13C NMR in Polymer Quantitative Analyses

9 13

C NMR in Polymer Quantitative Analyses

Downloaded by SUNY STONY BROOK on December 17, 2014 | http://pubs.acs.org Publication Date: March 28, 1984 | doi: 10.1021/bk-1984-0247.ch009

J. C. RANDALL and E. T. HSIEH

Phillips Petroleum Company, Research and Development, Bartlesville, OK 74004 Carbon 13 nuclear magnetic resonance can be used quantitatively in analyses of polymers to measure conveniently comonomer concentrations, average sequence lengths, run numbers and comonomer triad distributions. The identification of both short chain and long chain branches in polyethylene at concentrations of 1 per 10,000 carbon atoms has become feasible with the availability of improved probes and improved computer hardware/ software capabilities. Reviewed in this chapter are the methods and computations as well as the basic requirements for sound quantitative analyses: namely, correct choice of solvent, a consideration of concentration effect on line widths and satisfying nuclear Overhauser effects and spin lattice relaxation time requirements. Finally, the NMR generated structural information is put to use in correlations with polyethylene physical properties and measurements of number average molecular weight. T h e i m p o r t a n c e of 1 3 c N M R i n d e t e r m i n i n g p o l y m e r s t r u c t u r e has b e c o m e c l e a r l y e v i d e n t t h r o u g h the abundance of l i t e r a t u r e on the subject d u r i n g the past s e v e r a l y e a r s (1)(2). Sequence d i s t r i b u t i o n s i n v o l v i n g as many as s e v e n c o n t i g u o u s m o n o m e r u n i t s have been r e p o r t e d i n studies o f p o l y m e r c o n f i g u r a t i o n (3)(4) and t r i a d i n f o r m a t i o n has been c o n v e n i e n t l y e x t r a c t e d f r o m 1 3 c N M R s p e c t r a o f many c o p o l y m e r s (5-9). M u c h o f the e a r l y emphasis i n 1 3 c N M R studies o f p o l y m e r s was on c h e m i c a l shift assignments (2)(10)(11). A f t e r a l l , t h i s was an e s s e n t i a l p r e r e q u i s i t e t o any p o l y m e r 1 3 c N M R s t r u c t u r a l a n a l y s i s . R e l a x a t i o n t i m e and n u c l e a r O v e r h a u s e r e f f e c t m e a s u r e m e n t s w e r e also o f c o n s i d e r a b l e i n t e r e s t because o f the i n f o r m a t i o n p r o v i d e d about p o l y m e r d y n a m i c s and r e q u i s i t e e x p e r i m e n t a l c o n s i d e r a t i o n s . T h e last a r e a i n N M R studies t o be f u l l y e x p l o i t e d was the q u a n t i t a t i v e m e t h o d . It is n a t u r a l f o r the p o l y m e r c h e m i s t t o e x p e c t q u a n t i t a t i v e i n f o r m a t i o n once the p o w e r o f the m e t h o d i n d e c i p h e r i n g p o l y m e r s t r u c t u r e has b e c o m e e v i d e n t .

0097-6156/ 84/ 0247-0131 $06.25/ 0 © 1984 American Chemical Society In NMR and Macromolecules; Randall, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


132

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MACROMOLECULES

Improvements in instrumentation have led to increased s p e c t r a l s e n s i t i v i t y and t o m o r e r e l i a b l e methods f o r d a t a i n t e g r a t i o n . These i m p r o v e m e n t s c o u p l e d w i t h l a r g e r , more e f f i c i e n t and f a s t e r c o m p u t e r s have g i v e n q u a n t i t a t i v e 1 3 c N M R a p p l i c a t i o n s the boost r e q u i r e d f o r b r o a d useage. A s e n s i t i v i t y o f at least one s t r u c t u r a l unit p e r 10,000 c a r b o n a t o m s is r e q u i r e d f o r useful m o l e c u l a r w e i g h t and b r a n c h i n g m e a s u r e m e n t s and t o a l l o w s p e c i e s p r o d u c e d by o x i d a t i o n and i r r a d i a t i o n (12)(13) t o be d e t e c t e d e a r l y i n the p r o c e s s . T h e r e are a n u m b e r o f c o n s i d e r a t i o n s t h a t must be addressed w h e n f o r m u l a t i n g q u a n t i t a t i v e 1 3 c N M R p r o c e d u r e s - these i n c l u d e s o l v e n t e f f e c t s , s p e c t r a l o v e r l a p , l i n e w i d t h s , d y n a m i c and n u c l e a r O v e r h a u s e r e f f e c t s and d e t a i l e d a s s i g n m e n t s . T h e steps r e q u i r e d t o d e v e l o p sound q u a n t i t a t i v e methods w i l l be the subject o f t h i s c h a p t e r . It is i m p e r a t i v e t h a t e x c e l l e n t q u a n t i t a t i v e methods be e s t a b l i s h e d so t h a t NMR c a n be u t i l i z e d i n studies o f p o l y m e r structure-property r e l a t i o n s h i p s . P o l y m e r m o l e c u l a r s t r u c t u r e needs t o be r e l a t e d t o the i n c i p i e n t s o l i d s t a t e s t r u c t u r e and u l t i m a t e l y t o o b s e r v e d s o l i d s t a t e p h y s i c a l p r o p e r t i e s such as d e n s i t y , f l e x u r a l m o d u l i , e n v i r o n m e n t a l stress c r a c k i n g b e h a v i o r , t o n a m e a f e w . T h e usefulness o f 1 3 c N M R i n q u a n t i t a t i v e analyses o f p o l y m e r s is q u i t e b r o a d and c o v e r s a w i d e number of b o t h a d d i t i o n and c o n d e n s a t i o n p o l y m e r s . F o r the sake o f b r e v i t y , the p r e s e n t d i s c u s s i o n c o n c e r n i n g t h e d e v e l o p m e n t o f q u a n t i t a t i v e p r o c e d u r e s w i l l be l i m i t e d t o e t h y l e n e - 1 o l e f i n c o p o l y m e r s and a f e w high d e n s i t y p o l y e t h y l e n e s . These p o l y e t h y l e n e s a l l o w the t y p i c a l N M R p r o b l e m s e n c o u n t e r e d t o be e x p l o r e d w h i l e , at the s a m e t i m e , they s e v e r e l y t e s t the i n s t r u m e n t a l d y n a m i c range c a p a b i l i t i e s . A l t h o u g h p o l y e t h y l e n e s have a r e l a t i v e l y s i m p l e r e p e a t unit s t r u c t u r e , t h e y may c o n t a i n a v a r i e t y o f short c h a i n branches, l o n g c h a i n branches and d i f f e r e n t t y p e s o f end groups. In l i n e a r l o w d e n s i t y p o l y e t h y l e n e s , the c o m o n o m e r s e q u e n c i n g may o f f e r o v e r l a p and a s s i g n m e n t p r o b l e m s . The p r i n c i p l e s e s t a b l i s h e d i n t h e s e q u a n t i t a t i v e studies c a n be u s e f u l l y a p p l i e d t o o t h e r p o l y m e r s y s t e m s o f i n t e r e s t because t h e s a m e b a s i c c o n s i d e r a t i o n s must be addressed. In the f o l l o w i n g s e c t i o n s , the c h o i c e of s o l v e n t , c o n c e n t r a t i o n e f f e c t s , d y n a m i c e f f e c t s , w a y s t o a v o i d assignment p r o b l e m s , sequence analyses, l o n g c h a i n b r a n c h i n g and m o l e c u l a r w e i g h t m e a s u r e m e n t s w i l l be discussed in d e t a i l . E x p e r i m e n t a l V a r i a b l e s i n Q u a n t i t a t i v e N M R Studies o f P o l y m e r s C h o i c e o f S o l v e n t . T h e most a p p r o p r i a t e s o l v e n t f o r N M R studies o f p o l y m e r s w o u l d a l l o w a range of p o l y m e r c o n c e n t r a t i o n s t o be i n v e s t i g a t e d , be f r e e o f o v e r l a p p r o b l e m s and h o p e f u l l y p r o v i d e a s i g n a l for i n t e r n a l l o c k . N o t a l l o f these c o n d i t i o n s c a n u s u a l l y be met as m a n y h i g h m o l e c u l a r w e i g h t p o l y m e r s pose s o l u b i l i t y p r o b l e m s and c a n be e x a m i n e d in o n l y a l i m i t e d n u m b e r of s o l v e n t s . D e u t e r i u m resonance is the t y p i c a l c h o i c e f o r an i n t e r n a l l o c k s i g n a l on most m o d e r n N M R spectrometers. U n f o r t u n a t e l y , the m a j o r i t y o f a v a i l a b l e d e u t e r a t e d s o l v e n t s are poor s o l v e n t s f o r many a d d i t i o n p o l y m e r s such as the p o l y o l e f i n s w h i l e i t is g e n e r a l l y possible t o f i n d a n u m b e r o f a p p r o p r i a t e d e u t e r a t e d s o l v e n t s f o r many o f the c o n d e n s a t i o n p o l y m e r s . The

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


9.

R A N D A L L A N D HSIEH

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C NMR in Polymer Quantitative

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c h l o r i n a t e d b e n z e n e s a r e t h e best s o l v e n t s f o r N M R s t u d i e s o f p o l y e t h y l e n e s a n d v i n y l p o l y m e r s . In p a r t i c u l a r , 1 , 2 , 4 - t r i c h l o r o b e n z e n e has a h i g h b o i l i n g point (213.5 C ) , poses no o v e r l a p p r o b l e m s i n t h e 0-100 ppm (TMS) range, allows a significant range o f p o l y e t h y l e n e c o n c e n t r a t i o n s t o be i n v e s t i g a t e d and is s t a b l e o v e r t h e l o n g d a t a acquisition periods required for 1 3 c N M R analyses. Usually a small a m o u n t o f p e r d e u t e r o b e n z e n e c a n be added t o 1 , 2 , 4 - t r i c h l o r o b e n z e n e t o p r o v i d e a s i g n a l f o r l o c k purposes. If o n l y a f e w hours o f d a t a a c c u m u l a t i o n a r e r e q u i r e d , m a n y s u p e r c o n d u c t i n g m a g n e t s y s t e m s have s u f f i c i e n t s t a b i l i t y t o g i v e e x c e l l e n t s p e c t r a w i t h o u t u s i n g any l o c k s o l v e n t . F o r o v e r n i g h t and l o n g e r runs, a l o c k s o l v e n t is p r e f e r r e d . Effects of P o l y m e r Concentration. A normal approach when attempting t o d e t e c t l o w q u a n t i t i e s o f s t r u c t u r a l units i n p o l y o l e f i n s is t o p r e p a r e a s o l u t i o n as c o n c e n t r a t e d as possible f o r t h e N M R s t u d y . T h e p o l y m e r signal strength p e r free induction decay will i m p r o v e as t h e concentration increases. A f a c t o r not o f t e n c o n s i d e r e d t o be o f i m p o r t a n c e i n q u a n t i t a t i v e N M R studies o f p o l y m e r s is t h e e f f e c t o f c o n c e n t r a t i o n upon r e s o n a n c e l i n e w i d t h . F o r most p o l y o l e f i n s , t h e resonance l i n e w i d t h i n c r e a s e s as the c o n c e n t r a t i o n o f t h e p o l y m e r is increased. C a r b o n 13 N M R s p e c t r a o f a M a r l e x 6003 h i g h d e n s i t y p o l y e t h y l e n e ( M = 140,000, M = 20,000) a r e shown i n F i g u r e l a at a c o n c e n t r a t i o n o f 5 0 % by w e i g h t i n 1 , 2 , 4 - t r i c h l o r o b e n z e n e and i n F i g u r e l b at a c o n c e n t r a t i o n o f 15% by w e i g h t w i t h t h e r e m a i n d e r o f t h e experimental conditions being the same. (The n o m e n c l a t u r e is g i v e n under S e c t i o n 3.) T h e r e s o n a n c e l i n e w i d t h at o n e - h a l f h e i g h t f o r t h e major recurring methylene resonance, δ+δ+ , changed from a p p r o x i m a t e l y 1.0 H z at 15 w e i g h t % t o a p p r o x i m a t e l y 10 H z at 50 w e i g h t % . A l t h o u g h t h e end groups r e s o n a n c e s have l a r g e r a r e a s at a c o n c e n t r a t i o n o f 5 0 % r e l a t i v e t o 15%, t h e s i g n a l t o noise is a c t u a l l y b e t t e r f o r t h e s p e c t r u m w i t h t h e more n a r r o w l i n e w i d t h s . T h e s p e c t r u m in F i g u r e l a was o b t a i n e d a f t e r 4,992 F I D s w h i l e t h a t i n F i g u r e l b was o b t a i n e d a f t e r 5,518 F I D ' s . If the F I D a c c u m u l a t i o n f o r t h e 15% by w e i g h t s a m p l e is a l l o w e d t o c o n t i n u e t o 20,683, t h e s p e c t r u m shown i n F i g u r e 2 is o b t a i n e d . T h e s i g n a l t o noise is now s u c h t h a t one s t r u c t u r a l unit i n 10,000 c a r b o n a t o m s c a n be d e t e c t e d . Evidence that indicates the p r e s e n c e o f l o n g c h a i n b r a n c h i n g has now been o b s e r v e d as i n d i c a t e d by t h e weak r e s o n a n c e s f o r t h e m e t h i n e , and β δ + c a r b o n s f o r a l o n g chain branch. w

n

f

βδ + « δ +

α

δ + βδ+

-CH2-CH2-CH-CH2-CH2-

CH2

CH

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Methine

38.19 p p m ( f r o m T M S ) 34.55

βδ +

27.20



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Figure 1

50.3 M H z 13c N M R S p e c t r a o f 6003 P E at 1 2 5 ° C at (a) 50% by Weight and (b) 15% by Weight i n 1,2,4-Triehlorobenzene

3s

40 Figure 2

!

30

20

PPM, TMS

A 50.3 M H z 1 3 c N M R S p e c t r u m o f 6003 P E at 1 2 5 ° C at 15% Weight i n 1 , 2 , 4 - T r i e h l o r o b e n z e n e a f t e r an A c c u m u l a t i o n o f 20,683 FID's



9.

13



135

F r o m a c o m p a r i s o n o f peak heights, t h e l o n g c h a i n b r a n c h i n g i s a p p r o x i m a t e l y 1 per 10,000 carbons. Another desirable result from spectra obtained with narrow line w i d t h s is t h a t o v e r l a p is r e d u c e d not o n l y f o r c l o s e l y s p a c e d r e s o n a n c e s but also f r o m t h e L o r e n t z i a n " t a i l s " w h i c h i n f l u e n c e s m a l l r e s o n a n c e s w i t h i n 10 p p m o f a major resonance s u c h as the δ+δ + r e s o n a n c e i n p o l y e t h y l e n e . I n t e g r a t i o n o f the end group r e s o n a n c e s i n F i g u r e 2 leads to a n u m b e r a v e r a g e m o l e c u l a r w e i g h t o f 18,900. A t polymer c o n c e n t r a t i o n s above 1 5 % by weight, t h e m e a s u r e d number a v e r a g e m o l e c u l a r w e i g h t b e c o m e s a f u n c t i o n o f c o n c e n t r a t i o n as t h e o v e r l a p f r o m the s t r o n g t a i l o f the δ+δ+ r e s o n a n c e b e c o m e s m o r e severe w i t h increasing concentration. F o r p o l y e t h y l e n e s , 1 5 % by weight is a n o p t i m u m c o n c e n t r a t i o n i n t e r m s o f line w i d t h versus s i g n a l s t r e n g t h . F o r o t h e r p o l y m e r s , t h e c o n c e n t r a t i o n e f f e c t s w o u l d have t o be e s t a b l i s h e d independently d e p e n d i n g upon the p r o x i m i t y o f the v a r i o u s resonances a n d the r e l a t i v e s i g n a l s t r e n g t h s . A c c o r d i n g l y , i t may w e l l be possible t o w o r k s u c c e s s f u l l y w i t h h i g h e r c o n c e n t r a t i o n s i n p o l y m e r systems other than polyethylene. R e l a x a t i o n T i m e a n d N u c l e a r O v e r h a u s e r E f f e c t s . It is w e l l k n o w n t h a t f o r 90° pulse angles, a pulse s p a c i n g of 5 χ Τχ w i l l ensure t h a t 99% o f r f e x c i t e d n u c l e i w i l l be f u l l y r e l a x e d b e t w e e n pulses (14). S u c h pulse spacings w i l l ensure r e l i a b l e q u a n t i t a t i v e r e s u l t s a n d are r e c o m m e n d e d a l t h o u g h i t is possible to o b t a i n q u a n t i t a t i v e l y r e l i a b l e r e s u l t s w i t h l o w e r pulse angles. T h e f o l l o w i n g r e l a x a t i o n t i m e (Τχ) d a t a was o b t a i n e d f o r a 97/3 ethylene-1-hexene c o p o l y m e r : γδ+ βδ+ « δ + 1.7 1.1 1.3 1.3 1.8 2.9 8 8 7 ~CH2-CH2-CH2-CH-CH2-CH2-CH2-(CH2)n-CH2-CH2-CH2-CH3 4B4

CH

2

1.1

2

2.0

δ + δ+

4s

3s

2s

Is

I

3B4

2B4 IB4

CH

I C H 2 4.4 I CH3 7

It is evident f r o m these d a t a that use o f only t h e p o l y m e r b a c k b o n e c a r b o n resonances i n a q u a n t i t a t i v e t r e a t m e n t c o u l d l e a d t o an e f f i c i e n t NMR experiment. The spin-lattice relaxation times increase p r o g r e s s i v e l y f o r carbons t o w a r d s the ends of the b u t y l b r a n c h e s and the c h a i n ends. S i m i l a r s p i n - l a t t i c e r e l a x a t i o n t i m e d a t a was o b t a i n e d f r o m the i n v e r s i o n r e c o v e r y (15) and p r o g r e s s i v e s a t u r a t i o n (16) methods. T h e p e r f e c t 9 0 ° pulse angle e x p e r i m e n t w o u l d have a pulse s p a c i n g o f 40 seconds. T h i s p a r t i c u l a r ethylene-1-hexene c o p o l y m e r h a d a n u m b e r a v e r a g e m o l e c u l a r weight o f 5,300 and a w e i g h t a v e r a g e m o l e c u l a r weight o f 31,100 w h i c h a f f o r d e d an o p p o r t u n i t y t o o b t a i n the end group s p i n - l a t t i c e r e l a x a t i o n times. N u c l e a r O v e r h a u s e r e f f e c t s (14) c a n a r i s e f r o m energy t r a n s f e r f r o m the p r o t o n n u c l e a r spin r e s e r v o i r t o the 1 3 c n u c l e a r spin r e s e r v o i r



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d u r i n g p r o t o n h e t e r o n u c l e a r spin d e c o u p l i n g . It is possible f o r the N O E t o v a r y a m o n g v a r i o u s types o f p o l y m e r carbons, p a r t i c u l a r l y f o r p r o t o n a t e d versus n o n p r o t o n a t e d c a r b o n s i n s y s t e m s h a v i n g h i g h l y restricted molecular mobilities. Q u i t e o f t e n f u l l N O E ' s (-3.0) are o b s e r v e d f o r p o l y m e r s because the d i p o l e - d i p o l e r e l a x a t i o n m e c h a n i s m (14) w i l l p r e d o m i n a t e because t h e r e is a r e s t r i c t e d o v e r a l l m o l e c u l a r m o b i l i t y y e t t h e r e are s u f f i c i e n t i n t e r n a l s e g m e n t a l m o t i o n s t o a l l o w f o r a f u l l N O E . When N O E ' s do v a r y a m o n g p o l y m e r carbons of d i f f e r e n t t y p e s , t h e r e are s e v e r a l courses o f a c t i o n . T h e N O E c a n be m e a s u r e d d i r e c t l y t h r o u g h g a t e d d e c o u p l i n g e x p e r i m e n t s (17) and e a c h o b s e r v e d resonance i n t e n s i t y c a n t h e n be c o r r e c t e d f o r N O E d i f f e r e n c e s . If the s i g n a l t o noise r a t i o is such t h a t a c c u r a t e N O E ' s cannot be o b t a i n e d , it w o u l d be w i s e to u t i l i z e g a t e d d e c o u p l i n g t o p r e c l u d e the p o s s i b i l i t y o f differences in N O E contributions. N u c l e a r O v e r h a u s e r e f f e c t m e a s u r e m e n t s w e r e made f o r the 97/3 ethylene-l-hexene copolymer through gated decoupling experiments (17). T h e N O E was f u l l (-3.0) f o r a l l c a r b o n s i n c l u d i n g the ends o f branches and end groups. T h i s r e s u l t i n d i c a t e s t h a t the d i p o l e - d i p o l e r e l a x a t i o n m e c h a n i s m p r e d o m i n a t e s s u g g e s t i n g , once a g a i n , a l i m i t e d o v e r a l l m o l e c u l a r m o b i l i t y but indicating appreciable segmental m o t i o n s . T h i s is a most f o r t u n a t e o c c u r r e n c e s i n c e f u l l N O E ' s g r e a t l y s i m p l i f y the n u m b e r o f steps r e q u i r e d i n p e r f o r m i n g q u a n t i t a t i v e analyses on 1 3 c N M R d a t a f r o m p o l y m e r s . W i t h N O E and Τ χ i n f o r m a t i o n about the ethylene-l-hexene c o p o l y m e r , we are i n a p o s i t i o n t o d e s i g n an e f f i c i e n t e x p e r i m e n t f o r free i n d u c t i o n d e c a y (FID) a c c u m u l a t i o n s . Two experiments were p e r f o r m e d : (a) a 9 0 ° pulse a n g l e , (pulse w i d t h = 10.5 ps) a 15 s e c o n d pulse d e l a y and a 1.0 s e c o n d a c q u i s i t i o n t i m e , and (b) - 3 0 ° pulse a n g l e , (pulse w i d t h = 3.5 ps) no pulse d e l a y and a 1.0 s e c o n d a c q u i s i t i o n t i m e . C a r b o n 13 N M R s p e c t r a are p r e s e n t e d i n F i g u r e 3a f o r the 9 0 ° pulse angle e x p e r i m e n t and i n F i g u r e 3b f o r the - 3 0 ° pulse angle e x p e r i m e n t . The o b s e r v e d peak heights and r e l a t i v e areas are p r e s e n t e d in T a b l e I. The n u m b e r of F I D ' s a c c u m u l a t e d f o r the - 3 0 ° pulse angle e x p e r i m e n t was 14,400 w h i l e o n l y 1,441 w e r e a c c u m u l a t e d for the 9 0 ° pulse angle e x p e r i m e n t . T h e - 3 0 ° pulse angle e x p e r i m e n t r e q u i r e d four hours o f i n s t r u m e n t t i m e whereas the 9 0 ° pulse angle e x p e r i m e n t r a n f o r 6.4 hours. S i g n i f i c a n t t i m e savings c a n be r e a l i z e d w i t h l o w e r pulse angles when 1 3 c n u c l e i are present w i t h s u b s t a n t i a l l y l o n g r e l a x a t i o n t i m e s . A s c a n r e a d i l y be seen i n F i g u r e 3a, the s i g n a l t o noise r a t i o is m u c h b e t t e r f o r the 9 0 ° pulse angle s p e c t r u m . (The pulse angle at 3.5 ps is l i k e l y less t h a n 3 0 ° . A p p r o x i m a t e l y 1 ps is r e q u i r e d t o t u r n the pulse on.) T h e m o l e p e r c e n t 1-hexene, u s i n g a m e t h o d w h i c h w i l l be p r e s e n t e d s h o r t l y , was 3.1 f o r the 9 0 ° pulse angle e x p e r i m e n t and 3.5 f o r the - 3 0 ° pulse angle e x p e r i m e n t . T h e m e t h y l resonance w i t h the longest Τ χ was not used i n t h i s d e t e r m i n a t i o n o f m o l e p e r c e n t 1-hexene a l t h o u g h 2 B 4 was u t i l i z e d . If one only uses resonances w h i c h have a Τ χ less t h a n 2.0 seconds, s i m i l a r r e s u l t s are o b t a i n e d w i t h a v a l u e of 3.3 m o l e p e r c e n t for the 9 0 ° e x p e r i m e n t and 3.8 m o l e p e r c e n t f o r the - 3 0 ° e x p e r i m e n t . These r e s u l t s i n d i c a t e t h a t a pulse s p a c i n g o f less t h a n 5 χ Τχ w i l l not l e a d t o serious e r r o r s i f the pulse s p a c i n g is s t i l l g r e a t e r t h a n 3 χ Τ χ f o r the s l o w e s t r e l a x i n g nucleus and most o f the Ï 3 c n u c l e i are t o t a l l y


9.


B

C NMR in Polymer Quantitative

137

Analyses


Table I R e l a t i v e P e a k H e i g h t s and I n t e g r a t e d A r e a s f r o m 50.3 M H z 1 3 c N M R Spectra of a 97/3 E t h y l e n e - l - H e x e n e Copolymer U t i l i z i n g Different P u l s e A n g l e s and P u l s e S p a c i n g s .

Line

PPM.TMS

Areas

Areas

0.014

0.015

0.048

0.054

0.008

0.011

0.860

0.844

1.

38.15

2.

34.55

3.

34.15

4.

32.17

5.

30.47

6.

29.98

7.

29.55

8.

29.52

9.

27.28

0.029

0.031

10.

23.36

0.014

0.016

11.

22.86

0.008

0.008

12.

14.08 0.020

0.022

13.

14.03



138


satisfied dynamically by the experimental pulse spacing. Quantitatively, the results are similar for the two experiments utilizing different pulse angles. The determination of mole percent 1-hexene in the previous example was based on relative areas measured by spectral integration. The use of peak heights gives mole percents 1-hexene of 2.8 from the 9 0 ° data and 2.3 from the - 3 0 ° data. Relative areas are normally recommended because any difference in resonance line widths will be taken into consideration through a use of relative areas. Once a reliable 13c N M R spectrum of a polymer is obtained, one must still face problems associated with resonance overlap and detailed assignments. If the polymer solution is too concentrated and strong signals are present, overlap from Lorentzian "tails" can lead to large errors in a quantitative determination. A satisfactory way to avoid some of these problems is given in the following section. Method of Collective Assignments. One way to avoid errors introduced into a quantitative analysis of 13c N M R data of polymers by spectral overlap and complex fine structure is to divide the observed spectrum into spectral regions sufficiently broad to contain overlapping resonances and fine structure. Closely spaced resonances will often result from a high chemical shift sensitivity to either configuration or comonomer sequences. Configurational sensitivities often lead to observable, but incompletely resolved pentads and heptads where the various pentads and heptads are grouped according to common central triads which are sufficiently resolved as separate multiplets. The methyl region of polypropylene is a good example of such behavior. As shown in Figure 4, sequence sensitivity is to both pentads and heptads yet the three basic triad centers are well resolved as separate complex multiplets. Under such circumstances a triad assignment is all that is required because of the necessary triad-pentad and pentad-heptad relationships. Two examples are given below: mm = mmmm + m mmr + rmmr

(1)

mmmm - mmmmmm + mmmmmr + rmmmmr

(2)

The same principle applies to comonomer sequence chemical shifts. One needs only to identify the basic dyad in tetrads, hexads, etc., or the basic triad in pentads and heptads, etc. The final equations can be expressed in any adjacent complete distribution as desired through use of the appropriate necessary relationships (2) and as dictated by the number of independent spectral observations. The spectral region to be defined should be broad enough to include any unusual chemical shift behavior. A n example best illustrates such an approach; the 97/3 ethylene-l-hexene copolymer shown in Figure 3 and an 83/17 ethylene-l-hexene copolymer shown in Figure 5 serve as useful prototypes. The spectral regions are defined below in terms of spectral range, sequence assignments and contributing carbons. The nomenclature is that used throughout where Greek symbols are used to denote the nearest branched carbons in both directions from a


9.

13

R A N D A L L A N D H Si E H


139

(A)

βδ3B

2B,

S


4s

(B)

40

20

30

PPM, TMS

10

1 3

F i g u r e 3 50.3 MHz C NMR S p e c t r a o f a 97/3 E t h y l e n e - 1 - H e x e n e Copolymer a t 125° a n d 1 5 % b y W e i g h t i n 1 , 2 , 4 - T r i e h l o r o b e n z e n e a f t e r ( a ) a 90 P u l s e A n g l e and 16 s e c P u l s e I n t e r v a l a n d ( b ) a 30 P u l s e A n g l e a n d 1 s e c P u l s e I n t e r v a l

mmrm

22

21

+ rmrr

20

PPM, TMS

1 3

F i g u r e 4 A 50.3 MHz C NMR S p e c t r u m o f t h e M e t h y l R e g i o n o f an A t a c t i c P o l y p r o p y l e n e a t 125°C i n 1 , 2 , 4 - T r i e h l o r o b e n z e n e (Sample c o u r t e s y o f W a l t e r K a m i n s k y , U n i v . o f Hamburg, W.Germany) In NMR and Macromolecules; Randall, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

140


D


I

1

Ε

Η

A

h

1

30

40 Figure 5

20

PPM, TMS

10

A 50.3 MHz 13c N M R Spectrum of an 83/17 Ethylene-l-Hexene Copolymer at 1 2 5 ° C and 15% by Weight in 1,2,4-Triehlorobenzene


9.


I3


141

methylene carbon of interest. The backbone methylene carbons in poly(l-hexene), for example, are all « α . (For consistency, the final equations are expressed in terms of triads only.)


Region " A "

39.5 to 42 ppm

13C NMR in Polymer Quantitative Analyses

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