Ring-Opening Polymerization - ACS Publications

14 Stereoselective and Stereoelective Polymerization of Oxiranes and Thiiranes

Downloaded by PENNSYLVANIA STATE UNIV on May 31, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch014

NICOLAS SPASSKY Laboratoire de Chimie Macromoléculaire, Associé au CNRS, Universite Pierre et Marie Curie, 4, Place Jussieu, 75230 Paris Cedex 05, France

Ionic polymerization of oxiranes and thiiranes is usually divided in three main types : anionic, cationic and stereospecific-"coordinated". While anionic and cationic polymerization produce random amorphous polymers, at least when starting from racemic monomers, stereospecific initiators may give isotactic crystalline polymers. Depending on the initial reagents and resulting products, several types of stereospecific processes could be considered : - stereoselective polymerization, - stereoelective also called asymmetric-selective polymerization, - asymmetric-polymer synthesis. The first two processes are dealing with monomers which are a mixture of stereoisomers, while the last one considers symmetric monomers having two asymmetric carbons of opposite configuration. Most of the work in the field of stereoselective and stereoelective polymerization of oxiranes and thiiranes was carried out on monosubstituted monomers and was reviewed in some publications (1-6). The stereochemical aspects of the polymerization of some di-substituted oxiranes and thiiranes were described in the work of Vandenberg (7,8). The aim of this paper is to give a review of the more recent results concerning such reactions, to include some new unpublished data and to make proposals for mechanisms. 1 - Stereoselective polymerization A "stereoselective" polymerization is a process in which macromolecules containing only one type of configurational unit are formed by incorporation of one stereoisomer from a mixture into a growing polymer chain. There are as many different types of macromolecules as different stereoisomers present in the initial monomer mixture. In the case of cyclic compounds with one chiral center such 191

In Ring-Opening Polymerization; Saegusa, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

192

RING-OPENING POLYMERIZATION

a p e r f e c t process produces a s t e r e o r e g u l a r polymer composed o f two d i s t i n c t i s o t a c t i c poly R and poly S chains. For example i n the case of propylene oxide one must have :

CH r

racemi c

\

,3 "

0

H

., / h Downloaded by PENNSYLVANIA STATE UNIV on May 31, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch014

stereoselective initiator*

CH

>n

2

R

H

C — CH ^ 0 *

>

0

+

H

2

ι

( 0 - C - CH

2

)

n

poly S

CH Generally the process i s not as p e r f e c t and one obtains a strong predominance o f one type of c o n f i g u r a t i o n a l u n i t i n the chain. Since the f i r s t example described by P r u i t t & Baggett (9) i n 1955, many c a t a l y t i c systems have been used f o r s t e r e o s p e c i f i c po l y m e r i z a t i o n o f propylene oxide and others epoxides and some of them were r e c e n t l y reviewed (10,11). Most o f these c a t a l y t i c s y s tems contain metal-oxygen bounïï.~limong metals i r o n , magnesium, z i n c , aluminium, a l c a l i n e - e a r t h and boron were the most used. The s t e r e o s e l e c t i v i t y , i . e . the % o f c r y s t a l l i n e f r a c t i o n , could be evaluated using the c r i t e r i o n o f i n s o l u b i l i t y o f the i s o t a c t i c polymer i n a s o l v e n t o r determined from DTA o r X-Rays measurements. Several c a t a l y t i c systems derived from the r e a c t i o n of d i e thyl z i n c with compounds containing an a c t i v e hydrogen were e x t e n s i v e l y s t u d i e d , among them Z n E t - r U ) and ZnEt -CrL0H systems, and the r e s u l t s discussed and l a r g e l y reported (±>3*±j It was found t h a t the best s t e r e o s p e c i f i c i t y Ts obtained f o r ZnEt -H20 system when the i n i t i a t o r i s prepared i n s i t u i n a nonp o l a r s o l v e n t using equimolar amounts o f reagents (10). The e f f i ciency can be increased by f r e e z e - d r y i n g the c a t a l y s t (12). For a given c a t a l y t i c system, the s t e r e o s e l e c t i v i t y i s depending on the enantiomeric composition and on the nature o f the monomer. For example, propylene oxide of d i f f e r e n t o p t i c a l p u r i t i e s , was polymerized using ZnEt2-HoO (1 : 0.7) system prepared i n s i tu (13). As shown i n table I the % o f c r y s t a l l i n e f r a c t i o n as well as the t a c t i c i t y are increased with an increase of the o p t i c a l purity. On the other hand, with the same i n i t i a t o r , the s t e r e o s e l e c t i v i t y i s very d i f f e r e n t depending on the nature o f the monomer. Almost purely i s o t a c t i c products are obtained with t - b u t y l - t h i i r a n e while l e s s than 30 % o f c r y s t a l l i n e f r a c t i o n i s i s o l a t e d i n the case of propylene oxide ( t a b l e I). A mechanism assuming the existence of two groups o f e n a n t i o morphic s i t e s having more or l e s s R and S c h a r a c t e r was p r o posed i n order to e x p l a i n the formation o f polymers of d i f f e r e n t 3

?

2

2


14. SPASSKY

Polymerization

stereoregularities

of Oxiranes and

Thiiranes

193

(1). Table

I

S t e r e o s e l e c t i v i t y of ZnEt -rU0 (1/1) i n i t i a t o r depending on the nature o f the monomer used and i t s enantiomeric composition. 2


Monomer

o. p. i n i t i a l crystallinity: tacticity: réf. (dyads) . monomer : i % . % %

Methyl

oxirane

0 47.5 96

t-butyl

oxirane

0

:

90

thiirane

: 0 : 50

: : :

Methyl t-butyl

t h i i r a n e . 0-90

61 72 > 90

27 38 82

:

(13)

:

• (14)

60-80 > 70

: 76 • > 80

.

(

100

: > 90

:

(15)

5

)

The dépendance on the nature of the monomer and i t s e n a n t i o meric composition seems to i n d i c a t e t h a t c h i r a l a c t i v e s i t e s are formed a f t e r the r e a c t i o n o f the monomer with the i n i t i a t o r . The mechanistic aspects r e l a t e d to the s t e r e o s e l e c t i v i t y o f these i n i t i a t o r s are d i f f i c u l t to study owing to t h e i r i n s o l u b i l i t y and o v e r a l l low e f f i c i e n c y . An i n t e r e s t i n g approach was t r i e d i n the case o f propylene s u l f i d e using cadmium and z i n c t h i o l a t e s which gave homogeneous s o l u t i o n s when f u l l y consumed by the monomer (16). The p o l y m e r i z a t i o n i s o f " l i v i n g - t y p e " process,the polymers having one l i v i n g end per metal atom. Depending on the temperature and the s o l v e n t used, c r y s t a l l i n e o r amorphous polymers are o b t a i ned ( t a b l e II). Z i n c t h i o l a t e s i n contrary to z i n c a l c o h o l a t e s were unable t o give c r y s t a l l i n e products. The increase i n s t e r e o r e g u l a r i t y with lowering o f the temperature and the negative e f f e c t of p o l a r solvents (HMPA) may be e x p l a i n e d by c o o r d i n a t i o n e q u i l i b r i u m of the monomer on the a c t i v e s i t e . Cadmium s a l t s are the best c a t a l y s t s f o r the p o l y m e r i z a t i o n o f t h i i r a n e s g i v i n g polymers o f the highest s t e r e o r e g u l a r i t y , w h i l e they are unable to polymerize o x i r a n e s . T h i s behaviour can be e x p l a i n e d by the h a r d - s o f t acid-base c l a s s i f i c a t i o n i n which s u l f u r and cadmium are c l o s e r i n t h e i r c h a r a c t e r than oxygen which belongs to "hard elements".


194


Table

II

Influence of the temperature and of the solvents on the s t e r e o r e g u l a r i t y o f polymethyl t h i i r a n e s obtained by i n i t i a t i o n with z i n c and cadmium a l l y ! t h i o l a t e s .


T h i o l ate : initiator

solvent

temperature . c r y s t a l u n i t y : t a c t i c i t y i %(dyads) °C

;

: benzene toiuene toluene : t o i uene : : t e t r a h y d r o t h i ofen :tetrahydrofuran : : HMPA : :

Cd

Zn

: :

toiuene toiuene

:

20 20 10 0 -20 0 0 0

: : : : : : :

+ + + + + +

-

20 0

. : : : : : : :

> 90 50 58 75 84 78 76 50

: :

50 50

2 - S t e r e o e l e c t i v e polymerization A " s t e r e o e l e c t i v e " (17) o r " a s y m m e t r i c - s e l e c t i v e " (3) p o l y merization i s a process i n which a s i n g l e stereoisomer oT a mixture i s polymerized g i v i n g macromolecules containing one type o f c o n f i g u r â t ! o n a l base u n i t s . For example an o p t i c a l l y a c t i v e c a t a l y s t w i l l choose one enantiomer from a racemic mixture and form a macromolecule c o n t a i n i n g only one type o f enantiomeric u n i t s . Such an i d e a l r e a c t i o n should stop at 50 % y i e l d a f t e r consumpt i o n of the corresponding stereoisomer. optically racemic monomer (R=S)

active

initiator choosing

R

polymer *

p

o

l

y

R

unreacted monomer R/S

— *

0

In most o f the cases the choice i s not as p e r f e c t and one speaks of " s t e r e o e l e c t i v e " p r o c e s s when a p r e f e r e n t i a l p o l y m e r i z a t i o n of one of the enantiomers from a mixture i s observed. Thus, the enantiomorphic choice of the c a t a l y s t i s a predominant element i n t h i s process and should be defined by i t s e l e c t i v i t y and i t s s e l e c t i v i t y . The " s t e r e o e l e c t i v i t y " could be simply defined as the r e l a t i v e rate of consumption o f the enantiomers in the presence o f c h i r a l i n i t i a t o r . In the course of the r e a c t i o n the unreacted monomer i s continously enriched i n one enantiomer and t h e r e f o r e



14.

SPASSKY

Polymerization of Oxiranes and Thiiranes

195

the " s t e r e o e l e c t i v i t y " could be simply determined from the o p t i c a l p u r i t y of unreacted monomer a t a given conversion. On the c o n t r a r y , i n s p i t e of the p r e f e r e n t i a l constant choice o f one enantiomer by the c a t a l y s t , the o p t i c a l a c t i v i t y of the polymer i s decreasing with conversion due to progressive enrichment i n t o the opposite antipode. The " s t e r e o s e l e c t i v i t y " concerns the p o s s i b i l i t y o f i n t r o d u c t i o n i n t o the polymer chain o f only one type o f enantiomer or both o f them and informations on s t e r e o s e l e c t i v i t y are obtained from s t u d i e s of the polymer s t r u c t u r e . We s h a l l d i s c u s s both of these aspects and compare r e s u l t s obtained with oxiranes and t h i i r a n e s on the b a s i s o f recent works reported i n the l i t t é r a t u r e and based on our own experimental data. The i n t e r a c t i o n between the monomer and the o p t i c a l l y a c t i v e i n i t i a t o r i s determining the p r e f e r e n t i a l choice of the l a t t e r f o r one o f enantiomers. T h i s choice i s c h a r a c t e r i z e d by the s i g n , i . e . the c o n f i g u r â t ! o n o f the e l e c t e d antipode and by the magnitude i . e . the o p t i c a l p u r i t y of the resolved monomer. These parameters are depending mainly on the c o n f i g u r a t i o n and the nature o f the c h i r a l l i g a n d o f the i n i t i a t o r and t h e r e f o r e we s h a l l examine f i r s t the i n f l u e n c e o f the nature of the c a t a l y s t on s t e r e o e l e c t i v e processes. Then we s h a l l demonstrate that f o r a given i n i t i a t o r , the n a t u r e of monomer and i t s enantiomeric composition can deeply i n fluence the s t e r e o e l e c t i v i t y . Other parameters l i k e temperature and s o l v e n t e f f e c t s w i l l be also discussed. 2-1) Influence of the nature o f the i n i t i a t o r Initiators res u l t i n g from the r e a c t i o n between an o r g a n o m e t a l l i c d e r i v a t i v e and a c h i r a l compound c o n t a i n i n g an a c i d i c hydrogen were the most u s u a l l y employed i n the s t e r e o e l e c t i v e p o l y m e r i z a t i o n o f oxiranes and thiiranes. The c h i r a l l i g a n d a s s o c i a t e d to the m e t a l l i c atom i s p l a y i n g an important r o l e i n the c o n f i g u r a t i o n and the magnitude o f the enantiomeric c h o i c e . 2-1-1) E f f e c t on the c o n f i g u r â t ! o n a l choice Several types o f c h i r a l compounds c o n t a i n i n g an a c i d i c hydrogen such as a l c o h o l s , d i o l s , aminoacids were used as coreagents with organometallic compounds ( Z n E t , CdEtp and CdMe are the most employed). C o n f i g u r a t i o n a l r e l a t i o n s could be e s t a b l i s h e d i n s e v e r a l c a ses. I f one considers the absolute c o n f i g u r a t i o n o f the c h i r a l l i gand and t h a t o f the c y c l i c monomer, the choice o f the i n i t i a t o r would correspond to an "homosteric" type process i f the chosen enantiomer has the same c o n f i g u r a t i o n as the c h i r a l l i g a n d used i n the i n i t i a t o r (18). Homosteric c o n f i g u r a t i o n a l r e l a t i o n s are i l l u s t r a t e d i n the next scheme. 2

2


196

RING-OPENING

CH

V

POLYMERIZATION

2

/ CH

CH


R

R

]

3

monomer

Ligand

The f o l l o w i n g two s e r i e s were found t o give homosteric c o r relations Ref. = OH X = 0,S 1 = CH,0H (I) R our = alfcyl 4 = Me.Et.iPr.tBu group R,

(")

1

R« R,

= =

—

NH, COoH I

=

alRyl, aryl

X

=

0,S

R„ = Me

I t was a l s o p o s s i b l e t o e s t a b l i s h some c o r r e l a t i o n s with other s e r i e s of c h i r a l compounds e . g . a l c o h o l s , but one must be c a r e f u l i n the choice o f groups o f comparison f o r a given c o n f i g u r a t i o n . We t h i n k t h a t many r e s u l t s may be s a t i s f a c t o r y explained by these c o r r e l a t i o n s and some unknown c o n f i g u r a t i o n s p r e d i c t e d from s t e r e o e l e c t i v e experiments (21). The chemical composition o f the i n i t i a t o r could a l s o play a d e c i s i v e r o l e i n the enantiomeric c h o i c e . We were able to e s t a b l i s h i n the case o f m e t h y l t h i i r a n e t h a t when the i n i t i a t o r i s prepared i n such c o n d i t i o n s t h a t a l k y l a l c o hoiate species predominate over d i a l c o h o l a t e s p e c i e s , the i n i t i a t o r system e l e c t s the antipode the c o n f i g u r a t i o n o f which i s opp o s i t e t o that o f i t s c h i r a l l i g a n d . Such type o f e l e c t i o n was c a l led " a n t i s t e r i c " . The r e s u l t s were e s t a b l i s h e d i n the case o f 1,2 d i o l s o f s é r i e (I) and f o r several a l c o h o l s which were reacted with three d i f f e r e n t organometallic d e r i v a t i v e s (18). Thus the r a t i o I = R-M-0R*/R0-M-0R* had tcTEe considered (-0R* being the c h i r a l alcohol ate ligand) and the f o l l o w i n g r u l e was found: I, I_

s

3

a n t i s t e r i c choice

For example, when Z n E t i s reacted at room temperature with K ^ - ; t B u - CHOH-CHpOH i n (1:17 amount t h i s system chooses p r e f e r e n t i a l l y the d e x t r o r o t a t o r y m e t h y l t h i i r a n e (homosteric choice ; 2


14. SPASSKY

Polymerization

of Oxiranes and

Thiiranes

197

I = 0.44).When the same reagents are reacted i n (1:0,5) amounts the choice i s opposite (I = 4; antisteric process). The chemical composition i s t h e r e f o r e depending on the r e a c t i v i t y o f reagents and c o n d i t i o n s of p r e p a r a t i o n . The r e a c t i v i t y o f organometallic compounds decrease i n the order Z n E t > CdEt > CdMe and as example CdMe gives with R(-) tBu-CH0H-CH 0H i n * ( 1 : 1 ; conditions an a n t i s t e r i c i n i t i a t o r (I > 3). We have r e c e n t l y v e r i f i e d these f i n d i n g s on the example o f two other t h i i r a n e s ( C H and CH 0CH s u b s t i t u t e d ) . 2

2


2

2

5

3

?

2

2

We were able to i s o l a t e species of both types i n the case o f d i e t h y l z i n c - ( + ) 3,3 dimethyl 2 butanol i n i t i a t o r system. The a n t i s t e r i c species has a composition close to E t Z n ( 0 R ) o r Z n ( 0 R ) . (EtZnOR) (-OR being the 3,3 dimethyl 2 butoxy group), while an homosteric i n i t i a t o r had a Zn(0R) . EtZnOR composition. Both spec i e s were s o l u b l e i n benzene and were studied by H-NMR (18). It was p o s s i b l e to transform one specie i n t o the other by a c t i n g Z n E t o r by drying o r heating ( l o s s o f Z n E t and d i s p r o p o r t i o n a fi

ft

?

6

2

2

2

te;.

Recently Ishimori and al (22)showed t h a t Zn(0Me) .(EtZnOMe) have a centrosymmetric s t r u c t u r e formed pf two enantiomorphic d i s turbed cubes. T h i s complex had n o * ' y a c t i v i t y at room temper a t u r e , but polymerized methyloxirane at 8 0 ° . A process of d i s s o c i a t i o n at 80° could e x p l a i n such a r e a c t i v i t y . We s h a l l now consider only homosteric type i n i t i a t o r s f o r simplicity. 2

c a

a

g

t l c

2-1-2) E f f e c t on s t e r e o e l e c t i v i t y I f one considers i n i t i a tors prepared i n homosteric c o n d i t i o n s (I < 1) i t i s p o s s i b l e to compare the e f f i c i e n c y o f r e s o l u t i o n depending on the c h i r a l hydroxy l i g a n d associated with the organometallic compound. The o p t i c a l p u r i t y of recovered monomer at h a l f r e a c t i o n could be used as c r i t e r i o n o f e f f i c i e n c y . Thus, when racemic m e t h y l t h i i r a n e i s polymerized using d i f f e rent i n i t i a t o r s derived from the r e a c t i o n o f d i e t h y l z i n c with c h i r a l a l c o h o l s and g l y c o l s one f i n d s the f o l l o w i n g o r d e r o f e f f i c i e n cy : ligand : tBu-CH0H-CH 0H > tBu-CHOH-OL > tBu-CH0H-CH 0CH (a/a )x/2 : 30 % 12 % 2.5 % 9

L

9

6

c

Q

6

0

It was confirmed on several examples that c h i r a l 1,2 d i o l s gave the best r e s o l u t i o n r e s u l t s . This can be due t o the p o s s i b l e formation o f r i g i d c y c l i c o r perhaps polymeric species (18). In an homologous s é r i e o f c h i r a l hydroxy compounds "ïiïe b u l k i ness of the s u b s t i t u e n t i s f a v o r i s i n g s t e r e o e l e c t i v i t y . For example i n the 1,2 d i o l s é r i e associated to d i e t h y l z i n c one f i n d s f o r m e t h y l t h i i r a n e f o l l o w i n g e f f i c i e n c i e s :


198


1,2 d i o l substituent : (α/α )χ/2

:

0


tBu ~ i P r 30 %

>

Ph

28 %

>

Me

20 %

8 %

S i m i l a r r e s u l t s were obtained by Furukawa e t al (20) when using d i e t h y l z i n c - L - a m i n o a c i d systems i n polymerization o f methyl oxirane. Up to now we have found that the best s t e r e o e l e c t i v e i n i t i a t o r system f o r polymerization of oxiranes and t h i i r a n e s r e s u l t e d from the r e a c t i o n o f d i e t h y l z i n c and (-)3,3 dimethyl 1,2 butane d i o l (DMBD) taken i n (1:1) p r o p o r t i o n . 2-2) Influence o f the nature of the monomer Oxiranes and t h i iranes could be polymerized by the same type of i n i t i a t o r s which makes easy a way of comparison of t h e i r behaviour. We s h a l l now use our standard homosteric i n i t i a t o r ZnEt -(-)DMBD (1:1) and s t u dy the i n f l u e n c e of the nature of the monomer on the s t e r e o s e l e c t i v i t y and the s t e r e o e l e c t i v i t y of the process. 2

2-2-1) E f f e c t on s t e r e o e l e c t i v i t y Let us consider o p t i c a l y i e l d s obtained at h a l f r e a c t i o n with several t h i i r a n e s and o x i ~ ranes. Substituent : tBu ^ i P r > Et ^ Me > CH-CH 0 Thiiranes (α/α )χ/2 : 46 % 30 % 16 % ά

0

Substituent Oxi ranes

:

CH-O-LO 6

(α/α ) χ/2

:

σ

>

ά

CH. 6

25 %

20 %

It appears t h a t in general the s t e r e o e l e c t i v i t y i s higher f o r t h i i r a n e s than f o r o x i r a n e s , but a l s o that i n the case of the f o r mer the s t e r e o e l e c t i v i t y i s increased with the bulkiness o f the s u b s t i t u e n t . The o p t i c a l y i e l d at h a l f - r e a c t i o n allows a simple comparison between a l l types of monomers. However, t h i s value i s not r e f l e c t i n g the k i n e t i c scheme of r e s o l u t i o n and f o r t h i s p u r pose a study of the o p t i c a l p u r i t y of recovered monomer during f u l l course of polymerization i . e . on a l l conversion scale i s necessa ry. The experimental data taken on the whole range of conversion i n d i c a t e that there are d i f f e r e n c e s i n k i n e t i c behaviour between monomers. Two c l a s s e s of monomers could be defined corresponding to two types of t h e o r e t i c a l k i n e t i c equations. F i r s t order consumption equation A f i r s t c l a s s of monomers obeys equation with f i r s t order i n enantiomer consumption. One can write f o r each enantiomer : - d |R|/dt

= K

R

|R|

- d

|S|/ dt = \

IS ι


14. SPASSKY

Polymerization

dlRl

which give

K

=

d |S|

of Oxiranes and

R

| R |

Κ

=

| R |

r

|S|

Thiiranes

R R

— S

(1)

K and Κ are global rate constants r e l a t i v e to a c t i v e species afrd r tne s t e r e o e l e c t i v i t y r a t i o r e l a t i v e to R choice. r was found to be constant during the f u l l course o f polymerization and t h e r e f o r e equation (1) could be i n t e g r a t e d . I f one introduces experimental data which are α - o p t i c a l ac t i v i t y of recovered monomer, a - o p t i c a l a c t i v i t y of pure e n a n t i o mer, χ - conversion, | R | and |S| i n i t i a l concentrations of enantiomers D

ς

D

g


0

c

R|

- |s|

|R|

+ I si

0

+

|R|

|S|

α = α

One obtains

(l-x) "" 1

+

|R|o

isl

:

=

1

1 + (α/αο) — — Π " (a/a )|

2 " r

iRlo

r

0

which s i m p l i f i e s into :

|S| — ( mu + 1

r

— ι — Ν ο Γ

i f the i n i t i a l mixture i s racemic (|R|

0

, (i - χ )

Γ

=

1

-

( ) 2

1

=

1 + (a/oto) - r

I ι -

(a/a )|

|S| ) 0

(3) r

0

Experimental data found f o r racemic CH~(18) C HJ23),CH.CH 0 (24) t h i i r a n e s and CH (25),CH CH 0(24) o x i r a n e s , using our s t a n dard i n i t i a t o r were f i t t i n g with equation (3) with r e s p e c t i v e s t e r e o e l e c t i v i t y values (r) equal to 2.4, 2.4, 1.6, 1.8 and 2.0. In F i g . 1 are p l o t t e d experimental data f o r methyl t h i i r a n e . The r e s u l t s obtained with isopropyl and t - b u t y l t h i i r a n e were not f i t t i n g with equation (3)and t h e r e f o r e another k i n e t i c equat i o n o f second order was proposed (6,26). 9

?

?

é

3

3

2

Second order consumption equation The consumption i n enantiomer i s o f second order and the kinet i c equation becomes : d |R| d ιsι

|R|

2

|sr

K

which could be i n t e g r a t e d as p r e v i o u s l y a f t e r i n t r o d u c t i o n of and χ and gives f o r the racemic monomer : 1 (1-x)

(1 + ct/oo)

p

R

1

-

P

R

(1-χ)(1-α/α ) 0


(5)

α


200 where It optical α

0

p

i s the s t e r e o e l e c t i v i t y constant of second order. appears t h a t f o r a complete conversion the l i m i t value f o r a c t i v i t y i s no more a as i n the case o f equation (3) but R

Q

. ρ ~ j-

and t h e r e f o r e o p t i c a l l y pure monomer could not be ob

tained i n t h i s process. The experimental data found f o r t - b u t y l t h i i r a n e f i t t e d well with equation (5) as shown on F i g . 1 with p value equal to 8 at 20°C. Isopropyl t h i i r a n e gave almost the same value. P r e s e n t l y the d i s t i n c t i o n between both groups seems mainly based on the b u l k i ness on t h e i r s u b s t i t u e n t , the second order process being observed f o r the b u l k i e r compounds. Other examples are s t u d i e d f o r b e t t e r understanding. D


K

2-2-2) E f f e c t on s t e r e o s e l e c t i v i t y Informations on s t e r e o s e l e c t i v i t y are obtained by studying the s t e r e o r e g u l a r i t y o f p o l y mers. Generally they could be f r a c t i o n a t e d i n t o a c r y s t a l l i n e i s o t a c t i c f r a c t i o n and i n t o an amorphous h e t e r o t a c t i c f r a c t i o n , both o f them o p t i c a l l y a c t i v e . With our standard i n i t i a t o r at room temperature the p e r c e n t a ge of c r y s t a l l i n e i s o t a c t i c f r a c t i o n was 20 % f o r methyloxirane (25), (25), 35 % f o r methyl t h i i r a n e (2J[) and p r a c t i c a l l y 100 % f o r t butyl t h i i r a n e (27). The i s o t a c t i c f r a c t i o n comes from s i t e s o f almost pure R and S c h a r a c t e r and t h e r e f o r e the d i s t r i b u t i o n between d i f f e r e n t types of s i t e s f o r one t y p i c a l i n i t i a t o r i s again depending on the nature o f the monomer. It i s i n t e r e s t i n g to n o t i c e t h a t p o l y ( t - b u t y l t h i i r a n e ) ob t a i n e d i n s t e r e o e l e c t i v e experiments could be separated by s e l e c t i v e s o l u b i l i t y i n two f r a n c t i o n s , one o f which was i d e n t i f i e d as pure poly R polymer m.p. 157°C |α|£5 = + 164 (CHC1-) and the o t h e r , as the racemate (poly R + poly S) fn.p. = 204° (27)7 These f r a c t i o n s were compared with authentic samples prepared p r e v i o u s l y from pure l e v o r o t a t o r y monomer and racemic monomer (15). The l a t t e r r e s u l t s show that the s t e r e o e l e c t i v e p o l y m e r i z a t i o n i s a p o t e n t i a l method f o r obtention o f o p t i c a l l y pure p o l y mers from racemic monomers. 2-3) Influence of the enantiomeric composition o f the mono mer. Super s t e r e o e l e c t i v e processes. The enantiomeric composition of the i n i t i a l monomer may have a strong e f f e c t on the s t e r e o e l e c t i v i t y . It was shown on the example of methyl t h i i r a n e that the value of the s t e r e o e l e c t i v i t y r a t i o (r) could be r a i s e d up to 7 when using enriched monomers (28). The same phenomenon was r e c e n t l y observed with e t h y l t h i i r a n e (23)and methoxymethyl t h i i r a n e (29). The s t e r e o e l e c t i v i t y i s d i r e c t l y depending on the i n i t i a l R/S composition and obeys the general f i r s t - o r d e r equation (2). P r a c t i c a l l y , i n order to obtain more and more enriched mono mers the f o l l o w i n g procedure was used. The recovered unreacted mo nomer from one polymerization was reused as i n i t i a l monomer f o r


14.

SPASSKY

Polymerization

of Oxiranes

and

Thiiranes

201

the next step. As shown i n t a b l e III and F i g . 2 i t i s then p o s s i b l e i n 3-4 steps to i s o l a t e monomers having o p t i c a l p u r i t i e s h i g her than 95 %. Such a " s u p e r s t e r e o e l e c t i v e " process i s t h e r e f o r e i n t e r e s t i n g f o r the preparation o f small amounts o f almost o p t i c a l l y pure mo nomers from racemic mixtures. As an example methyl t h i i r a n e o.p. = 98 % was obtained i n three steps with an o v e r a l l y i e l d o f 11.5 % from the racemic compound ( t a b l e III).


Table

III

E f f e c t o f the enantiomeric p u r i t y of the roonoroer on the s t e r e o e l e c t i v i t y ZnEt -(-)DMBD (1:1) 2

Monomer

was used as i n i t i a t o r

i n i t i a l : Conversion; Recovered : monomer : monomer · : % : (α/α ) . (α/α ) :

:

0 0.35 0.68

: ethyl thiirane . (b) :

0 0.33 0.44 0.54

80 20 13 61

methoxymethyl thiirane (c)

0 0.36

74 24

(a) (b) (c)

: :

r

:

0

0

methyl t h i i r a n e (a)

system

• ' .

59 39 50

: :

; :

:

0.35 0.68 0.98

; 2.2 4.4 : 6.8

0.33 0.44 0.54 0.95

1.5 3.2 : 4.5 : 6.0

0.36 0.55

: 1.6 . 4.2

l : : : :

:

: : :

p o l y m e r i z a t i o n c a r r i e d out at room temperature i n t o luene s o l u t i o n . polym eri z a t i o n c a r r i e d out at -30°C i n bulk. p o l y m e r i z a t i o n c a r r i e d out a t room temperature i n bulk.

A monomer o f the same o p t i c a l p u r i t y could be obtained i n a simple s t e r e o e l e c t i v e experiment only a t conversions higher than 98 % i . e . with l e s s than 2 % y i e l d . One must add that such s u p e r s t e r e o e l e c t i v e processes are a l s o i n t e r e s t i n g as a source of polymers of high o p t i c a l p u r i t y . Indeed, i f one uses mixtures e n r i c h e d i n the enantiomer chosen i n the p r o c e s s , one can o b t a i n at low conversion (10 %) polymers o f high op t i c a l a c t i v i t y . For example, when using methyl t h i i r a n e or methyloxirane of 50 % o.p. ( i n R enantiomer) one gets polymers 90 % e n r i c h e d i n t h i s enantiomer (28). For monomers of the second group the s t e r e o e l e c t i v i t y was not a f f e c t e d by the i n i t i a l enantiomeric composition (26).




mon

Figure 1. Stereoelective polymerization of racemic thiiranes using ZnEt -(—)DMBD (1:1) initiator. ( ) first-order curve (·, exp. data for methylthiirane); ( ) second-order curve ( J , exp. data for tert hutylthiirane). g

mon

Figure 2. Superstereoelective procedure applied to the polymerization of methylthiirane using ZnEt (—)DMBD (1:1) initiator g


14.

SPASSKY

Polymerization

of Oxiranes

and

Thiiranes

203

2-4) E f f e c t o f the temperature on s t e r e o e l e c t i v i t y Few r e s u l t s only were reported on the e f f e c t of the temperature on s t e r e o e l e c t i o n . In the case of monomers o f f i r s t c l a s s the s t e r e o e l e c t i v i t y was not modified by changing the temperature, while the s t e r e o s e l e c t i v i t y o f the process increased by lowering the tempera ture o f p o l y m e r i z a t i o n as demonstrated i n the case of methyl o x i rane (25). The temperature showed a strong e f f e c t i n the po lymeriz a tio n o f t - b u t y l t h i i r a n e (26) The s t e r e o e l e c t i v i t y p doubled i n v a lue when temperature Towered from 20° to - 3 ° and on the c o n t r a r y P decreased with r a i s i n g of Τ and a t temperatures higher than 1T5° the choice o f the enantiomer was i n v e r t e d as shown i n t a b l e IV. The l i m i t value of the o p t i c a l p u r i t y of monomer was a l s o mo dified. A l i n e a r c o r r e l a t i o n was found between log p and 1/T and the o v e r a l l d i f f e r e n c e i n energy o f a c t i v a t i o n r e l a t i v e to the s t e r e o e l e c t i v e process f o r both enantiomers could be c a l c u l a t e d (5 K c a l / mol).


R

R

R

Table

IV

Influence o f the temperature on s t e r e o e l e c t i v i t y i n the p o l y m e r i z a t i o n of t - b u t y l t h i i r a n e with ZnEt -(-)DMBD (1:1) i n i t i a t o r system 2

(α/α ) recovered monomer: at χ % conversion : 0

: t°C

P

R .

: -3 : 20 : 63 :135 *)

: : :

14 8 2.5

χ = 50: l i m i t χ = 100 :

: :

58 48

;

!*>;

: :

87 78

: :

4 7*>

;

}

S enantiomer i s

4

preferentially elected

The d i f f e r e n c e i n temperature dépendance between both c l a s s o f monomers could be e x p l a i n e d i n terms o f mechanism o f p o l y m e r i z a t i o n as shown i n chapter 4. 2-5) E f f e c t o f solvents and a d d i t i v e s The r o l e o f s o l v e n t may be important i n such " a n i o n i c - c o o r d i n a t e d " p o l y m e r i z a t i o n s . It was shown f o r example i n the case o f methyl t h i i r a n e that a d d i t i o n of t e t r a h y d r o f u r a n decreased the s t e r e o e l e c t i v i t y , a competition o c c u r i n g between the monomer and the s o l v e n t f o r the c o o r d i n a t i o n on the m e t a l l i c atom (30). Very r e c e n t l y SepïïTchre (31) has shown the p o s s i b i l i t y to i n -


204


crease s u b s t a n t i a l l y the s t e r e o e l e c t i v i t y by modifying the i n i t i a t o r with c h i r a l a g e n t s ( o p t i c a l l y a c t i v e t h i o e t h e r s , amines) o r by using c h i r a l s o l v e n t s (limonene). Such a way seems very promising and s t u d i e s are i n progress i n our l a b o r a t o r y .


3 - Asymmetric polymer

synthesis

In the previous chapter we have seen that an o p t i c a l l y a c t i v e c a t a l y s t was able to make a p r e f e r e n t i a l choice between two s t e r e i s o m e r i c molecules d i f f e r e n t i a t e d by the opposite c o n f i g u r a t i o n o f t h e i r asymmetric c e n t e r s . Now we wish to r e p o r t our i n v e s t i g a t i o n s on the behaviour o f the same i n i t i a t o r s i n the presence o f symmetric monomers having two asymmetric centers of opposite c o n f i g u r a t i o n i n neighbouring position. C i s - 2 , 3 dimethyl t h i i r a n e (DMT) and cyclohexene s u l f i d e (CS) were s t u d i e d f o r t h i s purpose. A few p r e l i m i n a r y r e s u l t s concerning t h e i r p o l y m e r i z a t i o n are given i n table V. O p t i c a l l y a c t i v e c r y s t a l l i n e polymers were obt a i n e d which could be separated by s e l e c t i v e s o l u b i l i t y i n f r a c t i o n s o f d i f f e r e n t o p t i c a l a c t i v i t y and c r y s t a l l i n i t y . Table Asymmetric polymer s y n t h e s i s

V

using ZnEto-(-)DMBD. (1:1)

initiator

C i s 2,3 dimethyl t h i i r a n e (DMT) and cyclohexene s u l f i d e were polymerized i n bulk at room temperature.

Polymer

(CS)

fractions

: S o l . toluene room temp. : S o l . CHClj room temp. : Conversion

:

%

;

%

•Ref:

tCXp)

: : (CHC1 ) : 3

30

:

29

:

+20

m.p. °C 60

:DMT:

les : (a)

:

: % : (CHC1 ) : 3

71 :

m. p. °C

+50

: 125

+66

126

32 : 100

33

'.

+24

45

35

; +

3.8< \

+ 20 i n t r i c h l o r o b e n z e n e

40/90 . 67 a

65

: i o : +8.4( >; b

80

: : 33 :

(b) + 39 i n trichlorobenzene..

The d i r e c t i o n of ring-opening i s o r i e n t e d by the c h i r a l choice o f the c a t a l y s t which attacks p r e f e r e n t i a l l y one o f the asymmetric carbons with i n v e r s i o n o f c o n f i g u r a t i o n of the l a t t e r . The r e s u l t i n g polymer i s o p t i c a l l y a c t i v e due to the prevalence o f one type o f c o n f i g u r a t i o n a l u n i t s |for example E(-RR-)> z ( - S S - ) | .


14.

SPASSKY

Polymerization

of Oxiranes and

Thiiranes

205

According to Vandenberg ( 7 » 8 ) t h e ring-opening o f c i s compounds i n v o l v e s a process with i n v e r s i o n o f c o n f i g u r a t i o n o f the attacked carbon and depending on the type o f enchainment one can obtain d i i s o t a c t i c (I) o r d i s y n d i o t a c t i c ( I I ) s t r u c t u r e s . Structure C

C

\(R)

diisotactic

(I)

(S)/


c— c

/ \ HT

/ Χ

\ H

d i s y n d i o t a c t i c (II)

enchainment head-to-tail - RR - RR - SS - SS head-to-head - RR - SS -

X = 0,S According to such a scheme, the observed o p t i c a l a c t i v i t y must be due to the prevalence o f d i i s o t a c t i c chains (I) o f one type o f chirality. It was not p o s s i b l e up to now to a s c e r t a i n the o p t i c a l p u r i t y o f prepared polymers. Using '3c NMR i t was found t h a t r e s u l t i n g po lymers presented d i f f e r e n t types of stereosequences. In poly c i s 2,3 dimethyl t h i i r a n e the peak l o c a t e d a t 45.8 ppm (CDCU s o l v e n t , reference to TMS) was c l e a r l y assigned to the methine chain carbon o f d i i s o t a c t i c s t r u c t u r e as i t was d i r e c t l y i n c r e a s i n g with the o p t i c a l a c t i v i t y o f the polymer. Three other peaks corresponding to chain carbons were found, showing that other s t r u c t u r e s than the simple d i s y n d i o t a c t i c one (II) are a l s o p r e sent. An i n t e r e s t i n g source of informations on the s t r u c t u r e o f such polymers may be obtained from t h e i r c a t i o n i c degradation as r e p o r ted r e c e n t l y by Goethals (34). In the case o f c i s - d i m e t h y l t h i i r a nes c y c l i c oligomers o f d i f f e r e n t s t r u c t u r e were i d e n t i f i e d , name l y t r i t h i e p a n e s and tetramers(35). The degradation o f o p t i c a l l y a c t i v e poly c i s DMT produced op t i c a l l y a c t i v e t r i t h i e p a n e s and tetramers of opposite s i g n . The magnitude o f t h e i r o p t i c a l a c t i v i t i e s i s d i r e c t l y depending on the o p t i c a l a c t i v i t y o f the polymer used and, t h u s , r e f l e c t s i t s s t r u c t u r e . Further s t u d i e s are i n progress (36). The C NMR study of o p t i c a l l y a c t i v e poly(cyclohexene s u l f i d e ) showed a l s o a complicated s t r u c t u r e f o r the methine carbon o f the main chain with the presence o f f i v e or s i x peaks. S u r p r i s i n g l y , n e i t h e r c i s 2,3 dimethyl oxirane nor cyclohexene oxide have f u r n i s h e d products with s i g n i f i c a n t o p t i c a l a c t i v i t y when using the standard c h i r a l i n i t i a t o r . The reason o f t h i s be haviour i s not y e t known and s t r u c t u r e o f the polymers are under the study. One must mention that a poly c i s 2,3 dimethyl oxirane o f low o p t i c a l a c t i v i t y was obtained by Vandenberg (37) when using an aluminum i n i t i a t o r modified by 1-menthol. Thus, new types o f polymers could be obtained by means o f t h i s J


206


asymmetric synthesis where a c h i r a l enantiomeric polymeric m a t e r i a l i s created from an a c h i r a l monomer. Such a process may be c a l l e d " a c h i r a l e n a n t i o g e n i c " as suggested by P r o f e s s o r J . P . G u e t t é .


4 - Mechanistic aspects o f the s t e r e o s p e c i f i c polymerization o f oxiranes and t h i i r a n e s From the above considerations some general mechanistic f e a t u res could be proposed f o r s t e r e o s e l e c t i v e and s t e r e o e l e c t i v e p o l y merization using modified organometallic c a t a l y s t s . In a f i r s t step the monomer reacts with the i n i t i a t o r t o form a f u l l spectrum o f s i t e s having d i f f e r e n t R and S c h a r a c t e r . Some of these formed species have a complete s e l e c t i v i t y and p r o duce c r y s t a l l i n e i s o t a c t i c polymers. The proportion of such s e l e c t i v e species f o r a given i n i t i a t o r i s depending on the nature o f the monomer. We have seen that f o r monomers with bulky s u b s t i t u e n t s l i k e t - b u t y l t h i i r a n e almost a l l the s i t e s are purely s e l e c t i v e , while f o r other monomers l i k e methyl oxirane only 20 % o f the a c t i v e species are s e l e c t i v e . I f the i n i t i a t o r i s o p t i c a l l y a c t i v e there i s an unbalanced amount of R type and S type species and t h e r e f o r e s t e r e o e l e c t i o n w i l l occur when polymerizing a racemic monomer mixture. Other species have a much lower s e l e c t i v i t y and produce the amorphous p a r t o f the polymer. Again i f the i n i t i a t o r i s o p t i c a l l y a c t i v e , a predominance of one type o f species occurs and t h e r e f o r e an h e t e r o t a c t i c o p t i c a l l y a c t i v e polymer i s o b t a i n e d . In chapter 2 we have d i s t i n g u i s h e d two c l a s s e s of monomers a c cording t o t h e i r k i n e t i c behaviour i n s t e r e o e l e c t i o n . This d i f f e r e n c e i n behaviour can now be j u s t i f i e d by some mechanistic considerations. With monomers o f the f i r s t c l a s s , methyl oxirane f o r example, the a c t i v e s i t e s are formed i n an i r r e v e r s i b l e way a f t e r the r e a c t i o n (or very strong complexation) of the i n i t i a l monomer with the i n i t i a t o r . As a p r o o f , one f i n d s that the s t e r e o e l e c t i v i t y ( r ) , i . e . the enantiomorphic d i s t r i b u t i o n of s i t e s , i s not modified by a change of the temperature of p o l y m e r i z a t i o n , but (r) i s s t r o n g l y depending on the enantiomeric composition of the i n i t i a l monomer. On the c o n t r a r y , i n the case of monomers o f the second group ( t - b u t y l t h i i r a n e ) , the a c t i v e centers are formed a f t e r complexat i o n o f the monomer on the i n i t i a t o r s p e c i e s . Thus, the s t e r e o e l e c t i v i t y should depend on the temperature which i s indeed o b s e r ved i n a s i g n i f i c a n t way. The second order law could be explained by a two step process : the complexation - a r e v e r s i b l e s t e p , then the propagation step.


14.

SPASSKY

Polymerization

of Oxiranes and

Thiiranes

207

S i m p l i f i e d r e p r e s e n t a t i v e scheme o f both mechanisms i l l u s t r a t e d as f o l l o w s :

INITIATOR SITES highly \

n

STEREOREGULARiT

selective C

Uc Downloaded by PENNSYLVANIA STATE UNIV on May 31, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch014

could be

R

s

I §

R

>

η Poly R > m poly S s

isotactic

First group tfNJow

selectivity C

44-

Ρ

0 1

^

R

n' * S

heterotactic

+ R n|C.R|

η poly R

Second group

isotactic + S m|C.S|

-> m poly S

When C i s o p t i c a l l y a c t i v e η ^ m η ' Φ m'. There i s a mutual r e c o g n i t i o n between c h i r a l s i t e s and enan tiomers of s i m i l a r c o n f i g u r a t i o n . Moreover on the b a s i s o f asym m e t r i c s y n t h e s i s r e a c t i o n s i n v o l v i n g symmetric monomers, we may assume t h a t the c h i r a l i n i t i a t o r can d i s t i n g u i s h one p e c u l i a r asym m e t r i c carbon i n the molecule i n the course o f the ring-opening r e a c t i o n . T h i s ring-opening proceeds with i n v e r s i o n o f c o n f i g u r a t i o n at the carbon as p r e v i o u s l y e s t a b l i s h e d by Vandenberg (7). In the case o f monosubstituted monomers, the c a t a l y s t attacks the p r i m a ry methylenic carbon and t h i s does not a f f e c t the c o n f i g u r a t i o n o f the asymmetric carbon. For symmetrically d i s u b s t i t u t e d monomers, however, when using c h i r a l i n i t i a t o r s , o p t i c a l l y i n a c t i v e polymers are obtained i f s t a r t i n g from trans compounds and o p t i c a l l y a c t i v e products may be prepared from c i s compounds. More i n v e s t i g a t i o n s are s t i l l necessary f o r complete under standing o f s t e r e o s p e c i f i c processes. 5 -

Conclusion

A great v a r i e t y o f products could be prepared using s t e r e o s p e cific initiators. Three main d i r e c t i o n s seem promising : F i r s t , i s o t a c t i c o p t i c a l l y pure or h e t e r o t a c t i c products o f low o p t i c a l a c t i v i t y may be obtained s t a r t i n g from racemic mono-


208



mers. Second, monomers of high o p t i c a l p u r i t y could be i s o l a t e d i n l i m i t e d amounts s t a r t i n g from racemic mixtures. In such a case the s t e r e o e l e c t i v e p o l y m e r i z a t i o n can be considered as an o r i g i n a l r e s o l u t i o n method o f s p e c i a l i n t e r e s t f o r monomers which are not e a s i l y prepared by conventional s y n t h e t i c ways under t h e i r o p t i c a l l y a c t i v e form. Increase i n s t e r e o e l e c t i v i t y i s observed when using c h i r a l media, i . e . e n a n t i o m e r i c a l l y e n r i c h e d monomers or e x t e r n a l chiral additives. T h i r d , new o p t i c a l l y a c t i v e polymers are obtained by asymmetric transformation o f symmetric monomers. The author i s g r a t e f u l to Drs. Sepulchre, Dumas, MM. Coulon, D e f f i e u x , Khali 1, Momtaz, Pourdjavadi and Reix f o r t h e i r c o n t r i bution to t h i s work and communication o f unpublished data. The a u thor thanks P r o f e s s o r Sigwalt f o r the c r i t i c a l reading o f the ma nuscript and stimultating discussions. Literature Cited (1) Tsuruta T., Stereochemistry of Macromolecules, Ed. by Ketley A.D., Vol. 2, p. 177, M. Dekker, Inc., N.Y.,1967 (2) Sigwalt P., Int. J. Sulfur Chem. (1972) C7,83 (3) Tsuruta T., J. Polym. Sci. (1972) D, 179 (4) Tani H., Adv. Polym. Sci. (1973) 11, 57 (5) Spassky Ν., Dumas P., Sepulchre and Sigwalt P., J. Polym. Sci., Symposium n° 52 (1975) 327. (6) (7) (8) (9)

Sigwalt P., Pure and Applied Chemistry (197 ) (in press). Vandenberg E.J., J. Polym. Sci., (1969) A 1, 7, 529 Vandenberg E.J., J. Polym. Sci. (1972) A 1, 10, 329 Pruitt M.E. and Baggett I.M. (to Dow Chemical Co) U.S. Pat. 2, 706, 181 (1955) (10) Furukawa J. and Saegusa T., Polymerization of Aldehydes and Oxides, J. Wiley et Sons, N.Y. 1963 (11) Ishii Y. and Sakai S., Ring-Opening polymerization, Ed. by Frisch K.C. and Reegen S.L., Vol. 2, p. 13, M. Dekker, N.Y., 1969 (12) Oguni N., Watanabe S., Maki M. and Tani H., Macromolecules (1973) 6 (2), 195 (13) Inoue S. Tsukawa I., Kawaguchi M. and Tsuruta T., Makromol. Chem. (1967) 103, 151 (14) Price C.C., Akkapeddi M.K., Debona B.T. and Furie B.C. J. Amer. Chem. Soc. (1972) 94 (11), 3964 (15) Dumas P., Spassky N. and Sigwalt P., Makromol. Chem. (1972) 156, 55 (16) Guérin P., Boileau S. and Sigwalt P., European Pol. J. (1974) 10, 13 (17) Pino P., Adv. Polym. Sci. (1965) 4, 236 (18) Deffieux Α., Sépulchre Μ., Spassky N. and Sigwalt P., Makromol. Chem. (1974) 175/4, 339 (19) Furukawa J., Kawabata N. and Kato Α., J. Polym. Sci. (1967) B,5, 1073


14.

SPASSKY

Polymerization

of Oxiranes

and

Thiiranes


(20) Furukawa J., Kumata Y., Yamada K. and Fueno T., J. Polym. Sci. (1968) C (23), 711 (21) Sépulchre M. and Spassky Ν., to be published (22) Ishimori M., Hagiwara T., Tsuruta T., Kai Y., Yasuako N. and Kasai Ν., Bull. Chem. Soc. Jap. (1976) 49 (4), 1165 (23) Khalil Α., Sepulchre M. and Spassky N., to be published (24) Spassky Ν., Pourdjavadi A. and Sigwalt P., European Polym. J. (1977) (in press). (25) Coulon C., Spassky N. and Sigwalt P., Polymer (1976) 17, 821 (26) Dumas P., Spassky N. and Sigwalt P., to be published. (27) Dumas P., Spassky N. and Sigwalt P., J. Polymer Sci., Polymer Chem. Ed. (1974) 12, 1001 (28) Sépulchre M., Coulon C., Spassky N. and Sigwalt P., 1-rst International Symposium on Ring-Opening Polymerization, Jablonna (1975), Preprints p. 80 (29) Reix Μ., Sepulchre M. and Spassky N., to be published (30) Spassky N. and Sigwalt P., European Polym. J. (1971) 7, 7 (31) Sépulchre Μ., Sigwalt and Spassky N., IUPAC International Symposium on Macromolecules, Dublin (1977), Preprint (32) Momtaz A. and Spassky Ν., unpublished results. (33) Reix M. and Spassky N., unpublished results. (34) Goethals E.J., Adv. Polymer Sci. (1977) 23, 103 (35) Van Crayenest W. and Goethals E.J., European Polymer J. (1976) 12, 859 (36) Tan Crayenest W., Goethals E.J., Momtaz A. and Spassky N. Unpublished results, in collaboration. (37) Vandenberg E.J., J. Polymer Sci. (1964) B, 2, 1085


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Ring-Opening Polymerization - ACS Publications

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