Bull. Magn. Reson., 7 (1), 27-58 (1985)

ANALYSIS OF CHAIN MICROSTRUCTURE BY1H AND 1 3 C NMR SPECTROSCOPY

Yury E. Shapiro

NMR Laboratory Yaroslavl Polytechnic Institute USSR

Page I. Introduction 27 A. Microstructure of Macromolecules 27 B. Conformational Statistics and the Mechanism of Chain Growth 28

II. Analysis of Chain Microstructure by 1H NMR Spectroscopy 30 A. Assignment of NMR Signals in Accordance with the Dyad or Triad Theory 30 B. Expansion of x H NMR Spectroscopy Capabilities by Use of Superconducting Magnets. Assignment of Signals by Tetrad and Higher Order Theories 33 C. Polymer Chain Microstructure Influence on Segmental Mobility 37

III. Investigation of Chain Microstructure by x 3 C NMR Spectroscopy 38 A. Advantage of x 3 C NMR Compared with XH NMR in Microstructure Analysis 38 B. Nuclear Relaxation and the Nuclear Overhauser Effect 38 C. Microstructure Analysis of Macromolecules with the Aid of 1 3 C NMR Spectroscopy 40

IV. New Methods of Microstructure Analysis 49 A. Use of Shift Reagents for Chain Microstructure Analysis 49 B. Magic Angle Spinning and High Resolution NMR Spectroscopy in Solid Polymers 52

V. Conclusions 54

References 54

I. INTRODUCTION i.e., as triads. Use of superconducting magnets and 1 3 C NMR allows one to obtain information The aim of this review is to cover the contri- about the content of tetrads, pentads and higher bution of high resolution NMR spectroscopy to order sequences. the study of polymer microstructure, particularly The analysis of triads and tetrads has been after the years 1974-5, thus continuing the found to be very useful in the general interpreta- series of previous reviews (1-6). tion of polymer structural problems. More spe- The impact of stereospecific catalysts on the cific information is forthcoming which may be polymer world has created new demands for used in a special way, e.g., in correlations with methods of studying the stereochemical configu- statistical polymerization^ theory. ration of polymer chains. NMR spectroscopy has become a very important method in this field A. Microstructure of Macromolecules through its ability to discriminate between different structures in a quantitative manner. The 1. Vinyl Polymers study of polymer configuration involves consideration of the polymer chain as sequences of iso-, Vinyl monomers C(A)B=CH2 (where A and hetero- and syndiotactic monomer plasements, B are H or a substituent) have various

Vol. 7, No. 1 27 possibilities of joining into polymer chains: con- 1,2-structures have been discovered in iso- or figurations "head-to-tail, I, "head-to-head" and syndiotactic sequences CH, "tail-to-tail", 2; II * CH

A H A H A H A H H A A H CH I I I I I I I I I I I I II II CH H -C-C-C-C-C-C — -C-C-C-C-C-C - CH CH, I II I II I I I I I I 2 CH. B B B H B H B H H B B H iso- 9 syndio- 9 1,2-polybutadiene

3. Vinyl and Diene Copolymers formation of branching points, 3, and joining, 4, of chains is also possible. The different monomer units A and B form the following sequences in copolymer chains: A-C-B A H A H I I I I Dyads AA AB or BA BB H-C-H -C-C-C-C- A I A H I I I I I I II -C-C-C-C- B | B H I I I I H—C-H B I B B A-C-B A-C-B A I A H r I * i I I I I H-C-H -C-C-C-C- I I I I Triads AAA AAB or BAA BAB B H B H

mm • • • T Even stereoregular polymers are not always mr of very high stereochemical purity, and most t i polymers are composed of isotactic, 5, syndiotac- rr tic, 6, and heterotactic, 7, sequences or may be 1 regarded as copolymers of such units (1). An additional 10 triads formed by replace- ment of designations o and • are also possible. A H A H A H A H All of these and higher order sequences can 5 1 M I 1 1 1 1 be discriminated by NMR (4-6). B H B H B H B H A H B H A H B H B. Conformational Statistics and « 1 1 1 1 1 1 ) 1 Mechanism of Chain Growth B H A H B H A H A H A H B H B H A H 1. Homopolymerization

B H B H A H A H B H The possibility of determining vinyl polymer chain configuration by high resolution NMR is NMR spectroscopy has clarified these structures. based on the sensitivity of this method to mag- Asymmetric centers in these chains are netically nonequivalent nuclei in different actually pseudoasymmetrical (2) and such poly- sequences. Table 1 presents the designations of mers have no optical activity. dyads and triads. The quantities m, r, i, h and s as obtained 2. Diene Polymers from NMR spectra are usually normalized according to the equations (2) m + r = 1 and The diene forms l,4-(cis or trans), 8, and i + h + s = 1. 1,2- configurations, 9, by polymerization. The values of i, h and s have been coupled with various considerations of polymerization theories. Perhaps the simplest of these, according c = c % . / " • • to Bovey and Tiers (1), is the most generally - H 2 C C H 2 - - H 2 C applicable and involves the parameter P m , the cis-S trans- 8 probability that a polymer chain will add a 1,4-polybutadiene

28 Bulletin of Magnetic Resonance Table 1.

Designation Projection Bernoulli an Probability

Dyad meso, m ? 9

racemic, r T ' ^ l _ p m 9 9.9 Triad isotactic, i, mm I '' I *~r p m * 1 heterotactic, h, mr 4Y- '4 I- ' Jj 2 Pm(l - Pm) 2 syndiotactic, s, rr I* T • £ (1 - Pm) 3 Tetrad mmm I ' T ' I ' I P m

mmr Y ' T ' Y • j, 2 Pm> (1 - Pm) 1 2 I ' t ' ^ ' j; Pm (1 - Pm)

Pentad mmmm (isotactic) | ' I • | ' I ' +— ^m*

mmmr ? • ? • j, 2 V ( 1 - P r a ) • * • <

mmrm mmrr ? • ? • ? r m r m ( h e t e r o t a c t i c ) 1 1 J , J t — 1 * 1 2 p m 2 ^ ~

r m r r

m r r m

r r r m

r r r r ( s y n d i o t a c t i c ) 9 , t , 9 , I 1 Y

Vol. 7, No. 1 29 monomer unit to give the same configuration as Pm/m - 1 ~ Pm/r (3) that of the last unit at its growing end. In this case the chain growth process follows Bernoul- Pr/r = 1 ~ r/m (4) lian statistics (Table 1). Thus Markovian first-order statistics is con- Theoretical graphs of normalized i, h and s trolled by two independent parameters P m / r and against P m have been constructed and it has been found that methyl methacrylate free-radical m polymers give Bernoullian statistical propagation The average sequence lengths may be calcu- whereas anionic polymers show a different lated from the following equations (1-3, 7): behavior, which may be called non-Bernoullian < n m > = (5)

< n r > ^ (6) The Markovian second-order statistics have four independent probabilities. It takes into consideration the influence of the second unit from 0.8 the chain end (7). Non-Markovian processes are possible too. One of them is the Coleman-Fox process (8, 9), 0.6 which explains block configuration by chain growth. According to this model the block config- "•£0.4 urations come into being by formation of chain .a end and anti-ion chelate complexes with its sub- o sequent destruction.

" C L 2 2. Copolymerization

Assuming that the statistical copolymerization of monomers A and B is controlled by the Markovian first-order statistics, it holds for the low conversion limit (10):

Figure 1. Relationship between i, s, h -triad pos- PA/B = (1 + sibilities and P m . Points on the left of P m = 0.5 are for methyl methacrylate free-radical poly- PB/A = d + (7) mers; points on the right are for anionic polymers (4). r^ and rg represent copolymerization reactivity ratio parameters and [A] and [B] represent mole fractions of monomers in the initial system. The way in which the copolymer is synthesized markedly affects the copolymerization reac- propagation (Figure 1). tivity ratios r^ and rg and the coisotacticity The first-order Markovian sequence is parameter P m from the Bernoullian model (11). formed by chain growth, with the stereochemis- Plate and coworkers (12, 13) use only Markovian try of chain-growth end effects influencing con- first-order statisitics and introduce the two necting monomer units. We have now four con- parameters of coisotacticity PA/B an(* PB/A- P P ditional probabilities ^ r//mm> mm/r/>r r/rr/>r (P i II. ANALYSIS OF CHAIN which describe the connection process (P r / m is MICROSTRUCTURE BY1H NMR the probability of monomer unit connection in the SPECTROSCOPY m-configuration to the chain end with r-configuration). They are A. Assignment of NMR Signals in Accor- P = dance with the Dyad or Triad Theory m/r (mr)/[2(mm) + (mr)J [^ (1 - Pm)] (1)

P = (mr)/[2(rr) + (mr)] [a P ] (2) Two types of assignments may be distin- r / m m guished: those which follow purely from NMR

30 Bulletin of Magnetic Resonance spectra and those which are taken from non- NMR evidence. Although the interpretation of some polymer spectra is now relatively easy, in other cases serious complexities remain. Spin- spin decoupling has been used to simplify polymer spectra by eliminating the effect of spin coupling. Double irradiation becomes increasing difficult as the chemical shifts of the two coupled protons come closer together, as for example in polypropylene. Another method of NMR spectral simplification is to replace particular protons with other atoms that do not give an observable signal. One way of doing this is by deuteration, although substitution by halogen might also be considered. In this way, not only the signal of the substituted group removed from the spectrum, but also the spin-coupling effect of the substituted group is very much reduced. The interpretation of spectra of deuterated materials usually follows more readily than it does for decoupled spectra, and the assignments could be made with greater cer- tainty. Model compound of polymers have been used extensively in NMR spectroscopy in two main Figure 2. 1H NMR spectra of PMMA solution in ways. Firstly, to correlate chemical shifts found o-dichlorobenzene, 160° C; (a) predominantly iso- in polymers with those of well-defined small tactic polymer, (b) predominantly syndiotactic molecules in order to identify the presence of polymer (4). particular groupings. Secondly, the use of more sophisticated model compounds such as isomers of the 2,4-disubstituted pentane type as models of different configurations in the correlation of splitting patterns observed in polymer spectra. syndiotacticity was not solvent dependent, but its The study of the second type has been extended temperature dependence was illustrated (16). It to the 2,4,6-trisubstituted heptanes, and consid- was found that the P m value for free-radical erable insight into polymer conformational stud- MMA polymerization was 0.13 at -78° C and ies has been gained (14, 15). 0.36 at 250° C. Highly syndiotactic crystallizable samples of PMMA prepared by Ziegler catalysts 1. Homopoiymers were discussed (16). The new possibilities of conformational anal- One classical example of polymer micros- ysis of PMMA appear by the study of a model tructure investigation is NMR analysis of compound and MMA oligomers (17). It was ana- poly(methyl methacrylate) (PMMA) samples lyzed in relation to the stereospecific conforma- (1-4, 7, 16, 17). Bovey found that in' CDC13 tion, taking into account the structural end group solution three a-methyl peaks appeared at 0.91, effects and the characteristics of the geminal 1.05 and 1.22 ppm. They were due to syndio-, methylene proton signals. hetero-, and isotactic forms, respectively. Also, In polymer stereoisomerism investigations, the isotactic methylene signal was an AB-quartet serious difficulties can be found. The problems (J = -14.9 Hz), whereas the syndiotactic one arise from the necessity of summary spectra was a singlet (Figure 2). The observation of an separating subspectra with many components. AB quartet methylene signal is an absolute Therefore the number of polymers for which the determination of the presence of isotactic struc- microtacticity may be determined from *H NMR tures and is independent of any other type of spectra is now restricted (4). Recently increasing evidence. The stereoregularity of anionic pro- success has been achieved by use of partially duced polymers has been shown (7, 16) to be deuterated monomers (18). dependent on the solvent. For a free-radical-initi- The use of computer calculations for investi- ated polymer it has been claimed (1) that the gation of polymers is indispensable. First work in

Vol. 7, No. 1 31 this way has been performed for analysis of 1,2-polybutadienes. The methylene proton reso- poly(vinyl chloride) (PVC) 1 H spectrum (2). The nances can provide information about the meso spectrum of PVC methylene protons is the dyad. The chemical shifts of methylene type superposition of six mmm, mmr, rmr, mrm, rrm vinyl protons in iso and syndio forms differ by and rrr tetrad subspectra. Each of these sub- 0.14 ppm, indicating that the resonance of such spectra is a complex spin-spin multiplet. There- protons can provide information about the rela- fore a^-dj-PVC was synthesized for the stereoi- tive amounts of i, h, s-triads. Chemical shifts and somerism investigation (19). Figure 3 shows the coupling constants were estimated to obtain an tetrad assignment in this polymer. The same approximate fit of calculated lines to each spec- work was done for methine proton pentade shifts trum with the aid of the iterative program LAOCOON 3.

2. Copolymers

The methyl methacrylate-styrene and methyl methacrylate (MMA)-methacrylic acid (MAA) copolymers are the most investigated systems. Such copolymers have been mentioned in an ear- lier paper (1), where the effects of styrene blocks cleaving the aromatic signal and the random H styrene units dividing the methoxyl resonance were featured, but the a-methyl signal was not B resolved well enough for tacticity determination. Radical copolymers have been the subject of further detailed study (4), and the twelve triads involving composition and configuration in relation to a central MMA unit have been divided between the three methoxy resonances. The *H NMR spectra of random and alternating copolymers of styrene and methyl, ethyl, butyl and octyl methacrylates were analyzed in (11). The way in which the copolymerization mechanism follows therefrom markedly affect the magnitude of the parameter of coisotacticity P m : for all comonomer pairs under study, P m is higher when an organometallic catalyst is used (alternating) than in the case of a radical initiator (statistical). With increasing number of carbon atoms in the n-alkyl alcohol residue, P, decreases to a limiting value. • m 2.5 2.0 The chemical shifts of the methoxyl and a-methyl protons in the alternating MMA-styrene copolymer are calculated by taking into ppm account the contributions of the diamagnetic shielding and the magnetic anisotropy effect of the benzene rings in styrene units (22, 23). Figure 3. 100 MHz 1 H NMR spectrum of Three- and four-bond interaction parameters, poly(a-cis-3-d2-vinyl chloride) in CDC13. A, E - which are necessary for the calculation of con- rmr; B - mmr + mmm; C - mmm; D - mmr; F, formational probabilities of dyad sequences in a H - mrr; G - mrm + rrr (19). copolymer chain may be estimated from the parameters determined for the homopolymers. Klesper and Gronsky (24) investigated the monomer distribution and microtacticity of the MMA-MAA copolymers. By using model com- oW-d 2 -PVC (Figure 4). pounds they offered the well-founded assignment Zymonas and Harwood (21) interpreted the of twenty stereoisomeric triads to six components *H NMR spectra of iso-syndio- and heterotactic of the a-methyl spectrum.

32 Bulletin of Magnetic Resonance with the polar monomer units, sometimes longer sequences may be discovered. For example, the pentads were found in the 100 MHz spectra of the MMA-acrylonitrile copolymer solution in DMSO-d6 (28). Splitting of the MMA a-methyl signal on MMA pentad components shows the copolymers have block-sequences with the MMA units having more than three components.

B. Expansion ofxH NMR Spectroscopy Capabilities by Use of Superconducting Magnets. Assignment of Signals by Tetrad and Higher Order Theories

Development of high frequency spectrome- ters (220-600 MHz) with superconducting magnets has been of principal importance in polymer microstructure investigations. The major con- temporary technical difficulty stems from the fact that a high resolution NMR spectrum of a macromolecule is generally a broad, featureless, 4.4 4.2 and uninformative envelop of many overlapping lines of chemically related monomers and chain &. ppm sequences, even at the highest resolution availa- ble (29).

Figure 4. 100 MHz 1H{2H} NMR spectrum of 1. Homopolymers polyfp fi -d2 -vinyl chloride) in C2 HC15 1 - rrrr + rrrm + mrrm; 2 - rrmm + mrmm; 3 - rrmr + Figure 5 shows * H NMR spectra of the same rmrm; 4 - mmmm; 5 - rmmm; 6 - rmmr (20). samples of PMMA that are shown in Figure 2

The contribution of this investigation to polymer chemistry consists in the fact that these copolymers represent the best model for studying the unit distribution change and tacticity by copolymerization and chemical modification of copolymers. The copolymer list may be extended to the spectrally similar copolymers: phenylme- thacrylate-MMA (25), benzilmethacrylate-MAA and diphenyl methyl methacrylate-MAA (26), and at the expense of the systems, which turn into spectrally similar copolymers by analogous polymer reactions (MMA-ethyl, isopropyl, tert- Figure 5. 220 MHz J H NMR spectra of PMMA butyl, benzil, cc-methyl benzil, diphenyl, solution in o-dichlorobenzene (4); (a) predomi- 1,1-diphenylethyl, a,a-dimethylbenzil, trityl, nantly isotactic polymer, (b) predominantly syn- 1-naphtyl methacrylates) (27). The reactivities of diotactic polymer. the monomers have been explained in terms of the polar effect of the ester groups in radical and anionic copolymerizations. Coisotactic parameters have been determined by assuming the terminal model statistics. but at 220 MHz. The latent tetrad structure of Usually only dyad and triad sequences in methylene signals at 60 MHz is obvious at 220 polymer chains may be identified at frequencies MHz (2, 4). The pentad signals, which lead to lower than 200 MHz. However in copolymers the asymmetry of a-methyl signals at 220 MHz,

Vol. 7, No. 1 33 are distinguished beautifully at 300 MHz (Figure H H 6) (30). The fine hexad structure of CH2 -signals is visible as well at this frequency. Spectroscopy at 220 MHz is effective in the study of the ionic polymerization mechanism (31, Cl H H 32). The polymers of 1,2-butylene oxide and (m) TG ^ GT a-methylstyrene prepared under anionic polymerization have the Markovian first-order chain Tanaka and Sato (36) studied the distribution growth mechanism. The cationic poly(l,2- butyl- of cis- and trans-1,4 units in various kinds of ene oxide) and poly(a-methyl styrene) prepared 1,4-polybutadienes (PB) and -polyisoprenes at temperatures above -25° C follow Bernoullian including cis-trans equibinary PB and UV-isom- x statistics with P m =0.26. The information was erized PB by use of H NMR spectroscopy at 60, obtained from tetrad/pentad sequences (32). 100, 220, and 300 MHz. Poly(butadiene-2,3-d2) Anionic and cationic poly(p-isopropyl- was used for the peak assignment. The reso- a-methylstyrene)'s have the opposite chain nance of methylene protons in cis-trans linkage growth mechanisms (33). was described as an A2B2 system. Figure 8a The longest sequences in polyolefins were shows the spectra of poly(butadiene-2,3-d2) pre- obtained from 300 MHz spectra of predomi- pared by butyl lithium. The splitting of the signal nantly isotactic polystyrene. Flory (34) calcu- is identical with that of the methylene proton lated the chemical shifts for methine and aro- signal decoupled from the methine proton in PB matic protons of the central > CHAr group in the as shown in Figure 8b. In Figure 8a the centra! all-meso nonad mo and in the nonads rrm., peak due to the cis-trans linkages in the 60 and mrrm5, m2 r2 m4 , m3 r2 m2 containing a single 100 MHz spectra separates into two parts at racemic triad. The magnetic shielding by phenyl 220 MHz and 300 MHz. This indicates the exis- groups that were First and second neighbors tence of two types of methylene protons in cis- along the chain were computed according to their trans and trans:cis linkages. A computer simula- distances and orientations relative to the given tion was carried out as shown in Figure 8b and proton in each conformation of the chain using the intensity ratio of the peaks was chosen so as the ring current representation of the IT electrons to follow Bernoullian statistics. or, alternatively, the magnetic anisotropy of the The same results were obtained for phenyl group and the McConnell equation. The poly(2,3-dimethyl-l,3-butadiene) (PDMB). The resulting chemical shifts were averaged over all 220 MHz spectra of PDMB prepared by butyl conformations. The calculations for the methine lithium in cyclohexane were compared to those of proton are in excellent agreement with the 300 free-radical PDMB (37). By aid of measurements MHz spectrum. Treatment of the chemical shifts done on cis- and trans-1,4 PDMB prepared by for the aromatic protons in ortho and meta posi- Ziegler catalysts, it was determined that both tions is indecisive owing to the extraordinary polymers were Bernoullian with probabilities of sensitivity of the shielding to torsional angles in cis placement of 0.24 and 0.40, respectively. It the chain backbone. was shown that the anionic sample was largely For the first time the conformation of a poly- trans. Anionically prepared PB was predomi- mer chain was determined in Bovey's elegant nantly trans too (36) and had as much as 23% paper (35) with the aid of model spectral simula- cyclic structures (38). tion. Figure 7 shows the synthesis of the poly vinyl chloride (PVC) methylene and methine 220 2. Copolymers MHz spectra from the tetrad and pentad subspectra. The PVC spectra give moreover much NMR spectroscopy at high frequencies structural information, for example, the chemical (220-600 MHz) allows one to determine the shifts and spin-spin coupling constants. It was monomer unit distribution as well as the micro- ascertained that the m-dyads had the TG ^ GT tacticity of these units in copolymer chains. conformation in atactic PVC chain since their Investigations of a number of copolymers indi- vicinal spin-spin coupling constants were the cate that the copolymerization kinetics deviates same as for the model meso-dimer. r-Dyads had from the simple Mayo-Lewis scheme. In (39) 220 the TT conformation. Therefore the majority of MHz XH NMR spectra of some free-radically loops in PVC chain must occur in the mr link-up prepared MMA-chloroprene copolymers have of TGTT and GTTT forms and their mirror rep- been recorded. The intensities of the a-methyl resentations. signals are related to the relative proportions of various MMA centered triads and pentads. Triad

34 Bulletin of Magnetic Resonance rrrmm mrrsnmfn mmmmr mrrrinm rrmmm r rrrmr rmmmr mrmmr rrmmr mrmmr 2.4 2.2 2.0 1.4 1.2 0, ppm

Figure 6. 300 MHz x H NMR spectra of erythro-methylene (a) and a-methyl (b) protons of PMMA solution in o-dichlorobenzene, 120 C (30).

4.6 4.2 . ppm Figure 7. Computer synthesis of the PVC methylene and methine 220 MHz spectra from the tetrad and pentad subspectra (35). The top spectra are experimental (o-dichlorobenzene, 140 C).

Vol. 7, No. 1 35 method of reactivity ratio calculations is proposed, based on the use of specific values of the triad distribution functions and the Coleman-Fox

2.1 2.0 2JO ppm

Figure 8. *H NMR spectra of poly(butadiene- 2,3-d2) obtained at 60, 100, 220, and 300 MHz (a) and decoupled x H NMR spectra of isomerized polybutadiene (b) in the methylene region obtained at 100 and 300 MHz (36).

fractions indicate that the Mayo-Lewis scheme is not strictly applicable to this system and is in 1.5 good agreement with those calculated from the 5\ ppm penultimate reactivity ratios r 1 1 = 0.107, r 2 1 = 0.057, and r2 = 6.7 where MMA is monomer 1. However, although a small penultimate group effect is indicated, some deviation from the Figure 9. 250 MHz * H spectra of the a-methyl Mayo-Lewis scheme may be due to the occurence proton region of copolymer MMA and vinylidene of anomalous head-to-head and tail-to-tail chloride. [MMA] = 29% (a) and 81% (b). Pentad MMA-chloroprene linkages. A similar analysis decomposition is attributed (43). has been described for acrylic monomer-2-substituted-l,3-diene and alternating (40, 41) and conventional butadiene-d4-acrylonitrile (42) copolymers. Alternating copolymers were prepared under Et3Al2Cl3/VOCl3 or ZnCl2 model. It is possible to detect a penultimate catalysts. The random copolymers are in good effect for the vinylidene chloride-rich region. In agreement with the Markovian first-order statis- the same region, a change in tacticity of the tics. The reactivity ratios are r,, = 0.18, r2 = triads on the MMA sequences, as compared with 0.62 and r,, = 0.26, r2 = 0.63 (where MMA is homopolymers, is observed; it is suggested that monomer 1) for MMA-butadiehe and MMA-iso- the anomaly is caused by the competition of the prene, respectively (40, 41). depropagation reaction. It can by shown that the Constant composition copolymers of MMA or bulk copolymerization kinetics deviates from the methacrylonitrile and vinylidene chloride pro- Mayo-Lewis scheme (Figure 10). Small differ- duced by radical copolymerization were studied ences were found between the bulk and solution by 1H NMR at 60, 250 (43) and 220 MHz (44). copolymerization (44) since the bulk process was The monomer dyad/triad sequences and some of heterogeneous. This could indicate that solvation the tetrad/pentad sequences were obtained from effects were important. spectra (Figure 9). In (43) a new graphical When assigning sequences in NMR spectra of

36 Bulletin of Magnetic Resonance vinyl polymers, it is usually assumed that nearest-neighbor monomer units possess a larger influence on the chemical shifts of the central 1.0 unit than on monomer units further removed. Strasilla and Klesper (26, 45) studied the proton- cccc \ OCH3 resonances of MMA-methacrylic acid j (MAA) and MMA-diphenylmethyl-methacrylate copolymers. In fact, the differentiation of nearest MCCM neighbors appears to vanish in the present case, o ^ / y and within the limits of detection, only the units MCCC removed were responsible for resolving the 8 0.5 • -OCH, resonance of the MMA units into triad s peaks. The detection of such an effect by inten- o sity measurement is possible only with non-Ber- noullian copolymers, particularly with copolymers possessing a strong tendency toward - y^ alternation. In copolymers with alternating • i l l I IA/I character, the statistics of sequences composed of nearest neighbors differs much from the statis- 0 0.5 1.0 tics of sequences composed of next to nearest Mole fraction vinylidene chloride neighbors than in the case of copolymers of block-like character, e.g. in styrene-MMA copolymers (45a). An assignment of such "next to Figure 10. Measured tetrad distributions in bulk nearest neighbor" triads appears possible if it is prepared MAA-vinylidene chloride copolymers assumed that the syndiotactic chain is in an all- compared with distributions calculated from r trans conformation. 1 = 0.40 and r2 = 2.5 (solid lines) (44).

C. Polymer Chain Microstructure Influence on Segmental Mobility

The relationship between microstructure and Hatada and coworkers (48) have shown that segmental mobility of polymer chain may be bet- the tacticities of poly(alkyl methacrylates) can be ter studied with the aid of proton spin-lattice worked out in detail by using the large difference relaxation times than with 13C T, measure- in spin-lattice relaxation times of protons in a-Me ments. However, this is not correct since proton and ester groups to eliminate the ester group and carbon-13 T, values are the complement of resonance overlap with the a-Me signal which one another and are not always identical. normally obscures splittings due to tacticity. Accounts of nuclear magnetic relaxation and the Data were given for several C, -C5 -alkyl metha- theories of polymer chain motions can be found crylate homopolymers. In other work (49) in a number of reviews. The last among them is Hatada demonstrated that T1 values for isotactic (46). sequences were longer than for syndiotactic. Spevacek and Schneider (47) showed with Table 2 shows the correlation times for Me and the aid of a T, 1 H study that PMMA formed a-Me groups which were calculated from x H and 1 3 stereocompexes in CC16, CD3CN, toluene and C T1 -values for iso- and syndiotactic PMMA. benzene solution. The smallest syndiotactic With the aid of proton spin-lattice relaxation sequence length in complexes is 8 (in benzene measurements at 100 and 250 MHz, the seg- solution) or 3 (CC14, CD3 CN solution) monomer mental motion of poly(4-vinyl-pyridinium brom- blocks. The relationship between iso- and synd- ide) in methanol was studied (50). The necessary iosequences in a stereocomplex is 1:1.5. The geometrical parameters were received from the stereocomplexes between iso- and syndiotactic conformational calculation of hexads rrmrr PMMA have been formed by means of exchange assuming two models with and without Br" ion interaction between the ester groups. In dilute near pyridinium. For these two models of the solutions of s-PMMA a considerable portion charge distribution, the potential barriers of the (76%) of polymer segments are intramolecularly rm triad mobility have been calculated. The best associated. The motion of associated segments agreement between experimental data and appears as isotropic with an effective correlation temperature curves of T, was achieved by 7 13 frequency of 10' -10 Hz. AHRT = 6 kcal/mol, (T R ) 0 = 10" s for the

Vol. 7, No. 1 37 Table 2. XH and 13C Correlation Times for I so- and 1 1 Syndiotactic PMMA ( T C X 1 0 S) (57).

i s Group X3 1 C H 1 3 C

3 3 • 7 3-1 7-7 7-1 ™. 8 .8 8.it 16 22

aliphatic chain and AH g f = 2 kcal/mol, (T )0 = of the poly(2-vinylpyridine-0,3-d2) in D2SO4 1.4X10" 1X sfor pyridinium ion. In the temper- (Figure 12a) shows three peaks of methine pro- ature range of 250-350 K the vibrational ampli- tons, which are assigned to i, s and h triads. tude of the chain increases from 40° to 85° . Since the absorption peaks of hetero- and syndiotactic triads of methine protons overlap those of III. INVESTIGATION OF CHAIN methylene protons in nondeuterated polymers, MICROSTRUCTURE BY13C NMR only isotactic triad intensities can be obtained SPECTROSCOPY from 1 H NMR spectra of poly(2-vinylpyridine). The x 3 C signals (Figure 12b) split into a number A. Advantage ofX3C NMR compared with of peaks. This splitting may be due to pentad 1H NMR in Microstructure Analysis tacticity. The results (Table 4) show that poly(2-vinylpyridine) obtained by radical poly- Since the advent of commercial pulsed Four- merization (with AIBN as initiator) is an atactic ier transform 1 3 C NMR instrumentation, great polymer with Bernoullian statistics. The pentad advances have been made in the elucidation of tacticities of the isotactic polymer (prepared with polymer microstructure (51, 52). Firstly, the PhMgBr as initiator) were then calculated on the twenty-fold increase in chemical shift range over basis of a first-order Markovian process. 1 H NMR allows much better resolution of small Finally one must note that 1 3 C NMR spec- structural differences. Secondly, the relaxation troscopy allows one to obtain microstructure 3 times of * C nuclei in CHn groups (n > 0) are information inaccessible by other means. dominated by dipolar interaction with the attached protons. Since the C-H bond length B. Nuclear Relaxation and the Nuclear remains constant from one polymer to another, Overhauser Effect 1 3 C relaxation times are a reliable probe of molecular mobility. Noise decoupling in 1 3 C NMR spectroscopy Figure 1 la, the proton NMR spectrum for an aids assignment by collapsing multiplets to sin- isoprene-acrylonitrile copolymer, shows charac- glets, and in addition selectively enhances the teristic broad peaks and yields little structural signals through the nuclear Overhauser information. Figure lib, the proton decoupled enhancement (NOE). It has been found that the 1 3 C NMR spectrum for the same sample, gives intensities of carbons of similar hybridization and sharp peaks for each type of carbon atom, and is number of attached protons are directly corre- used with the coupled spectrum to assign the lated (46, 51). Carbons of different type are peaks (53). The peaks corresponding to CN-car- usually correlated by a single empirical NOE bon atoms are still not singlets in the decoupled factor measured directly from the spectra (49, spectrum. This is because of the microstructure 56-59). It has been found (46, 53, 56, 59, 60) effect which may be observed for other carbon that the NOE factor for the carbon-13 nucleus in atoms. Table 3 presents the structure composi- a main chain or near it is the same for a number tion of poly(isoprene-acrylonitrile)'s (54). of polymers in solution. This is proven by the x 13 Matsuzaki's poly(2-vinylpyridine) investiga- agreement of the H and C microstructural tion (55) may be cited as another example of the data. Recently a number of authors makes use of advantage of 13 C NMR. The * H NMR spectrum paramagnetic additions (nitroxil radicals (56), or

38 Bulletin of Magnetic Resonance Table 3. Structure Composition of Poly(isoprene-acrylonitrile)'s (62).

mass* Sequence LAJ, 18

Al*-tai1-to-tai1 0.i*58 0.638 Al -head-to-head 0.125 0.190 III ( I D 0.M7 0.172 IAI 0.85U 0.761 IAA 0.101 0.177 AAA 0.0i»5 0.062

* l-isoprene,

120 8 0 s ppm

B-2

Figure 11. *H NMR at 80 MHz and x 3 C{1H} at 20 MHz (b) spectra of isoprene-acrylonitrile 2 ppm 158 156 ppm copolymer (A content is 38 mol %) dissolved in CC14 and CDC13 (53).

acetylacetonate of Cr (60-62) and of Fe(III) (63) in order to decrease the NOE effect. The influence of stereochemistry on relaxa- Figure 12. *H at 100 MHz (2) and ^ C ^ H } at tion has been investigated for a few polymers. 25.1 MHz (b) NMR spectra of Isotactic PMMA is appreciably more mobile than poly(2-vinylpyridine) observed in D2SO4 at syndiotactic, the T, values being in the ratio 60° C. (Samples B-l and B-2 were prepared with 1:1.5 (see Table 2). Inoue et al. (64, 65, 66) AIBN and PhMgBr as initiator) (55). report a Tn-iso/T -syndio ratio of 2 for C6D6 solutions at 80 C. For polystyrene and poly(a-methylstyrene) (59, 65, 66) on the other hand, the isotactic form is slightly less mobile.

Vol. 7, No. 1 39 Table 4. Pentad Tacticity of 40 Poly (2-vinylpyr idine) (63). 3.0 2.0 h Pentad Compos ition Observed Calculated

mmmm 0.04 0.05 1.0 mnrnir 0.10 0.12 rmmr 0.07 0.06 mmrm 0.16 0.12 mtnrr 0.12* rmrm 0.32 0.12* mrrm 0.06* rmrr 0.11 0.14 rrrm 0.16 0.14 rrrr 0.04 0.07

* Total mmrr+rmrm+mrrm: 0.30 3.0 25 1/T°K *103

The T1 activation energies are independent of configuration. Randall (67) and Asakura (67a) have measured the 13C relaxation times of Figure 13. Arrhenius plot of syndiotactic (A and numerous stereochemical sequences in the CH,, isotactic (0) polypropylene methyl relaxation CH2 and CH3 regions of an atactic polypropoly- (67). lene sample. The carbons from isotactic sequences tended to exhibit the longest T, values, but the largest differences between iso- and syndiotactic units was 32% for CH carbons (Fig- ure 13). The activation energies for all T, val- It may be pointed out that the carbon ues were independent of configuration, as for relaxation study acquired greater significance polystyrene. The origin of the small stereochemi- than l H because of their simpler interpretation cal dependence of T, in polystyrene and polypro- and of a possibility of evaluating polymer seg- pylene is probably connected therefore with mental mobility in solids (71). slightly different values of the force constants (46). C. Microstructure Analysis of Macromolec- Gronski et al. have studied the dependence of ules with the Aid ofX3CNMR Spectroscopy n C T, values on sequence distribution in styrene-butadiene (68) and 1,4-1,2-butadiene (69, 1. Polyolefins 70) copolymers. In the styrene-butadiene system, the T, values for the para-phenyl carbon for two 1 3 C NMR has proven to be an informative samples with average block lengths of 1 and 6 technique for measuring stereochemical sequence are 0.56 and 0.33 s respectively in CHC13 at distributions in vinyl polymers. Chemical shift 53° C and 60 wt%. The comparable value for sensitivities to tetrad, pentad and hexad place- polystyrene is 0.11 s. The factor of 3 increase ments have been reported for 13C NMR spectra shown by the sample with < n g > = 6 is indica- of branched polyethylenes, polypropylene (PP), tive of segmental motions involving the coopera- polyvinylchloride (PVC), polyvinylalcohol (PVA), tion of perhaps three or four monomer units. and polystyrene (PS). Similar effects are observed in the 1,4-1,2 buta- Pulsed FT 1 3 C NMR studies clearly demon- diene copolymer. For example, the T1 value for strated the presence of ethyl, butyl and longer the CH of a 1,2-butadiene unit is 0.80 s when its chain branches in low density polyethylene neighbors are also 1,2-units, but 1.65 s when its (72-75). The concentrations of ethyl, n-butyl, and neighbors are cis-l,4-units. longer chain branches were determined as 3-4,

40 Bulletin of Magnetic Resonance 10-13, and 8-21 per 1000 carbon atoms accord- ingly. The methyl carbon of the ethyl branch was seen as three resonances. These were associated with isolated butene units (11.2 ppm) and adjacent butene-1 units as m and r dyads (10.8 and 10.4 ppm respectively). The same study was made for PVC (73, 73a) (2-4 branches per 1000 carbons). Randall (63, 67) made PP i 3 C resonance assignments with the aid of T, 's and the model compound study of Zambelli et al. (76) where only nine resonances were observed. Figure 14 shows *3 C NMR spectra of PP with the peak assignments. r i i Tonelli (77) demonstrated that the stereose- quence-dependent 1 3 C NMR chemical shifts observed in hydrocarbon polymers can be under- stood on the basis of the interaction between carbons separated by three bonds. A chemical inversion in PP chain was considered in papers (78-81). The sequence distributions of inverted propylene units were attributed to Bernoullian (79) or in an opposite view, to first-order Markovian (80) statistics. Isotactic PP was prepared in the presence of organometallic cocatalysts bearing * 3 C.-enriched methyl substituents (81). The enriched methyl carbon is detected, in stereoregular placement, on the end 29.2 27.2 groups and never undergoes transformation to methylene. Therefore it is unlikely that interme- diates are involved in the polymerization mechanism. In addition, since neither a chiral carbon nor a spiralized chain participates in the two addition steps, the steric control arises, unequi- vocally, from the chirality of the catalytic center. The effects of the tacticity on the 13C NMR spectra of PVC were calculated and observed in (82-84). Keller and coworkers showed in their investigations that 13C NMR spectroscopy allowed immediate determination of CC12 groups in chlorinated polyethylene, PP (85), and PVC 47.7 45.7 (86). By combination with proton resonance investigations the quantitative analysis of chlorinated polymers with respect to the constitution, 13 i.e., CH2, CHC1, and CC12 group content proved Figure 14. COH} NMR spectra of methyl (a), possible. The constitution curves obtained deviate methine (b), and methylene (c) carbons in PP at slightly from those calculated for the chlorination 120° C (67). of CH2 groups by Bernoullian statistics. The deviations can be sufficiently described by substitution statistics proposed by Frensdorff and Ekiner (87) for parameters X = 0.6 for chlorinated polyethylene and 0.9 or 1.6 for PVC, and PVA spectra at 67.9 MHz which was readily are discussed with respect to the chlorination assignable to pentad tacticity. Quantitative anal- model of Kolinski and coworkers (88). ysis of this spectra proved that stereoregularity By using the appropriate experimental con- of radical-initiated polymerization of vinyl ace- ditions (in DMSO solution) Wu (89) resolved the tate was almost atactic. The stereochemical methine carbon signal into a triplet of triplets in sequence distribution in the isopoly(vinyl alcohol)

Vol. 7, No. 1 41 derived from cationic polymerization conforms to carbon chemical shifts. The stereochemical first-order Markovian statistics. The conforma- sequence distribution in PS is in accord with tional aspects of poly(vinyl acetate) have been Bernoullian statistics (92a). By using the induced discussed in (90). currents approach the increments for the chemi- Randall (91) has made an assignment of sig- cal shift of a quarternary carbon due to diamag- nals in x 3 C NMR spectra of amorphous polys- netic screening by the neighboring aromatic sub- tyrene (PS) with the aid of model compounds (92) stituents for atactic (55, 93, 94) and regular and Paul-Grant calculations of the chemical conformations (95) of the iso- and syndiofrag- shifts (71). It has been found that ring currents ments of PS, poly-2-vinylpyridine and of neighboring phenyls influenced the methylene

T a b l e 5 . A s s i g n m e n t of 1 3 C NMR Signals of 1,4-PI (115)-

Chemical Shift (ppm from TMS) Carbon t r a n s - t r a n s t r a n s - c i s cis-trans cis-cis

CO) 39.67 39• 91 32.01 32.25 C(2) 134• 38 • 55 134.68 134. 85 26.69 26•55 26.45 26.36

Table 6. Sequence Distributions of 1,4-PI (115) -

Fractions of Dyad Sequence Sample trans-trans trans-cis cis-trans cis-cis Chicle 66.1 0 0 33.9 1somer ized qutta 60.1 12.4 15-3 6.3 percha (61.5)* (16.9)* (16.9) (4.7) 1somer ized ci s- 24.9 25-1 24.6 25.4 PI (25.0)* (25-0) (25.0) (25.0)

* The values in parantheses are calculated from Bernoullian statistics.

42 Bulletin of Magnetic Resonance poly-4-vinylpyridine were calculated. The temperature dependence of chemical shifts of the triads of quarternary carbon of atactic poly-2-vinylpyridine was studied from -20° to 50° C. On the basis of the theoretical and experimental data, a model of an atactic chain was presented for polymers with a different ampli- tude of torsional oscillations for different structures in the absence of free rotation. Conclusions concerning a conformational set of the irregular chain of macromolecules were made: the isofrag- ments were predominantly from the right-hand and left-hand spirals of the 3t type; the syndiof- ragments contained equal parts of trans-conformation and spiral structures 21 (95).

2. Polydienes

Recently several papers were published con- a cerning the sequence distribution study in polybutadiene (36, 52, 96-105) (PB) and polyisoprene (36, 106-109) (PI) by 1 3 C NMR spectroscopy. The * * C peak assignments were made with the aid of model compound spectra (96-98) and of the Grant-Paul additivity coefficient calculations. Each of the olefinic-carbon signals of the cis-1,4 and trans-1,4 units in PB were reported (36, 99) to split into two peaks which were tentatively assigned to the olefinic carbons of the central monomer unit in the triad sequences of cis-1,4 (C) and trans-1,4 (T) units (Figure 15). The ultrasonic irradiation of the polymer solution caused an enhancement of the resolution in *3 C NMR spectra as well as in the decoupled *H spectrum (Figure 15c). The observed dyad fractions fitted well to the theoretical curves calculated by assuming BernouUian statistics (Figure 16). It is in good agreement with those obtained ABCD by *H NMR and IR measurements. The distribution of cis and trans configurations in l,4-poly(2,3-dimethyl-l,3-butadiene) follows Ber- 132 130 128 nouUian statistics as well (100). 5,ppm The * 3 C NMR spectra of chickle PI and cis- trans isomerized 1,4-PI's were studied in the C,, 1 C2, and C4 carbon* signals of the isomerized Figure 15. ^CC !!} NMR spectra of a mixture Pi's. The new signals were assigned to the car- of cis-1,4 and trans-1,4 PB's (a), isomerized PB bon atoms in cis-trans linkages (Table 5). Table (b) and (c) ultrasonic-irradiated product of (b) 6 shows the fractions of the dyad sequences. It (36). was found that the cis-1,4 and trans-1,4 units were randomly distributed in the isomerized Pi's. Randall has shown (101) that the sequence distribution of 1,2- and 1,4-units in hydrogenated PB's conforms to the first-order Markovian

4C(l)Hj -C(2)C(5)H3 =C(3)H-C(4)H2

Vol. 7, No. 1 43 results obtained was confirmed by investigation of the carbon spectra of hydrogenated and deuterated Pi's which contain chain fragments with irregular addition of units. Samples of hydrogenated Pi's shown in Figure 17 give resonance lines that correspond to the methylene carbons at

0.5 trans - U -fraction

Figure 16. The dyad distributions of cis-1,4 and trans-1,4 units in isomerized PB's (106).

statistics. It may be explained by the steric dependence of a terminal 1,2-unit upon polymerization. Similar results were obtained from x 3 C NMR spectra of poly(2-phenyl-l,3-butadiene) (102). The consideration of position distributions of 1,2-units in the PB chain makes it possible to assign 64 various triads (103, 103a). The triad assignment of PB aliphatic carbons was made in (104, 105, 105a). Gronski and coworkers (107) and Beebe (108) published the 1 3 C NMR microstructure results of a binary PI with 3,4-cis-l,4 structural units and of a ternary PI with 3,4 and cis/ 30 20 10 trans-1,4 units. It has been shown that for all signals, the best agreements between predicted ppm and experimental intensities is found for the Markov model. Figure 17. Aliphatic part of the 13C NMR spec- Coleman (110) has studied polychloroprene tra at 67.88 MHz of Pi's (109). at 67.91 MHz. The dyad and triad microstructure was characterized. The back turning of trans-1,4 and cis-1,4, and isomerized 1,2- and 3,4 units was determined. 1 3 C NMR spectroscopic data obtained for 34.62 and 27.61 ppm, respectively. For the model compounds imitating regular and irregular deuterated Pi's, the methylene carbon reso- addition of monomer units in linear PI were nances of trans- and cis-units in head-to-head compared with the chemical shifts calculated addition was found at 38.6 and 31.4 ppm, with using the empirical regularities found for the those in the tail-to-tail addition of both isomers at branched alkanes and alkenes and a good corre- 28.4-28.8 ppm. The latter findings offer a prac- lation was established (109). The validity of the tical means of characterizing irregularities in PI.

44 Bulletin of Magnetic Resonance the isotactic regulation arises from the asymme- 3. Olefinic Copolymers tric spatial arrangement of the ligands in the catalytic centers, whereas the syndiotactic regu- Ethylene-propylene copolymers (EPC) have lation arises from the asymmetry of the last unit been well studied (111-114) with the aid of 1 3 C of the growing chain end; syndiotactic regulation NMR spectra of model alkanes (112, 113). Car- is therefore last whenever the last unit is achi- man and Elgert (111, 112) have developed a ral. mathematical model of EPC polymerization The dyad-tetrad sequence distribution in pro- which accurately accounts for the intensity of pylene-butene-1 copolymers was determined in each peak in any spectrum of EPC. This is a ter- (114-116). The monomer distribution is in good polymer model in which the propylene is added agreement with Bernoullian statistics (115). The by either primary or secondary insertion. Thus analysis of methine triads and tetrads of back- propylene inversion is determined from the ratio bone methylene carbons have been verified using of contiguous to isolated propylene sequences. first-order Markovian theory (116). Coisotactic The stereochemical environment of the isolated shift contributions also account for the reverse ethylene units, and the arrangement of the order of the propylene-centered from that pre- neighboring propylene units in EPC, prepared in dicted by the Grant-Paul equation. the presence of syndiotactic- and isotactic-specific Quite a number of authors (51, 89, 117, 118) catalysts were investigated (113, 115) by com- investigated ethylene-vinyl acetate copolymers. paring 1 3 C NMR spectra of selectively 1 3 The intensities of the methine and methylene C-enriched copolymers. The implications of peaks were related to the triad populations. The copolymer structure on polymerization mecha- plots of triad population variations with nisms are considered. In the presence of homo- monomer ratios are given in Figure 18. The geneous syndiotactic specific catalyst systems, similar triad splitting of the quarternary carbon, both the regiospecificity and stereospecificity are CN or CO groups was obtained in 1 3 C spectra of controlled by the last unit of the growing chain random styrene copolymers with acrylonitrile end. Stereoregulation is transmitted through (119, 120), acrylic acid (121) and MMA (51) and achiral ethylene units, but not in isotactic poly- alternating styrene-MMA copolymers (122). merization. The meaning of these facts is that

80 a b in \ vvv EEE A o / o 60 \ EVVrVVE - 3 Q. EEV+ VEE O / Q- 40 v EVE " • VEV O > > 20 v;

80 60 40 20 80 60 40 20 Vinyl acetate, mole %

Figure 18. Variation of (a) V-centered and (b) E-centered triad populations in ethylene-vinyl acetate copolymer with copolymer composition (51).

Vol. 7, No. 1 45 AmKfcnArNrAT ArNrArNrA* CH, r m r m | | m m r m r m rr ll — NmAmN r m r

U6 145 144 128 127 126 125 124 123 35 30 25 20 0, ppti

Figure 19. *3 C NMR spectrum of an alternating copolymer from a-methylstyrene and methacrylonitrile. Resonance regions: aromatic (a), nitrilic (b) and methyl (c) carbons (123).

Comparison of the 1 3 C NMR spectrum at off-resonance experiments and of calculations by 67.88 MHz of alternating methacryloni- the Grant and Paul scheme. trile-o:-methyls tyrene copolymer with those of the The 13 C NMR spectrum of butadiene-styrene syndiotactic homopolymers showed that the copolymer has 30 peaks at 25-46 ppm and 19 copolymer had the random configuration with peaks at 114-146 ppm assigned to 152 possible dominantly syndiotactic enchainment of monom- triads of 6 units: cis-1,4; trans-1,4; "head-to- ers (123), in contrast to 1 H NMR results. Fig- tail" and "head-to-head"-l,2-butadiene (B); ure 19 shows the triad and pentad peak assign- "head-to-tail" and "head-to-head" styrene (S) ments. The relative configurational enchainment (51, 127-129). In general, one would expect all of a-methylstyrene (A) and metha-acrylonitrile the styrene in samples to be in BSB triads and (N) in eyrthro-diisotactic structure is m, whereas would therefore expect a pattern of absorptions in threo-diisotactic is r. Slight deviation from (BS and SB) very similar to that of the vinyl exact alternating copolymerization was shown by units (Bv and vB), also expected to be randomly the presence of NNA triad or its corresponding distributed (51, 127). Styrene average block pentads. lengths were found to vary greatly (1.2—5.9 The radical copolymers of MMA with MAA units) while vinyl butadiene units showed no and a-methacrylophenone were studied (124, tendency to block together, cis units only a small 125). In this case Bernoullian statistics describes tendency (1.0—1.7) and trans units a moderate the chain growth too. The steric factors and high tendency (1.2—3.4). Styrene units display a ten- polarizability of aromatic keto-groups caused the dency to block with trans units whereas vinyl large values of P m = 0.40-0.43 (125). and cis units generally prefer to block with trans units (127, 129). 4.Diene Copolymers The butadiene-vinylchloride copolymers obtained by radiation copolymerization in chan- The first peak assignments for alternating nel complexes of urea have randomly distributed butadiene-propene, -acrylonitrile, isoprene-pro- units. Vinylchloride forms predominantly syndio- pene, -acrylonitrile and some polyalkenylenes tactic sequences (130). and polypentadienes has been obtained by Gatti Emulsion processed butadiene-acrylonitrile and Carbonaro (126) with the aid of (51) and isoprene-acrylonitrile (53, 54)

46 Bulletin of Magnetic Resonance copolymers were investigated. The vinyl and CN signals into two components corresponding to KII peaks were the most sensitive to environment and KIK, IKI and IKK triads demonstrates the and splitting into triad components. The struc- tendency of these copolymers to alternation at ture of these copolymers is highly alternating radical polymerization. The statistical treatment (see Table 3). At 28% acrylonitrile it is essen- of the data obtained shows (Table 7) that the tially block butadiene with short runs (1—3 character of polymer chain propagation follows units) of alternating A and B increasing to 7—8 first-order Markovian statistics, and the average unit lengths at 40% (51). With increasing con- length of alternated sections depends on the conversion the content of block-triads increases. The formation of the alkyl substituent in data prove that the theories of Medvedev and a^-unsaturated ketones (S-cis or S-trans). Simi- Smith-Ewart apply to emulsion polymerization lar alternation was discovered for radical poly- (54). merized butadiene-methacrylate copolymers We also studied the microstructure of copo- (131). lymers of or^-unsaturated ketones (K) with isoprene (I) by 13C NMR spectroscopy (60). Figure 20 shows the spectra of copolymers of isoprene Table 7. The Values of Conditional with alkylvinylketones CH2 =CH-COR (R = Probabilities, Average Lengths of Block and Alternating Unit KI(IK)>> and Isomeric Composition of Isoprene g.Jfl ho with Alkylvinylketones Copolymers (68) (MfO oo •8 8 R 5 Ps^ f Titnf* t f* IT 5CO I u! dlllG L v£ I o -CH3 -CH (CH3)2

[K] 0.480 0.485 0.5*5 P KK (1 I ) / K I (I K) 1 1 1 PKI (IK)/KK (1 1) 0.103 0.167 0.053 P KI (IK)/KI (1 K) 0.897 0.833 0.9*7 P KK(I O/KK (I 1) 0 0 0 < n KK(l l ) > 0.82 1.00 1.00 c _L _«»— < n KI (IK)> 9-67 5-99 19.0 trans-1,k 1 0.695 0.510 0.673 c i s - 1 , 4 1 O.2M 0.353 0.327 3 , * I 0.062 0.138 0 J L Microstructure of chloroprene-2,3-dichloro- butadiene copolymers prepared in free-radical- initiated systems have been studied (132). The 1 assignments were given in dyad form as combi- 220 140 100 40 20 0 nations of tail-to-head, head-to-tail, or cis-chloro- fr.ppm prene components. The calculated monomer reactivity ratio product, r, »r2 > 1, showed that the copolymers had a slight tendency toward Figure 20. 1 3 C NMR spectra of copolymers iso- blockiness. The monomer composition influenced prene with methyl- (a) isopropyl- (b) and tert-bu- microblockiness. tylvinylketone (c) in CDC13 (60). To analyze the effect of monomer composition on a microstructure of copolymers of piperylene with acrylonitrile obtained by the polymerization in DMSO, the l 3 C NMR at 67.88 MHz was used (61). The peak assignments in 1 3 CH3, CH(CH3)2 or C(CH8),). The splitting of C spectra (Figure 21) were made with the aid

Vol. 7, No. 1 47 of chemical shifts calculated by the additive chain show (Table 8) that the character of chain scheme of Lindeman and Adams. propagation accord with the first-order Marko- The data on a triad composition in copolymer vian statistics.

CM CO o o oo o

WO 20

Figure 21. ^Cl 1 !!} NMR spectra of polyacryl- onitrile (a) and copolymers piperylene with acryonitrile, containing 75.6 (b) and 51.5 (c) mol.% acrylonitrile in DMSO-d (61).

48 Bulletin of Magnetic Resonance Table 8. The Values of Conditional Probabilities, Average Lengths of Block < " A A (PP) "*a nc' Alternating Unit < nAP(PA) > » anc* Isomeric Composition of Piperyiene in Copoiymers with Acrylonitrile (69).

Acrylonitrile Content, mol Parameter 51.5 65.1 75-6 89.5

PAA(PP) /AP (PA) 1.00 0.680 0.397 0.104 PAP(PA)/AA(PP) 0.261 0.429 0.503 0.552 PAP(PA)/AP(PA) 0.739 0.571 0.497 0.448 PAA (PP) /AA (PP) 0 0.320 0.603 O.896 1.00 1.47 2.52 9.65 3.83 2.33 1.99 1.81 trans-1,4 P 0.873 O.876 0.888 0.925 cis-3,4 P 0.105 0.096 0.064 0

0.022 0.028 0.048 0.075 c-c-c-c(c)-

At more than 67 mol.% content acrylonitrile in functionality in the monomer unit were made for the copolymer, the chain is transformed from spectral simplification (133-143). It has been syndiolike into isolike. The increase of acryloni- reported that the use of Eu(dpm)3, Eu(fod)3 and trile content also increases the possibility of Pr(fod)3 improved the resolution in 100 and 220 1,4-trans-addition and decreases the possibility MHz spectra of PMMA, poly(vinyl methyl ether), of 3,4-cis and cyclic addition of piperylene. In poly(vinyl acetate), poly(propylene oxide), polysi- copolymer with 73.8 mol.% of acrylonitrile loxanes (64, 133, 134), polyethers (135, 136), obtained by emulsion polymerization, cyclic polyoles (137-139), polylactones (140), etc. Guil- structures are absent. Iet et al. (134) found that the order of shifts for The quantity of the above examples is large the various peaks in o-dichlorobenzene as solvent enough so as to be convinced of the considerable was s C-CH 3 > i C-CH 3 > h C-CH 3 > i 1 3 achievements of C NMR in microstructure OCH3 > s OCH3 > h OCH3 for the triad analysis of macromolecules. However, 13 C NMR sequence peaks of the methyl and methoxycar- has the same difficulties as that of 1 H NMR: bonyl signals. It had been found that in benzene limited precision of sequence analysis through solution at room temperature, the order of shifts line superposition and complexity of well-founded obtained was i C-CH 3 > h C-CH 3 > s line assignments. C-CH 3 > i OCH3 > h OCH3 > s 0CH3. The explanation of this is primarily a reflection of the IV. NEW METHODS OF dependence of polymer conformation on tacticity X MICROSTRUCTURE ANALYSIS (133-135). Figure 22 shows the H NMR spectrum of a sample of poly(vinyl acetate) in CDC13, A. Use of Shift Reagents for Chain and the effect of the addition of small quantities Microstructure Analysis of Eu(fod), and Pr(fod), to the solution. It can be seen that with both reagents the methoxycar- Recently, paramagnetic salts containing lan- bonyl protons are readily separated into the thanides such as europium or praseodymium absorptions for iso-, hetero- and syndiotactic have been effectively used for the investigation triads. The slopes of the lines in the diagram of polymer and copolymer microstructure and give a clear indication of the degree of shift and chain conformation. The first applications of it is noted that the Pr(fod)3 gives larger shifts paramagnetic shift reagents to a number of than Eu(fod)3 but in the former the broadening is polymers containing a basic lone-pair a bit greater.

Vol. 7, No. 1 49 concentration. The values can be related to the relative distance of protons from the coordinating site and they are dependent on solvent for syndiotactic PMMA. In general the flexible polymer chain in solution cannot always take the spe- cially fixed conformation. However, it has been shown that isotactic PMMA has the right-handed (10/1) helix conformation. The trans zig-zag conformation suggested for synditactic PMMA is a special case of glide-plane or heliocoidal conformations. The effect of paramagnetic shift reagents Eu(dpm)3 and Pr(dpm)3 on the 'HNMR spectra of atactic poly-4-vinylpyridine (P-4-VP) was studied in CDC13 solution in the temperature range 28-100° C (140). From the analysis of J H (360 MHz) and 1 3 C (22.63 MHz) spectra the microtacticity of radical P-4-VP was determined ppm and was well described by Bernoullian statistics with P m = 0.52. It was shown also that *H shifts of signals induced by Eu(dpm)3 were pseudocontact by nature. The values of the geo- Figure 22. Effect on the XH NMR spectrum of metrical factor were calculated for the fragments poly(vinyl acetate) of adding Eu(fod)3 and of regular conformations of iso- and syndiochains Pr(fod)3 (133). of a macromolecule. Based on a comparative analysis (according to a principle of maximum probability) of the experimental and calculated values of shifts induced by Eu(dpm)3, the conformational composition of the polymer was Slonim and coworkers (137) have shown that determined. It has been shown that the struc- the values of paramagnetic shifts depended on tural contents were [2,] = 47 mol.% (TTGG), the europium distribution between different coor- [Zs] = 28 mol.% (trans-zig-zag) and [3, ] = 25 dinating centers of polyethylene glycol and poly- mol. % (iso) which were in agreement with the formaldehyde chains. To determine the content model of P-4-VP microtacticity (55, 95). From of ordered trans-gauche-trans (TGT) conforma- the temperature relationship of the * H shifts of tion in polyethylene glycol x H NMR spectra were signals a conclusion was made about the varia- measured. The singlet peak at the lowest field tion of P-4-VP conformational set to the direction was assigned to the TGT conformation of the of the increase of the syndiotrans-form content. COCCOC sequence (138). The stereospecific con- We suggested an analytical method of inves- tact interactions in the NMR spectra of polyol- tigating microstructure of copolymers with the lanthanide (La3 + , Pr3*, Nd3*, Eu3 \ Tb3 + , use of shift reagents. The possibility of practical Yb3*) complexes were investigated (139). It has application of this method has been shown, the been shown that the contact increment in par- analysis of butadiene or isoprene and acryloni- amagnetic shifts is greatest if the chain is planar trile, MMA, or alkylvinylketone copolymers zig-zag. In other cases the isotropic proton shift being examples (54, 141-143). Figure 24 shows is pseudocontact predominantly. the resulting spectral effect of Eu(fod)3 addition Inoue and Konno (64) established the possi- to the isoprene-acrylonitrile (38 mol % A) copo- ble conformations in solution of iso- and syndio- lymer solution. The values of paramagnetic tactic PMMA, by comparing the observed values shifts of methyl, methylene, and vinylidene triad of pseudocontact shift with the values of the geo- signals decrease in AAA, AAI, LAI, and AIA, metric factor (1 — 3cos26)/r3, in the McConnell- All, III succession. Quantitative data were cal- Robertson equation, calculated for any glide culated from the content of triads in the copo- plane or heliocoidal chain conformations. Figure lymers obtained under emulsion copolymerization 23 shows the paramagnetically induced proton on sodium alkylsulfonate (ASS) and potassium shifts A<5 of the three groups, «-CH3, CH2, and abietate (AP) micelles, in mass or solution with OCH3 of iso- and syndiotactic PMMA in CDC13 different monomer content and conversion. It and CeD6 solutions with increasing Eu(dpm)3

50 Bulletin of Magnetic Resonance 1.2 b 4.0 0.S 1.6 0.8 2D 0.4 0.8 0.4 EO 0 0 / ^ 0 Q. Q. yt .1.6 0.4 0.8 <0.8 0.2 0.4 0.2 0 ^T*7. . 0 0 n 20 20 40 0 10 20 0 10 20

Figure 23. Variation of the paramagnetically induced proton shift with the amount of Eu(dpm)3 for isotactic (a, b, e, f) and syndiotactic (c, d, g, h) PMMA at 25° C (a-d) and 80° C (e-h) in solutions in CDC13 (a, c, e, g) and C6D6 (b, d,f, h). Concentration of polymer was 20 mg/0.4mL. (•): CH2; (o) -CH3; (A): OCH3 (64).

C71 E gi.0

0> f

U. ppm

Figure 24. Relationship between induced paramagnetic shift of triad-tetrad signals in the 1 H NMR spectra of isoprene-acrylonitrile copolymers and Eu(fod)3: copolymer (w/w) ratio (142).

was found that a blocking level of the copolymers micelles and micelles of fatty acids (144, 145) increased with an increase of local concentration with the relaxation probe Mn2 * allowed us to and a decrease of polar monomer molecule diffu- make a conclusion about the principal influence sion velocity in the micelle matrix (54, 143). The of detergent association in the micelle matrix stability analysis of aqueous mixed ASS-AP upon microstructure (54, 145). The isoprene with

Vol. 7, No. 1 51 acrylonitrile or MMA copolymers obtained on methyl and methine protons were tentatively matrices of different ASS-AP compositions have assigned to the triad sequences with a VA unit a maximum blocking level with a maximum sta- as a center, and their evaluated concentrations bility at 66.7 mol. % AP. Figure 25 shows the were compared with calculated concentrations by means of the random copolymerization theory (147). Similar investigations were made for chloroprene-MMA copolymer with aid of Eu(dpm)3 (148). The investigation of microstructure of the unsaturated polymers and copolymers without polar groups is performed by the use of the com- posite complex: double bond—AgNO3—Eu(fod)3 (148a). 0.4 The examples show that use of shift reagents for microstructure analysis and for the study of the polymerization mechanism of polymers having unresolved monomer unit spectra is particularly effective.

B. Magic Angle Spinning and High Resolu- N tion NMR Spectroscopy in Solid Polymers T Two sorts of line-narrowing techniques are - io -£ employed to obtained high-resolution, natural o abundance 13C NMR spectra of solid polymers. Dipolar broadening of the 13 C lines by protons is removed by strong resonant decoupling (referred . 6 to as dipolar decoupling) by using XH decoupling rf fields of about 10 G (149). These decoupling fields are comparable to the proton linewidth. In most cases the resulting 13C NMR spectra are still severely complicated by chemical shift ani- - 2 sotropies, so that, in general, only a few broad lines are observed. A dramatic improvement of the resolution can be achieved by additional 20 60 100 mechanical spinning of polymer samples at the lAA) , mol % so-called "magic angle" with a frequency some- what greater than the dispersion of chemical shifts (149, 150). Figure 25. Relationship between triad contents Even with the resolution achieved by a com- 1 and Ai> % (CH2)n in H NMR spectra of ASS-AP bination of dipolar decoupling and magic angle micelles and AP contents in them (145). spinning, a FT experiment on a solid polymer still has a serious limitation. Namely, a delay time of several 13C spin-lattice relaxation times (T1) must be tolerated before data sampling can be repeated. These repetitions are necessary to relationship between the matrix stability cri- provide a suitably strong signal by a time-aver- 3 terion (Avu of the (CH2 ) n signal in the spectrum aging process. Since some * C T, 's for solid of mixed micelles) and triad content. The chain- polymers are on the order of tens of seconds growth statistics of emulsion copolymers are (149), the time-averaging process becomes tedi- first- or second-order Markovian (145). The same ous. These delays can be avoided, however, by results were obtained for styrene-MMA copo- performing a matched spin-lock (or Hartmann- lymers investigated without shift reagents (146). Hahn) cross-polarization (CP) experiment (151). The x H NMR spectra of ethylene- and vinyl With this technique, polarization of the carbons chloride-vinyl acetate copolymers with low vinyl is achieved by a polarization transfer from acetate (VA) content were measured by use of nearby protons, spin-locked in their own rf field, via static dipolar interactions in a time TQJJ(SL). Eu(fod)3. The shifted signals of the acetate

52 Bulletin of Magnetic Resonance This polarization transfer is a spin-spin or T2-type process and generally requires no more than 100 MS (149). Most important, the CP transfer can be repeated and more data accumu- lated after allowing the protons to repolarize in the static field. For glassy polymers near room temperature this is more efficient than 13 C repolarization by spin-lattice processes and generally occurs in less than a half second (152). 0.56 kHz Schaefer (149, 153) has discovered that CP and magic angle spinning are compatible. As shown in Figure 26, high resolution in the dipo- 1 3 lar-decoupled C spectrum of solid PMMA is 0.77 achieved with quite low spinning frequencies, of the order of 500 Hz. This is true despite the fact that the chemical shift dispersion of the low-field resonance arising from the carbonyl carbon is about 3 KHz (154). 1.05 With separate lines resolved for individual carbons, a variety of relaxation experiments can be performed and interpreted in terms of the motions of the polymers in the solid state. It is 1.25 well known that a *H rotating-frame relaxation time (Tjn) is sensitive to motions associated with frequencies in the 10-100 KHz range. 1 3 Figure 27 shows the results of C T l p experiments for solid polycarbonate, both with 1.55 and without magic angle spinning, at 3 KHz (149). Five lines are well resolved. The lowest field line arises from the overlap of the carboxyl carbon resonance with that of the nonprotonated 2.10 aromatic carbons. This line has the longest T^p. The two lines just to higher field arise from the protonated aromatic carbons. These lines have the same relaxation behavior as one another and 3.20 are characterized by a short T ^ . The two lines at high field are due to the quarternary and methyl carbons. The methyl carbon T±p is inter- mediate between that of the low field combina- Figure 26. 10 G dipolar-decoupled 1 3 C NMR tion line, and that of the protonated aromatic- spectra of PMMA magic angle rotor, as a func- carbon line. This behavior simply reflects the tion of spinning frequency. The CP was per- weaker coupling of nonprotonated carbons to formed with spin-temperature alternation to more distant protons, as determined by the remove artifacts (154). inverse sixth power dependence on the internu- clear separation common to all dipolar interac- about the motions in the 5—30 MHz regions. tions. Interpretation of the T l p 's of these polymers 1 3 The C T l p and TCH(SL) have been meas- emphasises the dynamic heterogeneity of the ured also for poly(phenylene oxide), polystyrene, glassy state. Details of the relaxation processes polysulfone, poly(ether sulfone), PVC (149, 155), establish the short-range nature of certain low polybutadiene and poly(ethylene oxide) (156). frequency side-group motions, while clearly 1 3 The C T l p at 32 KHz is dominated by defining the long-range cooperative nature or spin-lattice processes rather than spin-spin pro- some of the main-chain motions, the latter not cesses. This means that the T ^ ' s contain infor- consistent with a local-mode interpretation of mation about the motions of the polymers in the motion. For instance, polystyrene the TQJJ and 10—50 KHz region, while the TQJJ'S contain T l p values are 1.2 and 3.5 ms for aromatic information about the near static interactions. rings, 0.5 and 3.6—4.1 ms for main chain car- The T, 's and NOE factors contain information bons (155). These motions involve cooperative

Vol. 7, No. 1 53 torsional oscillations within conformations rather NMR. However, actually all of these methods than another (149, 153). The ratio of T Q H to will not be able to secure the complete resolution Tjp for protonated carbons in the main chain of of signals in the 1 H and 1 3 C NMR spectra of polymers. Therefore the extraction of spectral information about microtacticity and conforma- 100 ppm tion of the chain with the aid of computers is widely practiced. We may expect increased understanding of the microstructures of mainly linear polymers and copolymers as experimental techniques improve. In turn this should lead to better without spinning understanding and hence control of the chain growth mechanism through variation of the con- ditions of synthesis. Great possibilities then open up for the design of polymers of specific physical with spinning characteristics for industrial application leading to increased cost effectiveness and quality rotating frame 0.025 12 improvement. It is hoped that this review will time, msec have shown that this subject offers considerable intellectual challenge together with its potential economic importance. Figure 27. CP 1 3 C NMR spectra of polycarbonate with and without magic angle spinning, as a REFERENCES function of the time the carbon magnetization was held in the rotating frame without CP con- 1F. A. Bovey and G. V. D. Tiers, J. Polym, tact (149). Sci. 44, 173 (1960); Fortschr. Hochpo- lym.-Forsch, 3, 139 (1963); F. A. Bovey, Pure and Appl Chem. 12, 525 (1966). 2 F. A. Bovey, High Resolution NMR of Macro- molecules, Academic Press, New York, 1972. each polymer is found to have a direct correla- 3 1. D. Robb, in Nuclear Magnetic Resonance, tion with the toughness or impact strength for all Vol. 5, Academic Press, London, 1976, p. 205. studied polymers (149). This empirical correla- 4 N. A. Plate and L. B. Stroganov, VysokomoL tion was rationalized in terms of energy dissipa- Soed. (USSR) 18, 955 (1976). tion for chains in the amorphous state in which 5 F. Heatley, in Nuclear Magnetic Resonance, low-frequency cooperative motions were deter- Vol. 9, Academic Press, London, 1980, p. 204. mined by the same inter- and intrachain steric

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58 Bulletin of Magnetic Resonance