1. No category

European Polymer Journal 49 (2013) 1760–1772
Contents lists available at SciVerse ScienceDirect
European Polymer Journal
journal homepage: www.elsevier.com/locate/europolj
Copolymerization of isoprene with polar vinyl monomers:
Reactivity ratios, characterization and thermal properties
David Contreras-López a, Enrique Saldívar-Guerra a,⇑, Gabriel Luna-Bárcenas b
a
b
Centro de Investigación en Química Aplicada (CIQA), Blvd Enrique Reyna Hermosillo 140, Saltillo, Coah. 25294, Mexico
CINVESTAV, Unidad Querétaro, Libramiento Norponiente 2000, Fracc Real de Juriquilla Queretaro, Qro. 76230, Mexico
a r t i c l e
i n f o
Article history:
Received 10 November 2012
Received in revised form 18 March 2013
Accepted 22 March 2013
Available online 10 April 2013
Keywords:
Isoprene
Maleic anhydride
Glycidyl methacrylate
Polar monomer
Reactivity ratios
Copolymer
a b s t r a c t
Reactivity ratios for the copolymers isoprene (IP)-co-maleic anhydride (MAH) and isoprene
(IP)-co-glycidyl methacrylate (GMA) are reported. Copolymers were prepared by free radical polymerization using benzoyl peroxide (BPO) as initiator at 70 °C. These copolymers
were characterized by FTIR and 1H NMR. The monomer compositions in the copolymer
were determined by 1H NMR and the reactivity ratios (ri) were calculated applying diverse
linear methods, namely Finemann–Ross (FR), Inverted Finemann–Ross (IFR), Kelen–Tüdös
(KT), Extended Kelen–Tüdös (EKT) and the nonlinear method of Tidwell–Mortimer (TM). By
using the latter procedure, the values of the reactivity ratios were estimated as 0.119 and
0.248 for the system IP (1) and GMA (2) respectively; whereas for the IP and MAH system
were 0.057 and 0.078 respectively. These values suggest the formation of nearly-alternating copolymers in both systems. Molecular weights were determined by gel permeation
chromatography (GPC). Glass transition temperatures (Tg) of the copolymers were obtained
by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Good
agreement is obtained between experimental Tg values and the model of Couchman. Tg
increases when the concentration of polar monomer in the copolymer increases.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Plastics industry is one of the most active sectors in the
world’s economy for the sustained demand of its products
in many technological applications; this includes polymers, copolymers and their blends [1]. Polymerization of
1,3-dienes, such as isoprene, remains an active research
area in both industry and academia [2–7]. Derivatives of
this type of diene monomers are of great commercial
importance. Polymerizations of these monomers are usually carried out by anionic [8,9], cationic [10,11] and coordination [5,12–14] techniques. However, these schemes
are highly sensitive to impurities and to the presence of
many functional groups.
⇑ Corresponding author. Tel.: +52 844 438 9830; fax: +52 844 438
9463.
E-mail address: [email protected] (E. Saldívar-Guerra).
0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.eurpolymj.2013.03.030
On the other hand, the inherent robustness of free radical polymerizations can be advantageously applied to the
polymerization of 1,3-dienes [15–17] in order to diversify
the variety and applications of this family of polymers
[2,18,19].
The polymeric derivatives belonging to this family can
be used as compatibilizers in blends containing natural
rubber [20,21]. They can also be used as impact modifiers
of rigid thermoplastics containing polar groups, such as acrylic polymers for medical applications [22,23].
Isoprene is a conjugated diene that is industrially
attractive because it is available at low cost; yet it poses
many challenges due to the presence of a pendant methyl
group. This methyl group leads to various possible configurations in the polymer backbone depending upon the
polymerization mechanism.
Polymerization of isoprene generates various microstructures that exhibit different mechanical and physicochemical properties [24,25] (Scheme 1). For instance,
D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
1761
2. Theory
The structure of a copolymer depends on the time of
reaction and the relative concentrations of the comonomer
and its reactivity (polarity, resonance and steric factors)
[30].
Mayo and Lewis [31] introduced the first kinetic model
of a free radical copolymerization, the terminal kinetic model, in which the reactivity of a radical depends only on the
last monomeric unit in the chain. According to this model,
the instantaneous relative consumption of the two monomers is given by:
dm1 m1 M 1 r1 M1 þ M2
¼
¼
dm2 m2 M 2 M1 þ r 2 M2
Scheme
1. Polymerization
microstructures.
of
isoprene
generating
several
trans-1,4- and 3,4-configurations are semicrystalline polymers with a Tg higher than that of the cis-1,4-isomer. Considering the limited abundance of natural rubber and the
increasing demand for various high performance synthetic
rubbers, isoprene-based elastomers are therefore of great
importance in the market.
Literature reports on the free radical copolymerization
of isoprene with polar monomers are very limited. Particularly in the case of maleic anhydride, it seems that the formation of a Diels–Alder cycloadduct [26–28], that
competes with the polymerization reaction, has precluded
further development of these copolymers. Hence, it is
important to deepen the understanding of the monomer
incorporation into the copolymer growing chain as this
will greatly influence the thermal and mechanical properties of the final copolymer. Indeed, there is only one report
of the reactivity ratios for the copolymerization system IP–
GMA [29] and none found for the IP–MAH system.
The spirit of our study is to further the knowledge of the
free radical copolymerization of isoprene with polar monomers to create more advanced and controlled structures,
including random or tapered blocks in the polymer chain.
These could be achieved by using controlled free radical
polymerization (CRP) techniques, in particular nitroxide
mediated polymerization and reversible addition fragmentation chain transfer (RAFT) polymerization. However, the
effective applications of these synthesis methods requires
prior knowledge of the reactivity ratios which are known
to remain fairly constant when going from conventional
free radical polymerizations to CRPs. At present, our group
is focusing on this type of approach.
The objectives of this work are: (1) to find the proper
synthesis conditions to copolymerize isoprene with two
readily available polar monomers: maleic anhydride and
glycidyl methacrylate; (2) to estimate the reactivity ratios
in these copolymerizations, and (3) to study the thermal
properties of the final copolymers. To accomplish the
above objectives, FTIR, NMR, GPC, DMA and DSC analyses
are performed.
ð1Þ
where m1 and m2 are the mole fractions of the two copolymerized monomers, M1 and M2 are the mole fractions of
the monomers in the feed, r1 and r2 are the reactivity ratios
of the species involved. The reactivity ratios are assumed
constant independent of the conversion; however, they
are usually determined at low conversions [30].
Numerous methods exist to determine the reactivity ratios, such as the methods of Finemann–Ross (FR) [32], Inverted Finemann–Ross (IFR) [32], Kelen–Tüdös (KT) [33],
Extended Kelen–Tüdös (EKT) [34] and Tidwell–Mortimer
(TM) [35], which have been used for determining ri at
low conversions. The first four use linear estimation techniques and only the last one (TM) uses a non-linear estimation technique.
3. Experimental
3.1. Materials
Isoprene (IP, Sigma–Aldrich) and glycidyl methacrylate
(GMA, Sigma–Aldrich) monomers were washed twice with
a solution of 5% NaOH, twice with distilled water, and dried
on Na2SO4. Such washed monomers were distilled prior to
a polymerization reaction; isoprene was distilled at atmospheric pressure and GMA was distilled under vacuum.
Maleic anhydride (MAH, Fluka) and benzoyl peroxide
(BPO, Sigma–Aldrich) were both recrystallized from chloroform solutions. The solvents N, N0 -dimethyl formamide
(DMF), chloroform, hexane, methanol and acetone (reagent
grade) were used as received.
3.2. Instrumentation
1
H NMR spectra were carried out on an Eclipse 300 Jeol
Instrument at 300 MHz using deuterated chloroform
(CDCl3) for IP–GMA copolymers and deuterated acetone
(Ac-d6) for IP–MAH copolymers at room temperature.
The Delta NMR Processing software Version 4.3.6 [Windows NT] by JEOL was used to analyze the spectra. In both
cases 16 scans were used with samples of 25 mg in 0.35 mL
of deuterated solvent. FTIR spectra were obtained on a
Nexus 470 Spectrometer in the 4000–400 cm1 range
using 32 scans and 4 cm1 resolution. Molecular weights
relative to polystyrene standards were determined by gel
permeation chromatography (GPC) using UltrastyragelR
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D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
columns in tetrahydrofuran (THF) at 40 °C and a solvent
flow rate of 1 mL/min in a Waters 410 apparatus. Thermal
analysis of polymeric materials was carried out by differential scanning calorimetry (DSC) in a Perkin Elmer 7 series
instrument. The heating rate was set to 10 °C/min and a
temperature range of 100 to 100 °C. Dynamic mechanical
analysis (DMA) was performed on a TA Instrument
DMAQ800 with a clamp voltage at a frequency of 1 Hz
and strain amplitude of 20 lm. The heating rate was
5 °C/min and the temperature range was from 100 to
90 °C.
3.3. General procedure for the copolymerization kinetics of IP–
GMA
Copolymerization reactions were carried out in glass
vials (8.5 cm length, 23 mm outside diameter) sealed with
aluminum caps and a rubber septum.
The monomers and initiator were mixed and split into
several 2 mL aliquots. These aliquots were kept in sealed
glass vials under nitrogen atmosphere. Vials were immersed into an ice bath for 20 min to promote degassing.
At this stage, more nitrogen was bubbled for 20 min in
the reaction mixture in order to remove oxygen. This operation was repeated thrice. Care was taken to ensure that
essentially no monomer loss occurred by venting during
this stage. Vials were immersed in an oil bath at
70 ± 0.5 °C to start the copolymerization. Vials were taken
out the oil bath at regular intervals to quench the reaction
by adding a 2% hydroquinone solution in an ice bath. Once
the desired conversion (<15%) was achieved, 15 mL of chloroform were added to each of the remaining vials to dilute
the solution. This solution was slowly poured into 60 mL of
methanol to precipitate the copolymer. This dissolution–
precipitation procedure was repeated five times. The fibrous copolymer precipitated was finally filtered and
washed with hexane. All samples were dried in a vacuum
oven at 40 °C for 24 h and analyzed by FTIR, 1H NMR,
GPC and DSC. The dried product had a whitish appearance.
3.4. General procedure for the copolymerization and kinetics
of IP–MAH
Copolymerization reactions were carried out in a fume
hood and performed at different compositions of the system IP/MAH.
A jacketed two-necked glass flask equipped with magnetic stirring and a condenser was charged with a solution
of recrystallized MAH and DMF using a ratio of 5% by
weight relative to the MAH. DMF was used to fully dissolve
the MAH in the reaction medium. It is noteworthy that in
the absence of DMF, the solubility of MAH in the isoprene
is limited to ca. 3–5% by weight similar to that of MAH in
styrene [36]. Since the amount of DMF used in the experiments is very small, the polymerization can be considered,
in practical terms, a bulk polymerization. The solution was
deoxygenated with nitrogen (UP grade) for 20 min.
Another solution containing distilled isoprene and BPO
(1.5 mol% with respect to IP) was deoxygenated with nitrogen (UP grade) for 20 min to remove impurities such as
oxygen or moisture. During this operation the solution
container was immersed in an ice bath to prevent loss of
monomer.
The MAH/DMF mixture was heated to 70 ± 0.5 °C by
means of a recirculating bath oil under magnetic stirring.
Once the temperature reached 70 °C, the IP/BPO mixture
was carefully added through an addition funnel that was
previously connected to the jacketed two-necked glass
flask over a period varying in the range 5–20 min, depending on the monomer composition used. Reaction time was
recorded from the initial addition of the IP/BPO mixture.
This procedure allows one to favor polymerization over
Diels–Alder cycloaddition [26].
Samples were taken at regular intervals (typically every
1 h) to measure the conversion gravimetrically. After
reaching the desired overall conversion (<15%), 20 mL of
acetone were added to the reacting mixture to dissolve
the copolymer. This mixture was slowly poured into
100 mL of hexane under stirring to precipitate the copolymer. The excess of solvent was removed by decantation.
This dissolution–decantation procedure was repeated five
times. The copolymer was recovered as a fibrous bundle.
All samples were finally filtered and washed one more
time with hexane and then dried in a vacuum oven at
40 °C for 24 h. The dried product had a yellowish-tobrownish appearance depending upon the copolymer composition. Samples were analyzed by FTIR, 1H NMR, GPC and
DSC.
4. Results and discussion
4.1. IP–GMA copolymer
The copolymerization of IP with GMA in bulk (Scheme
2) was studied by varying the mole fractions of the diene
from 0.1 to 0.9 in the feed.
FT-IR. Characteristic bands for the IP–GMA copolymer
are observed in Fig. 1. The copolymers exhibit similar characteristic absorption bands as those observed in the homopolymers; the bands at 2992 and 2926 cm1 are associated
to the stretching of ACH2A and ACH3 of the copolymers
with different IP/GMA compositions. The bands at 742,
1130, 1315 and 1325 cm1 are associated with vibrations
of the AC(CH3)@CHA group. Out of these, the 1130 cm1
band appears to be due to a vibration of the AC(CH3) in this
group, while the 1315 cm1 band corresponds to the vibration of the CAH group in cis conformation, while the band
in 1325 cm1 corresponds to the vibration of the same
group in trans conformation. Additionally, the band at
1666 cm1 is for 1,4-addition and that at 1645 cm1 for
3,4- addition of the isoprene units. The bands at 889 and
Scheme 2. Copolymerization reaction of isoprene–GMA.
D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
1763
Fig. 1. FT-IR spectra of IP–GMA copolymers in different ratios in mol% in the feed. (A): PGMA, (B): IP–GMA 20:80 copolymer, (C): IP–GMA 50:50 copolymer,
(D): IP–GMA 80:20 copolymer, (E): PIP.
837 cm1 are due to stretching vibrations of the unsaturation present [37]. On the other hand, the strong absorption
at 1733 cm1 is due to stretching of the carbonyl group of
GMA and the band at 911 cm1 corresponds to the epoxy
group of GMA.
1
H NMR (Copolymer composition). The 1H NMR spectrum of the IP–GMA copolymer is presented in Fig. 2. The
figure shows a comparison of spectra for different compositions of the copolymer (mol% in the feed), including those
of the homopolymers at the extremes. The 1H NMR spectrum of PGMA (Fig. 2, protons h and i) shows signals at
3.1 and 2.6 ppm due to methylene and methyne protons
of the epoxy group, respectively, and a peak of multiplets
at 4.3 and 3.8 assigned to the ACH2A group (g) between
the ester and the epoxy protons. The signal of the methyl
group (f) appears at 1.3 ppm.
The 1H NMR spectra of the IP–GMA copolymer (Fig. 2B–
D) show peaks around 4.9 ppm assigned to the proton (b)
from the isoprene unsaturation. Peaks at 2.0–2.2 ppm are
assigned to the methylene protons (d) of the main chain.
The protons from the CH3A (c) of the isoprene units are assigned to the peak at 1.7 ppm.
In order to determine the amount of each comonomer
incorporated into the copolymer formed, the integration
of selected peaks from the 1H NMR spectra was used for
this purpose. This technique is widely used for estimating
composition in both the industrial and the academic fields
[38,39]. The peak assignments were done according to ta-
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D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
Fig. 2. 1H NMR spectrum for the IP–GMA system. Left: Copolymer of IP–GMA in different ratios in mol% in the feed. (A): PGMA, (B): IP–GMA 20:80
copolymer, (C): IP–GMA 50:50 copolymer, (D): IP–GMA 80:20 copolymer, (E): PIP. Upper right: Amplified 1H NMR spectrum of IP–GMA 50:50 copolymer.
Lower right insert: Assignments of characteristic peaks of each unit involved (the numbers in the groups denote the chemical shifts (ppm)).
bles available in the literature [40]; some of them were
also confirmed via the software ACD/ChemSketch Version
4.55.
Reactivity ratios. In order to illustrate the calculations
involved in the determination of the copolymer composition, let us take as an example the 1H NMR spectrum of
the IP–GMA copolymer with a mole ratio of 50:50. It is possible to identify three regions in the spectrum (Fig. 2, upper
right): (i) the peaks corresponding to the protons in ACH
and ACH2ACH3 groups in the IP–GMA and isoprene units,
lying in the region of d = 0.9–2.3 ppm; (ii) the peaks assigned to protons in the glycidyl groups of GMA units, located in the region of d = 2.4–4.5 ppm and; (iii) the peaks
of protons adjacent to double bonds, AHC@and @CH2, of
isoprene units in the region of d = 4.6–5.3 ppm.Following
Rusakova et al.[29], we define the mole fractions of GMA
and isoprene present in the copolymer as m2 and (1m2)
respectively and, given that the GMA unit contains ten protons and the isoprene unit eight protons, the number of
protons which is proportional to the total area of the spectrum (ST) is:
ST a10m2 þ 8ð1 m2 Þ ¼ 8 þ 2m2
ð2Þ
On the other hand, the peaks in the range of 2.4–4.5 ppm
correspond to five protons of the glycidyl group, whose
area is denoted by SG and is proportional to 5m2, resulting
in:
SG
5m2
¼
;
ST 8 þ 2m2
m2 ¼
8SG
5ST 2SG
ð3Þ
Table 1 shows the copolymer molar compositions obtained by the above procedure for various isoprene–GMA
copolymerizations at low conversion, in which the monomer molar ratio in the feed was varied. Average molecular
weights by number (Mn) and weight (Mw) of the copolymers (determined by GPC) are also shown in Table 1. From
Table 1 it is apparent that the average molecular weight
decreases as the content of IP is increased in the copolymerization. This seems to be related to two factors: (i)
the lower average propagation constant as the content of
IP in the feed increases and (ii) a possible higher rate of
chain transfer to monomer as the IP content increases.
Although there is not enough kinetic parameter data available in the literature for these monomers (IP and GMA),
some rough estimations for the values of the relevant kinetic constants can be obtained from published data for
these or similar monomers. The only value that we found
reported for the propagation constant (kp) of isoprene is
2.8 L/(mol s) at 5 °C, while that for butadiene (similar diene
without the methyl group of isoprene) at the same temperature is 5.69 L/(mol s), that is, double the value for iso-
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D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
Table 1
Results of the copolymerization of IP with GMA.
% mol IPa
% mol GMAa
Composition of the
initial monomer (M1)a,c
Conversion (%)
Composition of the
copolymer (m1)b,c
Mn
Mw
Ð
10
20
30
40
50
60
70
80
90
90
80
70
60
50
40
30
20
10
0.11
0.21
0.29
0.38
0.50
0.61
0.72
0.80
0.93
9.9
11.1
8.0
11.5
8.7
13.3
12.1
14.1
14.7
0.23
0.32
0.43
0.45
0.52
0.52/0.54d
0.54
0.58
0.72
93,000
79,000
57,000
39,000
10,200
8900
9900
3400
5200
2,30,000
2,10,000
1,70,000
76,000
28,000
26,000
33,000
12,000
18,000
2.5
2.7
3.0
1.9
2.7
2.9
3.3
3.5
3.5
Temperature: 70 °C, initiator: BPO. 1: IP, 2: GMA The values of initial feed composition (mol %) of the first two columns are nominal values. Actual values in
column 3 (only monomer 1 shown) slightly differ from the nominal ones due to the experimental error at weighing.
a
Feed composition.
b
Determined by 1H NMR.
c
Molar composition.
d
This experiment was replicated.
prene, both values from the same old Ref. [41]. A more recent kp data for butadiene at 70 °C is 295 L/(mol s) [42]. The
kp value for GMA at 70 °C is reported as 1939 L/(mol s) [43],
around 6–7 times larger than that of butadiene at the same
temperature, and presumably some 12–14 times larger
than that of IP if we use all of the above data and make
simple extrapolations. These values are consistent with
the trend shown by our own data if we do simple linear
calculations based on the conversion–time data from Table
1; roughly, the polymerization rate for the 10/90 IP/GMA
copolymerization is three times larger than that for the
90/10 IP/GMA reaction. A plot not shown (available in Supporting Information) of the estimated polymerization rate
with varying monomer composition in the feed, using all
the data in Table 1, shows a consistently descending polymerization rate as the IP content is increased in the feed.
The change in kp and polymerization rate with monomer
composition will definitely contribute to generate lower
Mn copolymer as the content of IP increases in the copolymerization, but does not seem to be enough to explain the
dramatic change on the molecular weight with composition. On the other hand, for transfer to monomer rate constants (ktr) we found the value of 1.32 L/(mol s) for
butadiene at 70 °C [42], while no value was found for
GMA; however, for another methacrylate (methyl, MMA)
the ktr value reported at 50 °C is 0.0334 L/(mol s) [44].
Based on the kp of MMA at 50 °C, which can be estimated
as 651 or 1,157 L/(mol s) depending on the source
[43,45], the transfer constant CM (ktr/kp) for MMA at 50 °C
is 5.13 105 or 2.9 105 L/(mol s), respectively. Similarly, the value estimated for CM of butadiene at 70 °C
would be 4.5 103 L/(mol s), around two orders of magnitude larger than that of MMA. This suggests that the value of CM for isoprene could also be significantly larger than
that of GMA, also contributing to lower the Mn of the
copolymers for increasing contents of IP in the feed.
These polymeric materials were soluble in DMF, THF,
DMSO, acetone, toluene and chloroform, but insoluble in
methanol, ethanol, water and hexane.
Fig. 3 shows the curve of instantaneous copolymer composition (mole fraction of isoprene) obtained for a given
feed monomer composition for the IP–GMA system. Iso-
prene (IP) and glycidyl methacrylate (GMA) are designated
as monomers 1 and 2, respectively. The graph shows that
this pair of monomers exhibits an azeotropic point (where
the instantaneous composition of the copolymer is equal to
the composition of the monomer feed) at 50.7 mol % of IP.
The reactivity ratios and the compositions of glycidyl
methacrylate copolymer and isoprene were determined
by the linearized methods of Finemann–Ross (FR) [32], Inverted Finemann–Ross (IFR) [32], Kelen–Tüdös (KT) [33],
Extended Kelen–Tüdös (KTE) [34] and also by the non-linear method of Tidwell–Mortimer (TM) [35]. Some of the
older methods (e.g. FR and KT) are nowadays mostly used
to get initial estimates for the non-linear estimation techniques. The linear techniques use some form of linear
regression or fitting to a linear model. Both the ‘‘independent’’ and ‘‘dependent’’ variables involve the copolymer
composition, which is actually a response measured with
an inherent error. Statistically, this violates the assumptions required by the least squares method, so no valid
inference can be made about the fitted parameters. Strictly
speaking, a method of nonlinear least squares (NLLS)
should be used in order to avoid these problems. Tidwell
and Mortimer (TM) were among the first to use such a
technique to estimate reactivity ratios [35]. It must be
pointed out that the TM method still suffers from a shortcoming due to the fact that the independent variable is not
measured without error (a requirement for the application
of NLLS), especially when the data are taken at relatively
high conversions.
The experimental error was calculated using a pool-variance estimate. On the one hand, a measure of the experimental error was obtained by the sum of squared residuals
of the model (assuming the model is a correct one). In
order to have a second source of estimation for the experimental error, two experiments (one for each copolymerization system) were replicated. The two sources were
combined to provide an estimation of the experimental error (standard deviation) which, for the system IP–GMA,
was r = 0.028 with 8 degree of freedom. As a result, the
estimated 95% confidence intervals for the reactivity ratios
(TM technique) are r1 = 0.119 ± 0.048 and r2 = 0.248 ±
0.161
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D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
Fig. 3. Copolymer composition vs. feed composition for the copolymerization system IP–GMA. Experimental data points are compared with the prediction
using the Mayo–Lewis equation (solid line) with the reactivity ratios of the TM technique.
Table 2
Results of reactivity ratios calculated using various methods for the IP/GMA
system.
Method
r1
r2
r1 r2
FR
IFR
KT
EKT
TM
0.115
0.238
0.121
0.124
0.119
0.206
0.316
0.224
0.198
0.248
0.0236
0.0754
0.0270
0.0246
0.0295
r1: Reactivity ratio of IP. r2: Reactivity ratio of GMA.
The reactivity ratios of the monomers obtained by various methods are summarized in Table 2.
According to the values in Table 2, except for the IFR
method, the product r1r2 is less than 0.03, indicating that
the IP/GMA copolymerization exhibits a strong tendency
to alternate [46].
For this system the only other values of reactivity ratios
found in the literature were reported by Rusakova et al.
[29] as r1 = 0.135 and r2 = 0.195, confirming that the
copolymer is nearly alternate; however, they do not report
details of the method used for their estimation. These values are in good agreement with those calculated in this
work by the FR method.
It is also interesting to note that reported values of reactivity ratios for the copolymerization of isoprene with another methacrylate (IP = 1, methyl methacrylate,
MMA, = 2), r1 = 0.65 and r2 = 0.26, significantly differ from
our system in the value of r1. This seems to be related to
a relatively greater thermodynamic affinity of GMA than
MMA to polyisoprene, as measured by the square difference of the appropriate solubility parameters: 0.49 cal/
cm3 for GMA-polyisoprene compared to 1.69 cal/cm3 for
MMA-polyisoprene, estimations based on [47].1
1
The solubility parameters were estimated using the group contribution
formula (qR Gi/M), where Gi is the molar attraction constant of group i, qGi
is the sum for all the atoms and groupings in the molecules, q is the
polymer density and M is the polymer molecular weight.
Notice that for the calculation of the reactivity ratios it
is desirable to use samples at conversions as low as possible so the error introduced by the composition drift is minimized. In this regard, Penlidis et al. [48] discuss that if
conversions are higher than about 15–20% the reactivity
ratios should be obtained by integrating the copolymerization equation to take into account the composition drift.
DSC and DMA. Fig. 4 shows the glass transition temperatures (Tg) obtained by differential scanning calorimetry
(DSC) as a function of comonomer concentration. We
tested several popular Tg models for copolymers which include those of Fox, Pochan,Wood and Couchman. The best
correlation with our experimental data was obtained by a
model based on the Couchman theory [49,50]:
%wt2 C p2 Ln
T g2
T g1
%wt1 C p1 þ%wt2 C p2
T g ¼ T g1 e
ð4Þ
where Tgi is the glass temperature of the homopolymer i, %
wti is the weight fraction of monomer i in the copolymer
and Cpi is the heat capacity of the homopolymer i. The better fit obtained with the Couchman model is attributed
mainly to the fact that the this model follows rigorously
the thermodynamic theory of glass transition, including
the effect of the heat capacities; other models are less rigorous than this one. It is noteworthy that the experimental
data are in excellent agreement with the Couchman’s model. One can observe that the higher the concentration of
isoprene, the lower the Tg. One can also note that all thermograms showed a single glass transition temperature,
which implies that a copolymer of homogeneous composition was formed.
Tgs of the copolymer were also determined by dynamic
mechanical analysis (DMA). These independent measurements confirm the trend shown by DSC. DMA results for
the 90:10 IP-co-GMA are shown in Fig. 5.
Table 3 shows the comparison of Tg values obtained by
the DSC and the DMA techniques. It is noteworthy that
increasing the concentration of GMA in the copolymer
D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
1767
Fig. 4. Tg behavior of the IP–GMA copolymer as a function of weight concentration of IP. The continuous line represents the prediction of the Couchman
model.
Scheme 3. Copolymerization reaction of IP with MAH.
Fig. 5. DMA of IP–GMA 90:10 copolymer.
Table 3
Summary of DSC/DMA Tg values for the IP/GMA copolymer system.
% w/w IP
DSC
Tg (°C)
DMA
Tg (°C)
12
40
55
74.4
48.7
30.3
71.3
52.6
33.7
the value of its Tg increases. This effect is mainly due to the
presence of the polar comonomer GMA.
4.2. IP–MAH copolymer
The copolymerization of MAH-IP (Scheme 3) was studied by varying the mole fractions of the diene from 0.4 to
0.9 in the initial feed. This composition range was chosen
such that the formation of the cycloadduct could be minimized. This cycloadduct is the main reaction product when
the composition of the initial feed is below 0.4 mol fraction. To satisfy the Mayo–Lewis model [31], conversions
below 15 wt.% were experimentally studied. The copolymerization reaction was carried out in bulk and BPO, the
free radical initiator, was used in relatively large amount
(1.5 mol % with respect to the diene) to favor the polymerization with respect to the corresponding cycloaddition
Diels–Alder reaction [26]. The compositions in the monomer feed and in the copolymers are presented in Table 4.
The copolymers were soluble in DMF, THF, DMSO and acetone, but were insoluble in methanol, ethanol, chloroform,
toluene, water and hexane.
FTIR. IR spectra of the IP–MAH copolymer were qualitatively analyzed and they are shown in Fig. 6. It can be observed that the copolymers have similar characteristic
absorption bands that are related to the homopolymeric
units; the broad bands at 3077, 1666, 1645, 889 and
837 cm1 are due to stretching of isoprene with different
microstructures. The strong absorption bands at 1854
and 1780 cm1 confirm the presence of two carbonyl
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D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
Table 4
Results of the copolymerization of IP with MAH.
mol% IPa
mol % MAHa
Composition of the
initial monomer (M1)a,c
Conversion (%)
Composition of the
copolymer (m1)b,c
Mn
Mw
PDI
90
80
70
60
50
40
10
20
30
40
50
60
0.91
0.80
0.71
0.61
0.50
0.40
13.3
8.7
9.8
8.0
11.1
6.6
0.61
0.58
0.54/0.52d
0.52
0.49
0.48
5900
5600
4500
4700
3500
2900
17000
8800
7400
7500
6800
6300
2.9
1.6
1.6
1.6
1.9
2.2
Temperature: 70 °C, initiator: BPO; 1: IP, 2: MAH. The values of initial feed composition (mol %) of the first two columns are nominal values. Actual values in
column 3 (only monomer 1 shown) slightly differ from the nominal ones due to the experimental error at weighing.
a
Feed composition.
b
Determined by 1H NMR.
c
Molar composition.
d
This experiment was replicated.
Fig. 6. FT-IR spectra of IP–MAH copolymers in different ratios in mol% in the feed. (A): MAH, (B): IP–MAH 40:60 copolymer, (C): IP–MAH 50:50 copolymer,
(D): IP–MAH 80:20 copolymer, (E): PIP.
groups in the MAH unit and the bands at 1242 and
1059 cm1 are indicative of the presence of the CAOAC
group, which is associated to the MAH ring.
1
H NMR (Copolymer composition). Fig. 7 shows a comparison of the 1H NMR spectra of IP/MAH copolymers of
varying compositions, including the corresponding homo-
D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
1769
Fig. 7. 1H NMR spectrum for the IP–MAH system. Left: Copolymer of IP/MAH in different ratios in mol % in the feed. (A): IP–MAH 40:60 copolymer, (B): IP–
MAH 50:50 copolymer, (C): IP–MAH 80:20 copolymer, (D): PIP. Upper right: Amplified 1H NMR spectrum of the IP–MAH 50:50 copolymer. Lower right insert:
Assignments of characteristic peaks of each unit involved (the numbers in the groups denote the chemical shifts (ppm)).
polymers at the extremes. The spectra of the copolymers
(Fig. 7A, B and C) exhibit peaks at 5.6 ppm attributed to
proton b from the isoprene units. The signals at 3.15–
3.0 ppm are due to the protons e and f of the ACH group
in MAH and those at 2.6–2.8 ppm are assigned to protons
a and d in the ACH2 group. It is considered that the protons
c of the ACH3 group in the IP units correspond to the peaks
at 1.7 ppm.
As in the case of the spectrum for the IP–GMA copolymer, in order to estimate the copolymer composition, it
is possible to divide the 1H NMR spectrum of the IP–MAH
copolymer in three regions (Fig. 7): (i) the proton signals
in the region of d = 1.0–2.8 ppm, belonging to groups
ACH, ACH2A and ACH3 of the isoprene units; (ii) the proton peaks in the region of d = 3.0–3.3 ppm of the methyne
groups in the MAH units and; (iii) peaks in the region of
d = 4.5–5.9 ppm from the ACH@ and @CH2 groups in isoprene units.
If the molar fractions of MAH and IP in the copolymer
are designated as m2 and (1m2) respectively, and since
the number of protons in the monomer units are 2 and 8
for MAH and isoprene respectively, the total area of the
peaks in the spectrum (ST) is in this case proportional to:
ST a2m2 þ 8ð1 m2 Þ ¼ 8 6m2
ð5Þ
The area (SM) corresponding to the protons of the anhydride group of MAH in the range of 3.0–3.3 ppm is proportional to 2m2; therefore:
SM
2m2
8SM
¼
; m2 ¼
ST
8 6m2
2ST þ 6SM
ð6Þ
From Eq. (6) it is possible to estimate the copolymer composition. Table 4 shows the values of the mole fractions of
MAH in the copolymer estimated in this way from the 1H
NMR spectra.
Average molecular weights by number (Mn) and weight
(Mw) of the copolymers are also shown in Table 1 (determined by GPC). Some of the polydispersity values are
rather low, but these are generally associated with conversions below 10%. The largest polydispersity value was obtained for the experiment with 90% IP which also
proceeded up to 13% conversion. It seems that at lower
conversions the polydispersity is close to the ideal one
(1.5); however, as the content of isoprene and the conversion increases, some initial branching may occur due to the
residual double bonds in the dead polymer chains, leading
to increased polydispersity
Fig. 8 shows the copolymer composition curve generated by the corresponding comonomer feed compositions.
This graph shows that this pair of monomers forms an azeotropic mixture containing 49 mol%.
Reactivity ratios. The reactivity ratios were determined
by the same techniques used for the IP–GMA system. The
reactivity ratios of MAH and IP obtained by various methods are presented in Table 5. Values of the product
r1r2 < 0.03 in column four indicate that the IP/MAH system
copolymerizes in an alternate fashion. The donor–acceptor
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Fig. 8. Curve feed composition. vs copolymer composition in the polymer system for IP-co-MAH. Experimental data points are compared with the
prediction using the Mayo–Lewis equation with the reactivity ratios of the TM technique.
Table 5
Results of reactivity ratios calculated using various methods for the IP/MAH
system.
Method
r1
r2
r1 r2
FR
FRI
KT
KTE
TM
0.053
0.067
0.067
0.063
0.057
0.061
0.038
0.050
0.043
0.078
0.0032
0.0025
0.0034
0.0027
0.0044
r1: Reactivity ratios for the IP, r2: Reactivity ratios for the MAH.
character of the monomers could play a role in the alternating nature of these two copolymerization systems
(especially in the case of the IP–MAH system). The work
of Hall and Padias has shed light on the mechanisms occurring in this kind of copolymerization systems [51].
Although the slow addition of IP to the reaction mixture
could in principle have some influence on the determination of the reactivity ratios, the time to complete the addition was kept to a minimum (5–20 min) with respect to
the total reaction time necessary to reach the desired conversion in order to minimize this effect. This procedure
was necessary to avoid the formation of the Diels Alder adduct of isoprene and MAH.
The experimental error (standard deviation) estimated
for this system was r = 0.019 with 5 degrees of freedom.
The estimated 95 % confidence intervals for the reactivity
ratios (TM technique) are r1 = 0.057 ± 0.071 and
r2 = 0.078 ± 0.046. Notice that zero is included in the confidence interval for r1, which is due to the alternating nature
of this system.
DSC and DMA. Fig. 9 shows the glass transition temperatures (Tg) obtained by DSC in which a tendency is observed with respect to the concentration of comonomer
present in the polymeric material. One can see that the
higher the concentration of isoprene, the lower the Tg. It
is noteworthy that this family of copolymers exhibits a sin-
gle Tg. which clearly indicates the random nature of the
copolymers formed.
The values obtained for the glass transition temperatures by DSC were confirmed using DMA studies. Fig. 10
illustrates the behavior exhibited by the IP–MAH 90:10
copolymer.
Table 6 shows a comparative summary of the glass
transition temperatures obtained by means of the DSC
and DMA techniques. It shows that by decreasing the concentration of isoprene in the copolymer, the Tg increases
due to the presence of the polar maleic anhydride
monomer.
5. Conclusions
In this work, reactivity ratios for a commercially-important diene, isoprene (IP), with two polar monomers, glycidyl methacrylate (GMA) and maleic anhydride (MAH)
were reported. For the case of IP–GMA copolymer, there
is only one study in the literature that dates back to
1974 in which the reactivity ratio is reported [29]. In the
case of IP–MAH, experimental details are only given for a
50:50 feed composition ratio and their reactivity ratios
are not provided at all [26]. In our study, we report the
reactivity ratios for the systems IP–GMA and IP–MAH that
were estimated using different methods of calculation,
namely Finemann–Ross (FR), Inverted Finemann–Ross
(IFR), Kelen–Tüdös (KT), Extended Kelen–Tüdös (EKT) and
the nonlinear method of Tidwell–Mortimer (TM). By using
the latter procedure, the values of the reactivity ratios
were estimated as 0.119 and 0.248 for the system IP and
GMA respectively; whereas for the IP and MAH system
were 0.057 and 0.078 respectively. These values suggest
the formation of nearly-alternating copolymers in both
systems. Increased polar monomer content in the copolymer (MAH or GMA) increases the Tg of the polymeric material. The Couchman´s model for predicting Tg values is in
D. Contreras-López et al. / European Polymer Journal 49 (2013) 1760–1772
1771
Fig. 9. Tg behavior with respect to the weight concentration of IP in IP–MAH copolymers. The continuous line represents the prediction of the Couchman
model.
edges the financial support of this research by CONACYT
Grant 101670.
References
Fig. 10. DMA of the IP–MAH 90:10 copolymer.
Table 6
Summary of DSC/DMA Tg values for the IP–MAH copolymer system.
% w/w IP
DSC
Tg (°C)
DMA
Tg (°C)
40
49
52
99.2
72.2
65.9
100.9
75.0
69.8
good agreement with experimental data using DSC and
DMA techniques for both IP–GMA and IP–MAH systems.
Acknowledgements
The authors are thankful to Dr. Román Torres Lubián for
his valuable help in the interpretation of the NMR spectra.
We also thank Prof. Scott Parent of Queen´s University
for helpful comments. Enrique Saldívar-Guerra acknowl-
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