Relationship of Anion Pair Structure to Stereospecificity of

5 Relationship of Anion Pair Structure to Stereospecificity of Polymerization S.

BYWATER

Downloaded by COLUMBIA UNIV on March 16, 2013 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch005

Division of Chemistry, National Research Council of Canada, Ottawa, CanadaK1AO R 9

Spectroscopic measurements can be made on the active centers in anionic polymerization. Oligomeric model compounds, for example, of butadiene or isoprene show the existence of variable proportions of cis and trans forms at equilibrium dependent on counter-ion and solvent. The new active center formed immediately on monomer addition may not, however, be in its equilibrium configuration. In this case, the rate of isomerization to the stable form becomes important in microstructure determination if it is assumed that the structure is "frozen-in" at the next monomer addition. Charge distribution and counter-ion position also change with reaction conditions, which must have some influence on the proportion of 1,4 and vinyl units in the polymer. Particular examples of correlations between ion-pair properties and polymer microstructure are discussed.

One of the i n t e r e s t i n g features of anionic polymerization i s the v a r i a t i o n in m i c r o s t r u c t u r e of the polymers formed under different conditions. Both v i n y l and diene monomers show these e f f e c t s , most markedly with a c r y l a t e s , butadiene and isoprene and p a r t i c u l a r l y with l i t h i u m as counter-ion. E a r l y models for s t e r e o s p e c i f i c i t y are e s s e n t i a l l y s t a t i c ones, for example with the 1ithium/hydrocarbon/isoprene system, the formation of a s i x membered intermediate with c o o r d i n a t i o n o f the C-Li bond to the incoming monomer i n i t s c i s form ( 1 ) . The c o n f i g u r a t i o n of each terminal unit would then be determined i n the r e a c t i o n leading to i t s formation with no subsequent changes p o s s i b l e . S i m i l a r l y with a c r y l a t e s , complexation of the l i t h i u m with carbonyl groups on incoming monomer and u n i t s of the polymer chain i s the dominant feature of mechanisms for i s o t a c t i c polymer formation (2). Probably the f i r s t departure from these s t a t i c models was 0097-6156/81/0166-0071$05.00/0 Published 1981 A m e r i c a n Chemical Society

In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


72

ANIONIC POLYMERIZATION

s u g g e s t e d by Bovey e t a l (3_) i n a c r y l a t e p o l y m e r i z a t i o n . They were a b l e t o show u s i n g s p e c i f i c a l l y d e u t e r a t e d monomers t h a t a l t h o u g h i n h y d r o c a r b o n s o l v e n t s monomer a t t a c k d i d o c c u r so as t o m a i n t a i n a l i g n e d e s t e r - g r o u p s i n monomer and p o l y m e r , w i t h t r a c e s o f e t h e r s p r e s e n t , t h e monomer c o u l d a p p r o a c h w i t h e s t e r g r o u p s opposed. T h i s would normally produce a racemic d i a d . Subsequent r o t a t i o n o f the t e r m i n a l g r o u p t o m a x i m i z e e l e c t r o s t a t i c i n t e r a c t i o n s and hence b r i n g b a c k a meso c o n f i g u r a t i o n was found t o occur. I n i s o p r e n e p o l y m e r i z a t i o n t o o , t h e r e were i n d i c a t i o n s t h a t the s i m p l e p i c t u r e was not a d e q u a t e , f o r t h e c i s - 1 , 4 c o n t e n t was found t o be dependent on r e a c t i o n c o n d i t i o n s , p a r t i c u l a r l y on the i n i t i a t o r c o n c e n t r a t i o n ( 4 ) . The f a c t t h a t b u t a d i e n e u n d e r s i m i l a r c o n d i t i o n s gave a m i x e d c i s / t r a n s p o l y m e r a l s o s u g g e s t e d the mechanism was more c o m p l e x . B o t h t h e s e o b s e r v a t i o n s s u g g e s t t h a t k i n e t i c as w e l l as thermodynamic f a c t o r s are i m p o r t a n t i n s t e r e o s t r u c t u r e déterminât i o n . In order to i n c r e a s e the u n d e r s t a n d i n g of these p r o c e s s e s , a v a l u a b l e t o o l i s the s t u d y o f c o n f i g u r a t i o n s and i s o m e r i z a t i o n r a t e s o f low m o l e c u l a r w e i g h t m o d e l s o f the p o l y m e r c h a i n . The a b i l i t y t o s t u d y such compounds i s one o f t h e m a j o r a d v a n t a g e s o f studies in anionic polymerization. The a c t i v e c e n t e r s a r e s t a b l e under many c o n d i t i o n s and can be s t u d i e d a t l e i s u r e by a number o f t e c h n i q u e s , p a r t i c u l a r l y NMR and o p t i c a l s p e c t r o s c o p y . The d i e n e s p r o v i d e a good e x a m p l e . A d d i t i o n o f s e c . o r t - b u t y l 1 i t h i u m t o i s o p r e n e o r b u t a d i e n e u n d e r c o n t r o l l e d c o n d i t i o n s y i e l d s a model one unit, a c t i v e c h a i n u s u a l l y w i t h s m a l l amounts o f two u n i t m a t e r i a l (5^,6). The l a t t e r can be removed i f n e c e s s a r y by c o n v e r t i n g t o t h e m e r c u r y compound w h i c h can be d i s t i l l e d t o remove d i m e r . The m e r c u r y compound can s u b s e q u e n t l y be r e c o n v e r t e d t o t h e l i t h i u m ( o r o t h e r a l k a l i m e t a l ) d e r i v a t i v e by s i m p l e t r e a t m e n t w i t h an a l k a l i m e t a l ( 7 ) . W i t h t h e a c r y l a t e s , due t o e x t e n s i v e a t t a c k o f l i t h i u m a l k y l s on t h e e s t e r g r o u p , t h i s a p p r o a c h c a n n o t be u s e d , but m o d e l s can be made by m e t a l 1 a t i o n o f s u i t a b l e compounds such as m e t h y l i s o b u t y r a t e (8). N.M.R. s t u d i e s on d i e n e models show t h a t t h e p r e f e r r e d c o n f i g u r a t i o n o f the l i t h i u m compound i n h y d r o c a r b o n s i s t r a n s a l t h o u g h some c i s s t r u c t u r e does e x i s t i n e q u i l i b r i u m i . e . t h e a c t i v e c e n t e r i t s e l f does e x i s t i n two forms ( 9 , 1 0 ) .

H

CH

H

H

C—C

Crr.C

\

2

H

(trans) +

M (cis) In p o l a r s o l v e n t s

s u c h as THF,

g e n e r a l l y t h e c i s form i s more



5.

BYWATER

Anion

Pair

73

Structure

s t a b l e f o r t h e whole s e r i e s o f a l k a l i m e t a l s ( 1 1 ) ( t h e l i t h i u m d e r i v a t i v e o f b u t a d i e n e i n d i e t h y l e t h e r i s an e x c e p t i o n ) . I f the l i t h i u m i s o p r e n e model i s t r a n s f e r r e d from h y d r o c a r b o n s o l v e n t s t o THF a t low t e m p e r a t u r e , i t s c o n f i g u r a t i o n i s f r o z e n i n t h e f o r m stable i n hydrocarbons. On warming h o w e v e r , a t -40° a s l o w i s o m e r i z a t i o n o c c u r s t o t h e form s t a b l e i n THF ( 1 2 ) . This gives an e s t i m a t e o f t h e i s o m e r i z a t i o n r a t e under a s p e c i f i c s e t o f conditions. More i m p o r t a n t a r e t h e r a t e s i n h y d r o c a r b o n s o l v e n t s . These c a n be d e t e r m i n e d i n a d i f f e r e n t way. D i - i s o p r e n y l m e r c u r y as p r e p a r e d by t h e t e c h n i q u e d e s c r i b e d above i s m a i n l y i n i t s c i s form and when r e a c t e d w i t h l i t h i u m s u s p e n s i o n i n h y d r o c a r b o n s o l v e n t s at low temperatures g i v e s c i s - i s o p r e n y l l i t h i u m which slowly isomerizes to the e q u i l i b r i u m t r a n s - r i c h mixture. Rates can be measured between -20° and 0° ( 1 3 ) . E s t i m a t e s c a n a l s o be made o f t h e i s o m e r i z a t i o n r a t e s o f a two u n i t a c t i v e c h a i n w h i c h t u r n out t o be somewhat d i f f e r e n t . These l a t t e r r a t e s a r e probably c l o s e r t o that o f high polymer. The m i c r o s t r u c t u r e o f p o l y i s o p r e n e p r e p a r e d by l i t h i u m i n i t i a t i o n i n h y d r o c a r b o n s i s 9 5 % 1,4 u n d e r a l l c o n d i t i o n s . The t r a n s 1,4 c o n t e n t however f a l l s f r o m about 20% t o z e r o as t h e m o n o m e r / i n i t i a t o r r a t i o i n c r e a s e s l e a d i n g f i n a l l y t o a 9 5 % c i s 1,4 p o l y m e r . T h i s v a r i a t i o n c a n be e x p l a i n e d w i t h t h e f o l l o w i n g scheme.

J'

:rans*

+ M

» ~

trans,

cis*

where ~ ~ c i s * r e p r e s e n t s a c i s a c t i v e c e n t e r , an i n t e r n a l u n i t appearing unstarred. The newly formed a c t i v e c e n t e r i s e n t i r e l y i n t h e c i s form i . e t h e r e a c t i o n i s s t e r e o s p e c i f i c . T h i s p o i n t can a c t u a l l y be p r o v e d w i t h i n e x p e r i m e n t a l e r r o r . When new monomer a d d s , a c i s c e n t e r i s c o n v e r t e d t o a c i s c h a i n u n i t and o f c o u r s e a t r a n s one t o a t r a n s u n i t . I t i s u n l i k e l y t h a t monomer a d d i t i o n w o u l d change i t s c o n f i g u r a t i o n . Now i f monomer a d d i t i o n i s s l o w , t h e i n i t i a l l y formed c i s a c t i v e c e n t e r may have t i m e t o r e a r r a n g e t o t h e more t h e r m o d y n a m i c a 1 l y s t a b l e t r a n s form and i n the l i m i t o f very slow p o l y m e r i z a t i o n r a t e , t h e p o p u l a t i o n d i s t r i b u t i o n w i l l be t h e e q u i l i b r i u m o n e . I f however t h e r a t e o f monomer a d d i t i o n i s f a s t compared t o i s o m e r i z a t i o n t h e w h o l e a c t i v e c e n t e r p o p u l a t i o n w i l l r e m a i n c i s and w i t h i t t h e p o l y m e r configuration. On t h i s s i m p l e p i c t u r e t h e l o w r a t e p l a t e a u w o u l d g i v e a 66% t r a n s c o n t e n t i n t h e p o l y m e r and t h e h i g h r a t e l i m i t z e r o p e r c e n t i f r a t e s o f a d d i t i o n o f monomer t o t h e two t y p e s o f center are equal. I n f a c t i t c a n be shown t h a t t h e y a r e n o t , t h e c i s a c t i v e c e n t e r s a d d i n g monomer e i g h t t i m e s f a s t e r t h a n t h e trans centers. Knowing t h e r e l a t i v e monomer a d d i t i o n r a t e s , i s o m e r i z a t i o n r a t e s and e q u i l i b r i u m p o p u l a t i o n s i t i s p o s s i b l e t o c a l c u l a t e t h e c i s - t r a n s r a t i o i n t h e p o l y m e r s and compare i t w i t h


74

ANIONIC POLYMERIZATION


e x p e r i m e n t . Good agreement i s f o u n d ; t h e i m p o r t a n c e o f k i n e t i c and thermodynamic f a c t o r s i n t h i s c a s e i s w e l l e s t a b l i s h e d . A t t e m p t s t o r e p e a t t h e s e measurements on b u t a d i e n e p o l y m e r i z a t i o n l e d t o the r e s u l t t h a t t h e i s o m e r i z a t i o n r a t e s were f a s t e r - t o o f a s t t o measure by t h e t e c h n i q u e s i n use. No d e t a i l e d e x a m i n a t i o n was t h e r e f o r e p o s s i b l e but q u a l i t a t i v e l y i t i s c l e a r t h a t t h i s w i l l r e s u l t i n a m i c r o s t r u c t u r e much c l o s e r t o t h e e q u i l i b r i u m p o p u l a t i o n o f a c t i v e c e n t e r c o n f i g u r a t i o n s i . e . l e s s c i s 1,4 u n i t s in the polymer than w i t h isoprene although t h i s again i s probably a cis-stereospecificreaction. P o l y m e r i z a t i o n o f d i e n e s i n p o l a r s o l v e n t s i s a more c o m p l e x p r o b l e m f o r l a r g e amounts o f v i n y l u n s a t u r a t i o n o c c u r i n t h e polymer. Some 1,4 s t r u c t u r e s do p e r s i s t p a r t i c u l a r l y i n t h e e t h e r s o f l o w e r d i e l e c t r i c c o n s t a n t so i t i s o f i n t e r e s t t o i n q u i r e i f c i s / t r a n s i s o m e r i z a t i o n of the a c t i v e c e n t e r s i s again o f i m p o r t a n c e . T h i s can be shown t o be t h e c a s e i n b u t a d i e n e p o l y m e r i z a t i o n i n THF. O p t i c a l s p e c t r o s c o p y p r o v i d e s the needed tool. C i s and t r a n s a c t i v e c e n t e r s b o t h show t h e near-UV a b s o r p t i o n s c h a r a c t e r i s t i c o f d e l o c a l i z e d a l l y l i c i o n s and t h e i r pairs. The a b s o r p t i o n maxima a r e however s h i f t e d , t h e t r a n s c e n t e r s a b s o r b i n g at l o n g e r w a v e l e n g t h ( 1 4 , 1 5 ) . I f at -40° a r e l a t i v e l y l a r g e amount o f monomer i s added t o a p o l y b u t a d i e n y l sodium s o l u t i o n at e q u i l i b r i u m , an a l m o s t i n s t a n t a n e o u s s h i f t o f the a b s o r p t i o n maximum o c c u r s t o l o n g e r w a v e l e n g t h , w h i c h s l o w l y r e t u r n s t o i t s o r i g i n a l p o s i t i o n when p o l y m e r i z a t i o n i s o v e r . It appears t h a t i n t h i s s o l v e n t t r a n s c e n t e r s are p r e f e r e n t i a l l y formed on monomer a d d i t i o n w h i c h c a n n o t r e l a x t o t h e more s t a b l e c i s form at t h i s t e m p e r a t u r e . The s i t u a t i o n i s e x a c t l y o p p o s i t e to that observed i n non-polar s o l v e n t s . At h i g h e r t e m p e r a t u r e s however e q u i l i b r i u m i s m a i n t a i n e d . A g a i n we a r e d e a l i n g w i t h a r e a c t i o n w h i c h has a p r e f e r e n t i a l p r o d u c t w h i c h can be d i f f e r e n t from t h e t h e e q u i l i b r i u m f o r m , i n t h i s c a s e i n d i c a t e d by l o w e r i n g t h e t e m p e r a t u r e r a t h e r t h a n i n c r e a s i n g t h e monomer c o n c e n t r a t i o n . P a r a l l e l measurements on p o l y m e r m i c r o s t r u c t u r e show t h a t t h e s e changes a r e r e f l e c t e d i n t h e p r o d u c t . A l t h o u g h the t o t a l 1,4 c o n t e n t i s low, i t i s a l m o s t e n t i r e l y t r a n s at low t e m p e r a t u r e s (16). D i f f e r i n g r a t e s o f monomer a d d i t i o n t o c i s and t r a n s c e n t e r s a r e a l s o i n d i c a t e d by a s h a r p change o f the t e m p e r a t u r e c o e f f i c i e n t o f p o l y m e r i z a t i o n r a t e at -30° where " f r e e z i n g - i n o f s t r u c t u r e f i r s t becomes i m p o r t a n t - t h e t r a n s c e n t e r s add monomer faster in this solvent. In view o f these o b s e r v a t i o n s care should be t a k e n i n i n t e r p r e t a t i o n o f l i t e r a t u r e d a t a on m i c r o s t r u c t u r e , o f t e n d e t e r m i n e d at one p a r t i c u l a r t e m p e r a t u r e and monomer/ initiator ratio. H i e r e s u l t s may not be t y p i c a l o f t h e r e a c t i o n under a l l c o n d i t i o n s . , f

I n p o l a r s o l v e n t s t h e o t h e r i m p o r t a n t v a r i a b l e i s t h e 1,4 t o vinyl ratio. V i n y l u n s a t u r a t i o n i s a l w a y s an i m p o r t a n t p a r t o f t h e p o l y m e r s t r u c t u r e but i t s amount does depend on c o u n t e r - i o n . I t i s t e m p t i n g t o c o r r e l a t e t h i s w i t h c h a r g e d i s t r i b u t i o n a t α and y p o s i t i o n s o f the d e l o c a l i z e d a l l y l i c a c t i v e c e n t e r s . In C NMR 13


5.

BYWATER

Anion

Pair

75

Structure

s p e c t r a o f t h e 1:1 a d d u c t o f t - b u t y l 1 i t h i u m and b u t a d i e n e f o r example t h e r e s o n a n c e o f t h e γ-carbon moves u p f i e I d and t h a t o f t h e ot-carbon moves down f i e l d on m o v i n g from a h y d r o c a r b o n t o an e t h e r s o l v e n t . I n THF f u r t h e r s h i f t s o f t h e same t y p e o c c u r on c h a n g i n g t h e c o u n t e r - i o n from L i t o K ( 1 1 ) . These chemical s h i f t changes c a n be i n t e r p r e t e d as b e i n g c a u s e d by r e d i s t r i b u t i o n o f c h a r g e from α t o γ p o s i t i o n s i n t h e i o n p a i r s ( T a b l e 1 ) . +

+

Table

I


Charge d i s t r i b u t i o n on b u t a d i e n e one u n i t model and % 1,2 structure i n polybutadiene. % of charge at γ L i (benzene) L i (THF) Na (THF) K (THF) Cs (THF)

3

diethyl ether

22 37 43 47 50

73 58 44

a) as p e r c e n t a g e o f t o t a l

charge at

% 1 ,2 dioxane

87 85 55 41

THF

96 91 83 74

οΗ-γ

I t w i l l be seen t h a t t h e r e i s an a l m o s t e q u a l d i s t r i b u t i o n o f t h e c h a r g e between α and γ p o s i t i o n s i n THF f o r t h e h e a v i e r a l k a l i metal c o u n t e r - i o n s . I f we suppose t h a t i n c r e a s e d c h a r g e p r o d u c e s an i n c r e a s e d r e a c t i v i t y a t a g i v e n p o s i t i o n , t h e n more v i n y l u n s a t u r a t i o n w i l l be p r o d u c e d i n THF t h a n i n h y d r o c a r b o n s o l v e n t s and t h e h i g h e s t v i n y l c o n t e n t w i t h h e a v i e r a l k a l i metal c o u n t e r i o n s . The o r d e r i n THF i s however r e v e r s e d , i . e . t h e h i g h e s t v i n y l s t r u c t u r e s a r e p r o d u c e d by l i t h i u m c a t a l y s i s ( 1 7 ) a l t h o u g h m i c r o s t r u c t u r e determinations i n t h i s solvent normally apply t o r e a c t i o n s w i t h an a p p r e c i a b l e f r e e a n i o n c o n t r i b u t i o n and h e n c e c a n n o t be s i m p l y i n t e r p r e t e d . In d i o x a n e ( 1 8 ) and d i e t h y l e t h e r ( 1 9 ) however t h i s c o m p l i c a t i o n s h o u l d be absent and c h a r g e d i s t r i b u t i o n s u r e l y must f o l l o w t h e same p a t t e r n w i t h c o u n t e r - i o n . A g a i n t h e h i g h e s t 1,2 c o n t e n t i n t h e p o l y m e r o c c u r s w i t h l i t h i u m as c o u n t e r - i o n . The l a r g e c o u n t e r - i o n s p r o d u c e a f a i r l y e v e n l y b a l a n c e d 1,4/1,2 r a t i o as m i g h t be e x p e c t e d from an even a/ y c h a r g e d i s t r i b u t i o n and a l a r g e c o u n t e r i o n t e n d i n g t o b l o c k b o t h positions. I t seems p l a u s i b l e t o s u p p o s e t h a t a h i g h l y s o l v a t e d l i t h i u m c a t i o n and m o d e r a t e l y s o l v a t e d s o d i u m c a t i o n i n e t h e r s o l v e n t s s i t u a t e d c l o s e r t o t h e α-position e f f e c t i v e l y b l o c k t h e terminal p o s i t i o n l e a v i n g p r e f e r e n t i a l attack at the γ p o s i t i o n . Butadiene i s a r e l a t i v e l y simple case being a symmetrical monomer. W i t h i s o p r e n e on t h e o t h e r hand a f u r t h e r c o m p l i c a t i o n a r i s e s b e c a u s e t h e monomer c a n add a t i t s 1 o r 4 p o s i t i o n g i v i n g ζ ÇH 4,1 a c t i v e c e n t e r ( ^H -CH-C-CH'" K ) a ) o r a 1 ,4 c e n t e r 3

+

2

2


76

ANIONIC POLYMERIZATION +

(-CH2-C-CH-CH X ) b ) . As t h e a d d i t i o n i s i n p r a c t i c e not r e v e r s i b l e t h e mode o f monomer a d d i t i o n d e c i d e s i m m e d i a t e l y i f t h e a c t i v e c e n t r e w i l l p r o d u c e a) 1,4 o r 3,4 b) 4,1 o r 1,2 u n i t s , t h e f i n a l i n t e r n a l c h o i c e i n each case o n l y being d e c i d e d at t h e next monomer a d d i t i o n . A l l e v i d e n c e on configurâtional p r e f e r e n c e so f a r o b t a i n e d has been on t y p e a) c e n t r e s n o r m a l l y formed a l m o s t e n t i r e l y by r e a c t i o n o f b u t y l l i t h i u m w i t h monomer i n h y d r o c a r b o n s o l v e n t s ( 2 0 ) . The p r e s e n c e o f b o t h 1,2 and 3,4 u n i t s i n p o l y m e r s formed i n p o l a r s o l v e n t s shows t h a t h e r e b o t h modes o f a d d i t i o n o c c u r ( T a b l e t l ) . Between 20 and 30% o f t h e v i n y l u n i t s h a v e a Downloaded by COLUMBIA UNIV on March 16, 2013 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch005

2

Table I I Total

v i n y l u n s a t u r a t i o n and p r o p o r t i o n o f 1,2 s t r u c t u r e i n polyisoprenes.

Counter-ion Li Na Κ Cs

total

diethyl vinyl

65% 83% 62% 47%

a) P e r c e n t a g e o f t o t a l

ether % l,2 20 26 32 33

a

total

dioxane vinyl % 1,2

93% 91% 62% 53%

18 13 19 23

vinyl

1,2 s t r u c t u r e i n d i e t h y l e t h e r and d i o x a n e . With the free anion the p r o p o r t i o n r i s e s t o ~40% a c c o r d i n g t o r e s u l t s o b t a i n e d i n d i m e t h o x y e t h a n e where i t a p p e a r s t o be r e s p o n s i b l e f o r most polymer f o r m a t i o n . I t s h o u l d be n o t e d t h a t t h e s e f i g u r e s c a n n o t be used t o c a l c u l a t e t h e p r o p o r t i o n o f t h e two t y p e s o f a c t i v e c e n t r e s p r e s e n t b e c a u s e a g a i n t h e i r r e a c t i v i t i e s w i t h monomer may be d i f f e r e n t . T h e i r r e a c t i v i t i e s a t α and γ p o s i t i o n s may be a l s o d i f f e r e n t l e a d i n g t o d i f f e r e n t 1,4 ( 4 , 1 ) t o v i n y l r a t i o s . S t r u c t u r e s o f t y p e b) form have n o t so f a r been p r e p a r e d although t h e r e i s some e v i d e n c e t h a t t h e y p r e f e r t o be i n t h e t r a n s form even i n THF i n c o n t r a s t t o t y p e a) c e n t r e s ( 1 9 ) . Isolation of models o f type b) i s r e q u i r e d t o c o n f i r m t h e i r c o n f i g u r a t i o n a l p r e f e r e n c e and t o measure t h e d i f f e r e n t r a t e s o f monomer a d d i t i o n o f t h e two b e f o r e a good u n d e r s t a n d i n g o f t h e mechanism o f i s o p r e n e p o l y m e r i z a t i o n c a n be o b t a i n e d . W h i l e f o r t h e d i e n e s we have seen much i n f o r m a t i o n c a n be o b t a i n e d from s i m p l e one u n i t a c t i v e p o l y m e r c h a i n m o d e l s , t h i s i s l e s s h e l p f u l i n t h e c a s e o f v i n y l monomers where m i c r o s t r u c t u r e depends on t h e r e l a t i o n s h i p between two o r more monomer u n i t s i n t h e c h a i n and o r i e n t a t i o n o f t h e i n c o m i n g monomer. L i t t l e work has been done i n t h i s f i e l d ( 1 8 ) . A mechanism h a s , h o w e v e r , been proposed t o e x p l a i n the changes i n m i c r o s t r u c t u r e observed i n p o l y α-methylstyrene formed by l i t h i u m c a t a l y s i s i n THF w h i c h does



5.

BYWATER

Anion

Pair

Structure

77

a t t e m p t t o e x p l a i n t h e s e changes i n terms o f i s o m e r i z a t i o n o f t h e t e r m i n a l u n i t w i t h r e s p e c t t o t h e p e n u l t i m a t e one ( 2 1 ) . At l o w monomer c o n c e n t r a t i o n and h i g h t e m p e r a t u r e , e q u i l i b r i u m between t e r m i n a l meso and r a c e m i c d i a d s w o u l d be m a i n t a i n e d and m i c r o s t r u c t u r e d e t e r m i n e d by t h e thermodynamic s t a b i l i t y o f t h e meso and r a c e m i c u n i t s . The r a c e m i c form i s s u p p o s e d t o be more s t a b l e by 700 c a l / m o l e , i t s c o n c e n t r a t i o n i n c r e a s i n g from 7 2 % a t 0° t o 87% a t -100° a t e q u i l i b r i u m . At h i g h e r monomer c o n c e n t r a t i o n s and l o w e r t e m p e r a t u r e s k i n e t i c f a c t o r s were c l a i m e d t o d o m i n a t e w i t h the meso form b e i n g l e s s e a s i l y formed w i t h a h i g h e r a c t i v a t i o n energy. Once a g a i n t h e i m p o r t a n c e o f b o t h k i n e t i c and t h e r m o dynamic e f f e c t s i s e m p h a s i z e d-,i l l u s t r a t i n g t h e w i d e s p r e a d i m p o r t a n c e o f b o t h o f them i n s t e r e o s p e c i f i c p o l y m e r i z a t i o n m o d e l s .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Stearns, R.S. and Forman, L.E. J . Polym. Sci. (1959) 41, 381. Glusker, D.L., Lysloff, I. and Stiles, E. J . Polym. Sci., (1961) 49, 315. Fowells, W., Schuerch, C., Bovey, F. A. and Hood, F.P. J . Am. Chem. Soc. (1967) 89, 1396. Gebert, W., Hinz, J. and Sinn, H. Makromol. Chem. (1971) 144, 97. Schué, F., Worsfold, D.J. and Bywater, S. J . Polym. S c i . (Pol. Lett. Ed.) (1969) 7, 821. Glaze, W.H. and Jones, P.C. Chem. Commun. (1969) 1434. Bywater, S., Lachance, P. and Worsfold, D.J. J. Phys. Chem. (1975) 72, 2148. Lochmann, L. and Lim, D. J. Organometal . Chem. (1973) 50, 9. Brownstein, S., Bywater, S., and Worsfold, D.J. Macromolecules (1973) 6, 715. Glaze, W.H., Hanicak,J.E.,Moore, M.L. and Chaudhuri, J. J. Organometal Chem. (1977) 44, 39. Bywater, S. and Worsfold, D.J. J. Organometal Chem. (1978) 159, 229. Schué, F., Worsfold, D.J. and Bywater, S. Macromolecules (1970) 3, 509. Worsfold, D.J. and Bywater, S. Macromolecules (1978) 11, 582. Garton, A. and Bywater, S. Macromolecules (1975) 8, 694. Garton, Α., Chaplin, R.P. and Bywater, S. Eur. Polym. J . (1976), 12, 697. Garton, A. and Bywater, S. Macromolecules (1975) 8, 697. Rembaum, Α., E l l s , F.R., Morrow, R.C. and Tobolsky, A.V. J. Polym. Sci. (1962) 61, 166. Salle, R. and Pham, Q.T. J. Polymer Sci. (Chem. Ed.) (1977) 15, 1799. Dyball, D.J., Worsfold, D.J. and Bywater, S. Macromolecules (1979) 12 819. Schué, F. and Bywater, S. Bull. Soc. Chem. Fr. (1970) 271. Wicke, R. and Elgert, K.F. Makromol. Chem. (1977) 178, 3085.

Received February 12, 1981.


Relationship of Anion Pair Structure to Stereospecificity of

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