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KINETIC ANALYSIS OF NMP: HOW TO OBTAIN HIGH MOLAR
MASSES LIVING CONTROLLED POLYSTYRENE
experimental living fraction of polymers initiated by SG1-based
alkoxyamines at 90 °C and 120 °C for different targeted Mn.
Yohann Guillaneuf 1, Pierre-Emmanuel Dufils1, Benoit Luneau1, Olivier
Guerret2, Didier Gigmes1, Sylvain R. A. Marque1, Denis Bertin1,* and Paul
Tordo1
Experimental
Materials. Alkoxyamines Monams and MAMA-SG1 (available under
the trademark BlocBuilderTM) were provided by ARKEMA. Styrene was
purchased from Aldrich and used as received.
Typical polymerization experiment. A degassed (20 min. nitrogen
bubbling) bulk solution of styrene and the alkoxyamine was heated up to 90
°C or 120 °C and up to 60% - 80% monomer conversion. Sampling was
performed to follow the monomer conversion and molar mass.
Analytical techniques. Conversion was estimated by 1H NMR
experiments on 300 Avance Bruker spectrometer (CDCl3 as solvent, 300
MHz). Number average molecular weight, Mn, and polydispersity indexes
PDI, were determined by GPC using a Waters 515 HPLC gel permeation
chromatography equipped with 3 “styragel” columns (HR 3, 4 and 5) and
UV/visible (Waters 486) and RI (Waters 2414). Measurements were
performed in THF solvent at room temperature with a 1 mL/min flow and
calibration based on Polystyrene standards.
Living fraction (LF) estimation procedure. Non degassed t-BuPh
10-4 M solutions of polymer and TEMPO standard were filled in o.d. 5mm
glass tubes. Tubes were sealed off and a blank scan at room temperature
was recorded for the polymer sample. This sample was heated up to 120 °C
for two hours, then cooled down to room temperature. A spectrum was
recorded and compared to the TEMPO standard. By experience, such
procedure gives an absolute error smaller than 5%.
Kinetic modelling procedure using Predici11. The kinetic model is
described in scheme 3. In our model, it was assumed that the rate constants
were chain length independent although such assumption is not true for kd,
kp, kt, and certainly kc. The values of kinetic constant9,13,14 given in the
following table are determined for 90 °C and MAMA-SG1.
1
2
UMR 6517, CNRS et Université d’Aix-Marseille, Avenue Esc.
Normandie-Niemen, Marseille 13397 Cedex 20, France
* [email protected]
ARKEMA, Groupement de Recherche de Lacq, 64170 Lacq, France
Introduction
Since Rizzardo et al.1 showed that it was possible to prepare well
controlled and living polystyrene by radical polymerization in the presence
of nitroxyl radical as controlling agent, numerous studies on the Nitroxide
Mediated Polymerization (NMP) have been carried out2. NMP is grounded
on the Persistent Radical Effect3 (PRE) and its principle is shown in
Scheme 1.
self termination products
kdism
kd1
Y O N
kc1
1
nM
kadd
N O + Y
2
+M
kd
Y Mn O N
4
Y Mn O N
living chains
3
kp
Y Mn-1 + O N
kc
5
kt
kd1
R SG1
dead chains
R + SG1
Scheme 1. Mechanism of Nitroxide Mediated Polymerization.
(1)
R SG1
(2)
2 R
(3)
R R
(4)
RM
(5)
RMn
kini
3 M
Nitroxide 2 and the initiating radical 3 are produced by the reversible
homolysis of alkoxyamine 1. The permanently increasing concentration of
2, due to alkyl self-termination at the early stage of the polymerization,
increase the probability of recombination instead of self-termination, which
becomes slower and slower with time. A majority of dormant living chains
4 can then grow until the monomer is depleted, producing a polymer with a
narrow distribution and a large living character.
The development of alicyclic nitroxides like TIPNO4 and SG15 (Ntert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide) as counter
radical allowed to extend this technique to other monomers such as
acrylates, than styrenic derivatives and stimulated the interest of NMP.
R + SG1
kdism
R +
R
R +
M
kadd
RM
+ (n-1) M
RMn
+ SG1
R Mn SG1
kd1
1.7 10-2 s-1
kc1
5.0 106 M-1s-1
kini
8.8 10-11 M-2s-1
kadd
8.0 103 M-1s-1
kdism
2.0 109 M-1s-1
(6)
kp
9.0 10-2 M-1s-1
(7)
kd
3.1 10-4 s-1
(8)
kc
2.6 105 M-1s-1
kt
1.5 108 M-1s-1
kc1
kp
kc
R Mn SG1
kd
RMn
+ SG1
kt
RMn + RMn
Dead Polymers
(9)
Scheme 3
N
O
P(O)(OEt)2
COOH
MAMA-SG1
N
O
P(O)(OEt)2
N
O
P(O)(OEt)2
COOMe
MONAMS
SG1
Scheme 2
Nevertheless, most of the polymers produced by this method have
molecular weight significantly below 100 000 g.mol-1. In particular it is
very difficult to synthesize high molar mass controlled living polystyrene
because at the classical reaction temperature (T>100°C for NMP), autoinitiation by Mayo mechanism6 continuously produce alkyl radicals.
Torkelson has examined theoretically7a and experimentally7b the possibility
to obtain high molar masses with very low alkoxyamine concentration at
low temperature, but in his case the conversion remained very low and has
no practical interest. New nitroxides8 have been developed to decrease the
reaction temperature but no real investigation for the synthesis of living
high molar masses has been realized.
With the new tertiary SG1-based alkoxyamine introduced by Tordo9,
the styrene polymerization can be carried out at 90 °C with a better
controlled and living character, but no analysis have been done to quantify
the influence of targeted molar mass on living character and therefore the
influence of the temperature.
Since a complete kinetic modelling has been realized on NMP
polymerization10, in this work, we have compared modellings and
Results and Discussion
Fischer12 has demonstrated that controlled polymerization does not
involve livingness and vice-versa. Many papers4b are published about the
preparation of controlled and living homopolymers with the evolution of
the Mn as function of the conversion and the kinetics but nothing about the
living character. Reinitiating procedures for the preparation of block
copolymers are the standard proof of the livingness of the initiating
polymer but the shift of the GPC trace cannot quantify this living character.
The livingness of a polymer is easily and merely determined by ESR
experiments as exemplified with PS-SG1 and PBA-SG1 polymers by
Bertin et al.13.
In a previous study10, we have shown that polymerizations of styrene
at 90 °C initiated with two different SG1-based alkoxyamines present
different evolutions of Mn versus conversion. With MAMA-SG1, control is
obtained during all the polymerization whereas with Monams, due to its too
low kd1, no linear evolution of the Mn with conversion can be observed. But
what about the livingness? Here, we provided the amount of living
polymer for the experiments at 120 °C and 90 °C initiated by the two
alkoxyamines (Figure 1). These values are obtained from the released
amount of SG1 after heating a t-butyl benzene solution of polymer in the
presence of oxygen as alkyl radical scavenger and comparing the ESR
signal with a standard.
Polymer Preprints 2005 , 46(2),
270
Mn = 73 500 g/mol
PDI = 1.15
110 hours, α = 0.8, LF =90 %
Living Fraction
0.9
Mn = 9 800 g/mol
PDI = 1.2
31 hours, α = 0.57, LF = 98 %
Mn = 210 000 g/mol
PDI = 1.6
147 hours, α = 0.6,
LF = 60 %
0.6
residual
styrene
MAMA-SG1 120 °C
MONAMS 120 °C
MAMA-SG1 90 °C
MONAMS 90 °C
0.3
0.0
0
20
40
60
80
20
Conversion
25
30
35
Retention time (min)
Figure 1. Comparison of modelling (lines) and experimental (symbols)
living fraction for styrene polymerization using MAMA-SG1 and Monams
(Targeted Mn = 20 000 g.mol-1)
For each experiment, the livingness is unambiguously above 70 %
whatever the initiator and the temperature, confirming the non-relationship
between livingness and controlled character. Furthermore the values
determined experimentally are in very good agreement with the theoretical
ones resulted from the modelling.
This analysis has demonstrated the ability to obtain control and living
polystyrene with SG1 at 90 °C if we use the appropriate alkoxyamine. We
use therefore the same modelling to investigate the possibility to obtain
high molar masses living polystyrene.
1.0
Figure 3. Comparison of the raw GPC chromatograms from the styrene
polymerization at 90 °C with targeted Mn respectively 20 000, 100 000 and
500 000 g.mol-1
Conclusion
The livingness of a polystyrene during the NMP polymerization
process has been followed by ESR experiments and compared with
modellings. These results confirm the non relationship between the
livingness and the controlled character as stated by Fischer12. Secondly we
use the modellings validated previously to investigate the synthesis of high
molar masses living polystyrene. From this analysis three polymerizations
have been carried out. Polystyrene with Mn up to 200 000 g.mol-1 has been
obtained with a living fraction close to 60 %.
Acknowledgements.
The authors would like to thank ARKEMA,
University of Provence and CNRS for financial support of this research.
Living Fraction
0.8
References
0.6
(1)
(2)
0.4
(3)
(4)
-1
0.2
0.0
0.0
20 000 g.mol
-1
100 000 g.mol
-1
200 000 g.mol
-1
500 000 g.mol
0.1
0.2
0.3
(5)
0.4
0.5
0.6
0.7
0.8
Conversion
(6)
(7)
Figure 2. Comparison of modelling (lines) and experimental (symbols)
living fraction for styrene polymerization using MAMA-SG1 at different
temperatures and targeted Mn ; 120 °C dashed line; 90 °C full line
(8)
At 120 °C (Figure 2 dashed line), we could observe that the livingness
decreased rapidly when high molar masses are targeted. A polymer with a
targeted Mn of 500 000 g.mol-1 has only 35 % of living chains.
Nevertheless at lower temperature i. e. 90 °C, the living character seems
more conversion dependent and high molar mass living polystyrene seemed
to be synthesized. For the same polymer than previously we predicted 60 %
of livingness.
These modelling prompted us to check experimentally if high molar
masses living polystyrenes could be obtained. Three polymerizations have
been carried out at 90 °C with an alkoxyamine concentration (MAMASG1) of 5.0 10-2, 1.0 10-2 and 2.25 10-3 mol.L-1. The raw GPC traces of
these 3 polystyrenes confirm our modellings and show that high molar
masses (Mn closed to 200 000 g.mol-1) living (LF = 0,6) and controlled
(PDI = 1.6) polystyrene can be synthesized with our tertiary SG1-based
alkoxyamine.
(9)
(10)
(11)
(12)
(13)
(14)
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