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Solvent effect in the cationic polymerization of ethylene oxide by trityl salts
Eur. Polvm. J. Vol. 22. No. 3, pp, 243 244, 1986 Printed in Great Britain. All rights reserved
0014-3057 86 S3.00 ~ 0 0 0 Copyrighl ~ 1986 Pergamon Press Lid
SOLVENT EFFECT IN THE CATIONIC POLYMERIZATION OF ETHYLENE OXIDE BY TRITYL SALTS A . M A R T I N E Z , F. MIJANGOS a n d L. M. LEON* Grupo de Quimica Macromolecular, Departamento de Quimica-Fisica, Facultad de Ciencias. Universidad del Pals Vasco. Apdo. 644. Bilbao. Spain
(Received 1 July 1985; in revisedjorm 26 September 1985)
Abstract--The kinetics of the cationic polymerization of ethylene oxide by trityl salts have been studied in dichloromethane at 285 K and chlorobenzene at 298 K. As previously reported for the same polymerization in nitrobenzene [I] (J. Polym. Sci. Lett. Ed. 21,885, 1983L the internal order is unity, the paired and unpaired ions having the same reactivity. The lack of dependency of kp on g. is interpreted as due to solvation of the oxonium ions.
INTRODUCTION
EXPERIMENTAl.
The cationic polymerization of ethylene oxide in nitrobenzene using trityl salts, P h 3 C t A (A = AsF6 -, S b C 1 6 . . . ) , as initiators follows a rate e q u a t i o n c o r r e s p o n d i n g to a dieidic system:
Purification of reagents has been described [4]. Methylene dichloride was stirred with conc. H2SO4 for several days, washed with water and dilute alkali, and dried with CaH2; after distilling, it was stored over fresh CaH. on the vacuum line. Chlorobenzene was twice distilled from CaH~ and stored over Call 2. It was distilled before use. The experimental technique has been described [4]: the automatic manometer was calibrated with the solvents used, verifying that in both cases it operated linearly (with methylene dichloride some modifications were introduced [5] in order to allow for its high vapour pressure and it v~as found preferable to work at 285 K with this solvent).
r =v* +v -+=[k~+kp(l-~)][I][M],
(1)
where ~ is the degree of dissociation of paired macroions into macrocations, i.e. ,,~P,+A
~ ,,,~P+ + A - .
A change in the dielectric c o n s t a n t of the solvent may affect the polymerization rate in two distinct ways (note that when c o m p a r i n g the rate in media of different polarities, we assume that the, e is no change in the m e c h a n i s m of polymerization); (I) The effect on the polymerization rate constant, which will be modified as deduced from the transition state theory [2], viz. an increase in the polarity of solvent, will stabilize the initial state more t h a n the transition state if the latter is less polar thus raising the activation energy and lowering the rate constant. (2) The effect on the degree of dissociation of ion-pairs since the higher the dielectric constant, the more dissociated are the species [3]. In the field of cationic polymerization of heterocycles [3], e q u a t i o n [1] has been well tested allowing the conclusion that k~ ± ki~. In the polymerization of cyclic ethers via an o x o n i u m ion, R~O +, due to extensive solvation of the p r o p a g a t i n g cations, little change in the rate c o n s t a n t is expected on changing the solvent apart from the small effect on the transition state. In order to confirm this idea, we have studied the polymerization of ethylene oxide by trityl salts (AsF~ and SbC16 as counterions) in methylene dichloride and chlorobenzene as solvents and we have c o m p a r e d these results with those previously reported using nitrobenzene [4].
R E S U I , T S AND D I S C U S S I O N
Kinetics in b o t h solvents were similar to those previously reported for nitrobenzene [4]: viz. reactions are first order with respect to m o n o m e r as shown in Fig. 1. The p r o p a g a t i o n stage remained linear up to the end of reaction for all experiments and so it may be inferred that there is no loss of active centres. The fact that in all experiments the initial trace is regained at the end of the reaction d e m o n strates that m o n o m e r conversion is total. The p r o p a g a t i o n rate c o n s t a n t was obtained in each case by dividing the slope of the linear portion
21i
e/ . / .J j;
7-, J
Jm /
e/°
0
an/
i
I
I
I O0
5OO
5OO
t ( min )
Fig. I. First order plot (O--Sb0-6; •
*To whom correspondence should be addressed. 243
As0-6).
A. MARTINEZ el al.
244
Table I. Initial conditions and kr values in CIC~H, at 298 K [M],, " [I}. x 103 kr x 102 Exp. (moldm ~) (moldm 3) (moldm 3) ~sec As0- I As0-2 Asl-2 As0-3 As0-4 As0-5 As0-6 Asl-6 As0-7 As 1-7 Sb0- I
0.149 0.161 0.195 0.121 0.246 0.245 0.161 0.195 0.161 0.246 0.151
2.44 1.71 1.71 1.10 0.206 0.404 0.830 0.830 0.681 0.681 0.567
7.03 6.13 6.72 6.35 6.42 5.78 6.78 6.97 6.75 6.57 6.65
Table 2. Initial conditions and kp values in CH2CI2 at 285 K [M]0 [I]0 x 103 kp x 102 Exp. (moldm 3) (moldm 3) (moldm 3) i sec I Sb0-2 Sb0-3 Sb0-4 Sb0-5 Sb0-6 As0-8
0.892 0.905 0.896 1.35 0.896 0.862
9.41 0.830 1.38 5.10 3.14 1.04
2.43 1.93 1.67 1.27 2.12 1.60
of the first order plot by the initial initiator concentration; Tables 1 and 2 show the results for both solvents; constancy in the kp values for each solvent is satisfactory indicating that the reaction follows the rate equation: v = kp[I] [M] with
indicated that kp did n o t vary significantly after o n e o r m o r e m o n o m e r a d d i t i o n s (in Table 1 the label As I-2 m e a n s the first m o n o m e r a d d i t i o n after the original As0-2 h a d been c o m p l e t e d ) . For both methylene dichloride and chlorobenzene, the c o u n t e r i o n has n o influence on the polym e r i z a t i o n rate; n o r d o the kp values in either solvent d e p e n d on the initial initiator c o n c e n t r a t i o n . This b e h a v i o u r is similar to that f o u n d for n i t r o b e n z e n e [4, 5] a n d indicates that k~ = k~ a n d that the m e c h a n i s m d o e s n o t c h a n g e on c h a n g i n g the solvent. Values o f ko in m e t h y l e n e d i c h l o r i d e (at 285 K) a n d c h l o r o b e n z e n e are very close a n d s h o w no great difference f r o m that in n i t r o b e n z e n e [kp = 4.78 x 10 2 (tool d m - 3 ) - t sec-J at 298 K]. This is logical since the reactivities o f b o t h u n p a i r e d a n d p a i r e d ions are similar, a n d a n y effect o n the p o l y m e r i z a t i o n rate o f different degree o f d i s s o c i a t i o n due to c h a n g i n g solvent p o l a r i t y w o u l d be u n d e t e c t a b l e . T h e small differences in kp values in n i t r o b e n z e n e a n d c h l o r o b e n z e n e merely reflect slight differences in the t r a n s i t i o n states due to solvation o f the o x o n i u m ions f r o m ethylene oxide.
Acknowledgements--One of us (A.M.) gratefully acknowledges financial support from Departamento de Universidades e Investigaci6n del Gobierno Vasco. We are also grateful to Dr Matilde Rodriguez for her collaboration. This work has been carried out under the financial support of Comisi6n Asesora de Investigaci6n Cientifica y T6cnica (CAICYT) from the Spanish Government.
kp = 1.77 x 10 : ( m o l d m - 3 ) -1 sec -1 in CH2Clz at 285 K,
kp=6.56x10
2(moldm 3)-isec-i in C1C6H 5 at 298 K.
With chlorobenzene as solvent, the absence of
termination reactions was also confirmed from kinetic measurements with successive monomer additions after the first reaction was complete; results
REFERENCES
1. F. Mijangos and L. M. Ledn, J. Polym. Sci. Lett. Ed. 21, 885 (1983). 2. P. H. Plesch, Br. Polym. J. 5, 1 (1973). 3. S. Penczek, D. Kubisa and K. Matyjaszewski, Adv. Polym. Sci. 37, 1 (1980). 4. F. Mijangos and L. M. Le6n, Eur. Polym. J. 19, 29 (1983). 5. A. Martinez. To be published.
0014-3057 86 S3.00 ~ 0 0 0 Copyrighl ~ 1986 Pergamon Press Lid
SOLVENT EFFECT IN THE CATIONIC POLYMERIZATION OF ETHYLENE OXIDE BY TRITYL SALTS A . M A R T I N E Z , F. MIJANGOS a n d L. M. LEON* Grupo de Quimica Macromolecular, Departamento de Quimica-Fisica, Facultad de Ciencias. Universidad del Pals Vasco. Apdo. 644. Bilbao. Spain
(Received 1 July 1985; in revisedjorm 26 September 1985)
Abstract--The kinetics of the cationic polymerization of ethylene oxide by trityl salts have been studied in dichloromethane at 285 K and chlorobenzene at 298 K. As previously reported for the same polymerization in nitrobenzene [I] (J. Polym. Sci. Lett. Ed. 21,885, 1983L the internal order is unity, the paired and unpaired ions having the same reactivity. The lack of dependency of kp on g. is interpreted as due to solvation of the oxonium ions.
INTRODUCTION
EXPERIMENTAl.
The cationic polymerization of ethylene oxide in nitrobenzene using trityl salts, P h 3 C t A (A = AsF6 -, S b C 1 6 . . . ) , as initiators follows a rate e q u a t i o n c o r r e s p o n d i n g to a dieidic system:
Purification of reagents has been described [4]. Methylene dichloride was stirred with conc. H2SO4 for several days, washed with water and dilute alkali, and dried with CaH2; after distilling, it was stored over fresh CaH. on the vacuum line. Chlorobenzene was twice distilled from CaH~ and stored over Call 2. It was distilled before use. The experimental technique has been described [4]: the automatic manometer was calibrated with the solvents used, verifying that in both cases it operated linearly (with methylene dichloride some modifications were introduced [5] in order to allow for its high vapour pressure and it v~as found preferable to work at 285 K with this solvent).
r =v* +v -+=[k~+kp(l-~)][I][M],
(1)
where ~ is the degree of dissociation of paired macroions into macrocations, i.e. ,,~P,+A
~ ,,,~P+ + A - .
A change in the dielectric c o n s t a n t of the solvent may affect the polymerization rate in two distinct ways (note that when c o m p a r i n g the rate in media of different polarities, we assume that the, e is no change in the m e c h a n i s m of polymerization); (I) The effect on the polymerization rate constant, which will be modified as deduced from the transition state theory [2], viz. an increase in the polarity of solvent, will stabilize the initial state more t h a n the transition state if the latter is less polar thus raising the activation energy and lowering the rate constant. (2) The effect on the degree of dissociation of ion-pairs since the higher the dielectric constant, the more dissociated are the species [3]. In the field of cationic polymerization of heterocycles [3], e q u a t i o n [1] has been well tested allowing the conclusion that k~ ± ki~. In the polymerization of cyclic ethers via an o x o n i u m ion, R~O +, due to extensive solvation of the p r o p a g a t i n g cations, little change in the rate c o n s t a n t is expected on changing the solvent apart from the small effect on the transition state. In order to confirm this idea, we have studied the polymerization of ethylene oxide by trityl salts (AsF~ and SbC16 as counterions) in methylene dichloride and chlorobenzene as solvents and we have c o m p a r e d these results with those previously reported using nitrobenzene [4].
R E S U I , T S AND D I S C U S S I O N
Kinetics in b o t h solvents were similar to those previously reported for nitrobenzene [4]: viz. reactions are first order with respect to m o n o m e r as shown in Fig. 1. The p r o p a g a t i o n stage remained linear up to the end of reaction for all experiments and so it may be inferred that there is no loss of active centres. The fact that in all experiments the initial trace is regained at the end of the reaction d e m o n strates that m o n o m e r conversion is total. The p r o p a g a t i o n rate c o n s t a n t was obtained in each case by dividing the slope of the linear portion
21i
e/ . / .J j;
7-, J
Jm /
e/°
0
an/
i
I
I
I O0
5OO
5OO
t ( min )
Fig. I. First order plot (O--Sb0-6; •
*To whom correspondence should be addressed. 243
As0-6).
A. MARTINEZ el al.
244
Table I. Initial conditions and kr values in CIC~H, at 298 K [M],, " [I}. x 103 kr x 102 Exp. (moldm ~) (moldm 3) (moldm 3) ~sec As0- I As0-2 Asl-2 As0-3 As0-4 As0-5 As0-6 Asl-6 As0-7 As 1-7 Sb0- I
0.149 0.161 0.195 0.121 0.246 0.245 0.161 0.195 0.161 0.246 0.151
2.44 1.71 1.71 1.10 0.206 0.404 0.830 0.830 0.681 0.681 0.567
7.03 6.13 6.72 6.35 6.42 5.78 6.78 6.97 6.75 6.57 6.65
Table 2. Initial conditions and kp values in CH2CI2 at 285 K [M]0 [I]0 x 103 kp x 102 Exp. (moldm 3) (moldm 3) (moldm 3) i sec I Sb0-2 Sb0-3 Sb0-4 Sb0-5 Sb0-6 As0-8
0.892 0.905 0.896 1.35 0.896 0.862
9.41 0.830 1.38 5.10 3.14 1.04
2.43 1.93 1.67 1.27 2.12 1.60
of the first order plot by the initial initiator concentration; Tables 1 and 2 show the results for both solvents; constancy in the kp values for each solvent is satisfactory indicating that the reaction follows the rate equation: v = kp[I] [M] with
indicated that kp did n o t vary significantly after o n e o r m o r e m o n o m e r a d d i t i o n s (in Table 1 the label As I-2 m e a n s the first m o n o m e r a d d i t i o n after the original As0-2 h a d been c o m p l e t e d ) . For both methylene dichloride and chlorobenzene, the c o u n t e r i o n has n o influence on the polym e r i z a t i o n rate; n o r d o the kp values in either solvent d e p e n d on the initial initiator c o n c e n t r a t i o n . This b e h a v i o u r is similar to that f o u n d for n i t r o b e n z e n e [4, 5] a n d indicates that k~ = k~ a n d that the m e c h a n i s m d o e s n o t c h a n g e on c h a n g i n g the solvent. Values o f ko in m e t h y l e n e d i c h l o r i d e (at 285 K) a n d c h l o r o b e n z e n e are very close a n d s h o w no great difference f r o m that in n i t r o b e n z e n e [kp = 4.78 x 10 2 (tool d m - 3 ) - t sec-J at 298 K]. This is logical since the reactivities o f b o t h u n p a i r e d a n d p a i r e d ions are similar, a n d a n y effect o n the p o l y m e r i z a t i o n rate o f different degree o f d i s s o c i a t i o n due to c h a n g i n g solvent p o l a r i t y w o u l d be u n d e t e c t a b l e . T h e small differences in kp values in n i t r o b e n z e n e a n d c h l o r o b e n z e n e merely reflect slight differences in the t r a n s i t i o n states due to solvation o f the o x o n i u m ions f r o m ethylene oxide.
Acknowledgements--One of us (A.M.) gratefully acknowledges financial support from Departamento de Universidades e Investigaci6n del Gobierno Vasco. We are also grateful to Dr Matilde Rodriguez for her collaboration. This work has been carried out under the financial support of Comisi6n Asesora de Investigaci6n Cientifica y T6cnica (CAICYT) from the Spanish Government.
kp = 1.77 x 10 : ( m o l d m - 3 ) -1 sec -1 in CH2Clz at 285 K,
kp=6.56x10
2(moldm 3)-isec-i in C1C6H 5 at 298 K.
With chlorobenzene as solvent, the absence of
termination reactions was also confirmed from kinetic measurements with successive monomer additions after the first reaction was complete; results
REFERENCES
1. F. Mijangos and L. M. Ledn, J. Polym. Sci. Lett. Ed. 21, 885 (1983). 2. P. H. Plesch, Br. Polym. J. 5, 1 (1973). 3. S. Penczek, D. Kubisa and K. Matyjaszewski, Adv. Polym. Sci. 37, 1 (1980). 4. F. Mijangos and L. M. Le6n, Eur. Polym. J. 19, 29 (1983). 5. A. Martinez. To be published.