Carbon-13 NMR and Polymer Stereochemical Configuration

14 Carbon-13 N M R and Polymer Stereochemical Configuration JAMES C. R A N D A L L

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: July 2, 1979 | doi: 10.1021/bk-1979-0103.ch014

Phillips Petroleum Company, Bartlesville, O K 74004

With the discovery of crystalline polypropylene in the early 1950's, polymer stereochemical configuration was established as a property fundamental to formulating both polymer physical charac teristics and mechanical behavior. Although molecular asymmetry was well understood, polymer asymmetry presented a new type of problem. Both a description and measurement of polymer asymmetry were essential for an understanding of the polymer structure. Technically, each methine carbon in a poly(l-olefin) is asym metric; however, this asymmetry cannot be observed because two of the attached groups are essentially equivalent for long chains. Thus a specific polymer unit configuration can be converted into its opposite configuration by simple end-to-end rotation and sub sequent translation. It is possible, however, to specify relative configurational differences and Natta introduced the terms isotactic to describe adjacent units with the same configurations and syndiotactic to describe adjacent units with opposite configurations (1). Although originally used to describe dyad configura tions, isotactic now describes a polymer sequence of any number of like configurations and syndiotactic describes any number of alternating configurations. Dyad configurations are called mesο i f they are alike and racemic if they are unlike (2). Thus from a configurational standpoint, a poly(l-olefin) can be viewed as a copolymer of meso and racemic dyads. The measurement of polymer configuration was difficult and sometimes speculative until the early 1960's when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configuration al information appeared available but was not unambiguously ac cessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers ap peared on the subject of polypropylene tacticity but none totally resolved the problem (5). 0-8412-0505-l/79/47-103-291$07.00/0 © 1979 A m e r i c a n C h e m i c a l Society

In Carbon-13 NMR in Polymer Science; Pasika, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

292

CARBON-13 NMR IN POLYMER SCIENCE


f

With the advent of C-13 NMR i n the e a r l y 1970 s, the measurement of polymer stereochemical c o n f i g u r a t i o n became r o u t i n e and reasona b l y unambiguous. The advantages of C-13 NMR i n measurements of polymer s t e r e o chemical c o n f i g u r a t i o n a r i s e p r i m a r i l y from a u s e f u l chemical s h i f t range which i s approximately 20 times that of proton NMR. The s t r u c t u r a l s e n s i t i v i t y i s enhanced through an e x i s t e n c e of w e l l separated resonances f o r d i f f e r e n t types of carbon atoms. Overlap i s g e n e r a l l y not a l i m i t i n g problem. The low n a t u r a l abundance (*wl%) of C-13 n u c l e i i s another favorably c o n t r i b u t i n g f a c t o r . S p i n - s p i n i n t e r a c t i o n s among C-13 n u c l e i can be s a f e l y neglected and proton i n t e r a c t i o n s can be e l i m i n a t e d e n t i r e l y through heteronuclear decoupling. Thus each resonance i n a C-13 NMR spectrum represents the carbon chemical s h i f t of a p a r t i c u l a r polymer moiety. In t h i s r e s p e c t , C-13 NMR resembles mass spectrometry because each s i g n a l represents some fragment of the whole polymer molecule. F i n a l l y , carbon chemical s h i f t s are w e l l behaved from an a n a l y t i c a l viewpoint because each can be d i s s e c t e d , i n a s t r i c t l y a d d i t i v e manner, i n t o c o n t r i b u t i o n s from neighboring carbon atoms and c o n s t i t u e n t s . This a d d i t i v e behavior l e d to the Grant and Paul r u l e s ( 6 ) , which have been u s e f u l l y a p p l i e d i n polymer analyses, f o r p r e d i c t i n g alkane carbon chemical s h i f t s . The advantages so c l e a r l y evident when a p p l y i n g C-13 NMR to polymer c o n f i g u r a t i o n a l analyses are not devoid of d i f f i c u l t i e s . The s e n s i t i v i t y of C-13 NMR to s u b t l e changes i n molecular s t r u c t u r e c r e a t e * a wealth o f chemical s h i f t - s t r u c t u r a l i n f o r m a t i o n which must be " s o r t e d out". Extensive assignments are r e q u i r e d because the chemical s h i f t s r e l a t e to sequences from three to seven u n i t s i n l e n g t h . Model compounds, which are o f t e n used i n C-13 analyses, must be very c l o s e s t r u c t u r a l l y to the polymer moiety reproduced. For t h i s reason, appropriate model compounds are d i f f i c u l t to o b t a i n . A model compound found u s e f u l i n p o l y propylene c o n f i g u r a t i o n a l assignments was a heptamethylheptadecane where the r e l a t i v e c o n f i g u r a t i o n s were known ( 7 ) . To be comp l e t e l y accurate, the model compounds should reproduce the c o n f o r mational as w e l l as the c o n f i g u r a t i o n a l polymer s t r u c t u r e . Thus r e f e r e n c e polymers such as predominantly i s o t a c t i c and syndiot a c t i c polymers form the best model systems. Even when a v a i l a b l e , only two assignments are obtained from these p a r t i c u l a r polymers. Pure r e f e r e n c e polymers can be used to generate other assignments as w i l l be discussed l a t e r ( 5 ) . To o b t a i n good q u a n t i t a t i v e C-13 NMR data, one must understand the dynamic c h a r a c t e r i s t i c s of the polymer under study. F o u r i e r transform techniques combined with s i g n a l averaging are normally used to o b t a i n C-13 NMR s p e c t r a . E q u i l i b r i u m c o n d i t i o n s must be e s t a b l i s h e d during s i g n a l averaging to ensure that the experimental c o n d i t i o n s have not l e d to d i s t o r t e d s p e c t r a l informat i o n . The n u c l e a r Overhauser e f f e c t (NOE), which a r i s e s from H - l , C-13 heteronuclear decoupling d u r i n g data a c q u i s i t i o n , must a l s o be considered.


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293

Energy t r a n s f e r , o c c u r r i n g between the H - l and C-13 n u c l e a r energy l e v e l s d u r i n g s p i n decoupling, can l e a d to enhancements o f the C-13 resonances by f a c t o r s between 1 and 3. Thus the s p e c t r a l r e l a t i v e i n t e n s i t i e s w i l l only r e f l e c t the polymerVmoiety concent r a t i o n s i f the NOE's are equal or e l s e taken i n t o c o n s i d e r a t i o n . Experience has shown that polymer NOE s are g e n e r a l l y maximal, and consequently equal, because o f a p o l y m e r i r e s t r i c t e d m o b i l i t y (8) ( 9 ) . To be sure, one should examine the polymer NOE's through e i t h e r gated decoupling or paramagnetic quenching and thereby avoid any m i s i n t e r p r e t a t i o n of the s p e c t r a l i n t e n s i t y data. Let us now d i s c u s s C-13 NMR s p e c t r a from a s e r i e s o f v i n y l polymers to survey the i n f o r m a t i o n a v a i l a b l e concerning polymer stereochemical c o n f i g u r a t i o n . We w i l l l a t e r r e t u r n to the t o p i c s of how C-13 s t r u c t u r a l s e n s i t i v i t y i s e s t a b l i s h e d and how a s s i g n ments are made. As mentioned e a r l i e r , the C-13 c o n f i g u r a t i o n a l s e n s i t i v i t y f a l l s w i t h i n a range from t r i a d to pentad f o r most v i n y l polymers. In n o n c r y s t a l l i n e polypropylenes, three d i s t i n c t regions corresponding to methylene (%/46 ppm) methine (^28 ppm) and methyl (**>20 ppm) carbons are observed i n the C-13 NMR spectrum. (Throughout t h i s d i s c u s s i o n and i n the ensuing d i s c u s s i o n s , the chemical s h i f t s are reported with respect to an i n t e r n a l t e t r a m e t h y l s i l a n e (TMS) standard.) The C-13 spectrum of a t y p i c a l amorphous polypropylene i s shown i n F i g u r e l . Although a c o n f i g u r a t i o n a l s e n s i t i v i t y i s shown by a l l three s p e c t r a l r e gions, the methyl r e g i o n e x h i b i t s by f a r the g r e a t e s t s e n s i t i v i t y and i s consequently o f the most v a l u e . At l e a s t ten resonances, assigned to the unique pentad sequences, a r e observed i n order, mmmm, miiniir, rmmr, mmrr, mmrm, rmrr, mrmr, r r r r , r r r m and mrrm, from low to h i g h f i e l d (7) (10) (11). These assignments w i l l be d i s c u s s e d i n more d e t a i l l a t e r .


1

a

The C-13 spectrum o f p o l y s t y r e n e , shown i n F i g u r e s 2 and 3, c o n t a i n s two regions where stereochemical i n f o r m a t i o n can be ext r a c t e d . There are nine methylene resonances and a t l e a s t 20-22 aromatic quaternary carbon resonances. No other carbons i n p o l y styrene e x h i b i t a c o n f i g u r a t i o n a l s e n s i t i v i t y . Tentative assignments have been made f o r the methylene carbons based on an assumed B e r n o u l l i a n behavior (12). As shown i n F i g u r e 4, the methylene and methine carbons of p o l y v i n y l c h l o r i d e show a s e n s i t i v i t y toward c o n f i g u r a t i o n and comp l e t e , i n t e r n a l l y c o n s i s t e n t assignments have been given by Carman (13). Likewise a s i m i l a r s i t u a t i o n e x i s t s i n the C-13 NMR spectrum of p o l y v i n y l a l c o h o l , shown i n F i g u r e 5, where assignments have been given by Wu and Ovenall (14). The general trend among v i n y l polymers i s f o r the methylene carbons to e x h i b i t a greater c o n f i g u r a t i o n a l s e n s i t i v i t y than the methine carbons. T h i s spectrum and others shown i n t h i s paper were taken from polymers d i s s o l v e d i n 1,2,4-trichlorobenzene a t 125°C.




294


In Carbon-13 NMR in Polymer Science; Pasika, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. Academic

Press

Figure 2. The methylene and methine regions of the C-13 NMR spectrum of PS prepared with a free radical initiator. The internal standard is HMDS, which occurs at 2.03 ppm with respect to TMS (30).


to CO


296


Figure 3.

The aromatic quaternary resonances from a free radical PS shown in Figure 2 (30)



14. RANDALL

Figure 4.

Polymer

Stereochemical

Configuration

C-13 NMR spectrum at 25.2 MHz of PVC (30). See Figure 2 for an explanation of the scale.


297


Figure 5. C-13 NMR spectrum at 25.2 MHz of PVC (SO). See Figure 2 for an explanation of the scale.



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Configuration

299

Exceptions do e x i s t as shown i n the spectrum o f p o l y a c r y l o n i t r i l e i n F i g u r e 6. The methine resonances show a d i s t i n c t t r i p l e t with very l i t t l e s p l i t t i n g e x h i b i t e d by the methylene resonances. The g r e a t e s t s e n s i t i v i t y toward c o n f i g u r a t i o n occurs f o r the n i t r i l e resonances where an almost i d e a l B e r n o u l l i a n d i s t r i b u t i o n i s observed . In general, the trends observed among c o n f i g u r a t i o n a l a s s i g n ments cannot be used to make assignments i n a system where the a s signments are unknown. C e r t a i n l y , s i m i l a r i t i e s do e x i s t as shown by the data i n Tables I, I I and I I I ; however, there are no obvious explanations f o r some o f the g l a r i n g e x c e p t i o n s . The methine r e s onances i n p o l y v i n y l c h l o r i d e occur as r r , mr and mm from low t o high f i e l d . A s i m i l a r trend appears reasonable f o r p o l y a c r y l o n i t r i l e ; however the methine resonances i n p o l y v i n y l a l c o h o l have been shown to occur i n the i n v e r s e o r d e r , mm, mr and r r . In an analogous manner, the methine carbon resonances from polypropylene show a pentad s e n s i t i v i t y with the mmmm pentad o c c u r r i n g a t low field. In t h i s i n s t a n c e , the observed pentads may occur i n an order s i m i l a r t o the methyl pentad resonances (15). An example where independent, but s i m i l a r , assignments were obtained occurs f o r the s i d e - c h a i n c a r b o n y l carbons i n polymethylmethacrylate (16) and the quaternary aromatic carbons o f poly-^Cmethylstyrene (17). Both are sp2 carbons ; otherwise they are i n completely d i f f e r e n t environments. The assignments, as they occur from low to h i g h f i e l d , a r e : ^0=0

polymethylmethacrylate

mrrm, rrrm, r r r r , rmrm+mmrm, rmrr-hnmrr, mmmm, mmmr, rmmr -C=

poly-^C-methylstyrene

mrrm, rrrm, r r r r , rmrr, mmrm, rmrm+mmrr, mmmm +nnnmr +rmmr L i s t e d i n Tables I, I I and I I I are r e p r e s e n t a t i v e examples o f the chemical s h i f t behavior f o r both the backbone methine, methylene carbons and f o r the v a r i o u s pendant s i d e - c h a i n carbons. In almost a l l o f the v i n y l polymers examined so f a r , an unambiguous source f o r c o n f i g u r a t i o n a l i n f o r m a t i o n has been found (18). For example, the backbone methylene resonances f o r the v a r i o u s p o l y v i n y l e t h e r s show a b a s i c dyad s e n s i t i v i t y with the r resonance o c c u r r i n g downfield from the m resonance (19) (20). In p o l y ( m e t h y l a c r y l o n i t r i l e ) one can u t i l i z e the methyl resonances (21). The backbone carbons y i e l d c o n f i g u r a t i o n a l i n f o r m a t i o n i n the C-13 s p e c t r a o f the v a r i o u s p o l y a l k y l a c r y l a t e s (22) (23). The e x t r a c t i o n o f c o n f i g u r a t i o n a l i n f o r m a t i o n , o f course, depends upon the a v a i l a b i l i t y o f c o r r e c t assignments i n s p e c t r a which a r e o f t e n det a i l e d and complex. Let us now t u r n t o the determination of the v a r i o u s s p e c t r a l s e n s i t i v i t i e s and the establishment o f c o n f i g u r a t i o n a l assignments.



CN


mr

50

40

30 PPM,

20

0

10

TMS

Academic Press Figure 6.

C-13 NMR spectrum at 25.2 MHz of PAN (SO). See Figure 2 for an explanation of the scale.


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301

Configuration

Table I The T r i a d Chemical S h i f t Sequence with Respect to an I n c r e a s i n g F i e l d Strength f o r Some Representative V i n y l Polymer Backbone Methine Carbon Resonances

6

-CH-


~H Poly(vinylchloride) Poly(isopropyl aerylate)

rr rr

mr mr

mm mm

Poly(vinyl alcohol) Polypropylene P o l y ( v i n y l acetate)

mm mm mm

mr mr (mr)

rr rr (rr)

Polymers with only S i n g l e t Backbone Methine Resonances Polystyrene P o l y ( e t h y l v i n y l ether) P o l y ( i s o b u t y l v i n y l ether) Poly(methyl a e r y l a t e ) Poly(methyl v i n y l ether)

Table I I The T r i a d Chemical S h i f t Sequence with Respect to an I n c r e a s i n g F i e l d Strength f o r Some Representative V i n y l Polymer Side-Chain Carbon Resonances 6

c H

Polyacrylonitrile (CN) , P o l y ( t e r t i a r y v i n y l ether)(-C-) Polypropylene P olystyrene

(C H3) (-C=)

Poly(methyl v i n y l ' e t h e r )

1

(OCH3)

mm mm mm mm rr

mr mr mr

rr rr rr

—

—

mr

mm



302 Table I I I

The Tetrad Chemical S h i f t Sequence with Respect to an Increasing F i e l d Strength f o r Some Representative V i n y l Polymer Backbone Methylene Carbon Resonances -CH ~

Poly(isopropyl aerylate)

2

Η

rrr

+

rmr,

mmr

+

mrm

+

mrr,

mmm

°-CH - Poly(methyl a c r y l a t e ) Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: July 2, 1979 | doi: 10.1021/bk-1979-0103.ch014

2

Η

rrr

rmr,

mmr -CH -

+

mrr,

mrm

+

mmm

Polypropylene

2

-> Η

mrm

rrr

mrr -CH 2

H

mrr

(rmr)

m

Polystyrene

mmr

-CH -

(mrm)

mmm

(rrr)

P o l y ( v i n y l acetate)

2

->>

Η

rrr

mrr

mrm

-CH mrr

+

-CH ~ 2

rrr

rmr

-CH 2

-CH ~ 2

-CH 2

rmr,

mmr

Poly(vinyl alcohol)

9

H

+

mrm,

rmr

+

mmr,

Poly(vinyl chloride) mrr

mrm

+

mmr,

P o l y ( e t h y l v i n y l ether)

P o l y ( i s o b u t y l v i n y l ether)

Poly(methyl v i n y l ether)



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Configuration

303

The v i n y l polymer s t u d i e d most thoroughly with r e s p e c t t o c o n f i g u r a t i o n has been polypropylene (5) (7) (10) (11) (24) (25) (26). The C-13 NMR spectrum o f a c r y s t a l l i n e polypropylene shown i n F i g u r e 7 contains only three l i n e s which can be i d e n t i f i e d as methylene, methine and methyl from low t o h i g h f i e l d by o f f - r e s o nance d e c o u p l i n g . An amorphous polypropylene e x h i b i t s a C-13 spectrum which contains not only these three l i n e s but a d d i t i o n a l resonances i n each o f the methyl, methine and methylene regions as shown p r e v i o u s l y i n F i g u r e 1. The c r y s t a l l i n e polypropylene must, t h e r e f o r e , be c h a r a c t e r i z e d by a s i n g l e type o f c o n f i g u r a t i o n a l structure. In t h i s case, the c r y s t a l l i n e polypropylene s t r u c t u r e i s predominantly i s o t a c t i c , thus the three l i n e s i n F i g u r e 7 must r e s u l t from some p a r t i c u l a r l e n g t h of meso sequences. This sequence l e n g t h i n f o r m a t i o n i s not a v a i l a b l e from the spectrum o f the c r y s t a l l i n e polymer but can be determined from a corresponding spectrum o f the amorphous polymer. To do so, one must examine the s t r u c t u r a l symmetry o f each carbon atom i n the v a r i o u s p o s s i b l e monomer sequences. L e t us begin by an i n s p e c t i o n o f the p o l y propylene methyl group i n t r i a d and pentad c o n f i g u r a t i o n a l e n v i ronments. (These arguments can be a p p l i e d , o f course, i n a r e l a t e d d i s c u s s i o n o f any v i n y l polymer.) I f the methyl group chemical s h i f t i s s e n s i t i v e t o j u s t nearest neighbor c o n f i g u r a t i o n s , then the simplest c o n f i g u r a t i o n a l s e n s i t i v i t y must be t r i a d , that i s ,

/

C

H

^CH

2

CH

CH

I CH

^CH CH

I 0

CH

^

I 0

CHo

There are only three unique t r i a d combinations, mm, mr and r r ; thus a methyl c o n f i g u r a t i o n a l s e n s i t i v i t y to j u s t n e a r e s t neighbor c o n f i g u r a t i o n s would produce only three resonance i n the methyl r e g i o n o f the C-13 spectrum. From an e a r l i e r spectrum o f the amorphous polymer, we noted a t l e a s t ten methyl resonances. We must t h e r e f o r e consider the s i t u a t i o n where the next-nearest as w e l l as nearest neighbor c o n f i g u r a t i o n s a r e a f f e c t i n g the chemical s h i f t , that i s ,

An i n s p e c t i o n o f the number o f unique c o n f i g u r a t i o n s taken f i v e a t a time (pentads) shows that there a r e t e n unique pentad c o n f i g u r a t i o n s : mmmm, mmmr, rmmr, mmrm, mmrr, rmr, rmrr, mrrm, mrrr and




304

CH3 CH

CH2

JL 50

40

30

PPM, TMS

HMDS

20

10

Academic Press Figure 7.

C-13 NMR spectrum at 25.2 MHz of crystalline PP (30). See Figure 2 for an explanation of the scale.


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Configuration

305

r r r r . Note that there a r e three pentads with common mm c e n t e r s , three with common r r c e n t e r s and four with common mr t r i a d cen t e r s . By now, we can see a p a t t e r n developing. The s i d e c h a i n groups o f a v i n y l polymer w i l l be c o n f i g u r a t i o n a l l y s e n s i t i v e to an odd sequence o f s t r u c t u r a l u n i t s ; that i s , three, f i v e , seven, e t c . The number o f unique combinations (N) f o r a p a r t i c u l a r s e quence l e n g t h (n) can be p r e d i c t e d w i t h the f o l l o w i n g equation (27): 2

η-2

+

2

(η-3)/2


Therefore, when η = 3,

Ν = 3

η = 5,

Ν = 10

η = 7,

Ν = 36

etc.

These same arguments apply t o the polymer backbone methine carbons where the s t r u c t u r a l s e n s i t i v i t y , r e q u i r e d by symmetry, w i l l be t o an odd number o f continuous u n i t s . In a s i m i l a r manner, i t can be shown that the backbone methylene carbons a r e s e n s i t i v e t o an even number o f s t r u c t u r a l u n i t s w i t h the simplest c o n f i g u r a t i o n a l s e n s i t i v i t y being dyad. The number o f unique combinations f o r a p a r t i c u l a r sequence l e n g t h i s g i v e n by (27):

=

2

n-2

+

2

(η-2)/2

where η = 2,

Ν = 2

η = 4,

Ν = 6

η = 6,

Ν = 20

etc.

I f the s t r u c t u r a l s e n s i t i v i t y e x h i b i t e d by C-13 NMR extends beyond next-nearest neighbor c o n f i g u r a t i o n s , an unwieldy number o f con f i g u r a t i o n s must be c o n s i d e r e d . Such has been the case f o r the aromatic quaternary carbon resonances i n p o l y s t y r e n e where 20-22 resonances a r e observed as shown i n F i g u r e 3. When making these assignments, one must c o n s i d e r t h i r t y - s i x p o s s i b l e heptads. Even " c l e a r c u t " analyses may be d e c e p t i v e l y simple. The methyl spec trum o f polypropylene apparently c o n s i s t s o f three t r i a d regions which a r e f u r t h e r subdivided i n t o pentads. However, some o f the pentads may be composites o f o v e r l a p p i n g heptads. This i s p a r t i c u l a r l y t r u e f o r the resonances from the r r - c e n t e r e d sequences.




306

Thus only the assignment problem faced i n C-13 NMR polymer conf i g u r a t i o n a l s t u d i e s can be defined by simply counting the number of observed resonances. F o r t u n a t e l y , f o r most v i n y l polymers, the c o n f i g u r a t i o n a l s e n s i t i v i t y i s predominantly t r i a d - p e n t a d f o r the backbone methine and pendant s i d e - c h a i n carbons and dyadt e t r a d f o r the backbone methylene carbons. Caution should be exe r c i s e d when making assignments because even with the e x i s t e n c e of only a few c o n f i g u r a t i o n a l resonances, the assignments are o f t e n n e i t h e r s t r a i g h t f o r w a r d nor unambiguous. Let us now examine the techniques used i n making c o n f i g u r a t i o n a l assignments i n polypropylene. In p r i n c i p l e , they could be a p p l i e d i n any c o n f i g u r a t i o n a l study. The polypropylene a s s i g n ments have been w e l l e s t a b l i s h e d and are probably the l e a s t e q u i v o c a l of any reported f o r v i n y l polymers. Two c o n f i g u r a t i o n a l assignments can be made without d i f f i c u l t y by comparing the amorphous polymer spectrum with those obtained from r e f e r e n c e polymers c o n s i s t i n g predominantly of i s o t a c t i c and s y n d i o t a c t i c sequences. Roberts, et a l . , i d e n t i f i e d both the mmmm and r r r r pentads i n the methyl r e g i o n u s i n g t h i s approach (24). Randall l a t e r c o r r e c t l y i d e n t i f i e d the c o n f i g u r a t i o n a l sequence of assignments through an extension of the Grant and Paul parameters to account f o r c o n f i g u r a t i o n a l d i f f e r e n c e s (10). F i n a l proof o f the assignments, however, came from a model compound study by Zambelli, et a l . , (7) and an e p i m e r i z a t i o n study by S t e h l i n g and Knox ( 5 ) . L e t ' s f i r s t consider the e p i m e r i z a t i o n study because i t can be more e a s i l y adapted to other polymer systems. An a d d i t i o n of 1% dicumylperoxide plus 4% tris(2,3-dibromopropyl)phosphate to a configurâtionally pure polypropylene r e s u l t s i n w e l l spaced i n v e r s i o n s i f the conversions are d e l i b e r a t e l y kept low. a

0 0 0 0 0 0 0 0 0 0 0 mmmmmmmmmm

e

p

l

m

e

r

i

z

a

t

i

o

£

0 0 0 0 0 1 0 0 0 0 0 mmmmrrmmmm

The new c o n f i g u r a t i o n a l sequences, mmmr, mmrr and mrrm w i l l be produced from the i s o t a c t i c polymer by i s o l a t e d i n v e r s i o n s o f c o n f i g u r a t i o n . The r e l a t i v e i n t e n s i t i e s f o r the mmmr, mmrr and mrrm pentads w i l l be 2:2:1 which allows a p o s i t i v e i d e n t i f i c a t i o n of the mrrm pentad. Correspondingly, the predominantly syndiot a c t i c polymer w i l l have mm t r i a d s i n s e r t e d i n t o r r r sequences by the i n v e r s i o n process. 0 1 0 1 0 1 0 1 0 1 0 r r r r r r r r r r

e

P

i m e r i 2

ation

Q l O l l l O l O l O r r r m m r r r r r

Consequently, the new pentads, mmrr, rrrm and rmmr w i l l be p r o duced i n a 2:2:1 r a t i o . Once again, the rmmr pentad can be i d e n t i f i e d by i t s unique r e l a t i v e i n t e n s i t y . The mmrr pentad i s a

M

A 0 " represents one monomer u n i t c o n f i g u r a t i o n , "1" i s i t s opposite configuration.


14.

RANDALL

Folymev

Stereochemical

307

Configuration

commonly produced between the two i n v e r s i o n experiments and t h e r e f o r e can be p o s i t i v e l y i d e n t i f i e d . The r r r m and mmmr pentads are assigned by d e f a u l t . The e p i m e r i z a t i o n experiment, t h e r e f o r e , leads to an i d e n t i f i c a t i o n o f a l l but the mrmr, mmrm and rmrr pen tads. Most importantly, the resonances from common t r i a d c e n t e r s , that i s , mm, mr and r r have been p o s i t i v e l y i d e n t i f i e d . This t r i a d i n f o r m a t i o n i s s u f f i c i e n t f o r a complete determination of the c o n f i g u r a t i o n a l s t r u c t u r e . The probable " o r d e r " o f the c o n f i g u r a t i o n a l assignments i n polypropylene have been e s t a b l i s h e d by Zambelli, et a l . , (7) and A. P r o v a s o l i and D. R. F e r r o (11) i n a study o f C-13 l a b e l l e d com pounds, 3(S) , 5(R) , 7(RS) , 9(RS) , l l f c S ) , 13(R) , 1 5 ( S ) 4 i e p t a methylheptadecane (compound A) and a mixture or A with 3 ( s ) 5(s) , 7(RS) , 9(RS) , l l ( R S ) ,13(R) > 15(s)-heptamethylheptadecane. The pentad assignments from low to h i g h f i e l d are mmmm, mmmr, rmmr, mmrr, mrrm, rrmr, mrmr, r r r r , mrrr and mrrm i n agreement with the e i g h t assignments by S t e h l i n g and Knox and the order p r e d i c t e d e a r l i e r u s i n g the NMR a d d i t i v i t y r e l a t i o n s h i p s . The polypropylene spectrum appears c o n s i s t e n t with the assignment order observed i n the model compounds; however^heptad s p l i t t i n g i n the mr and r r centered pentads could obscure the true s i t u a t i o n and l e a d to mis placed pentad assignments. I n s p i t e of t h i s d i f f i c u l t y , the t r i a d d i s t r i b u t i o n can be obtained with c o n f i d e n c e . Q u a n t i t a t i v e r e s u l t s can be acquired by e i t h e r i n t e g r a t i n g or curve f i t t i n g the methyl r e g i o n f o r the mm, mr and r r r e l a t i v e i n t e n s i t i e s (28). The most commonly used technique f o r making C-13 NMR s p e c t r a l assignments f o r v i n y l homopolymers has been through the use of B e r n o u l l i a n s t a t i s t i c s (29) (30). I f one knows the r e l a t i v e con c e n t r a t i o n s of e i t h e r m or r , any p a r t i c u l a r "n-ad" d i s t r i b u t i o n can be c a l c u l a t e d because


f

Ρ

=X m

(3)

1 - Ρ =Χ m r

(4)

m and

A c c o r d i n g l y , the r e l a t i v e t r i a d and t e t r a d c o n c e n t r a t i o n s are given by, Triad mm mr, rr

rm

Tetrad

Ρ m 2(1—P )P mm (1-P ) m 2

mmm mmr, rmm rmr mrm mrr, rrm ' rrr

Ρ « m 2P"(1-P ) m m« Ρ (1-P ) m m P (l-P ) m m« 2P (1-P ) nr m « (1-P ) m 2

7


308


I f the m and r mole f r a c t i o n s a r e unknown, one u s u a l l y i t e r a t e s over values f o r P u n t i l s a t i s f a c t o r y agreement i s obtained be tween the c a l c u l a t e d and observed n-ad d i s t r i b u t i o n s . The suc cess o f the method depends upon the existence o f d i s c r i m i n a t i n g d i f f e r e n c e s among the r e l a t i v e i n t e n s i t i e s and a w e l l - r e s o l v e d t r i a d , t e t r a d or "n-ad" d i s t r i b u t i o n . I t i s g e n e r a l l y best ap p l i e d when more than one s p e c t r a l r e g i o n ( f o r example, a t r i a d methine d i s t r i b u t i o n and a t e t r a d methylene d i s t r i b u t i o n ) a r e a v a i l a b l e f o r the c a l c u l a t e d versus observed f i t . Even under the best o f circumstances, unique f i t s may not be obtained. I f inde pendent p a r t i a l assignments a r e a v a i l a b l e ( f o r example, the i s o t a c t i c "n-ad" resonance) the method can be a p p l i e d with more con f i d e n c e . In cases where B e r n o u l l i a n f i t s cannot be obtained, one then r e s o r t s to higher order s t a t i s t i c a l analyses, that i s , f i r s t order Markov or Coleman-Fox (29). Care must be e x e r c i s e d , however, because more parameters are used to f i t the data,thereby reducing the number o f degrees o f freedom i n the a n a l y s i s . In s p i t e o f these d i f f i c u l t i e s , more assignments have been proposed u t i l i z i n g conformity to s t a t i s t i c a l behavior than any other t e c h nique. An example where good B e r n o u l l i a n f i t s were obtained be tween the methine and methylene regions has been reported by Carman f o r p o l y v i n y l c h l o r i d e (13).


m

Methine Resonances Triad

Observed

Calculated Ρ =0.45 m 0.303

rr

0.291

mr

0.520

0.496

mm

0.188

0.202

Methylene: Resonances Tetrad

Observed

Calculated Ρ =0.45 m 0.166

rrr

0.161

rmr

0.146

0.130

rrm

0.282

0.272

mmr+mrm

0.320

0.334

0.092

0.091

mmm

Once assignments a r e made, C-13 NMR "n-ad" d i s t r i b u t i o n s a r e a v a i l a b l e . In general, one would l i k e to o b t a i n a d i s t r i b u t i o n over the longest p o s s i b l e sequence l e n g t h . R e l a t i o n s h i p s , o f t e n r e f e r r e d t o as the "necessary r e l a t i o n s h i p s ' ^ e x i s t between n-ad sequences o f d i f f e r e n t l e n g t h s . I t i s p o s s i b l e to reduce any n-ad d i s t r i b u t i o n to m versus r , which correspond to the simplest comonomer d i s t r i b u t i o n but i s devoid o f any information concerning sequence l e n g t h .


14.

Polymer

RANDALL

Stereochemical

309

Configuration

A few of the necessary r e l a t i o n s h i p s , defined by Bovey dyad-triad

(31)^are

triad-tetrad

m = mm + 1/2

mr

mm

r = r r + 1/2

mr

mr = mrm + 1/2 mrr ^

— mmm

+ 1/2

r r = r r r +1/2

mmr rmr + 1/2

mmr

mrr


etc. These r e l a t i o n s h i p s are a l s o v a l u a b l e f o r t e s t i n g assignments i n d i f f e r e n t regions o f the same C-13 NMR polymer spectrum where d i f f e r e n t c o n f i g u r a t i o n a l s e n s i t i v i t i e s are shown. When measuring v i n y l polymer t a c t i c i t y , one p r e f e r s the longest complete n-ad d i s t r i b u t i o n a v a i l a b l e as w e l l as the t r a n s l a t e d simplest comonomer d i s t r i b u t i o n , p o s s i b l y m versus r . An a l t e r n a t i v e e x i s t s to the m versus r d i s t r i b u t i o n i n the form of number average or mean sequence l e n g t h s . I f any v i n y l homopolymer i s viewed c o n c e p t u a l l y as a copolymer of meso and racemic dyads, mean sequence lengths can be determined f o r continuous runs of both meso and racemic c o n f i g u r a t i o n s (32), that i s , i=n IN (mm) + l/2(mr) r(m) r (5) ±

l/2(mr) =!

r(m) r

ZÊ

m(r).m

±

=ο =η

(rr)

+

l/2(mr)

(6)

1 1/2 (mr)

In a d d i t i o n to viewing a v i n y l homopolymer c o n c e p t u a l l y as a co polymer of meso and racemic dyads, one may a l s o c o n s i d e r the mean sequence l e n g t h o f " l i k e " c o n f i g u r a t i o n s (28). In t h i s i n s t a n c e , the polymer c h a i n i s seen as a succession o f d i f f e r e n t lengths o f c o - o r i e n t e d c o n f i g u r a t i o n s from one to "n", the longest sequence of l i k e c o n f i g u r a t i o n s , that i s , r r m m r r r m m m r r m m r m r r r r r m r r r r r m m 1 0 1 1 1 0 1 0 0 0 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 0 1 1 1

\^

\ y

11

3

11

4

ν 1

3

ν

v

2 1 1 1 1 2 1 1 1 1

3

where, n

l

l

k

e

m

(13)1 + (2)2 + (3)3 + (1)4 13+2

i n the above

m

1

7

+ 3 + 1

example.


310


More g e n e r a l l y , n

_ like = 1

i

k

e

Σ" ι=ο

1 N

Ν


!?"

1(

0 ) ι

Σ™ i= ο

+

l(0).l ι

',Σ

+

1

1 N

0(1).0

(7)

i N

(D

0 ( 1 > i 0

As was true i n the case o f mean sequence lengths f o r meso and racemic dyads, the necessary r e l a t i o n s h i p s can be used to develop corresponding equations f o r any p a r t i c u l a r n-ad d i s t r i b u t i o n . A f a v o r a b l e point concerning the concept o f " l i k e " c o n f i g u r a t i o n s i s that i t attaches a p h y s i c a l s i g n i f i c a n c e t o the racemic d i s tribution. The mean sequence lengths may o f f e r a b e t t e r way to present the simple comonomer d i s t r i b u t i o n than meso, racemic d i s t r i b u t i o n s because they do r e f l e c t the polymer s e q u e n t i a l s t r u c t u r e . In a d d i t i o n , mean sequence lengths may be u s e f u l i n e v a l u a t i n g statistical fits. For example, η

m

=

ή r

=

ή, like

=

2.0

f o r i d e a l l y random B e r n o u l l i a n d i s t r i b u t i o n s when P Generally, f o r B e r n o u l l i a n d i s t r i b u t i o n s , %

-

"like

(8)

m

= 0.5.

(

"

9

)

and n. r

=

1/P m m

(10)

For f i r s t order Markov, ή n

5

m r

=

1/(P , ) m/r'

(11)

-

l/(P

(12)

like

v

-

1

+

r / m

)

» r / « > ^ » / r >

(

1

3

)

Mean sequence lengths can a l s o be used t o w r i t e average con f i g u r a t i o n a l s t r u c t u r e s , that i s , (m)

(r)

5

m

s

r


14.

RANDALL

Polymer

Stereochemical

Configuration

311

In the amorphous polypropylene, shown i n F i g u r e 1, the mean s e quence lengths a r e : n , like 1 4

=

η

2.0

=3.3 m


ή

=

r

3.4

The sequence d i s t r i b u t i o n s are c l e a r l y n o n - B e r n o u l l i a n i n t h i s case although they could s a t i s f y conformity to Markovian behavior as i n d i c a t e d by equations 11-13. The f o l l o w i n g average s t r u c t u r e i s suggested by these observed mean sequence l e n g t h s . 4

11

5

111

4

11

-( 0 0 0 1 0 1 1 1 1 1 0 1 0 1 1 1 1 0 1 0

) _ n

m m m r r r m m m m r r r r m m m r r r T h i s amorphous polypropylene, t h e r e f o r e , has a tendency toward short blocks of meso and racemic sequences. This s t r u c t u r a l con c l u s i o n i s not r e a d i l y apparent from a simple i n s p e c t i o n of the pentad d i s t r i b u t i o n although the dominant pentads are mmmm, mmmr, r r r r , rrrm and mmrr as i n d i c a t e d by the above formula f o r the average c o n f i g u r a t i o n a l s t r u c t u r e . In c o n c l u s i o n , we can see that C-13 NMR o f f e r s p o s s i b l y the best experimental approach now a v a i l a b l e to determine polymer t a c t i c i t y or stereochemical sequence d i s t r i b u t i o n s . The chemical s h i f t s e n s i t i v i t y i s g e n e r a l l y i n the i d e a l range o f dyad to pen tad. Higher s e n s i t i v i t i e s could l e a d to n-ad d i s t r i b u t i o n s which are unwieldy or d i f f i c u l t to a s s i g n . From the work r e p o r t e d i n the l i t e r a t u r e , we f i n d that f r e e r a d i c a l c a t a l y s t s g e n e r a l l y produce c o n f i g u r a t i o n a l d i s t r i b u t i o n s which conform to B e r n o u l l i a n s t a t i s t i c s (30). C a t i o n i c or a n i o n i c c a t a l y s t s or i n i t i a t o r s produce d i s t r i b u t i o n s conforming to e i t h e r Coleman-Fox or higher order Markovian models. B e r n o u l l i a n s t a t i s t i c s have proven to o f f e r a reasonable approach to assignments f o r polymers produced by f r e e r a d i c a l i n i t i a t o r s . The model compounds f o r polypropylene have apparently given the c o r r e c t pentad methyl a s signments and may o f f e r a u s e f u l approach to assignments i n other polymers. E p i m e r i z a t i o n i s one o f the more imaginative t e c h niques but so f a r has seen only a l i m i t e d a p p l i c a t i o n . When d e s c r i b i n g polymer t a c t i c i t y , one should attempt to ob t a i n the h i g h e s t complete "n-ad" d i s t r i b u t i o n a v a i l a b l e as w e l l as a simple "comonomer" d i s t r i b u t i o n . In connection w i t h such a measurement, the mean sequence lengths may o f f e r a v i a b l e a l t e r n a t i v e to the simple m versus r d i s t r i b u t i o n . Useful r e l a t i o n s h i p s , which are h e l p f u l i n e s t a b l i s h i n g p a r t i c u l a r s t a t i s t i c a l behaviors, are a v a i l a b l e .



312

I b e l i e v e we are j u s t on the threshold of the C-13 NMR ap p l i c a t i o n s i n s t u d i e s of polymer chemistry. D e t a i l e d s t r u c t u r e s are a v a i l a b l e f o r c o r r e l a t i o n s with p h y s i c a l p r o p e r t i e s . Dif ferences i n c o n f i g u r a t i o n a l s t r u c t u r e produced by v a r i o u s c a t a l y s t s can be a c c u r a t e l y determined. For example, the " i r r e g u l a r i t i e s " i n c r y s t a l l i n e polypropylene s t r u c t u r e have been shown to be 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 mmmmmmmmmmm r r mmmmmmmmmmm


as opposed to 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 mmmmmmmmmmmrmmmmmmmmmmmm Although not p r e s e n t l y reported, the l a t t e r s t r u c t u r e could be e a s i l y detected i f i t were, i n f a c t , produced by an appropriate c a t a l y s t system. F i n a l l y , the c o n f i g u r a t i o n a l s t r u c t u r e i n copolymers i s cur r e n t l y a t o p i c i n v e s t i g a t e d by many i n d u s t r i a l s c i e n t i s t s . I s c o n f i g u r a t i o n r e t a i n e d when i n s e r t i n g ethylene i n t o a sequence of i s o t a c t i c propylene u n i t s ? This question has been r e c e n t l y an swered f o r a p a r t i c u l a r c a t a l y s t system where i t was shown that c o n f i g u r a t i o n was r e t a i n e d and c a t a l y s t s t e r e o s p e c i f i c i t y was considered to be the c o n t r o l l i n g f a c t o r (33). Now that the groundwork has been l a i d , there should be more extensive and v a r i e d a p p l i c a t i o n s of C-13 NMR i n determining polymer s t r u c t u r e . C e r t a i n l y , an indispensable t o o l has become a v a i l a b l e f o r d e t e r mining polymer stereochemical c o n f i g u r a t i o n .

REFERENCES 1.

G. Natta and F. Danusso, J. Polym. Sci., XXXIV, 3 (1959).

2.

F. A. Bovey, "Polymer Conformation and Configuration", Academic Press, New York, Ν. Υ., 1969, p. 8.

3.

F. A. Bovey and G. V. D. Tiers, J. Polymer Sci., 44, 173 (1960).

4.

F. A. Bovey, "Polymer Conformation and Configuration", Academic Press, New York, Ν. Y., 1969, p. 32.

5.

F. C. Stehling and J. R. Knox, Macromolecules, 8, 595 (1975).

6.

D. M. Grant and E. G. Paul, J. Amer. Chem. Soc., 86, 2984 (1964).


14.

RANDALL

Polymer

Stereochemical

Configuration

313

7. A. Zambelli, P. Locatelli, G. Bajo, and F. A. Bovey, Macromolecules, 8, 687 (1975). 8.

J. Schaefer and D. F. S. Natusch, Macromolecules, 5, 416 (1972).

9.

D. E. Axelson, L. Mandelkern and G. C. Levy, Macromolecules, 10, 557 (1977).


10.

J. C. Randall, J. Polym. Sci., Polym. Phys. Ed., 12, 703 (1974).

11. A. Provasoli and D. R. Ferro, Macromolecules, 10, 874 (1977). 12.


13.

C. J. Carman, Macromolecules, 6, 725 (1973).

14.

T. K. Wu and D. W. Ovenall, Macromolecules, 6, 582 (1973).

15.


16.

I. R. Peat and W. F. Reynolds, Tetrahedron Letters, No. 14, 1359 (1972).

17. K. F. Elgert, R. Wicke, B. Stützel and W. Ritter, Polymer, 16, 465 (1975). 18.

K. Matsuzaki, H. Ito, T. Kawamura and T. Uryu, J. Polym. Sci., Polym. Chem. Ed., 11, 971 (1973).

19.

J. C. Randall, "Polymer Sequence Determination: Carbon-13 NMR Method", Academic Press, New York, Ν. Υ., 1977, Chapter 6.

20.

L. F. Johnson, F. Heatley and F. A. Bovey, Macromolecules, 3, 175 (1970).

21. Y. Inoue, K. Koyama, R. Chûjô and A. Nishioka, Makromol. Chem. 175, 277 (1975). 22. H. Girad and P. Monjol, C. R. Hebd. Seances Acad. Sci., Ser. C., 279(13), 553 (1974). 23. H. Matsuzaki, T. Kanai, T. Kawamura, S. Matsumoto and T. Uryu, J. Polym. Sci., Polym. Chem. Ed., 11, 961 (1973).



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24. W. D. Crain, Jr., A. Zambelli, and J. D. Roberts, Macromolecules, 3, 330 (1970). 25. Y. Inoue, A. Nishioka, and R. Chûjô, Makromol. Chem., 152, 15 (1972).


26. A. Zambelli, D. E. Dormann, A. I. Richard Brewster and F. A. Bovey, Macromolecules, 6, 925 (1973). 27.

F. A. Bovey, "Polymer Conformation and Configuration", Academic Press, New York, Ν. Y., 1969, p. 16.

28.


29.

F. A. Bovey, "Polymer Conformation and Configuration", Academic Press, New York, Ν. Υ., 1969, Chapter 2.

30.

J. C. Randall, "Polymer Sequence Determination: Carbon-13 NMR Method", Academic Press, New York, Ν. Y., 1977, Chapter 4.

31.

H. L. Frisch, C. L. Mallows, and F. A. Bovey, J. Chem. Phys. 45, 1565 (1966).

32.

J. C. Randall, "Polymer Sequence Determination: Carbon-13 NMR Method", Academic Press, New York, Ν. Υ., 1977, p. 35.

33.

J. M. Sanders and R. A. Komoroski, Macromolecules, 10, 1214 (1977). DISCUSSION

T. K. Wu, Du Pont de Nemours, Delaware: I have a minor comment concerning the p o l y v i n y l a l c o h o l spectrum. The one shown (Figure 5) i s s i m i l a r to one obtained i n our l a b o r a t o r y i n 1973. Last year, Dr. Lana Sheer obtained a p o l y v i n y l a l c o h o l spectrum where the methine resonance was resolved into a t r i p l e t of t r i p l e t s . By studying the e f f e c t of temperature upon the spectrum and by tuning the spectrometer c a r e f u l l y , one sometimes obtains b e t t e r hyperfine s t r u c t u r e . J . C. R a n d a l l : Have you obtained s a t i s f a c t o r y assignments?


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T. K.

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Stereochemical

315

Configuration

Wu:

Yes, the assignments (10, 529 (1977)).

have been p u b l i s h e d i n Macromolecules

J . C. R a n d a l l : The s p e c t r a shown here were obtained from our equipment so, i n p r i n c i p l e , s p e c t r a could be presented which were recorded under s i m i l a r o p e r a t i n g c o n d i t i o n s .


W. M. P a s i k a , L a u r e n t i a n U n i v e r s i t y , O n t a r i o : What d i c t a t e s how f a r down the chain c o n f i g u r a t i o n a l e f f e c t s are sensed by the resonance from your r e f e r e n c e carbon? J . C. R a n d a l l : I b e l i e v e the evidence p o i n t s to conformational f a c t o r s . Some people have r e c e n t l y observed chemical s h i f t e f f e c t s between carbons as f a r as s i x bonds away. O r i g i n a l l y , Grant and Paul (reference 6) d e f i n e d chemical s h i f t " c o n t r i b u t i o n s " among f i v e neighboring carbons, which were described as α through ε. I b e l i e v e i t i s conformational d i f f e r e n c e s which are r e s p o n s i b l e f o r the chemical s h i f t behavior observed f o r v a r i o u s c o n f i g u r a t i o n s . Depending upon the p a r t i c u l a r set of circumstances, these e f f e c t s can i n v o l v e f i v e to seven consecutive monomer u n i t s . Sometimes, these chemical s h i f t d i f f e r e n c e s can be t r e a t e d by an a d d i t i v e scheme which considers the c o n t r i b u t i o n s from each p o s s i b l e c o n f i g u r a t i o n a l a r r a y . W. M. P a s i k a : Do the observed d i f f e r e n c e s i n c o n f i g u r a t i o n a l s h i f t s disappear at higher temperatures?

chemical

J . C. R a n d a l l : There i s a temperature e f f e c t i n many systems. The peaks e i t h e r c o l l a p s e or separate. Again, t h i s goes back to the concept that i t i s the conformation of the c o n f i g u r a t i o n that you are d e a l i n g w i t h . T h i s i s why we see so much unusual chemical s h i f t behavior when we examine trends among v i n y l polymers. G. B a b b i t t , A l l i e d Chemical, New

Jersey:

With regard to branching i n p o l y e t h y l e n e , I was wondering i f you have arranged your experiments i n such a way that NOE's and T ' s are taken i n t o account so you can assume 1



316

that your areas represent accurate q u a n t i t a t i v e data. In s t u d i e s of polyethylene branching, there i s a strong major methylene resonance with much weaker methylene and methyl resonances. What i s the best way to handle such data q u a n t i t a t i v e l y as f a r as the area measurements are concerned?


J . C. R a n d a l l : In the systems that I have examined, I can s a t i s f y the dynamic requirements with a ten second pulse delay. The longest methyl Ί± may be 3 seconds. In general, the longer the side c h a i n , the longer w i l l be the methyl Τ·^. We w i l l hear more about t h i s subject l a t e r on. We need not be too concerned about NOE f a c t o r s because they are u s u a l l y f u l l under the experimental c o n d i t i o n s (T = 120-130°C) used f o r polymer q u a n t i t a t i v e measurements. The Τ·^ problem can be handled, even under none q u i l i b r i u m c o n d i t i o n s , by u t i l i z i n g resonances from the same types o f carbon atoms i n a q u a n t i t a t i v e treatment. Such an approach can sometimes lead to more e f f i c i e n t q u a n t i t a t i v e NMR measurements. Adequate pulse spacings w i l l have to be used whenever one wishes to u t i l i z e a l l of the observed resonances. Q u a n t i t a t i v e measurements i n branched polyethylenes are very d e s i r a b l e because t h i s i s one of the best a p p l i c a t i o n s of a n a l y t i c a l polymer C-13 NMR. G. B a b b i t t : I was wondering more about the p h y s i c a l problem of o b t a i n i n g areas, f o r example, c u t t i n g and weighing, when some of the methylene resonances may be truncated. J . C. R a n d a l l : The problems a s s o c i a t e d with dynamic range are ones which we would a l l l i k e to see s o l v e d . I am using peak heights with some success. I t may be the best method when the r e l a t i v e h e i g h t s are 500:1 or higher. In any event, one would l i k e to have an independent measurement or reference standard. I have checked w i t h i n f r a r e d analyses because i t can measure the methyl content. Unfortunately, IR does not d i s c r i m i n a t e the polymer methyl end group from methyls on s i d e - c h a i n short branches. Nevertheless, with t h i s c o n s i d e r a t i o n , I have obtained good c o r r e l a t i o n s between IR and NMR measurements using NMR peak h e i g h t s . Such a comparison can o f t e n serve as a g u i d e l i n e when measuring an i n t e r n a l branching d i s t r i b u t i o n . More confidence i s placed i n the NMR r e s u l t , i f , o v e r a l l , s i m i l a r r e s u l t s are achieved. Truncation, which occurs among resonances with r e l a t i v e l y low i n t e n s i t i e s , i s a constant worry. Another way of checking f o r p o s s i b l e t r u n c a t i o n i n a s p e c i f i c r a t i o range i s through number average molecular weight measurements on


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NBS standard polyethylenes where the i s known. Commercial polyethylenes may a l s o be u s e f u l because the M i s frequently i n the 10 to 15 thousand range which corresponds to the s e n s i t i v i t y a v a i l a b l e with most C-13 NMR spectrometers. n


D. E. Axelson, F l o r i d a State U n i v e r s i t y , F l o r i d a : We have been i n v e s t i g a t i n g branched polyethylenes i n s o l u t i o n and would l i k e to respond to a couple of questions which were brought up. In a recent paper i n Macromolecules (10, 557 (1977)), we showed that some methyl groups, p a r t i c u l a r l y b u t y l and longer, have T]/s as long as 7 seconds; hence 5 could be of the order of 35 seconds at 120°C i n s o l u t i o n . We have a l s o measured the NOE s of most of the small branches because of the high s e n s i t i v i t y of our 270 MHz spectrometer, and, as you i n d i c a t e d , the NOE enhancements have uniformly been f u l l . We have been able to measure peak areas because of our good s e n s i tivity. E l e c t r o n i c i n t e g r a t i o n s have not given s a t i s f a c t o r y r e s u l t s . More c o n s i s t e n t r e s u l t s have been obtained with a planimeter. Dr. Cudby of ICI i n England has been very k i n d to send us some comparisons between NMR and IR t o t a l methyl contents. Close agreement was obtained when the two experiments were done carefully. 1

J . C. R a n d a l l : Have you had an opportunity to evaluate the r e s u l t s from peak heights versus peak area measurements? D. E.

Axelson:

The peak heights worked out f a i r l y w e l l . I do not b e l i e v e you get i n t o a peak height versus peak area problem i f the r e s o l u t i o n i s s u f f i c i e n t l y high and overlap i s not a problem. We found that r e s u l t s from one spectrum to another were w i t h i n an order of one to two branches per thousand carbons as f a r as c o n s i s t e n c y was concerned. Spectra, which were obtained on other systems under l e s s than e q u i l i b r i u m c o n d i t i o n s , show that the t o t a l methyl content, s u r p r i s i n g l y , stays very constant. You f i n d that the low end i s enhanced and the long branches are saturated s l i g h t l y ; however, the t o t a l remains the same. J . C. R a n d a l l : Yes, I b e l i e v e more e f f i c i e n t NMR methods can be by working on systems w i t h known amounts of branching.

developed


318


D. E. Axelson: A comparison to IR r e s u l t s can be dangerous i n the absence of a good standard f o r the IR experiment. We ran i n t o a problem where the IR r e s u l t s were one-half of that obtained from NMR. This discrepancy can u s u a l l y be a t t r i b u t e d to the IR method. J . C. R a n d a l l :


Yes, s i n c e IR measures t o t a l methyl groups, problems can develop i f the branching content i s comparable to the end group concentration. RECEIVED M a r c h 13,

1979.


Carbon-13 NMR and Polymer Stereochemical Configuration

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