NMR Characterization of Ion Pair Structure - American Chemical Society

BB'X p a t t e r n , indicating that rotation about the C-C .... differ greatly from the conventionally written 1:4 addition product, although it is c...
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12 N M R Characterization of Ion Pair Structure D . J. W O R S F O L D

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National Research Council of Canada, Ottawa, Canada K 1 A

OR9

An account is given of the NMR methods which may be applied to study the structure of the ion pair at the active chain end in anionic polymerizations, and the results of some of these studies when applied to styrene and diene polymerizations. Studies by means of spectral patterns and coupling patterns, the displacement of the NMR lines from those in the parent hydrocarbon, carbon-carbon coupling constants, and coalescence temperatures are described. Some special e f f e c t s can be illustrated by the effect of selective deuteration. The picture that emerges is that although in most cases there is l i t t l e change in hybridization or delocalization of charge with change of counter ion, there is marked change in the r i g i d i t y of carbanion with different counter ions.

I o n i c p o l y m e r i z a t i o n s are remarkable i n the v a r i e t y o f p o l y m e r s t e r i c s t r u c t u r e s t h a t are p r o d u c e d b y v a r i a t i o n o f t h e s o l v e n t o r the c o u n t e r i o n . The l o n g l i v e d n a t u r e o f t h e a c t i v e c h a i n ends i n t h e a n i o n i c p o l y m e r i z a t i o n o f d i e n e and s t y r e n e t y p e monomers l e n d s i t s e l f t o s t u d i e s o f t h e i r s t r u c t u r e and p r o p e r t i e s w h i c h m i g h t have r e l e v a n c e t o the s t r u c t u r e o f t h e p o l y m e r p r o d u c e d when t h e s e c h a i n ends add f u r t h e r monomer. One o f t h e t o o l s t h a t may be used i n t h e c h a r a c t e r i z a t i o n o f t h e s e i o n p a i r s i s t h e NMR s p e c t r o m e t e r . However, i t s h o u l d a l w a y s be a p p r e c i a t e d t h a t t h e c o n d i t i o n s i n t h e NMR tube a r e f r e q u e n t l y f a r removed from t h o s e i n the a c t u a l p o l y m e r i z a t i o n . F u r t h e r m o r e NMR o b s e r v e s t h e e q u i l i b r i u m form on a l o n g t i m e s c a l e , and t h i s i s not n e c e s s a r i l y t h a t form p r e s e n t a t t h e moment o f p o l y m e r i z a t i o n . S e v e r a l NMR t e c h n i q u e s may be used i n t h e s e s t u d i e s i n c l u d i n g ( a ) s p e c t r a l p a t t e r n s (b) c h e m i c a l s h i f t s ( c ) c o a l e s c e n c e t e m p e r a t u r e (d) c o u p l i n g c o n s t a n t s ( e ) s p e c i a l e f f e c t s such as t h o s e c a u s e d b y d e u t e r a t i o n . Here t h e s e t e c h n i q u e s a r e a p p l i e d t o t h e 0097-6156/81/0166-0177$05.00/0 Published 1981 American Chemical Society

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

178

ANIONIC POLYMERIZATION

al 1 y l c a r b a n i o n , t h e p a r e n t o f t h e d i e n e p o l y m e r i z i n g c a r b a n i o n , and t h e r e s u l t s compared w i t h t h o s e f r o m m o d e l s o f t h e d i e n e and s t y r e n e p o l y m e r i z i n g systems. Al 1 y l a l k a l i m e t a l

Compounds

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1 3

( a ) The C s p e c t r a l p a t t e r n o f a l l t h e compounds, f r o m L i t o C s , i n THF s o l u t i o n c o n s i s t s o f two l i n e s , T a b l e lAl.>—>2? T h i s i n d i c a t e s t h a t t h e two end c a r b o n s a r e e q u i v a l e n t , and t h e compound i s e i t h e r a s y m m e t r i c a l i o n p a i r o r a r a p i d l y e q u i l i b r a t i n g m i x t u r e o f two i d e n t i c a l a s y m m e t r i c s p e c i e s . Table I 3

C Shifts of Allylalkalimetal

M

Li

6 δ

J

β

Na

Compounds i n THF a t 0°

Κ

Rb

Cs

50.8 146.0 146

48.7 145.1 148

52.5 143.8 148

54.5 143.9 149

58.7 143.8 145

133

132

132

131

132

C -H

Na+222 51.8 147.9

a

J

C -H 3

The *H s p e c t r a l p a t t e r n , T a b l e ΙΙ'Σ'Ε) i s more c o m p l e x and each o f t h e s e compounds h a s two f o r m s . There i s a h i g h e r t e m p e r a t u r e form w i t h an A X p a t t e r n i n d i c a t i n g e q u i v a l e n c e o f a l l 4 end p r o t o n s , and r a p i d r o t a t i o n about t h e C-C b o n d s . At l o w e r t e m p e r a t u r e t h e s p e c t r a l p a t t e r n becomes an AA BB'X p a t t e r n , i n d i c a t i n g t h a t r o t a t i o n about t h e C-C bonds i s s l o w on t h e NMR t i m e s c a l e , b u t t h a t t h e two end g r o u p s a r e s t i l l i d e n t i c a l . This l a t t e r form i s e x p l i c a b l e i n terms o f a p l a n a r m o l e c u l e w i t h e x t e n s i v e charge d e l o c a l i z a t i o n . 1

Table I I X

H Shifts of Allylalkalimetal Li12) w i t h t h e l a r g e s t f a c t o r b e i n g t h e bond o r d e r . The v a l u e s found h e r e a r e f a i r l y t y p i c a l o f d e l o c a l i z e d systems such as a r o m a t i c s y s t e m s . But a l s o t h e v a l u e s a r e c l o s e t o t h e mean o f a p a i r o f r a p i d l y e q u i l i b r a t i n g s i n g l e and d o u b l e b o n d s . α

Table V 13

C

Coupling Constants M J c

in Al1ylalkalimetal Li

Κ

55.9

C

59.8

Compounds a t 0°

Cs 61.1

( e ) A t t e m p t s have been made t o d i f f e r e n t i a t e b e t w e e n r a p i d l y e x c h a n g i n g s y m m e t r i c a l c a r b e n i u m i o n s and d e l o c a l i z e d i o n s by m e a s u r i n g t h e e f f e c t d e u t e r a t i o n o f one end w i l l h a v e on t h e * C s h i f t o f t h e o t h e r p r o t o n a t e d e n d . 2J.>JL?.'J-L In a delocalized s y s t e m such e f f e c t w o u l d be m i n i m a l , b u t i n an e x c h a n g i n g s y s t e m d e p e n d i n g on t h e K / K r a t i o q u i t e l a r g e e f f e c t s c o u l d be expected. The r e s u l t s found h e r e , T a b l e V I , a r e more c o n s i s t e n t w i t h t h o s e e x p e c t e d from a d e l o c a l i z e d s y s t e m , u n l e s s t h e K / Κ β r a t i o i s e x c e p t i o n a l l y l o w . U n f o r t u n a t e l y Kg/K]) not w e l l known f o r c a r b a n i o n s . 3

r

a

t

l

o

s

a

Table VI Effect of Deuteration i n A l l y l a l k a l i m e t a l M c

L i

α

Na

Compounds a t 0°C

Κ

ppm

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

r

e

12.

WORSFOLD

NMR

Model d i e n e p o l y m e r i z a t i o n

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181

Characterization

systems

NMR s p e c t r a have been t a k e n on d i e n e p o l y m e r i c systems by M o r t o n e t al AHï) > and o f m o d e l s formed by t h e a d d i t i o n o f t - b u t y l 1 i t h i u m t o d i e n e s by G l a z e e t a] Λ12) and B y w a t e r e t a l . (Jjj.) Compounds w i t h o t h e r c o u n t e r i o n s have been made from these. ( a ) The *H s p e c t r a l p a t t e r n s show t h a t t h e a d d i t i o n o f monomer t o t h e a l k a l i l i t h i u m compound i s made a t t h e 1 p o s i t i o n (4 f o r isoprene). The p o s i t i o n s o f t h e o t h e r NMR a b s o r p t i o n s , h o w e v e r , show t h a t t h e s e compounds d i f f e r g r e a t l y from t h e c o n v e n t i o n a l l y w r i t t e n 1:4 a d d i t i o n p r o d u c t , a l t h o u g h i t i s c o n v e n i e n t t o t a l k o f them i n t h a t way. Under some c o n d i t i o n s two forms can be i d e n t i f i e d i n t h e NMR s p e c t r a t h a t c a n be a s s o c i a t e d w i t h c i s and t r a n s i s o m e r i z a t i o n about the c e n t r a l bond o f t h e f o u r c a r b o n s k e l e t o n , T a b l e V I I . The r o t a t i o n about t h i s bond a p p e a r s a l w a y s s l o w on t h e NMR t i m e s c a l e ( e x c e p t f o r t h e magnesium c o m p o u n d s ) , a l t h o u g h i t can be shown t o be f a r more r a p i d t h a n t h a t a s s o c i a t e d w i t h a normal d o u b l e bond. T h e r e i s a l s o e v i d e n c e f o r r e s t r i c t e d r o t a t i o n about t h e end C-C bond. Table V I I % Trans

Structure

M Li Li Li Na Κ Rb Cs Residue

of 5,5-Dimethylhexen-2-yl Sol vent

DEE THF THF THF THF THF

M %

in Solution

Trans

77 75 35 23 c.a.lO