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 dif- ferent structures in a quantitative manner. The 1. Vinyl Polymers study of polymer configuration involves consid- eration 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 be discriminated by NMR (4-6). 5 1 M I 1 1 1 1 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 ) • * • <<K in i n rmmr . 9 9 9 . P 2 n _ P W 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 con- sideration 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 copolymeriza- tion 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 poly- mers (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 synthes- ized 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-configu- ration). 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.