI all fall within
this range, barring G2b-f21. The polymers (fully) quarternised with MeI, on the other hand,
show even lower surface resistivities (1010 - 1011 Ohms/sq) which classifies them as antistatic materials. Again these results are confirmed by static decay measurement which
show that in fact the polymers quarternised with MeI all show very short t 1/2, with many
dissipating the charge in less than a second. On the other hand, polymers quarternised
with FHI show longer decay half-lives (> 9 seconds) but are still considered acceptable for
anti-dust applications.
Higher degrees of quarternisation with FHI (G1a-f100) show a slight improvement
in the surface resistivity behaviour with values reaching 6.79 × 10 13 Ohms/sq, however it
does not approach the low values obtained using MeI as a quarternising agent with
similar degrees of quarternisation (Table 4). There is a distinct trend in surface resistivity
depending on the degree of quarternisation. Based on quarternisation of G 1a with MeI,
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Chapter 6
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
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Figure 6 shows the expected trend that with higher degrees of quarternisation lower
surface resistivities are measured.
Surface Resistivity//sq
1012
1011
1010
109
40
50
60
70
80
90
100
% Quarternisation
Figure 6. The effect of the degree of quarternisation of G1a on the surface resistivity at a film
thickness of 100 nm.
In this section, highly promising results have been obtained from surface resistivity
and static decay measurements for coated PC compared to untreated PC. Graft
copolymers quarternised with MeI show much better anti-static and anti-fogging
properties than polymers quarternised with FHI, and dry film thicknesses of only 100 nm
are sufficient to give good coverage of the PC substrate successfully modifying the
properties of the surface.
Anti-fogging Properties. The coated plaques, described in the previous section, were tested
for their anti-fogging properties using a simple test. By breathing on the coated plaques
and comparing to untreated PC it was very clear that the coated substrates show antifogging properties; the coated PC remained transparent whereas uncoated PC showed
signs of ‘fog’, turning opaque. In order to provide quantitative evidence of the anti-fog
properties, the contact angle of water on the substrate surface was measured.
Firstly, the contact angle was measured over time using a sessile drop method. An
initial contact angle of < 40º is desired for anti-fogging applications, although a contact
angle of < 20º is preferred. A list of the contact angles for many of the coatings is shown
in Table 5; the contact angle at the start of the measurement (t=0), as well as the time
until the contact angle reaches 40º and 20º are also displayed in the table. For reference,
PC is decidedly hydrophobic with an initial contact angle of 88º which does not fall below
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
40º even in excess of 160 seconds (at which point evaporation can start to influence the
measurement).
Table 5. Anti-fogging properties of quarternised polymers.
PC
G1a-m97a
G1a-m60a
G1a-m40a
G1b-m99a
G2a-m94b
G2b-m100b
G1a-f10c
G1a-f100c
G1b-f26c
G2a-f42d
G2b-f21d
Dry Film
Thickness/
nm
10
100
680
2000
10000
100
100
100
2000
100
2000
7000
10000
100
2000
7000
10000
1800
6300f
100
1800e
1800
1800g
Contact
angle at
t=0/º
88
34
52
63
48
58
81
78
46
57
44
52
51
45
43
45
42
53
114
117
95
83
85
93
Time until
Contact angle
<40º/sec
>160
1
0.3
1
9
76
0.5
0.5
0.1
5
0.1
7
6
7
0.1
2
1
1
>160
>160
>160
49
32
>160
Time until
Contact angle
<20º/sec
>160
10
5
21
80
>160
5
5
11
67
2.5
66
90
79
2
40
70
145
>160
>160
>160
90
>160
>160
Dynamic Contact
angle
(Adv./Rec.)/º
84/70
45/28
<20/<20
52/28
100/58
85/53
71/52
98/66
33/23
49/34
<20/<20
60/32
67/30
63/33
24/19
45/25
42/24
51/28
137/120
115/97
88/72
91/56
109/75
90/58
1 g polymer dissolved in 6 g water and 3 g ethanol; b 1 g polymer dissolved in 3 g ethanol and 6 g
water; c 1g polymer dissolved in 5 g water and 5 g ethanol; d 1 g polymer dissolved in 2.5 g water
and 7.5 g ethanol; e Minor cracking of film; f patchy coverage of coating; g hazy film
a
Initially, thicker coatings (20 – 100 µm) were made and tested. Figure 7a shows the
influence of film thickness on the contact angle for G 1a-m97 (Table 3). At all film
thicknesses, a contact angle of > 40º is observed directly after the droplet has been placed
on the coated substrate, although in all cases the contact angle is less than that of
uncoated PC. However, unlike PC which remains at 88º over time (disregarding the
effects of evaporation), the contact angle of water on the coated substrates decreases over
time as the droplet spreads out over the substrate surface. In fact, at a dry film thickness
of 20 µm, the contact angle of PC coated in G 1a-m97 decreases rapidly from 48º to 40º in 9
seconds, then falls further to 20º within 80 seconds (Figure 7a). It appears that with
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
increasing film thickness, the contact angle (over time) is higher as the rate of decrease of
contact angle is lower at higher film thicknesses. As the coatings are slightly soluble in
water, it is thought that the variation of increasing film thickness with higher contact
angles may be related to a change in surface tension of the water as the polymer is
solubilised by the water droplet. This is further confirmed by the measured contact angles
of a water droplet over time on PC coated with each of the polymers quarternised with
MeI (Figure 7b). Again, a decrease in contact angle over time is observed with all coatings
reaching a contact angle of < 40º within 20 seconds, and < 20º within 80 seconds.
However with increasing hydrophilic monomer content, the contact angle appears to
increase. This is surprising as with more DMAEA there would be more quarternised
amine groups, increasing the hydrophilicity of the surface, and thus decreasing the contact
angle. It is thought that again the solubility of the coating in water may play a role,
changing the surface tension of the water droplet. It should be noted here that although
the hydrophilic monomer content will play a large role in the properties of the polymers,
the structure of the graft copolymer (which in this case is determined by the hydrophilic
monomer content) can also influence the behaviour of the coating.
(a)
(b)
20 m
70 m
100 m
70
60
G1b-m99
60
G2a-m94
50
50
Increasing film thickness
40
30
20
Contact Angle/ o
Contact Angle/ o
G1a-m97
G2b-m100
40
Increasing DMAEA content
30
20
10
10
0
0
50
100
0
150
20
40
60
80
100
Time/Seconds
Time/Seconds
Figure 7. Contact angle over time for films (a) of G1a-m97 of varying thicknesses and (b) with
different hydrophilic monomer contents (film thickness = 20 µm).
Dynamic contact angle measurements were then used to measure the advancing
and receding contact angles of a water droplet on the coatings using a sessile drop (needle
in) method. It was thought that more information on the behaviour of the water droplet
on the substrate could be determined. For anti-fogging materials, an advancing contact
angle < 40º and a receding contact angle <12º are quoted to be acceptable. 9 Again,
polymers G1a-m97, G1b-m94, G2a-m94 and G2b-m100, which have similar degrees of
quarternisation but different DMAEA amounts, were used. For dry film thicknesses of 20
µm, a similar trend to that observed in Figure 7 is seen (Figure 8, black). However, on
decreasing the film thickness to 100 nm, a clearer trend is observed (Figure 8, red). Here,
149
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
a decrease in the advancing angle is observed, so much so that at > 0.5 mol fraction of
hydrophilic monomer the contact angle is too low (< 20º) to be accurately measured.
100
Advancing
Receding
90
Contact Angle/º
80
70
60
50
40
30
20
0.4
0.5
0.6
0.7
Hydrophilic Monomer/Mol Fraction
Figure 8. Dependence of advancing and receding contact angles on [hydrophilic monomer]. Dry
film thickness = 20 µm (black) and 100 nm (red).
Clearly, there is an influence of film thickness and this is illustrated by Figure 9a. A
distinct minimum contact angle can be seen when film thicknesses of 100 nm are used,
giving an advancing contact angle < 20º (which was too low to be measured). Increasing
film thickness does not improve the properties, in fact an increase in the contact angle is
observed presumably attributable to changes in the surface tension of the water droplet as
a result of dissolution. A film thickness of 10 nm, however, appears to be insufficient to
provide homogeneous coverage of the PC substrate resulting in a larger contact angle.
G1a-m97 has been quarternised with MeI in varying degrees, and this has a marked
effect on the advancing and receding contact angles (Figure 9b). At 40% quarternisation
contact angles comparable to uncoated PC are observed. However with increasing
quarternisation an almost linear trend towards lower contact angles is seen, in which at
100% quarternisation the contact angle is too low to be measured. This confirms that a
high degree of quarternisation is required to obtain the best anti-fogging properties from
the coating.
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
(a)
(b)
Advancing
Receding
100
100
80
Contact Angle/º
80
Contact Angle/º
Advancing
Receding
90
90
70
60
50
70
60
50
40
40
30
30
20
20
0
2000
4000
6000
8000
40
10000
50
60
70
80
90
100
% Quartenisation 100% quartenisation was too
Dry Film Thickness/nm
100 nm = Contact angle
too low to be measured
low to be measured (< 20º)
Figure 9. Influence of the (a) film thickness of G1a-m97 and (b) degree of quarternisation of G1a with
MeI on the advancing and receding contact angle.
The coatings made from graft copolymers quarternised with FHI show much
poorer anti-fogging behaviour as illustrated by the evolution of the contact angle over
time compared to untreated PC (Figure 10a). G1b-f26 and G2a-f42 have higher hydrophilic
monomer contents and correspondingly a larger decrease in contact angle is observed,
both falling below 40º within 50 seconds. G2a-f21 and G1a-f10 have very low hydrophilic
monomer contents and in fact G1a-f10 actually demonstrates superhydrophobic behaviour
with contact angles above 100º possibly due to free FHI which is highly hydrophobic in
nature. The trends observed in Figure 10a are confirmed by measurement of the
advancing and receding contact angles shown in Figure 10b.
(a)
(b)
140
G1a-f10
120
G1b-f26
G2a-f42
100
80
G2b-f21
120
Contact Angle PC
110
60
Contact Angle/º
Contact Angle/ o
Advancing
Receding
130
40
100
90
80
70
60
20
0
20
40
60
80
100
120
140
160
50
0.05
0.10
0.15
0.20
0.25
Hydrophilic Monomer/Mol Fraction
Time/Seconds
Figure 10. (a) Contact angle over time (film thickness = 20 um) and (b) advancing and receding
contact angles for films of polymers quarternised with FHI with varying hydrophilic monomer
contents.
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
Direct comparisons between the anti-fogging properties of MeI and FHIquarternised polymers are unfair due to the differences in degrees of quarternisation.
However, even at 100% quarternisation with FHI (G1a-f100), contact angles closer to
untreated PC are observed (Table 5). This observation may be explained by the combined
and conflicting effects of the hydrophobic fluorinated tail and the quarternary ammonium
moiety. The hydrophobic group assists in increasing the advancing contact angle, whereas
the hydrophilic group helps to decrease the advancing contact angle. 27 Therefore, in
systems containing these two groups these two effects can cancel out one another. 28
The use of MeI as a quarternising agent appears to be the most suitable choice in
order to obtain anti-fogging properties, in terms of contact angle measurements and
excluding the conflicting effects discussed above which influence the results based on FHI.
The reason for the poor behaviour of FHI compared to MeI as a quarternising agent may
be a result of slower rearrangement of the fluorinated tails on the surface. Alternatively,
the ammonium ion may well be more shielded when quarternised with FHI compared to
MeI. It should however be noted that the choice of FHI as a quarternisation agent was
based on its incompatibility with PC which may be important to self-segregation of the
polymeric additives after an extrusion process.
Despite difficulties resolving contact angle effects from changes in the surface
tension of water due to dissolution of the coatings, the anti-fogging properties of the
coatings look promising. Admittedly, the water solubility of the polymer is an issue, but it
is hoped that with future extrusion studies better contact and interaction between the
p(CHMA-co-MMA) grafts and PC will be achievable. As it stands, quarternisation with
MeI appears to be a much more effective route to synthesising anti-fogging materials both
in terms of reaction time and coating properties.
CONCLUSION
CCTP has been used to synthesise range of CHMA and MMA well-defined
macromonomers, which have then been copolymerised with DMAEA to form a series of
graft copolymers. These quarternised p(DMAEA-g-(CHMA-co-MMA)) graft copolymers
have shown promising results as both anti-static and anti-fogging agents, with the use of
MeI as a quarternising agent giving markedly improved properties compared to both
FHI-quarternised polymers and untreated PC.
Naturally, further studies are required to optimise the system for practical use.
Addition of these additives to PC during an extrusion process introduces additional
parameters which must be explored. Here, migrational factors will need to be considered.
For example, the ratio of hydrophobic (CHMA, MMA) content to hydrophilic units as
well as the polymer length will play a huge role in the tendency of the polymer to migrate
to the surface. Conversely, the chain length will also affect the ease at which the additive
is irreversibly removed from the material surface, and the hydrophilicity (as has been seen
in this work) directly affects the properties of the surface. A balance of these conflicting
properties and requirements is essential to achieving a successful anti-static or antifogging additive.
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GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
However, it has been shown in these studies that polymers of this nature are
extremely viable alternatives to small molecule anti-static and anti-fogging additives, and
further investigation into a more industrial setting would be greatly beneficial.
153
GRAFT COPOLYMERS AS ANTI-STATIC AND ANTI-FOGGING ADDITIVES
FOR POLYCARBONATE SUBSTRATES
Chapter 6
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154
ACKNOWLEDGEMENTS
During an internship at ICI Paints in UK, a colleague who knew that I was
thinking about doing a PhD, enquired as to whether I had ever considered doing a PhD
abroad. To be honest, the thought had never crossed my mind, but he suggested a
polymer group in the Dutch town of Eindhoven. I had never heard of Eindhoven (I’m not
much of a football fan…) but once the idea had been planted in my head (and after
checking out the research activities), moving to Eindhoven for my PhD seemed more and
more appealing…. So I guess my first ‘thank you’ goes to Dr Neal Williams, without
whom I would probably still not know that the city of Eindhoven even exists.
I must also thank Alex van Herk and Cor Koning for welcoming me into their
group and allowing me the opportunity to carry out my PhD in SPC. You started out as
both being my promoters, and you have both helped and supported me throughout these
four years. Hans and Rob, thank you for being my co-promoters and having faith in me
and the project from beginning to end. It started out a little rocky, but between us we
managed to turn the project around and into something that I hope you are as proud of as
I am. Thank you for all the time and effort you put in; I know how busy you both were,
particularly in the final months.
The Dutch Polymer Institute (DPI) is greatly acknowledged for the funding of this
project and for providing a platform in which to interact with other academic and
industrial members.
My committee members: Prof. Dave Haddleton, Dr Franck D’Agosto and Dr Theo
Hoeks, thank you for doing me the honour of being part of my committee and for taking
the time to read my thesis.
I would also like to thank my paranymphs, Timo Sciarone and Bas van
Ravensteijn, not only for their support during my defence but also for their help during
my PhD. I would not have had the thesis I have without you two. Timo, you were there
from the beginning and were always available for a brainstorming session or just some
good old fashioned advice on chemistry, life, music, etc. As the post-doc on my project,
you were termed a ‘mine-sweeper’, and thank goodness you were there because there were
a lot of mines! Bas, thank you for all your hard work for nine months (or so!) on the
epoxides chapter, your dedication and work ethic is an inspiration to us all and I wish you
all the best in your PhD and the rest of your career.
During my PhD I also had the opportunity to work with some great people and
groups which has not only enhanced my PhD work but has also been an excellent
experience and I’m grateful to be able to work with people from different fields. Franck,
Christophe and Jérôme, thank you for allowing me to spend two very interesting weeks in
your lab in Lyon. I thoroughly enjoyed it, even if none of the reactions worked! Theo,
thank you for all the fruitful discussions over the past few years and for showing an
interest in my project during DPI cluster meetings. Jan and Jan Henk, thanks for taking
the time for carrying out a lot of the anti-static and anti-fogging tests in lab at Sabic IP
and teaching me about the ins-and-outs these tests. Further, I would like to thank
Michelle Coote in Queensland, Australia for doing some quantum calculations for the
pSMA work. I would also like to thank my students: Kenny, Toon and Vicente.
156
Of course I cannot forget the analytic staff of SPC, who helped me no end over the
last four years and without whom I would have had a much tougher time. Martin, thank
you for all your help with SEC measurements and always make time for me and my
questions. Hanneke and Carin, thanks for teaching me how to use the MALDI and for
carrying out many many GPEC measurements for me.
Pleunie and Caroline, thank you so much for all your help over the last 4 years,
particularly with helping me sort out all forms and admin I needed for my defence after I
had already moved away.
I would also like to thank my office mates: Yingyuan and Dogan. We started
together and will end (more or less) together. I think we had the most peaceful and quiet
office in the whole corridor and all appreciated that the heating should be turned up full
all the time. I wish you all the best for the final stages of your PhDs.
Cacticians, past and present, even though I was never really an ‘organometallic
chemist’, I’m glad you accepted me and finally let me be your “Labor-Mama”! The Cactus
meetings were always educational (even if I never want to hear anything about MAO ever
again) and the Cactus evenings always fun and enlightening (even if the next day I never
wanted to drink beer ever again)! There are a few people who made it worth me going to
the lab, even on the darkest of experimental days. Raf, thank you for all kindness,
generosity and help in the lab and in chemistry in general – you are truly a fountain of
knowledge! T-meister, I’ve already mentioned you, but watch out, I’ll get my ice revenge
one day… Camille, my dear, the Cactus lab wouldn’t have been the same without you;
thank you for continually inviting me to things even when I was being incredibly boring
and stressed, sometimes I really needed it! Fabi, rule well and enjoy being “Labor-Papa”.
Shan the Man, I very much appreciate that you have successfully completed your PhD
without blowing me up! Pepels, one Cactus evening I asked Rob for a drinking, nonsmoking labmate and a few months later I got one! It’s been too short, but I hope you
enjoy our old apartment as much as we did!
There are a few other friends I made while in Eindhoven that I’d like to thank:
Bahar (DPD-roomie and culinary extraordinaire), Jérôme (gossip-buddy), MC (who’d a
thought we’d meet again), Inge and Martin O (the SPC helpdesk), Erik (long live the
PandA project), Judith (my fair-weather friend), Mark (HFL), Niels (CCT-addict), and
Shona, Vanessa and Nsem (with whom I tried to regain my ability to speak English).
Thank you to all my other friends, in the UK and further afield who helped me get
this far and always encouraged me.
Finally I’d like to thank Mum, Dad, Lu and the rest of my family, for supporting
me though all the long years of education which finally led me to this point. And Gijs,
thanks for motivating me when I needed it and for everything else.
Gemma
157
CURRICULUM VITAE
Gemma Claire Sanders was born on 30th July 1986 in Colchester, UK. After completing
her school education at King Edward VI School in Bury St Edmunds in 2004, she studied
Chemistry at Durham University. In 2007 she carried out a one year industrial placement
as part of her Master’s degree at ICI Paints in Slough, UK. Her research project was
entitled: ‘Novel Polymer Synthesis and Evaluation for Use in Waterborne Coatings’ and
was carried out under the supervision of Dr Manish Sakar and Dr Ezat Khosravi. In 2008
she graduated from Durham University with Master in Chemistry (Industrial Route)
awarded with a second class honours, upper division (2:1) degree. In September 2008,
Gemma commenced her PhD under the supervision of prof.dr. Cor Koning, prof.dr. Alex
van Herk, dr.ir Hans Heuts and dr. Rob Duchateau, the results of which are presented in
this thesis. Since June 2012, Gemma has been employed at BASF SE in Ludwigshafen,
Germany.
158
LIST OF PUBLICATIONS
Gemma C. Sanders, Timo J.J. Sciarone, Hanneke M.L. Lambermont-Thijs, Robbert
Duchateau, Johan P.A. Heuts; Methacrylic Stereoblock Copolymers via the Combination
of Catalytic Chain Transfer and Anionic Polymerization, Macromolecules, 2011, 44, 9517.
Gemma C. Sanders, Bas G.P. van Ravensteijn, Johan P.A. Heuts, Robbert Duchateau; The
Copolymerisation Behaviour of Epoxidised Catalytic Chain Transfer Polymerisationderived Macromonomers, Polymer Chemistry, 2012, 3, 2200.
Jérôme Mazzolini, Gemma C. Sanders, Vincent Monteil, Johan P. A. Heuts,
Robbert
Duchateau, Didier Gigmes, Denis Bertin, Franck D’Agosto, Christophe Boisson;
Polyethylene End-Functionalization Using Polyethylenes Containing Thiol EndFunctionalities, Polymer Chemistry, 2012, DOI: 10.1039/c2py20199b.
Gemma C. Sanders, Robbert Duchateau, Ching Yeh Lin, Michelle L. Coote, Johan P.A.
Heuts,
Styrene-Maleic
Anhydride
Copolymers
via
Catalytic
Chain
Transfer
Polymerisation; Macromolecules, 2012, DOI: 10.1021/ma301161u.
Gemma C. Sanders, Jérôme Mazzolini, Franck D’Agosto, Christophe Boisson, Johan P. A.
Heuts, Robert Duchateau, Block Copolymers using Thiol-terminated Polyethylenes as
Chain Transfer Agents, In preparation.
159