Intermolecular and Intramolecular Reactions of Substituted

PN and Parent Exo:PX) have unsubstituted norbornenyl rings and differ from each other ... phenyl substituent at a vinyl position, while CBX represents...
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5 Intermolecular and Intramolecular Reactions of Substituted Norbornenyl Imides CHAIMN.SUKENIK, VINAY MALHOTRA, and UDAY VARDE Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106 The present study reports the synthesis, characteriza­ tion and thermal reactions of phenyl and carbomethoxy substituted norbornenyl imides. These substrates were designed to model the reactive end-caps of the PMR-15 resin and allow an assessment of the effect that conju­ gating substituents would have on the high temperature cure of such systems. The effect of these substituents on both monomer isomerization and polymerization is reported and a possible use of the phenyl substituent as a probe of polymer structure is suggested. The t h e r m a l p o l y m e r i z a t i o n o f r e a c t i v e p o l y i m i d e o l i g o m e r s i s a c r i ­ t i c a l p a r t o f a number o f c u r r e n t l y i m p o r t a n t p o l y m e r s . Both t h e s y s t e m i n w h i c h we a r e i n t e r e s t e d , P M R - 1 5 , a n d o t h e r s l i k e i t ( L A R C 13, HR-600), a r e u s e f u l h i g h t e m p e r a t u r e r e s i n s . They a l s o s h a r e t h e f e a t u r e t h a t , w h i l e t h eb a s i c s t r u c t u r e and chemistry o f t h e i r imide p o r t i o n s i s w e l l d e f i n e d , t h e mode o f r e a c t i o n a n d u l t i m a t e l y t h e s t r u c t u r e s that r e s u l t from t h e i r t h e r m a l l y a c t i v a t e d end-groups i s not c l e a r . Since an understanding o f t h i s thermal cure would bean i m p o r t a n t step towards t h e improvement o f b o t h t h e c u r e p r o c e s s and t h e p r o p e r t i e s o f s u c h s y s t e m s , we h a v e a p p r o a c h e d o u r s t u d y o f P M R 15 w i t h a f o c u s o n l y o n t h i s h i g h e r t e m p e r a t u r e t h e r m a l c u r i n g p r o ­ cess. To t h i s e n d , we h a v e u s e d s m a l l m o l e c u l e m o d e l c o m p o u n d s w i t h pre-formed imide m o i e t i e s and have c o n c e n t r a t e d on t h e c h e m i s t r y o f the norbornenyl end-cap ( 1 ) . T h e s p e c i f i c c o m p o u n d s t h a t we u s e d a r e shown b e l o w . They c a n be grouped i n t h e f o l l o w i n g way. The s i m p l e s t models ( P a r e n t Endo: PN a n d P a r e n t E x o : P X ) h a v e u n s u b s t i t u t e d n o r b o r n e n y l r i n g s a n d d i f f e r from each other only i n t h e stereochemistry o f t h e i r r i n g f u s i o n . T h e s e compounds h a v e b e e n e l a b o r a t e d b y t h e i n c o r p o r a t i o n o f e i t h e r a p h e n y l (φ) o r a c a r b o m e t h o x y ( C ) g r o u p i n t h e b r i d g e h e a d ( B ) o r v i n y l (V) p o s i t i o n s . T h u s , t h e n o t a t i o n φν*Ν r e p r e s e n t s P N w i t h a p h e n y l s u b s t i t u e n t a t a v i n y l p o s i t i o n , w h i l e CBX r e p r e s e n t s P X w i t h a carbomethoxy s u b s t i t u e n t a t t h ebridgehead p o s i t i o n . The i s o m e r i z a t i o n a n d p o l y m e r i z a t i o n c h e m i s t r y o f t h e s e n i n e compounds ( P N , P X , φΒΝ, φ ν Ν , φν*Χ, C B N , C B X , C V N , C V X ) a r e t h e m a i n c o n c e r n s o f o u r w o r k .

0097-6156/85/0282-0053$06.00/0 © 1985 American Chemical Society

54

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P N ( X = H , Y = H) C B N ( X = H , Y = COOMe) C V N ( X = COOMe, Y - H)

P X ( X = H , Y = H) C B X ( X = H , Y = COOMe) C V X ( X = COOMe, Y = H)

φΒΝ(Χ - Η , Υ - φ ) φΥΝ(Χ = φ, Υ = Η)

φ ν Χ ( Χ = φ, Υ = Η)

Synthesis

and C h a r a c t e r i z a t i o n

The f i r s t p h a s e o f o u r e f f o r t s was t h e unambiguous s y n t h e s i s o f e a c h model s u b s t r a t e . PN and PX were a l r e a d y w e l l c h a r a c t e r i z e d m a t e r i a l s (1). W h i l e d i r e c t s y n t h e s i s o f t h e p h e n y l a n d c a r b o m e t h o x y compounds f r o m PN a n d / o r P X was a t t e m p t e d , t h i s a p p r o a c h was u n s u c c e s s f u l d u e t o t h e s l u g g i s h r e a c t i v i t y o f t h e n o r b o r n e n y l d o u b l e bonds i n t h e s e m o l e c u l e s ( 2 ) . A s u c c e s s f u l a p p r o a c h t o CBN a n d φΒΝ b a s e d o n N p h e n y l m a l e i m i d e (NPMI) t r a p p i n g o f t h e r e s p e c t i v e t h e r m o d y n a m i c a l l y f a v o r e d 1 - s u b s t i t u t e d c y c l o p e n t a d i e n e s i s shown i n E q u a t i o n 1 . Simi­ l a r l y , k i n e t i c t r a p p i n g of 2-phenyl cyclopentadiene, from t h e i n s i t u dehydration of 3-hydroxy, 3-phenyl cyclopentene, gives a clean y i e l d o f φνΝ ( E q u a t i o n 2 ) . T h e r e m a i n i n g p h e n y l i s o m e r ( φ ν Χ ) a n d t h e t h r e e o t h e r c a r b o m e t h o x y i s o m e r s ( C B X , C V N , CVX) w e r e a l l o b t a i n e d b y t h e thermal isomerization chemistry described i n the next section of t h i s paper. They were each i s o l a t e d i n p u r e f o r m by l i q u i d chromatography We w e r e u n a b l e t o o b t a i n a n y φΒΧ o r a n y o f t h e 7 - s u b s t i t u t e d i s o m e r s by any means.

CBN;X=COOMe

5.

SUKENIK ET A L .

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A l l compounds s t u d i e d w e r e s h a r p m e l t i n g s o l i d s s h o w i n g i m i d e c a r b o n y l s b e t w e e n 1702 a n d 1712 c m " . The b r i d g e h e a d s u b s t i t u t e d compounds w e r e d i s t i n g u i s h e d f r o m t h e i r v i n y l s u b s t i t u t e d i s o m e r s b y a shorter X i n t h e i r UV s p e c t r a a n d e a c h i s o m e r h a d d i s t i n c t i v e H-L a n d C - ^ NMR s p e c t r a . These s p e c t r a l d a t a ( f u l l y a s s i g n e d ) as w e l l as a d e l i n e a t i o n o f p a t t e r n s f o r d i s t i n g u i s h i n g s e t s o f isomers a r e reported i n d e t a i l elsewhere 03). For present purposes, i t w i l l s u f ­ f i c e to note that a l l isomers f o r a s i n g l e substituent are r e a d i l y s e p a r a b l e ( f o r b o t h p r e p a r a t i v e and a n a l y t i c a l p u r p o s e s ) and t h a t assignments of b o t h s u b s t i t u e n t r e g i o c h e m i s t r y and r i n g f u s i o n s t e r e o c h e m i s t r y h a v e b e e n made w i t h a h i g h d e g r e e o f c e r t a i n t y . 1

m a x

Monomer

Isomerization

One o f t h e c o m p l e x i t i e s i n t h e d i r e c t s t u d y o f t h e PMR c u r e i s t h e s u p e r p o s i t i o n o f monomer i s o m e r i z a t i o n o n t h e p o l y m e r i z a t i o n c h e m ­ i s t r y of i n t e r e s t . To e n s u r e o u r a b i l i t y t o d i s s e c t t h e s e t w o k i n d s o f p r o c e s s e s , we f i r s t s t u d i e d t h e t h e r m a l b e h a v i o r o f e a c h o f o u r m o d e l compounds i n t h e a b s e n c e o f a n y p o l y m e r f o r m i n g p r o c e s s : i n dilute solution. A l l s o l v e n t s f o r these s o l u t i o n t h e r m o l y s i s r e a c t i o n s were f r e s h l y d i s t i l l e d and a l l r e a c t i o n s were done i n s e a l e d g l a s s t u b e s heated i n a thermostatted oven. O v e r a w i d e r a n g e o f s o l v e n t s (DMF, n a p h t h a l e n e , diphenylmethane, benzene, t o l u e n e , and d e c a l i n ) t h e r e was n o s i g n i f i c a n t v a r i a t i o n i n e i t h e r i s o m e r i z a t i o n r a t e o r p r o d u c t composition. R e a c t i o n s were done a t 1 2 5 C , 155°C and 1 9 5 C and t h e o n l y l i m i t a t i o n w a s t h a t DMF c o u l d n o t b e u s e d a s t h e s o l v e n t i n r e a c t i o n s a t 195°C; i t l e d t o s u b s t a n t i a l s u b s t r a t e d e s t r u c t i o n (polymer forming r e a c t i o n s o f s u b s t r a t e w i t h DMF?). Isomer c o m p o s i ­ t i o n s w e r e a s c e r t a i n e d b o t h b y HPLC a n d b y NMR. The r e s u l t s o f t h e s e i s o m e r i z a t i o n s t u d i e s c a n be summarized a s follows. A t 125°C and 155°C b o t h p a r e n t i s o m e r s (PN and PX) and t h e v i n y l s u b s t i t u t e d c o m p o u n d s ( C V N , C V X , 97%) b y t h e v i n y l s u b s t i t u t e d isomers f o r b o t h t h e p h e n y l and carbomêthoxy s e r i e s . In a l l cases ( i n c l u d i n g t h e p a r e n t s y s t e m ) , a l t h o u g h t h e endo c o m p o u n d s w e r e p r o duced e x c l u s i v e l y by our D i e l s A l d e r s y n t h e t i c r o u t e s , t h e exo isomer d o m i n a t e s t h e f i n a l e q u i l i b r i u m m i x t u r e ( 5 6 % P X / 4 4 % P N ; 62% φνχ/37% φνΝ/1% φΒΝ; 7 1 % CVX/27% CVN/2% CBN/1% C B X ) . A l l o f t h e i s o m e r i z a t i o n d a t a shown above i s c o n s i s t e n t w i t h t h e normal e l e c t r o c y c l i c r e a c t i o n chemistry expected f o r such substrates (4). That s u c h f u s e d n o r b o r n e n y l systems undergo exo/endo i s o m e r i z a ­ t i o n v i a D i e l s A l d e r / r e t r o D i e l s A l d e r r e a c t i o n s has been e x p l i c i t l y p r o v e n f o r s i m p l e c y c l o p e n t a d i e n e - m a l e i c a n h y d r i d e a d d u c t s (5) and e

e

e

e

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should a p p l y here as w e l l . Moreover, our monosubstituted substrates p r o v i d e a d d i t i o n a l c o n f i r m a t i o n t h a t t h i s pathway i s o p e r a t i v e by t h e i r l o w t e m p e r a t u r e i s o m e r i z a t i o n o f BN t o V N . Their selective f o r m a t i o n of t h e k i n e t i c a l l y p r e f e r r e d (endo) D i e l s A l d e r p r o d u c t r a t h e r than VX, t h e t h e r m o d y n a m i c a l l y f a v o r e d product i s i n c o n s i s t e n t w i t h o t h e r p o s s i b l e r a d i c a l mechanisms (6). T h e o t h e r n e c e s s a r y r e a c t i o n f o r a BN t o VN i s o m e r i z a t i o n i s a w e l l p r e c e d e n t e d 1,5 H s h i f t t o c o n v e r t t h e l i n e a r l y c o n j u g a t e d s u b ­ s t i t u t e d c y c l o p e n t a d i e n e (LCC) i n t o t h e c r o s s c o n j u g a t e d c y c l o p e n t a diene (CCC). T h e r e l a t i v e l a b i l i t y o f BN r e l a t i v e t o V N i s t h u s a r e f l e c t i o n of the s t a b i l i z i n g conjugation of the s u b s t i t u e n t i n the v i n y l i s o m e r s a n d t h e f a c t t h a t t h e f o r m a t i o n o f LCC f r o m BN i s m o r e f a v o r a b l e t h a n t h e f o r m a t i o n o f CCC f r o m t h e r e t r o D i e l s A l d e r o f V N . The r e l a t i v e e n e r g e t i c s f o r a l l o f t h e s e p r o c e s s e s i s r e p r e s e n t e d i n a c o m b i n e d r e a c t i o n p r o f i l e d i a g r a m s h o w n i n F i g u r e 1. The c o n s t r u c t i o n o f t h i s o v e r a l l r e a c t i o n p r o f i l e i s b a s e d on the following considerations. T h e l o w e r e n e r g y (5-10 k c a l / m o l e ) o f VN a n d V X v e r s u s BN i s d u e t o t h e c o n j u g a t i o n o f t h e i r s u b s t i t u e n t w i t h the norbornenyl double bond. L i n e a r l y conjugated cyclopentadiene d e r i v a t i v e s a r e l o w e r i n e n e r g y (1-2 k c a l / m o l e ) t h a n t h e i r c r o s s c o n ­ jugated counterparts. VX i s s t e r i c a l l y l e s s c r o w d e d t h a n VN and i s thus of lower energy (M.kcal/mole). The o b s e r v e d k i n e t i c p r e f e r e n c e f o r e n d o D i e l s A l d e r a d d u c t s r e q u i r e s t h a t TS4 b e h i g h e r i n e n e r g y (2-4 k c a l / m o l e ) t h a n T S 3 . S i m i l a r l y , since the s e l e c t i v e generation o f CCC f o r χ = φ ( E q u a t i o n 2 a b o v e ) a l l o w e d t h e s e l e c t i v e s y n t h e s i s o f φνΝ, TS3 ( D i e l s A l d e r t o f o r m VN) m u s t b e somewhat l o w e r t h a n T S (1,5 Η s h i f t t o f o r m L C C ) . And l a s t l y , t h e s e l e c t i v e D i e l s A l d e r s y n t h e s i s o f BN f r o m LCC a n d NPMI ( E q u a t i o n 1 a b o v e ) r e q u i r e s t h a t T S i be l o w e r i n energy t h a n TS£. 2

A consequence of our o b s e r v a t i o n t h a t the bridgehead isomers are e a s i l y i s o m e r i z e d a n d t h e r m o d y n a m i c a l l y d i s f a v o r e d i s t h a t t h e y may b e l a r g e l y i r r e l e v a n t t o t h e a c t u a l PMR c u r e . Moreover, the approach o f e a c h s e t o f monomers t o a b o n a f i d e e q u i l i b r i u m u n d e r c o n d i t i o n s t h a t are m i l d e r than t y p i c a l t h e r m a l c u r i n g c o n d i t i o n s (see below) suggests that regardless of the isomeric composition present i n the PMR r e s i n i n i t s l o w t e m p e r a t u r e i m i d i z a t i o n s t a g e , t h e f i n a l h i g h temperature cure probably occurs w i t h a mixture that r e f l e c t s sub­ s t a n t i a l , i f not complete, isomer e q u i l i b r a t i o n . L a s t l y , the o b s e r ­ v a t i o n t h a t b o t h t h e p h e n y l a n d c a r b o m e t h o x y s u b s t i t u t e d monomers u n d e r g o e x o / e n d o e q u i l i b r a t i o n a t a b o u t t h e same r a t e a s t h e P N / P X s y s t e m , i n d i c a t e s t h a t t h e s e v i n y l s u b s t i t u e n t s h a v e l i t t l e o r no e f f e c t on t h e ease o f t h e r e t r o D i e l s A l d e r p r o c e s s . T h e i r e f f e c t on p o l y m e r i z a t i o n r a t e s w i l l be d e t a i l e d b e l o w , but i t i s i m p o r t a n t t o n o t e i n a d v a n c e t h a t a n y e f f e c t t h e s e s u b s t i t u e n t s do h a v e o n p o l y ­ m e r i z a t i o n i s not a r e f l e c t i o n o f a change i n t h e p r o p e n s i t y f o r retro Diels Alder chemistry. Despite the importance of D i e l s A l d e r r e a c t i o n s i n our i s o m e r i z a t i o n p r o c e s s e s , i t s r e l e v a n c e t o any p o l y ­ mer f o r m i n g r e a c t i o n s m u s t s t i l l b e c r i t i c a l l y e v a l u a t e d . Polymer

Formation

The p o l y m e r i z a t i o n o f o u r m o d e l s u b s t r a t e s was s t u d i e d by h e a t i n g n e a t s o l i d s a m p l e s o f e a c h monomer i n s e a l e d g l a s s t u b e s . No p r e t r e a t m e n t o f t h e g l a s s was n e e d e d t o a c h i e v e r e p r o d u c i b l e r e s u l t s a n d t h e s e a l e d

Substituted Norbornenyl Imides

SUKENIK ET A L .

Figure

1.

Combined r e a c t i o n p r o f i l e d i a g r a m

isomerization.

for

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t u b e s p r e v e n t e d any l o s s of v o l a t i l e s . A l l cured samples (heated t o 1 9 5 ° , 2 5 0 ° , o r 3 3 0 ° C ) w e r e c o m p l e t e l y s o l u b l e i n THF a n d i n c h l o r o ­ form. T h i s g r e a t l y f a c i l i t a t e d a n a l y s i s by b o t h s i z e e x c l u s i o n c h r o m a t o g r a p h y ( S E C ) a n d b y NMR. O u r f i r s t i n t e r e s t was t o a s s e s s t h e p o l y m e r f o r m i n g a b i l i t y o f e a c h o f o u r monomers. T h i s was d o n e b y a n a l y z i n g f u l l y c u r e d ( 2 5 0 C f o r 24 h r s o r 3 3 0 C f o r 2 h r s ) s a m p l e s o f e a c h m o n o m e r . In a l l cases no r e s i d u a l m o n o m e r i c s u b s t r a t e c o u l d b e d e t e c t e d a n d SEC r e s u l t s showed t h a t s u b s t a n t i a l o l i g o m e r i z a t i o n h a d o c c u r r e d . For a l l mono­ m e r s t h e b u l k o f t h e c u r e d m a t e r i a l was i n t h e f o r m o f o l i g o m e r s w i t h η = 5-8 a n d t h e l o n g e s t c h a i n s w e r e η = 2 4 + 3 . T h e SEC p r o ­ f i l e s f o r a l l t h r e e k i n d s o f p o l y m e r ( u n s u b s t i t u t e d , p h e n y l and c a r ­ bomêthoxy) were s i m i l a r enough t o j u s t i f y a c o m p a r i s o n o f t h e polymer f o r m i n g p r o c e s s f o r e a c h k i n d o f monomer. This comparison i s o u t l i n e d below. A more d e t a i l e d a n a l y s i s o f b o t h o l i g o m e r d i s t r i b u t i o n and s t r u c t u r e i s p r e s e n t l y underway and w i l l be r e p o r t e d i n f u l l elsewhere (7). e

e

M o n i t o r i n g t h e p o l y m e r i z a t i o n o f each s u b s t r a t e p r o v i d e d an i n f o r m a t i v e p i c t u r e of t h e e f f e c t o f b o t h s u b s t i t u e n t s and o f isomer d i s t r i b u t i o n on t h e c u r i n g p r o c e s s . We f i r s t a d d r e s s e d t h e q u e s t i o n o f t h e r e l a t i v e r a t e s o f s u b s t r a t e i s o m e r i z a t i o n and p o l y m e r i z a t i o n . We f o u n d t h a t , f o r t h e p a r e n t monomer ( P N a n d P X ) , t h e r a t e o f i s o m e r i z a t i o n g r e a t l y exceeds the r a t e of p o l y m e r i z a t i o n . Under c o n d i t i o n s w h e r e PN a n d P X a r e f u l l y e q u i l i b r a t e d ( 1 9 5 ° C / 1 5 h r s o r 2 5 0 / l h r ) t h e r e i s s t i l l l e s s t h a n 20% p o l y m e r f o r m a t i o n i n t h e n e a t sample. We c o n c l u d e t h a t f o r P N o r P X t h e c o m p o s i t i o n o f t h e m i x t u r e undergoing p o l y m e r i z a t i o n i s e s s e n t i a l l y independent of the s t a r t i n g isomer. T h e o b s e r v a t i o n t h a t f u l l y c u r e d s a m p l e s o f e i t h e r PN o r P X show i d e n t i c a l H a n d C ^ NMR s p e c t r a a n d i n d i s t i n g u i s h a b l e SEC analyses, is consistent with this contention. e

1

1

W h i l e e x o / e n d o i s o m e r i z a t i o n o f t h e s u b s t i t u t e d monomers h a d b e e n f o u n d t o be c o m p a r a b l e t o t h a t o f t h e p a r e n t monomer, p o l y m e r i ' z a t i o n o f t h e s u b s t i t u t e d m o n o m e r s was m u c h f a s t e r . Under t h e c o n d i t i o n s i n d i c a t e d a b o v e f o r 80% p o l y m e r formation. T h i s g r e a t l y c o m p l i c a t e s any a t t e m p t t o d i s s e c t i s o m e r i z a t i o n from p o l y m e r i z a t i o n i n these neat samples. N e v e r t h e l e s s , we were a b l e t o e s t a b l i s h t h a t , i n t h e lower temperature (195 C) p o l y m e r i z a t i o n , t h e i s o m e r i c m i x t u r e u n d e r g o i n g p o l y m e r i z a t i o n was t h e same ( f o r t h e c a r b o m ê t h o x y s y s t e m ) w h e t h e r we s t a r t e d f r o m CBN o r f r o m CVN ( 1 9 5 ° , 1 h r : 7 1 % V N , 2 3 % V X , 6% BN + B X ) . A similar result was o b t a i n e d b y c o m p a r i n g t h e p o l y m e r i z a t i o n o f 250°C), b o t h p o l y m e r i z a t i o n a n d i s o m e r i z a t i o n w e r e p r o c e e d i n g so r a p i d l y t h a t p r e ­ c i s e a n a l y s e s o f i s o m e r r a t i o s became d i f f i c u l t . E q u i l i b r a t i o n and p o l y m e r i z a t i o n were o c c u r r i n g a t c o m p e t i t i v e r a t e s . We do h a v e some t e n t a t i v e e v i d e n c e t h a t i n t h e h i g h e r t e m p e r a t u r e c u r e s t h e r e may b e some d i f f e r e n c e s i n t h e r a t e a t w h i c h p o l y m e r f o r m s d e p e n d i n g o n t h e s t a r t i n g isomer. This data i s currently being reinvestigated. Even i f s u c h a n e f f e c t i s v e r i f i e d , i t i s s m a l l a n d i n no way a f f e c t s o u r major c o n c l u s i o n t h a t s u b s t i t u t i o n g r e a t l y enhances t h e r a t e of p o l y ­ m e r i z a t i o n of norbornenyl imides. Moreover, s i n c e even samples of s u b s t i t u t e d i s o m e r i c i m i d e s c u r e d a t 3 3 0 C s h o w e d s i m i l a r SEC p r o ­ f i l e s , t h e p r o b a b i l i t y of m a j o r d i f f e r e n c e s as a f u n c t i o n of i s o m e r i c composition is small. e

5.

SUKENIK ET AL.

Substituted Norbornenyl Imides

59

I n terms o f an i n t e r p r e t a t i o n of t h e enhancement o f p o l y m e r i z a ­ t i o n r a t e p r o v i d e d b y o u r v i n y l s u b s t i t u e n t s , we c a n o n l y s p e c u l a t e . I t i s c l e a r t h a t any k i n d o f a d d i t i o n p o l y m e r i z a t i o n o f t h e n o r b o r ­ n e n y l d o u b l e bond w i l l b e n e f i t f r o m t h e e l e c t r o n i c s t a b i l i z a t i o n p r o ­ v i d e d by a c o n j u g a t i n g s u b s t i t u e n t . A simple r a d i c a l addition process s u c h a s i s known f o r b o t h s t y r e n e a n d a c r y l a t e monomers may b e a reasonable analogy to our system. Whether t h i s e f f e c t a l o n e i s enough to account f o r our observations i s not c l e a r . A possible additional e f f e c t , a t l e a s t i n t h e c a s e o f t h e p h e n y l s u b s t i t u t e d monomers, i s s u g g e s t e d b e l o w as p a r t of our work on polymer s t r u c t u r e . Polymer

Structure

H a v i n g e s t a b l i s h e d t h e e f f e c t o f s u b s t i t u t i o n on t h e r a t e s o f b o t h monomer i s o m e r i z a t i o n a n d p o l y m e r i z a t i o n , we a d d r e s s e d t h e q u e s t i o n of polymer s t r u c t u r e . S p e c i f i c a l l y , are norbornenyl imide u n i t s i n ­ corporated i n t o the f u l l y cured polymer w i t h t h e i r n o r b o r n y l r i n g s intact? I f so, does t h e polymer a l s o r e f l e c t t h e e q u i l i b r i u m r a t i o o f exo and endo r i n g f u s e d monomers? F o r o u r p a r e n t m o n o m e r s , PN a n d PX, t h i s q u e s t i o n has been u n a n s w e r a b l e . We h a v e n o t f o u n d a n y d i r e c t p r o b e t h a t a l l o w s an unambiguous assessment o f s p e c i f i c s u b ­ structures w i t h i n the cured polymer. We d o , h o w e v e r , h a v e some e v i ­ d e n c e b e a r i n g on t h i s q u e s t i o n f o r t h e p h e n y l s u b s t i t u t e d monomer. T h i s e v i d e n c e i s a t t r i b u t a b l e i n p a r t t o our d i s c o v e r y o f an u n e x ­ p e c t e d s i d e - r e a c t i o n i n t h e c u r e o f t h e p h e n y l s u b s t i t u t e d monomer, a n d i n p a r t t o t h e p r e s e n c e o f a u n i q u e NMR d i a g n o s t i c f o r p h e n y l s u b s t i t u t e d , endo n o r b o r n y l N - p h e n y l i m i d e s . Both of these r e s u l t s are d e t a i l e d below. In the course of our a n a l y s i s of cured samples of phenyl s u b s t i ­ t u t e d m o n o m e r s , we f o u n d t h a t many o f t h e s e s a m p l e s y i e l d e d a s much as 15-20% o f a monomeric m a t e r i a l i d e n t i f i e d as t h e h y d r o g e n a t e d a n a l o g o f φνχ ( H Y V X ) . Even more i n t e r e s t i n g l y , none o f t h e c o r r e s ­ p o n d i n g HYVN was f o u n d i n a n y s a m p l e . W h i l e we do n o t y e t f u l l y u n d e r s t a n d t h i s u n u s u a l p a i r o f o b s e r v a t i o n s , a number o f p o i n t s a r e clear. F i r s t o f a l l , t h e f o r m a t i o n o f HYVX r e q u i r e s a d o n o r o f hydrogen. We h a v e v e r i f i e d t h a t t h i s i s n o t a r i s i n g f r o m a d v e n t i ­ t i o u s contamination of our r e a c t i o n v e s s e l s . I t must t h e r e f o r e be t r u e t h a t some o f o u r p h e n y l monomer m o l e c u l e s a r e a c t i n g a s t h e h y ­ d r o g e n d o n o r s and a r e t h e m s e l v e s b e i n g o x i d i z e d . A p o s s i b l e avenue by w h i c h t h i s c o u l d o c c u r i s by i n c r e a s i n g u n s a t u r a t i o n and p o s s i b l e a r o m a t i z a t i o n o f some f r a c t i o n o f o u r n o r b o r n y l r i n g s . This p o s s i ­ b i l i t y i s d e p i c t e d i n t h e scheme b e l o w . It provides not only a s o u r c e o f h y d r o g e n f o r t h e p r o d u c t i o n o f HYVX, b u t a l s o s u g g e s t s a new way i n w h i c h t h e p h e n y l s u b s t i t u e n t c o u l d e n h a n c e t h e g e n e r a t i o n o f r a d i c a l i n i t i a t o r s and t h u s enhance t h e p o l y m e r i z a t i o n p r o c e s s as w e l l . T h e o b s e r v e d a b s e n c e o f a n y HYVN d e s p i t e t h e f o r m a t i o n o f HYVX i s i n t e r p r é t a b l e i n e i t h e r o f two w a y s . I t i s p o s s i b l e t h e (J)VN i s p o l y m e r i z i n g w h i l e t h e φνχ i s h y d r o g e n a t i n g a n d t h u s n o h y d r o g e n a t e d