12 N M R Characterization of Ion Pair Structure D . J. W O R S F O L D
Downloaded by UNIV OF MISSOURI COLUMBIA on March 16, 2013 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch012
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
Downloaded by UNIV OF MISSOURI COLUMBIA on March 16, 2013 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch012
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
Downloaded by UNIV OF MISSOURI COLUMBIA on March 16, 2013 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch012
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
