75-MHz 13C NMR Studies on Polystyrene and Epimerized Isotactic

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75-MHz C NMR Studies on Polystyrene and Epimerized Isotactic Polystyrenes 1

Downloaded by CORNELL UNIV on August 3, 2016 | http://pubs.acs.org Publication Date: March 28, 1984 | doi: 10.1021/bk-1984-0247.ch013

H. JAMES HARWOOD, TENG-KO CHEN, and FU-TYAN LIN

Institute of Polymer Science, The University of Akron, Akron, OH 44325 Isotactic polystyrene was epimerized to various extents by reaction with KOtBu i n hexamethylphosphoramide solution at 100°. The C-NMR spectra of the epimerized samples and of polystyrene i n 9:1 trichlorobenzene:nitrobenzene-d solution at 150° were recorded and analyzed using stereosequence distributions that were calculated for the samples by Monte Carlo simulation of the epimerization process. Triad, pentad and heptad assignments were developed for the aromatic C-1 carbon resonance patterns and t r i a d assignments previously proposed for the methine carbon resonances of polystyrene were v e r i fied. Based on the assignments developed, the C-NMR spectrum of polystyrene indicates it to be almost a t a c t i c , P(m)=0.45. 13

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P o l y s t y r e n e was one o f t h e f i r s t p o l y m e r s t o be s t u d i e d b y h i g h r e s o l u t i o n NMR s p e c t r o s c o p y C O , b u t i t s s p e c t r a a r e i n c o m p l e t e l y u n d e r s t o o d e v e n t o d a y . The H-NMR s p e c t r a o f p o l y s t y r e n e a r e p o o r l y d e f i n e d ; t h e methine and ortho aromatic p r o t o n s p e c t r a a r e i n f l u e n c e d s i g n i f i c a n t l y by p e n t a d o r l a r g e r s t e r e o s e q u e n c e e f f e c t s but they c o n t a i n o n l y three o r f o u r b r o a d , b a d l y overlapped r e s o n ances. I n c o n t r a s t , t h e C-NMR s p e c t r a o f p o l y s t y r e n e a r e r i c h i n d e t a i l , b e i n g s e n s i t i v e t o hexad and h e p t a d s t e r e o s e q u e n c e effects. S i n c e t h e r e a r e 20 d i s t i n g u i s h a b l e h e x a d c o m b i n a t i o n s and 36 d i s t i n g u i s h a b l e h e p t a d c o m b i n a t i o n s , i t h a s been e x t r e m e l y d i f f i c u l t t o a s s i g n t h e v a r i o u s r e s o n a n c e s o b s e r v e d i n t h e C-NMR spectrum o f p o l y s t y r e n e . The d i f f i c u l t y h a s b e e n compounded b y the f a c t t h a t f r e e r a d i c a l i n i t i a t e d p o l y m e r i z a t i o n s o f s t y r e n e y i e l d p o l y m e r h a v i n g t h e same m i c r o s t r u c t u r e , r e g a r d l e s s o f p o l y m e r i z a t i o n t e m p e r a t u r e . Many i o n i c p o l y m e r i z a t i o n s o f s t y r e n e a l s o y i e l d p o l y m e r s t h a t have t h e same m i c r o s t r u c t u r e a s t h o s e p r e p a r e d 1

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Current address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15261. 0097 6156/84/0247 0197$07.50/0 © 1984 American Chemical Society Randall; NMR and Macromolecules ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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by f r e e r a d i c a l p o l y m e r i z a t i o n . Thus, i t has not been p o s s i b l e t o p r e p a r e , by d i r e c t p o l y m e r i z a t i o n , a s e r i e s o f p o l y s t y r e n e s a m p l e s h a v i n g m i c r o s t r u c t u r e s t h a t v a r i e d i n some r a t i o n a l way. Studies on t h e s p e c t r a o f s u c h s a m p l e s , i n w h i c h t h e i n t e n s i t i e s o f t h e v a r i o u s s i g n a l s w o u l d v a r y as t h e p o l y m e r s t r u c t u r e c h a n g e d , w o u l d have been v e r y u s e f u l f o r making resonance a s s i g n m e n t s . Cationic and some a n i o n i c p o l y m e r i z a t i o n s o f s t y r e n e h a v e y i e l d e d p o l y m e r s t h a t have s p e c t r a t h a t d i f f e r from the spectrum of f r e e r a d i c a l p o l y m e r i z e d p o l y s t y r e n e ( 2 - _ 3 ) . However, t h e u s e f u l n e s s o f s u c h s a m p l e s was dampened by t h e p o s s i b i l i t y t h a t s i d e r e a c t i o n s accomp a n i e d t h e c a t i o n i c p o l y m e r i z a t i o n s and by t h e p o s s i b i l i t y t h a t m u l t i - s t a t e p r o p a g a t i o n mechanisms w e r e i n v o l v e d i n some o f t h e anionic polymerization reactions. Our r e c e n t u n d e r s t a n d i n g o f t h e NMR s p e c t r a and s t r u c t u r e o f p o l y s t y r e n e i s b a s e d on t h e r e s u l t s o f model compound s t u d i e s ( 4 - 1 1 ) , t h e o r e t i c a l c a l c u l a t i o n s ( 1 2 - 1 4 ) , s t u d i e s on p o l y s t y r e n e d e r i v a t i v e s (15) and a n a l o g u e s ( 1 6 ^ ) , s t u d i e s on t h e NMR s p e c t r a o f e p i m e r i z e d i s o t a c t i c p o l y s t y r e n e s a m p l e s ( 1 7 - 1 9 ) and on r e s u l t s o b t a i n e d u s i n g other approaches(20-22). The m o d e l compound a p p r o a c h , e x e m p l i f i e d by t h e e x c e l l e n t p a p e r c o n t a i n e d i n t h i s v o l u m e ( 1 1 ) , i n v o l v e d i s o l a t i n g p o l y s t y r e n e o l i g o m e r s o f known c o n f i g u r a t i o n and r e c o r d i n g t h e i r NMR s p e c t r a . C h e m i c a l s h i f t a s s i g n m e n t s made f o r t h e o l i g o mers a r e t h e n u s e d i n m a k i n g a s s i g n m e n t s f o r p o l y s t y r e n e . This a p p r o a c h r e q u i r e s t h a t a l a r g e number o f d i f f e r e n t o l i g o m e r s be s e p a r a t e d and s t u d i e d i f a s s i g n m e n t s a r e t o be made a t p e n t a d o r h i g h e r l e v e l s and c a r e must be t a k e n t o d e s i g n t h e o l i g o m e r s so t h a t t h e y a d o p t c o n f o r m a t i o n s s i m i l a r t o t h o s e a d o p t e d by t h e p o l y m e r . T h i s t e d i o u s a p p r o a c h has b e e n v e r y f r u i t f u l and rewarding(11). E p i m e r i z a t i o n of s t e r e o r e g u l a r polymers(17-19,23-33) p r o v i d e s a way t o o b t a i n p o l y m e r s h a v i n g c o n t r o l l e d s e q u e n c e d i s t r i b u t i o n s . The p r o d u c t s o f p o l y m e r e p i m e r i z a t i o n r e a c t i o n s can be u s e f u l f o r e s t a b l i s h i n g NMR a s s i g n m e n t s b e c a u s e t h e i r s t r u c t u r e s can be c a l c u l a t e d by s i m u l a t i n g t h e e p i m e r i z a t i o n p r o c e s s . They c a n t h u s serve as models f o r s t r u c t u r e assignment purposes. A l t h o u g h f r e e r a d i c a l c h e m i s t r y h a s b e e n u s e d f o r some p o l y m e r e p i m e r i z a t i o n experiments(30), s t r o n g bases are g e n e r a l l y used to epimerize p o l ymers. S h e p h e r d ( 3 4 ) , made a m a j o r c o n t r i b u t i o n t o t h e s t u d y o f p o l y s t y r e n e s t r u c t u r e by s h o w i n g t h a t i s o t a c t i c p o l y s t y r e n e c o u l d be e p i m e r i z e d by p o t a s s i u m t - b u t o x i d e i n h e x a m e t h y l p h o s p h o r a m i d e s o l u t i o n at m i l d temperatures. T h i s p r o c e s s c a n be e n v i s i o n e d a s shown by t h e e q u a t i o n s g i v e n on t h e f o l l o w i n g page. These e q u a t i o n s show how a mmmm s e q u e n c e i n p o l y s t y r e n e c a n be c o n v e r t e d i n t o a mrrm s e q u e n c e by a s i m p l e e p i m e r i z a t i o n event. Should the c o n f i g u r a t i o n of the f o u r t h carbon from the l e f t i n t h e l a s t s t r u c t u r e a l s o be a l t e r e d , a rmrm p e n t a d w o u l d result. T h u s , by a s e r i e s o f e p i m e r i z a t i o n s t e p s i t i s p o s s i b l e t o change i s o t a c t i c p o l y s t y r e n e g r a d u a l l y i n t o a p o l y m e r t h a t e x h i b i t s t h e same NMR s p e c t r a a s t h e p o l y s t y r e n e t h a t was p r e p a r e d by f r e e r a d i c a l p o l y m e r i z a t i o n ( 1 7 - 1 9 ) . A Monte C a r l o p r o g r a m h a s

Randall; NMR and Macromolecules ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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Polystyrene and Epimerized Isotactic Polystyrenes H

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b e e n w r i t t e n t o s i m u l a t e t h i s e p i m e r i z a t i o n p r o c e s s and t o c a l c u l a t e the r e l a t i v e concentrations o f stereosequences present i n the p o l y m e r s a f t e r v a r i o u s e x t e n t s o f epimerization(17)· By c o r r e l a t i n g t h e resonance p a t t e r n s o f e p i m e r i z e d p o l y s t y r e n e s t h a t were prepared by Shepherd s procedure w i t h stereosequence d i s t r i b u t i o n s c a l c u l a t e d f o r t h e p o l y m e r s u s i n g t h e Monte C a r l o m e t h o d , a s s i g n ments have b e e n d e v e l o p e d f o r t h e m e t h i n e p r o t o n ( 1 _ 7 ) , m e t h i n e c a r bon (18) , m e t h y l e n e c a r b o n ( 1 8 ) a n d a r o m a t i c C - l c a r b o n r e s o n a n c e s of p o l y s t y r e n e ( 1 9 ) . These a s s i g n m e n t s agree v e r y w e l l w i t h a s s i g n ments d e v e l o p e d i n t h e m o d e l compound s t u d i e s , a n d t h e y l e a d t o the c o n c l u s i o n t h a t p o l y s t y r e n e , a s prepared by f r e e r a d i c a l polym e r i z a t i o n methods, h a s a B e r n o u l l i a n s t e r e o s e q u e n c e d i s t r i b u t i o n w i t h a meso c o n t e n t , P(m) o f a b o u t 0.46. I n o t h e r w o r d s , t h e polymer i s almost a t a c t i c . Our p r e v i o u s r e p o r t s c o n c e r n e d 20 MHz C-NMR s p e c t r a o f p a r t i a l l y epimerized polystyrenes. The a r o m a t i c C - l c a r b o n r e s o n a n c e s w e r e r e c o r d e d a t room t e m p e r a t u r e a n d w e r e p o o r l y d e f i n e d when t h e e x t e n t o f e p i m e r i z a t i o n was h i g h . A need f o r r e m e a s u r i n g these resonances a t higher temperature, using higher f i e l d spectro m e t e r s was c l e a r l y e v i d e n t . This paper i s t h e r e f o r e concerned w i t h t h e 75 MHz C-NMR s p e c r r a o f p a r t i a l l y e p i m e r i z e d i s o t a c t i c polystyrenes. 1

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Experimental Isotactic Polystyrene. I s o t a c t i c p o l y s t y r e n e was p r e p a r e d by h e a t i n g a 1 0 % s o l u t i o n o f s t y r e n e i n b e n z e n e a t 60° f o r 24 h o u r s i n t h e p r e s e n c e o f a c a t a l y s t p r e p a r e d i n t h e p r e s e n c e o f monomer f r o m e q u i m o l a r amounts o f ( h e p t a n e s o l u t i o n ) a n d T1CI3-AA (heptane suspension). The r e a c t i o n m i x t u r e was d i l u t e d w i t h b e n zene a n d p o u r e d i n t o a n e x c e s s o f i s o p r o p a n o l . The r e s u l t i n g p r e c i p i t a t e was d i s s o l v e d i n warm m e t h y l e n e c h l o r i d e . The s o l u t i o n was f i l t e r e d a n d added t o h o t m e t h y l e t h y l k e t o n e . The s o l u t i o n was c o n c e n t r a t e d a n d c o o l e d t o o b t a i n t h e i s o t a c t i c p o l y m e r i n t h e f o r m o f a w h i t e powder t h a t was washed w i t h m e t h y l e t h y l k e t o n e and d r i e d u n d e r vacuum. F i n a l p u r i f i c a t i o n was a c h i e v e d b y r e p r e c i p i t a t i o n o f the polymer from benzene s o l u t i o n i n t o methanol, f o l l o w e d b y d r y i n g a t 60° u n d e r vacuum.


(C2H5)3AI

E p i m e r i z a t i o n Procedure. P o t a s s i u m t - b u t o x i d e t h a t had been heated u n d e r vacuum a t 130°C f o r t h r e e h o u r s a n d h e x a m e t h y I p h o s p h o r a m i d e t h a t h a d been d r i e d o v e r C a H a t 60° f o r 24 h o u r s a n d t h e n d i s t i l l e d from L i A l H i * under reduced p r e s s u r e were used. S o l u t i o n s cont a i n i n g 10g. i s o t a c t i c p o l y s t y r e n e a n d 56g. (0.5 m o l e ) KOtBu p e r l i t e r o f hexamethyIphosphoramide ( C a u t i o n - suspected carcinogen) w e r e p r e p a r e d u n d e r n i t r o g e n a n d were h e a t e d a t 100°C f o r p e r i o d s t h a t r a n g e d f r o m 1 t o 20 h o u r s . The r e a c t i o n m i x t u r e s w e r e i n i t i a l l y p i n k and u l t i m a t e l y developed a v i o l e t c o l o r t h a t i n t e n s i f i e d w i t h r e a c t i o n time. The r e a c t i o n m i x t u r e s w e r e p o u r e d i n t o m e t h a n o l t o i s o l a t e t h e p o l y m e r s . These w e r e r e p r e c i p i t a t e d f r o m b e n z e n e s o l u t i o n i n t o m e t h a n o l and were d r i e d u n d e r vacuum a t 40°C. The u s e o f t e m p e r a t u r e s h i g h e r t h a n 100°C c a u s e s e x t e n s i v e m o l e c u l a r w e i g h t r e d u c t i o n . The s a m p l e s employed i n t h i s s t u d y a r e t h e same o n e s u s e d i n o u r p r e v i o u s C-NMR s t u d i e s . NMR M e a s u r e m e n t s . In a l l cases, ten percent s o l u t i o n s of the p o l y s t y r e n e s a m p l e s d i s s o l v e d i n a 9:1 1 , 2 , 4 - t r i c h l o r o b e n z e n e m i t r o b e n z e n e - d m i x t u r e were u s e d f o r t h e NMR s t u d i e s . 75 MHz C-NMR """H-decoupled s p e c t r a o f t h e e p i m e r i z e d p o l y s t y r e n e s a m p l e s w e r e r e c o r d e d a t 150-160° u s i n g a B r u k e r WH-300 NMR S p e c t r o m e t e r . A 70° p u l s e w i d t h , a n a c q u i s i t i o n t i m e o f 0.82 s e c o n d s w i t h a 16K d a t a s i z e , a n d a p u l s e d e l a y o f 0.1 s e c o n d w e r e e m p l o y e d . The number o f t r a n s i e n t s c o l l e c t e d v a r i e d f r o m 3000 t o 10,000 a n d t h e d a t a w e r e p r o c e s s e d w i t h a l i n e b r o a d e n i n g o f 0.8-1.0 Hz. A T s t u d y done on t h e a r o m a t i c C - l a n d a l i p h a t i c c a r b o n r e s o n a n c e s o f p o l y s t y r e n e a t 200°C, u s i n g a V a r i a n XL-400 NMR S p e c t r o m e t e r , r e v e a l e d t h a t w i t h i n e x p e r i m e n t a l e r r o r t h e i n d i v i d u a l components o f t h e s e r e s o n a n c e p a t t e r n s h a d t h e same r e l a x a t i o n t i m e ( 3 5 ) . This i n d i c a t e s t h a t t h e c o n d i t i o n s d e s c r i b e d above a r e a p p r o p r i a t e f o r o b t a i n i n g resonance p a t t e r n s t h a t c o u l d be a n a l y z e d q u a n t i t a t i v e l y . R e s o n a n c e a r e a measurements were made b y c u t t i n g a n d w e i g h i n g s p e c t r a t r a c i n g s . S p e c t r a s i m u l a t i o n s w e r e done u s i n g a Calcomp p l o t t e r w i t h t h e a i d o f a p r o g r a m w r i t t e n by D r . B.L. B r u n e r o f the U n i v e r s i t y o f Kentucky, u s i n g stereosequence d i s t r i b u t i o n s c a l c u l a t e d f o r t h e p o l y m e r s b y Monte C a r l o s i m u l a t i o n s o f t h e e p i m e r i 2

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Polystyrene and Epimerized Isotactic Polystyrenes

201

z a t i o n p r o c e s s and u s i n g c h e m i c a l s h i f t s o b t a i n e d by methods t o be d e s c r i b e d l a t e r i n t h i s p a p e r . A l i n e shape t h a t was a c o m b i n a t i o n o f L o r e n t i z i a n and G a u s s i a n f u n c t i o n s was assumed f o r e a c h line. T h i s combined f u n c t i o n h a s worked w e l l i n o u r p r e v i o u s s i m u l a t i o n s t u d i e s . A c o n s t a n t l i n e - w i d t h was u s e d f o r a l l l i n e s w i t h i n a g i v e n spectrum. Monte C a r l o S i m u l a t i o n s . The e p i m e r i z a t i o n r e a c t i o n was s i m u l a t e d u s i n g t h e Monte C a r l o p r o g r a m we d e s c r i b e d e a r l i e r ( 1 7 ) . A 5000 e l e m e n t a r r a y was a l l o c a t e d t o s t o r e i n f o r m a t i o n a b o u t t h e c o n f i g u r a t i o n s o f monomer u n i t s a t v a r i o u s p o s i t i o n s i n a 5000 u n i t p o l y mer c h a i n . The p o s i t i o n s w e r e i n d e x e d i n s u c h a way t h a t t h e p o l y m e r c o u l d be c o n s i d e r e d c y c l i c . T h i s was done t o a v o i d e n d group e f f e c t s . The c o n f i g u r a t i o n s ( R o r S) a t i n d i v i d u a l s i t e s w e r e i n d i c a t e d by 0 o r 1 v a l u e s . The p o l y m e r c h a i n was made i s o t a c t i c b y g i v i n g a l l e l e m e n t s o f t h e a r r a y i n i t i a l v a l u e s o f 0. The c h a i n was t h e n " e p i m e r i z e d " b y c o n d u c t i n g a s e r i e s o f " e p i m e r i z a t i o n e v e n t s , " each o f which i n v o l v e d t h e f o l l o w i n g s t e p s ; 1.

A s i t e o n t h e c h a i n was s e l e c t e d t o e x p e r i e n c e a n e p i m e r i z a t i o n e v e n t b y c a l l i n g a random number t h a t r a n g e d f r o m 1 t o 5000. 2. The s t r u c t u r e o f t h e s i t e s e l e c t e d , i n c l u d i n g s t r u c t u r e s o f n e a r e s t n e i g h b o r s was t h e n d e t e r m i n e d . The p o s s i b l e s t r u c t u r e s w e r e 0 0 0 , 0 0 1 , 1 0 1 , 1 1 0 , 011 a n d 1 1 1 . 3. D e p e n d i n g on t h e s t r u c t u r e f o u n d , a p r o b a b i l i t y ( P ) t h a t t h e s i t e s e l e c t e d would have c o n f i g u r a t i o n 1 a t t h e c o m p l e t i o n o f t h e e p i m e r i z a t i o n e v e n t was e s t a b l i s h e d b y u s e o f T a b l e I . The V v a l u e i n t h i s t a b l e (R i n p r e v i o u s p a p e r s ) h a s a range o f 0 - 1 and i s t h e o n l y parameter r e q u i r e d f o r t h e simulation. 4. A s e c o n d random number h a v i n g a r a n g e o f 0 - 1 was t h e n c a l l e d a n d compared t o P. When t h i s number was l e s s t h a n o r e q u a l t o Ρ, t h e a r r a y e l e m e n t r e p r e s e n t i n g t h e s i t e was g i v e n a v a l u e o f 1. O t h e r w i s e i t was g i v e n a v a l u e o f 0. 5. An e p i m e r i z a t i o n e v e n t c o u n t e r was t h e n i n c r e m e n t e d and compared t o a n o u t p u t s c h e d u l e . I f o u t p u t was d e s i r e d a f t e r t h i s e v e n t , t h e c h a i n ( a r r a y ) was analyzed f o r a l l p o s s i b l e dyad, t r i a d , t e t r a d , p e n t a d , hexad and heptad s t e r e o s e q u e n c e s and t h e r e s u l t s were s t o r e d . E p i m e r i z a t i o n e v e n t s were r e p e a t e d u n t i l t h e c h a i n r e a c h e d equilibrium. The c a l c u l a t i o n was r e p e a t e d t e n t i m e s a n d t h e s t e r e o s e q u e n c e c o n c e n t r a t i o n s e v a l u a t e d a f t e r s p e c i f i e d numbers o f e p i m e r i z a t i o n e v e n t s w e r e t h e n a v e r a g e d , p r i n t e d a n d punched f o r use i n s p e c t r a s i m u l a t i o n s . I n r e l a t i n g Monte C a r l o and e x p e r i m e n t a l r e s u l t s , t h e v a l u e o f t h e e p i m e r i z a t i o n event counter can be used a s a q u a n t i t y t h a t i s


202

NMR

T a b l e 1:

P r o b a b i l i t i e s That t h e S e l e c t e d S i t e Has C o n f i g u r a t i o n 1 a t The End o f a n E p i m e r i z a t i o n E v e n t S t r u c t u r e Found 000 o r 100 101 111 o r 011 010

001

110


AND MACROMOLECULES

P r o b a b i l i t y (P) V 0.5 1-V 1-V 0.5 V

p r o p o r t i o n a l t o time i n a c t u a l e p i m e r i z a t i o n experiments. In the p r e s e n t s t u d y , h o w e v e r , Monte C a r l o r e s u l t s w e r e c o r r e l a t e d w i t h e x p e r i m e n t a l r e s u l t s by m a t c h i n g mm-contents c a l c u l a t e d b y t h e Monte C a r l o method t o mm-contents e v a l u a t e d f r o m t h e 300 MHz meth ine proton resonances of the polymers. Two d i f f e r e n t V v a l u e s w e r e u s e d f o r t h e Monte C a r l o c a l c u l a tions. A v a l u e o f 0.5 was u s e d t o p r e d i c t t h e e f f e c t o f a com p l e t e l y random e p i m e r i z a t i o n p r o c e s s t h a t y i e l d s a p e r f e c t l y a t a c t i c p o l y m e r when e q u i l i b r a t i o n i s c o m p l e t e . Our p r e v i o u s work employed a V v a l u e o f 0.65 a n d t h i s was a l s o u s e d h e r e . The 0.65 v a l u e , w h i c h y i e l d s a s i i g h t l y s y n d i o t a c t i c p o l y m e r (P(m)=0.43), was s e l e c t e d i n i t i a l l y b e c a u s e i t p r e d i c t s a r r - c o n t e n t f o r t h e completely e q u i l i b r a t e d polymer that agrees w i t h t h e o r e t i c a l pre d i c t i o n s o f F l o r y and W i l l j a m s ( 3 6 ) . R e s u l t s and D i s c u s s i o n A r o m a t i c C - l R e s o n a n c e . F i g u r e s 1-6 show a r o m a t i c C - l c a r b o n r e sonance p a t t e r n s o b s e r v e d f o r v a r i o u s e p i m e r i z e d i s o t a c t i c p o l y s t y r e n e s a m p l e s . The p a t t e r n s a r e e a s i l y d i v i d e d i n t o s i x r e s o n a n c e a r e a s , t h a t a r e d e s i g n a t e d A-F i n o r d e r o f i n c r e a s i n g f i e l d . A r e a s A, C a n d Ε a r e c l e a r i n t h e s e F i g u r e s w h i l e a r e a s B, D a n d F a r e cross-hatched. Except f o r the resonance p a t t e r n observed f o r atac t i c p o l y s t y r e n e ( F i g u r e 6 ) , t h e resonances appear t o be a s s i g n a b l e as f o l l o w s : A - mmmm Β - (mmmr + rmmm) C - rmmr D - (mmrr 4- rrmm) + (mmrm + mrmm) Ε - (rmrm + mrmr) + ( r m r r + r r m r ) F - rr T h u s , t r i a d s e q u e n c e d i s t r i b u t i o n s c a n b e measured b y c o m b i n i n g t h e s e f r a c t i o n a l r e s o n a n c e a r e a s : mm=A+B+C; (mr+rm)=D+E; r r = F . I n t h e c a s e o f t h e a t a c t i c p o l y s t y r e n e p a t t e r n ( F i g u r e 6) t h e demar c a t i o n between t h e D and Ε a r e a s i s n o t c l e a r and i t i s e a s i e r t o d i s t i n g u i s h D a n d Ε a r e a s t h a t may b e a s s i g n e d a s f o l l o w s : f

1



Polystyrène

and Epimerized Isotactic Polystyrenes 203


13.

147

145

146 ôc(ppm)

F i g u r e 1.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l Carbon Resonance P a t t e r n o f an E p i m e r i z e d Isot a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content of 0.70.


204


NMR AND MACROMOLECULES

F i g u r e 2.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f a n E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content o f 0.59.





13.

F i g u r e 3.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f a n E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content o f 0 . 5 5 .


205


F i g u r e 4.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f an E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content o f 0.41.




13.

A

147


B

O

D

E

146

F

145 ôc(ppm)

F i g u r e 5.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f an E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content of 0.34.


207

208



F i g u r e 6.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r b o n R e s o n a n c e P a t t e r n o f P o l y s t y r e n e (mm M3.20).


13.


Polystyrene and Epimerized isotactic Polystyrenes


D Ε

1

1

209

- (mmrr + rrmm) - (mmrm + mrmm) + (rmrm + mrmr) + (rmrr + rrmr).

In e v a l u a t i n g t r i a d stereosequence d i s t r i b u t i o n s f o r a t a c t i c p o l y styrene, then, the f o l l o w i n g combinations of f r a c t i o n a l resonance a r e a s c a n b e u s e d : mm=A+B+C, (mr+rm)=D'+Ε'; r r - F . T a b l e I I com pares t r i a d stereosequence d i s t r i b u t i o n s evaluated a s described above w i t h v a l u e s c a l c u l a t e d f o r t h e p o l y m e r s b y Monte C a r l o s i m u l a t i o n o f t h e e p i m e r i z a t i o n p r o c e s s . C a l c u l a t e d r e s u l t s b a s e d on V v a l u e s o f 0.50 a n d 0.65 a r e p r o v i d e d f o r c o m p a r i s o n . T h e r e i s l i t t l e d i f f e r e n c e b e t w e e n c a l c u l a t e d r e s u l t s b a s e d o n V=0.50 o r 0.65 when mm>0.35, b u t when mm A r r r > Amrm. F i g u r e 9 may h e l p t o i l l u s t r a t e t h i s a p p r o a c h . Once a l l s e v e n A x y z p a r a m e t e r s h a v e b e e n e v a l u a t e d , c h e m i c a l s h i f t s f o r t h e r e m a i n i n g 21 h e p t a d s c a n b e c a l c u l a t e d and u s e d f o r s i m u l a t i n g the s p e c t r a o f polymers epimerized t o h i g h e r e x t e n t s , as w e l l a s t h e s p e c t r u m o f p o l y s t y r e n e . M i n o r a d j u s t m e n t s c a n t h e n b e made i n c a l c u l a t e d c h e m i c a l s h i f t s t o f i l l i n o r deepen v a l l e y s o r t o e l i m i n a t e d i s t o r t i o n s caused by c o i n c i d e n c e s o f c a l culated chemical s h i f t s . T h i s approach worked r e a s o n a b l y success f u l l y when a p p l i e d t o q u a t e r n a r y a r o m a t i c c a r b o n r e s o n a n c e s o b s e r ved f o r e p i m e r i z e d i s o t a c t i c p o l y s t y r e n e s a s measured a t 20 MHz and room t e m p e r a t u r e ( 1 9 ) . I t proved n e c e s s a r y , however, t o s h i f t heptads c o n t a i n i n g a c e n t r a l rmrr (or rrmr) pentad u p f i e l d by 0.256 ppm t o o b t a i n good a g r e e m e n t b e t w e e n o b s e r v e d and s i m u l a t e d s p e c t r a . T h i s s u g g e s t s t h a t t h e s h i e l d i n g e x p e r i e n c e d by a n u c l e u s f r o m one d i r e c t i o n o f t h e p o l y m e r c h a i n may n o t b e e n t i r e l y i n d e pendent o f t h e s t r u c t u r e o f t h e c h a i n t h a t proceeds i n the opposite d i r e c t i o n . T h i s same g e n e r a l a p p r o a c h was u s e d i n t h e p r e s e n t s t u d y t o d e v e l o p A x y z v a l u e s f o r p o l y s t y r e n e s p e c t r a r e c o r d e d a t 150°. The values o b t a i n e d , together w i t h those developed p r e v i o u s l y f o r s p e c t r a r e c o r d e d a t room temper a ture(]L9) a r e g i v e n i n T a b l e V. The most s i g n i f i c a n t d i f f e r e n c e b e t w e e n t h e two s e t s o f v a l u e s i s the dramatic i n c r e a s e i n A r r r w i t h an i n c r e a s e i n temperature. S i n c e Armm, A r r m , Armr and A r r r a l l h a v e s i m i l a r and l a r g e n e g a t i v e v a l u e s compared t o t h e o t h e r A x y z v a l u e s a t 150°, i t i s u n d e r s t a n d able t h a t p o l y s t y r e n e s p e c t r a recorded a t h i g h temperature can p r o v i d e r e l i a b l e measures o f r r - t r i a d c o n c e n t r a t i o n s . M i n o r a d j u s t m e n t s were made i n c h e m i c a l sh i f t s c a l c u l a t e d u s i n g Axyz v a l u e s t o improve the q u a l i t y o f f i t between s i m u l a t e d and o b s e r v e d s p e c t r a . The h e p t a d c h e m i c a l s h i f t s u s e d t o s i m u l a t e




-p| mmmmmm - mmmmmr

=-Ammr


I mmmmmm - mmmmrr = - A m r r

mmmmmm - mmmrrm = - A r r m

mmmmmm - mmrrmm = - 2 A r m m

•

I—*\ I

>j

h Group F i g u r e 9.

I—*| (

A

xxxmmm - xxxmrm =-Δmrm *|

^ I

^calculated line

^|

xxxmmm - xxxrrr = - Δ π τ xxxmmm - xxxrmr =-Δ rmr

B

E s t i m a t i o n o f Aabc P a r a m e t e r s f r o m t h e S p e c t r a o f E p i m e r i z e d P o l y m e r s H a v i n g H i g h mm-Contents.


216

NMR A N D MACROMOLECULES

T a b l e V.

E m p i r i c a l Chemical S h i f t Parameters f o r P o l y s t y r e n e a t 20° and 150°


Parameter Δ mmr Δπιπιι Armm Amrr Arrm Armr Arrr rmrr

correction

(Axyz) E v a l u a t e d

V a l u e (ppm) 20°

150°

-0.080 -0.288 -0.752 -0.192 -0.695 -0.621 -0.496

-0.077 -0.266 -0.728 -0.196 -0.637 -0.665 -0.616

-0.256

-0.112

the s p e c t r a a r e g i v e n i n Table V I , along w i t h those c a l c u l a t e d u s i n g t h e p a r a m e t e r s l i s t e d i n T a b l e V. T a b l e V I a l s o compares the r e l a t i v e order o f these heptad resonance assignments w i t h t h e r e l a t i v e order o f methyl carbon heptad resonances c a l c u l a t e d f o r p o l y p r o p y l e n e by S c h i l l i n g a n d T o n e l l i ( 3 7 ) , u s i n g t h e r o t a t i o n a l i s o m e r i c s t a t e m o d e l . The c o r r e s p o n d e n c e b e t w e e n t h e two s e t s o f a s s i g n m e n t s i s s u r p r i s i n g l y good. A l t h o u g h some a r o m a t i c C - l c a r bon p e n t a d r e s o n a n c e p a t t e r n s o v e r l a p i n t h e p o l y s t y r e n e s p e c t r a , the r e l a t i v e o r d e r i n g o f the pentad resonance p a t t e r n s e x a c t l y matches t h a t o f t h e methyl carbon pentad resonances o f p o l y p r o p y l e n e , a s c a l c u l a t e d b y S c h i l l i n g a n d T o n e l l i ( 3 7 ) [mmmm, (mmmr + rmmm), rmmr, (mmrr + rrmm), (mmrm + mrmm), ( r m r r + r r m r ) , (rmrm + mrmr), r r r r , ( m r r r + r r r m ) , mrrm, i n o r d e r o f i n c r e a s i n g f i e l d ] . T h i s o r d e r i n g i s a l s o i n agreement w i t h t h e a s s i g n m e n t s d e v e l o p e d f o r t h e a r o m a t i c C - l r e s o n a n c e s o f p o l y s t y r e n e by S a t o a n d Tanaka (11). I t i s i n o n l y f a i r agreement w i t h o r d e r i n g b a s e d on T o n e l l i s recent c a l c u l a t i o n s f o r p o l y s t y r e n e ( 3 8 ) , which, i n con t r a s t t o t h e p o l y p r o p y l e n e c a l c u l a t i o n s ( 3 7 ) , t e n d t o group t h e l i n e s i n t o f o u r g e n e r a l a r e a s . R e s o n a n c e s o f mm-centered p e n t a d s a r e c a l c u l a t e d t o o c c u r i n t h r e e o f t h e s e a r e a s a n d t h e r e i s more e x t e n s i v e o v e r l a p p i n g o f mm- and ( m r + r m ) - c e n t e r e d p e n t a d r e s o n a n c e r e g i o n s t h a n seems r e a s o n a b l e b a s e d on o u r s i m u l a t i o n s t u d i e s . F i g u r e s 1 - 6 compare o b s e r v e d a r o m a t i c C - l c a r b o n r e s o n a n c e s p e c t r a w i t h s i m u l a t e d s p e c t r a based on t h e heptad c h e m i c a l s h i f t s g i v e n i n T a b l e V I and on heptad stereosequence c o n c e n t r a t i o n s c a l c u l a t e d by Monte C a r l o s i m u l a t i o n o f t h e e p i m e r i z a t i o n p r o c e s s , u s i n g V=0.65. The s i m u l a t i o n s p e c t r a r e p r o d u c e t h e g e n e r a l f e a t u r e s o f t h e o b s e r v e d s p e c t r a v e r y w e l l a n d c a n be c o n s i d e r e d t o be i n a t l e a s t s e m i - q u a n t i t a t i v e agreement w i t h t h e o b s e r v e d s p e c tra. The agreement b e t w e e n o b s e r v e d and s i m u l a t e d s p e c t r a m i g h t be i m p r o v e d i f s p e c t r a w i t h h i g h e r S/N r a t i o s were employed a n d i f a d d i t i o n a l p a r a m e t e r a d j u s t m e n t s w e r e made. I t seems, h o w e v e r , t h a t t h e h e p t a d a s s i g n m e n t s d e v e l o p e d i n t h i s work a r e r e a s o n a b l y correct. f


13.



111

Table V I . Heptad Assignments f o r A r o m a t i c C - l Carbon Resonances of P o l y s t y r e n e Heptad

6 (Calc) c

Polypropylene C


3

b

mmmmmm rmmmmm + mmmmmr rmmmmr

146.40 146.32 146.25

146.40 146.32 146.25

l 2 3

l 2 3

1 2 3

+ + + +

mmmmrm rmmmrm mmmmrr rmmmrr

146.13 146.06 146.20 146.13

146.13 146.06 146.20 146.18

6 7 4 5

9 11-13 7,8 10

5 7 4 6

mrmmrm rrmmrm + mrmmrr rrmmrr

145.87 145.94 146.01

145.91 145.92 146.01

10 9 8

18 17 16

10,11 9 8

mmmrrm rmmrrm mmmrrr rmmrrr

+ + + +

mrrmmm mrrmmr rrrmmm rrrmmr

145.76 145.69 145.78 145.71

145.77 145.70 145.85 145.76

12 14 11 13

11-13 14,15 11-13 14,15

mmmrmm rmmrmm mmmrmr rmmrmr

+ + + +

mmrmmm mmrmmr rmrmmm rmr mmr

145.67 145.59 145.74 145.66

145.67 145.60 145.74 145.63

16 18 15 17

5 7,8 4 6

16 18 15 17

mrmrmm rrmrmm mrmrmr rrmrmr

+ + + +

mmrmrm mmrmrr rmr mrm rmrmrr

145.41 145.48 145.47 145.54

145.36 145.41 145.50 145.53

25 24 20 19

22 20,21 20,21 19

26 24 25 23

mrmrrm rrmrrm mrmrrr rrmrrr

+ + + +

mrrmrm mrrmrr rrrmrm rrrmrr

145.39 145.46 145.41 145.48

145.35 145.45 145.43 145.48

26 22 23 21

26 23,24 25 23,24

21 19 22 20

mrrrrm rrrrrm + mrrrrr rrrrrr

145.13 145.15 145.17

145.27 145.15 145.20

27 29 28

27 28 29

27 28 29

+ + + +

mmrrrm mmrrrr rmrrrm rmr r r r

145.04 145.06 145.10 145.12

145.07 145.06 145.11 145.12

32 34 31 30

32 33 31 30

32 33 30 31

mmrrmm rmrrmm + mmr rmr rmr rmr

144.94 145.01 145.07

144.94 145.00 145.08

36 35 33

36 35 34

36 35 34

mrmmmm mrmmmr rrmmmm rrmmmr

mrrrmm rrrrmm mrrrmr rrrrmr

a

b

10,11 13 12 14

(a) B a s e d o n l i n e s u s e d f o r s i m u l a t i o n s shown i n F i g u r e s 1-6. (b) B a s e d o n c a l c u l a t i o n s o f T o n e l l i ( 3 8 ) . (c) Based on c a l c u l a t i o n s o f T o n e l l i and S c h i l l i n g ( 3 7 ) .


1


218

NMR A N D MACROMOLECULES

S p e c t r a s i m u l a t i o n s b a s e d on s t e r e o s e q u e n c e distributions c a l c u l a t e d f o r t h e p o l y m e r s u s i n g V=0.50 a l s o matched t h e e x p e r i mental s p e c t r a very w e l l , except f o r p o l y s t y r e n e ( o r the completely epimerized polymer). I t was n o t p o s s i b l e t o d e v e l o p h e p t a d r e s o n ance a s s i g n m e n t s t h a t a f f o r d e d u n i f o r m agreement b e t w e e n o b s e r v e d and s i m u l a t e d s p e c t r a f o r a l l t h e s a m p l e s s t u d i e d when s t e r e o s e quence d i s t r i b u t i o n s b a s e d on V=0.50 w e r e u s e d . Since resonance a s s i g n m e n t s c a n be made u s i n g e p i m e r i z e d p o l y m e r s w i t h h i g h mm c o n t e n t s u s i n g s t e r e o s e q u e n c e c o n c e n t r a t i o n s b a s e d on e i t h e r V=0.50 o r V=0.65, t h e s e a s s i g n m e n t s c o u l d be u s e d t o d e t e r m i n e what V v a l u e was a p p r o p r i a t e f o r p o l y s t y r e n e ( o r t h e c o m p l e t e l y e p i m e r i z e d polymer). V v a l u e s r a n g i n g f r o m 0.62 t o 0.65 a f f o r d e d good a g r e e ment b e t w e e n s i m u l a t e d and o b s e r v e d s p e c t r a o f p o l y s t y r e n e . Assuming B e r n o u l l i a n s t a t i s t i c s , ν=(1-σ) /σ , where σ i s t h e p r o b a b i l i t y o f a meso p l a c e m e n t i n t h e c o m p l e t e l y e q u i l i b r a t e d p o l y m e r ( o r i n p o l y s t y r e n e ) . V v a l u e s o f 0.62-0.65 t h u s i m p l y t h a t p o l y s t y r e n e c a n be c h a r a c t e r i z e d b y a σ v a l u e o f 0.44. One d i s t u r b i n g a s p e c t o f t h i s p o r t i o n o f t h e s t u d y i s t h e f a c t t h a t t h e mmrrmm s i g n a l ( 6 = v L 4 5 . 3 ppm) i s n o t a s i n t e n s e i n some o b s e r v e d s p e c t r a ( F i g u r e s 1-4) a s i t s h o u l d be b a s e d on s i m u l a t i o n and on a r g u m e n t s p r e s e n t e d e a r l i e r i n t h i s p a p e r . T h i s may i n d i c a t e an i n f l u e n c e o f n o n a d s , o r a d i m i n i s h e d s e n s i t i v i t y o f c a r b o n s i n t h i s e n v i r o n m e n t due t o r e l a x a t i o n t i m e o r NOE d i f f e r ences. A d d i t i o n a l study of t h i s p o i n t i s merited. M e t h y l e n e and M e t h i n e C a r b o n R e s o n a n c e s . F i g u r e 10 compares t h e m e t h y l e n e and m e t h i n e c a r b o n r e s o n a n c e p a t t e r n s o b s e r v e d f o r p o l y s t y r e n e and t h e e p i m e r i z e d p o l y m e r s . The m e t h y l e n e c a r b o n s p e c t r a are too noisy t o j u s t i f y q u a n t i t a t i v e study. An a n a l y s i s o f s p e c t r a r e c o r d e d u s i n g a l a r g e r number o f FID a c c u m u l a t i o n s w i l l be reported subsequently. However, t h e p a t t e r n s o b s e r v e d a r e q u a l i t a t i v e l y s i m i l a r t o those d i s c u s s e d e a r l i e r t h a t were r e c o r d e d w i t h a 20 MHz s p e c t r o m e t e r ( 1 8 ) . The m e t h i n e c a r b o n r e s o n a n c e p a t t e r n s o c c u r r e d o v e r a s m a l l c h e m i c a l s h i f t r a n g e a n d were t h e r e f o r e adequately d e f i n e d f o r q u a n t i t a t i v e study. We r e p o r t e d p r e v i o u s l y that the methine carbon resonance of these polymers occurs i n two g e n e r a l a r e a s a n d a s s i g n e d t h e l o w e r f i e l d a r e a t o m m - t r i a d s . F i g u r e 11 compares t h e p r o p o r t i o n o f m e t h i n e c a r b o n r e s o n a n c e o b s e r v e d i n t h i s l o w e r a r e a w i t h mm-contents m e a s u r e d f o r t h e p o l y mers f r o m t h e i r m e t h i n e p r o t o n r e s o n a n c e p a t t e r n s . I t c a n be s e e n t h a t t h e r e i s a 1:1 c o r r e s p o n d e n c e b e t w e e n t h e s e two q u a n t i t i e s , t h u s p r o v i n g t h a t t h e l o w e r f i e l d m e t h i n e c a r b o n r e s o n a n c e i s due to mm-triads. 2

2

c

Conclusions Although the aromatic C - l carbon resonance of p o l y s t y r e n e i s very complex, r e l a t i v e l y simple aromatic C - l carbon resonance s p e c t r a are observed f o r p a r t i a l l y epimerized i s o t a c t i c p o l y s t y r e n e s . S t u d i e s on s u c h "model p o l y s t y r e n e s " p r o v i d e t h e i n f o r m a t i o n needed to i n t e r p r e t t h e spectrum o f p o l y s t y r e n e i t s e l f . B a s e d on a s s i g n -





F i g u r e 10.

75 MHz M e t h y l e n e and M e t h i n e C a r b o n R e s o n a n c e o f P o l y s t y r e n e and o f E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Samples.


219


220 NMR AND MACROMOLECULES


13.



221

1

merits made i n t h i s p a p e r a n d i n o t h e r s i n t h i s s e r i e s , t h e H and C-NMR s p e c t r a o f p o l y s t y r e n e i n d i c a t e t h a t i t c a n be c h a r a c t e r i z e d by a σ v a l u e o f ^ 0 . 4 5 . 13

A c k n o w l e dgment s T h i s s t u d y was s u p p o r t e d i n p a r t b y a g r a n t f r o m t h e N a t i o n a l S c i e n c e F o u n d a t i o n (DMA-80-10709).

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RECEIVED

November 3, 1983


75-MHz 13C NMR Studies on Polystyrene and Epimerized Isotactic

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