analysis of chain microstructure by h and c nmr … · analysis of chain microstructure by 1h and 1...

ANALYSIS OF CHAIN MICROSTRUCTURE BY1H AND 1 3 C NMR

SPECTROSCOPY

Yury E. Shapiro

NMR LaboratoryYaroslavl Polytechnic Institute

USSR

PageI. Introduction 27

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

II. Analysis of Chain Microstructure by 1H NMR Spectroscopy 30A. Assignment of NMR Signals in Accordance with the Dyad or Triad Theory 30B. Expansion of x H NMR Spectroscopy Capabilities by Use of

Superconducting Magnets. Assignment of Signals by Tetrad and HigherOrder Theories 33

C. Polymer Chain Microstructure Influence on Segmental Mobility 37

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

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

in Solid Polymers 52

V. Conclusions 54

References 54

I. INTRODUCTION

The aim of this review is to cover the contri-bution of high resolution NMR spectroscopy tothe study of polymer microstructure, particularlyafter the years 1974-5, thus continuing theseries of previous reviews (1-6).

The impact of stereospecific catalysts on thepolymer world has created new demands formethods of studying the stereochemical configu-ration of polymer chains. NMR spectroscopy hasbecome a very important method in this fieldthrough its ability to discriminate between dif-ferent structures in a quantitative manner. Thestudy of polymer configuration involves consid-eration of the polymer chain as sequences of iso-,hetero- and syndiotactic monomer plasements,

i.e., as triads. Use of superconducting magnetsand 1 3 C NMR allows one to obtain informationabout the content of tetrads, pentads and higherorder sequences.

The analysis of triads and tetrads has beenfound to be very useful in the general interpreta-tion of polymer structural problems. More spe-cific information is forthcoming which may beused in a special way, e.g., in correlations withstatistical polymerization^ theory.

A. Microstructure of Macromolecules

1. Vinyl Polymers

BVinyl monomers C(A)B=CH2are H or a substituent) have

(where A andvarious

Vol. 7, No. 1 27

possibilities of joining into polymer chains: con-figurations "head-to-tail, I, "head-to-head" and"tail-to-tail", 2;

A H A H A HI I I I I I

-C-C-C-C-C-C —I II I IIB B B H B H

A H H A A HI I I I I I

-C-C-C-C-C-C -I I I I I IB H H B B H

formation of branching points, 3, and joining, 4,of chains is also possible.

A-C-BH-C-HA I A HI I I I

-C-C-C-C-I I I IB I B BA-C-BH-C-H

A H A HI I I I

-C-C-C-C-I I IIB | B HH—C-HA-C-BA I A HI I I I

-C-C-C-C-I I I IB H B H

Even stereoregular polymers are not alwaysof very high stereochemical purity, and mostpolymers are composed of isotactic, 5, syndiotac-tic, 6, and heterotactic, 7, sequences or may beregarded as copolymers of such units (1).

A H A H A H A H

5 1 M I 1 1 1 1B H B H B H B HA H B H A H B H« 1 1 1 1 1 1 ) 1B H A H B H A H

A H A H B H B H A H

B H B H A H A H B H

NMR spectroscopy has clarified these structures.Asymmetric centers in these chains are

actually pseudoasymmetrical (2) and such poly-mers have no optical activity.

2. Diene Polymers

The diene forms l,4-(cis or trans), 8, and1,2- configurations, 9, by polymerization.

c = c % . / " • •- H 2 C C H 2 - - H 2 C

cis-S trans- 81,4-polybutadiene

1,2-structures have been discovered in iso- orsyndiotactic sequences

CHIICH

IICH,

CH

CH.

CH,II *CH

H

2

iso- 9 syndio- 91,2-polybutadiene

3. Vinyl and Diene Copolymers

The different monomer units A and B formthe following sequences in copolymer chains:

Dyads AA AB or BA

r I * i

Triads AAA AAB or BAA

BB

BAB

mm

mr

rr

• ••

t

Ti

1An additional 10 triads formed by replace-

ment of designations o and • are also possible.All of these and higher order sequences can

be discriminated by NMR (4-6).

B. Conformational Statistics andMechanism of Chain Growth

1. Homopolymerization

The possibility of determining vinyl polymerchain configuration by high resolution NMR isbased on the sensitivity of this method to mag-netically nonequivalent nuclei in differentsequences. Table 1 presents the designations ofdyads and triads.

The quantities m, r, i, h and s as obtainedfrom NMR spectra are usually normalizedaccording to the equations (2) m + r = 1 andi + h + s = 1.

The values of i, h and s have been coupledwith various considerations of polymerizationtheories. Perhaps the simplest of these, accordingto Bovey and Tiers (1), is the most generallyapplicable and involves the parameter P m , theprobability that a polymer chain will add a

28 Bulletin of Magnetic Resonance

Table 1.

Designation Projection Bernoulli anProbability

? 9Dyad meso, m

racemic, r T ' l _ pm

9 9.9Triad isotactic, i, mm I '' I *~r pm*

4-4-heterotactic, h, mr Y ' I1 ' Jj 2 Pm(l - Pm)

syndiotactic, s, rr I* T • £ (1 - Pm) 2

Tetrad mmm I ' T ' I ' I Pm3

mmr Y ' T ' Y • j, 2 Pm> (1 - Pm)

1I ' t ' ' j; Pm

2 (1 - Pm)

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

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

rmmr . 9 9 9 . P 2 n _ P W

mmrm

mmrr ? • ? • ?rmrm (heterotactic) 1 1 J , J t — 1 * 1 2 p m 2 ^ ~

rmrr

m r r m

rrrm

rrrr ( 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 asthat of the last unit at its growing end. In thiscase the chain growth process follows Bernoul-lian statistics (Table 1).

Theoretical graphs of normalized i, h and sagainst P m have been constructed and it hasbeen found that methyl methacrylate free-radicalpolymers give Bernoullian statistical propagationwhereas anionic polymers show a differentbehavior, which may be called non-Bernoullian

Pm/m - 1 ~ Pm/r

0.8

0.6

"•£0.4.ao

" C L 2

Figure 1. Relationship between i, s, h -triad pos-sibilities and P m . Points on the left of P m = 0.5are for methyl methacrylate free-radical poly-mers; points on the right are for anionic poly-mers (4).

propagation (Figure 1).The first-order Markovian sequence is

formed by chain growth, with the stereochemis-try of chain-growth end effects influencing con-necting monomer units. We have now four con-ditional probabilities r/m> Pm/r> Pr/r>

(P i^ r/m m/r r/rwhich describe the connection process (Pr /m isthe probability of monomer unit connection in them-configuration to the chain end with r-configu-ration). They are

Pm/r = (mr)/[2(mm) + (mr)J [ (1 - Pm)] (1)

P r / m = (mr)/[2(rr) + (mr)] [a Pm] (2)

Pr/r = 1 ~ r/m

(3)

(4)

Thus Markovian first-order statistics is con-trolled by two independent parameters P m / r and

mThe average sequence lengths may be calcu-

lated from the following equations (1-3, 7):< n m > =

< n r > ^

(5)

(6)

The Markovian second-order statistics havefour independent probabilities. It takes into con-sideration the influence of the second unit fromthe chain end (7).

Non-Markovian processes are possible too.One of them is the Coleman-Fox process (8, 9),which explains block configuration by chaingrowth. According to this model the block config-urations come into being by formation of chainend and anti-ion chelate complexes with its sub-sequent destruction.

2. Copolymerization

Assuming that the statistical copolymeriza-tion of monomers A and B is controlled by theMarkovian first-order statistics, it holds for thelow conversion limit (10):

PA/B = (1 +PB/A = d + (7)

r^ and rg represent copolymerization reactivityratio parameters and [A] and [B] represent molefractions of monomers in the initial system.

The way in which the copolymer is synthes-ized markedly affects the copolymerization reac-tivity ratios r ^ and rg and the coisotacticityparameter P m from the Bernoullian model (11).Plate and coworkers (12, 13) use only Markovianfirst-order statisitics and introduce the twoparameters of coisotacticity PA/B an(* PB/A-

II. ANALYSIS OF CHAINMICROSTRUCTURE BY1H NMR

SPECTROSCOPY

A. Assignment of NMR Signals in Accor-dance with the Dyad or Triad Theory

Two types of assignments may be distin-guished: those which follow purely from NMR


spectra and those which are taken from non-NMR evidence. Although the interpretation ofsome polymer spectra is now relatively easy, inother cases serious complexities remain. Spin-spin decoupling has been used to simplify poly-mer spectra by eliminating the effect of spincoupling. Double irradiation becomes increasingdifficult as the chemical shifts of the two coupledprotons come closer together, as for example inpolypropylene.

Another method of NMR spectral simplifica-tion is to replace particular protons with otheratoms that do not give an observable signal. Oneway of doing this is by deuteration, althoughsubstitution by halogen might also be considered.In this way, not only the signal of the substitutedgroup removed from the spectrum, but also thespin-coupling effect of the substituted group isvery much reduced. The interpretation of spectraof deuterated materials usually follows morereadily than it does for decoupled spectra, andthe assignments could be made with greater cer-tainty.

Model compound of polymers have been usedextensively in NMR spectroscopy in two mainways. Firstly, to correlate chemical shifts foundin polymers with those of well-defined smallmolecules in order to identify the presence ofparticular groupings. Secondly, the use of moresophisticated model compounds such as isomersof the 2,4-disubstituted pentane type as modelsof different configurations in the correlation ofsplitting patterns observed in polymer spectra.The study of the second type has been extendedto the 2,4,6-trisubstituted heptanes, and consid-erable insight into polymer conformational stud-ies has been gained (14, 15).

1. Homopoiymers

One classical example of polymer micros-tructure investigation is NMR analysis ofpoly(methyl methacrylate) (PMMA) samples(1-4, 7, 16, 17). Bovey found that in' CDC13solution three a-methyl peaks appeared at 0.91,1.05 and 1.22 ppm. They were due to syndio-,hetero-, and isotactic forms, respectively. Also,the isotactic methylene signal was an AB-quartet(J = -14.9 Hz), whereas the syndiotactic onewas a singlet (Figure 2). The observation of anAB quartet methylene signal is an absolutedetermination of the presence of isotactic struc-tures and is independent of any other type ofevidence. The stereoregularity of anionic pro-duced polymers has been shown (7, 16) to bedependent on the solvent. For a free-radical-initi-ated polymer it has been claimed (1) that the

Figure 2. 1H NMR spectra of PMMA solution ino-dichlorobenzene, 160° C; (a) predominantly iso-tactic polymer, (b) predominantly syndiotacticpolymer (4).

syndiotacticity was not solvent dependent, but itstemperature dependence was illustrated (16). Itwas found that the P m value for free-radicalMMA polymerization was 0.13 at -78° C and0.36 at 250° C. Highly syndiotactic crystallizablesamples of PMMA prepared by Ziegler catalystswere discussed (16).

The new possibilities of conformational anal-ysis of PMMA appear by the study of a modelcompound and MMA oligomers (17). It was ana-lyzed in relation to the stereospecific conforma-tion, taking into account the structural end groupeffects and the characteristics of the geminalmethylene proton signals.

In polymer stereoisomerism investigations,serious difficulties can be found. The problemsarise from the necessity of summary spectraseparating subspectra with many components.Therefore the number of polymers for which themicrotacticity may be determined from *H NMRspectra is now restricted (4). Recently increasingsuccess has been achieved by use of partiallydeuterated monomers (18).

The use of computer calculations for investi-gation of polymers is indispensable. First work in

Vol. 7, No. 1 31

this way has been performed for analysis ofpoly(vinyl chloride) (PVC) 1 H spectrum (2). Thespectrum of PVC methylene protons is thesuperposition of six mmm, mmr, rmr, mrm, rrmand rrr tetrad subspectra. Each of these sub-spectra is a complex spin-spin multiplet. There-fore a^-dj-PVC was synthesized for the stereoi-somerism investigation (19). Figure 3 shows thetetrad assignment in this polymer. The samework was done for methine proton pentade shifts

HB

2.5 2.0

ppm

Figure 3. 100 MHz 1 H NMR spectrum ofpoly(a-cis-3-d2-vinyl chloride) in CDC13. A, E -rmr; B - mmr + mmm; C - mmm; D - mmr; F,H - mrr; G - mrm + rrr (19).

oW-d2-PVC (Figure 4).Zymonas and Harwood (21) interpreted the

*H NMR spectra of iso-syndio- and heterotactic

1,2-polybutadienes. The methylene proton reso-nances can provide information about the mesodyad. The chemical shifts of methylene typevinyl protons in iso and syndio forms differ by0.14 ppm, indicating that the resonance of suchprotons can provide information about the rela-tive amounts of i, h, s-triads. Chemical shifts andcoupling constants were estimated to obtain anapproximate fit of calculated lines to each spec-trum with the aid of the iterative programLAOCOON 3.

2. Copolymers

The methyl methacrylate-styrene and methylmethacrylate (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 blockscleaving the aromatic signal and the randomstyrene units dividing the methoxyl resonancewere featured, but the a-methyl signal was notresolved well enough for tacticity determination.Radical copolymers have been the subject of fur-ther detailed study (4), and the twelve triadsinvolving composition and configuration in rela-tion to a central MMA unit have been dividedbetween the three methoxy resonances. The *HNMR spectra of random and alternating copo-lymers of styrene and methyl, ethyl, butyl andoctyl methacrylates were analyzed in (11). Theway in which the copolymerization mechanismfollows therefrom markedly affect the magnitudeof the parameter of coisotacticity P m : for allcomonomer pairs under study, P m is higherwhen an organometallic catalyst is used (alter-nating) than in the case of a radical initiator(statistical). With increasing number of carbonatoms in the n-alkyl alcohol residue, P,decreases to a limiting value.

The chemical shifts of the methoxyl anda-methyl protons in the alternating MMA-styr-ene copolymer are calculated by taking intoaccount the contributions of the diamagneticshielding and the magnetic anisotropy effect ofthe benzene rings in styrene units (22, 23).Three- and four-bond interaction parameters,which are necessary for the calculation of con-formational probabilities of dyad sequences in acopolymer chain may be estimated from theparameters determined for the homopolymers.

Klesper and Gronsky (24) investigated themonomer distribution and microtacticity of theMMA-MAA copolymers. By using model com-pounds they offered the well-founded assignmentof twenty stereoisomeric triads to six componentsof the a-methyl spectrum.

• m


4.4 4.2

&. ppm

Figure 4. 100 MHz 1H{2H} NMR spectrum ofpolyfp fi -d2 -vinyl chloride) in C2 HC15 1 - rrrr +rrrm + mrrm; 2 - rrmm + mrmm; 3 - rrmr +rmrm; 4 - mmmm; 5 - rmmm; 6 - rmmr (20).

with the polar monomer units, sometimes longersequences may be discovered. For example, thepentads were found in the 100 MHz spectra ofthe MMA-acrylonitrile copolymer solution inDMSO-d6 (28). Splitting of the MMA a-methylsignal on MMA pentad components shows thecopolymers have block-sequences with the MMAunits having more than three components.

B. Expansion ofxH NMR SpectroscopyCapabilities by Use of Superconducting

Magnets. Assignment of Signalsby Tetrad and Higher Order Theories

Development of high frequency spectrome-ters (220-600 MHz) with superconducting mag-nets has been of principal importance in polymermicrostructure investigations. The major con-temporary technical difficulty stems from thefact that a high resolution NMR spectrum of amacromolecule is generally a broad, featureless,and uninformative envelop of many overlappinglines of chemically related monomers and chainsequences, even at the highest resolution availa-ble (29).

1. Homopolymers

Figure 5 shows * H NMR spectra of the samesamples of PMMA that are shown in Figure 2

The contribution of this investigation topolymer chemistry consists in the fact that thesecopolymers represent the best model for studyingthe unit distribution change and tacticity bycopolymerization and chemical modification ofcopolymers. The copolymer list may be extendedto the spectrally similar copolymers: phenylme-thacrylate-MMA (25), benzilmethacrylate-MAAand diphenyl methyl methacrylate-MAA (26),and at the expense of the systems, which turninto spectrally similar copolymers by analogouspolymer reactions (MMA-ethyl, isopropyl, tert-butyl, benzil, cc-methyl benzil, diphenyl,1,1-diphenylethyl, a,a-dimethylbenzil, trityl,1-naphtyl methacrylates) (27). The reactivities ofthe monomers have been explained in terms ofthe polar effect of the ester groups in radical andanionic copolymerizations. Coisotactic parame-ters have been determined by assuming the ter-minal model statistics.

Usually only dyad and triad sequences inpolymer chains may be identified at frequencieslower than 200 MHz. However in copolymers

Figure 5. 220 MHz J H NMR spectra of PMMAsolution in o-dichlorobenzene (4); (a) predomi-nantly isotactic polymer, (b) predominantly syn-diotactic polymer.

but at 220 MHz. The latent tetrad structure ofmethylene signals at 60 MHz is obvious at 220MHz (2, 4). The pentad signals, which lead tothe asymmetry of a-methyl signals at 220 MHz,

Vol. 7, No. 1 33

are distinguished beautifully at 300 MHz (Figure6) (30). The fine hexad structure of CH2 -signalsis visible as well at this frequency.

Spectroscopy at 220 MHz is effective in thestudy of the ionic polymerization mechanism (31,32). The polymers of 1,2-butylene oxide anda-methylstyrene prepared under anionic poly-merization have the Markovian first-order chaingrowth mechanism. The cationic poly(l,2- butyl-ene oxide) and poly(a-methyl styrene) preparedat temperatures above -25° C follow Bernoullianstatistics with Pm=0.26. The information wasobtained from tetrad/pentad sequences (32).Anionic and cationic poly(p-isopropyl-a-methylstyrene)'s have the opposite chaingrowth mechanisms (33).

The longest sequences in polyolefins wereobtained from 300 MHz spectra of predomi-nantly isotactic polystyrene. Flory (34) calcu-lated the chemical shifts for methine and aro-matic protons of the central > CHAr group in theall-meso nonad mo and in the nonads rrm.,mrrm5, m2 r2 m4 , m3 r2 m2 containing a singleracemic triad. The magnetic shielding by phenylgroups that were First and second neighborsalong the chain were computed according to theirdistances and orientations relative to the givenproton in each conformation of the chain usingthe ring current representation of the IT electronsor, alternatively, the magnetic anisotropy of thephenyl group and the McConnell equation. Theresulting chemical shifts were averaged over allconformations. The calculations for the methineproton are in excellent agreement with the 300MHz spectrum. Treatment of the chemical shiftsfor the aromatic protons in ortho and meta posi-tions is indecisive owing to the extraordinarysensitivity of the shielding to torsional angles inthe chain backbone.

For the first time the conformation of a poly-mer chain was determined in Bovey's elegantpaper (35) with the aid of model spectral simula-tion. Figure 7 shows the synthesis of the poly vi-nyl chloride (PVC) methylene and methine 220MHz spectra from the tetrad and pentad sub-spectra. The PVC spectra give moreover muchstructural information, for example, the chemicalshifts and spin-spin coupling constants. It wasascertained that the m-dyads had the TG ^ GTconformation in atactic PVC chain since theirvicinal spin-spin coupling constants were thesame as for the model meso-dimer. r-Dyads hadthe TT conformation. Therefore the majority ofloops in PVC chain must occur in the mr link-upof TGTT and GTTT forms and their mirror rep-resentations.

H H

Cl H H

(m) TG ^ GT

Tanaka and Sato (36) studied the distributionof cis- and trans-1,4 units in various kinds of1,4-polybutadienes (PB) and -polyisoprenesincluding cis-trans equibinary PB and UV-isom-erized PB by use of x H NMR spectroscopy at 60,100, 220, and 300 MHz. Poly(butadiene-2,3-d2)was used for the peak assignment. The reso-nance of methylene protons in cis-trans linkagewas described as an A2B2 system. Figure 8ashows the spectra of poly(butadiene-2,3-d2) pre-pared by butyl lithium. The splitting of the signalis identical with that of the methylene protonsignal decoupled from the methine proton in PBas shown in Figure 8b. In Figure 8a the centra!peak due to the cis-trans linkages in the 60 and100 MHz spectra separates into two parts at220 MHz and 300 MHz. This indicates the exis-tence of two types of methylene protons in cis-trans and trans:cis linkages. A computer simula-tion was carried out as shown in Figure 8b andthe intensity ratio of the peaks was chosen so asto follow Bernoullian statistics.

The same results were obtained forpoly(2,3-dimethyl-l,3-butadiene) (PDMB). The220 MHz spectra of PDMB prepared by butyllithium in cyclohexane were compared to those offree-radical PDMB (37). By aid of measurementsdone on cis- and trans-1,4 PDMB prepared byZiegler catalysts, it was determined that bothpolymers were Bernoullian with probabilities ofcis placement of 0.24 and 0.40, respectively. Itwas shown that the anionic sample was largelytrans. Anionically prepared PB was predomi-nantly trans too (36) and had as much as 23%cyclic structures (38).

2. Copolymers

NMR spectroscopy at high frequencies(220-600 MHz) allows one to determine themonomer unit distribution as well as the micro-tacticity of these units in copolymer chains.Investigations of a number of copolymers indi-cate that the copolymerization kinetics deviatesfrom the simple Mayo-Lewis scheme. In (39) 220MHz XH NMR spectra of some free-radicallyprepared MMA-chloroprene copolymers havebeen recorded. The intensities of the a-methylsignals are related to the relative proportions ofvarious MMA centered triads and pentads. Triad


mrrsnmfn mmmmr mrrrinm rrmmm rrmmmr mrmmr rrmmr mrmmr

rrrmmrrrmr

2.4 2.2 2.0

0, ppm

1.4 1.2

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

CH

4.6 4.2. ppm

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

Vol. 7, No. 1 35

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 isomerizedpolybutadiene (b) in the methylene regionobtained at 100 and 300 MHz (36).

fractions indicate that the Mayo-Lewis scheme isnot strictly applicable to this system and is ingood agreement with those calculated from thepenultimate reactivity ratios r1 1 = 0.107, r2 1= 0.057, and r2 = 6.7 where MMA is monomer1. However, although a small penultimate groupeffect is indicated, some deviation from theMayo-Lewis scheme may be due to the occurenceof anomalous head-to-head and tail-to-tailMMA-chloroprene linkages. A similar analysishas been described for acrylic monom-er-2-substituted-l,3-diene and alternating (40,41) and conventional butadiene-d4-acrylonitrile(42) copolymers. Alternating copolymers wereprepared under Et3Al2Cl3/VOCl3 or ZnCl2catalysts. The random copolymers are in goodagreement with the Markovian first-order statis-tics. The reactivity ratios are r , , = 0.18, r2 =0.62 and r , , = 0.26, r2 = 0.63 (where MMA ismonomer 1) for MMA-butadiehe and MMA-iso-prene, respectively (40, 41).

Constant composition copolymers of MMA ormethacrylonitrile and vinylidene chloride pro-duced by radical copolymerization were studiedby 1H NMR at 60, 250 (43) and 220 MHz (44).The monomer dyad/triad sequences and some ofthe tetrad/pentad sequences were obtained fromspectra (Figure 9). In (43) a new graphical

method of reactivity ratio calculations is pro-posed, based on the use of specific values of thetriad distribution functions and the Coleman-Fox

1.55\ ppm

Figure 9. 250 MHz * H spectra of the a-methylproton region of copolymer MMA and vinylidenechloride. [MMA] = 29% (a) and 81% (b). Pentaddecomposition is attributed (43).

model. It is possible to detect a penultimateeffect for the vinylidene chloride-rich region. Inthe same region, a change in tacticity of thetriads on the MMA sequences, as compared withhomopolymers, is observed; it is suggested thatthe anomaly is caused by the competition of thedepropagation reaction. It can by shown that thebulk copolymerization kinetics deviates from theMayo-Lewis scheme (Figure 10). Small differ-ences were found between the bulk and solutioncopolymerization (44) since the bulk process washeterogeneous. This could indicate that solvationeffects were important.

When assigning sequences in NMR spectra of


vinyl polymers, it is usually assumed that near-est-neighbor monomer units possess a largerinfluence on the chemical shifts of the centralunit than on monomer units further removed.Strasilla and Klesper (26, 45) studied the proton-OCH3 resonances of MMA-methacrylic acid(MAA) and MMA-diphenylmethyl-methacrylatecopolymers. In fact, the differentiation of nearestneighbors appears to vanish in the present case,and within the limits of detection, only the unitsremoved were responsible for resolving the-OCH, resonance of the MMA units into triadpeaks. The detection of such an effect by inten-sity measurement is possible only with non-Ber-noullian copolymers, particularly with copolym-ers possessing a strong tendency towardalternation. In copolymers with alternatingcharacter, the statistics of sequences composed ofnearest neighbors differs much from the statis-tics of sequences composed of next to nearestneighbors than in the case of copolymers ofblock-like character, e.g. in styrene-MMA copo-lymers (45a). An assignment of such "next tonearest neighbor" triads appears possible if it isassumed that the syndiotactic chain is in an all-trans conformation.

C. Polymer Chain Microstructure Influenceon Segmental Mobility

The relationship between microstructure andsegmental mobility of polymer chain may be bet-ter studied with the aid of proton spin-latticerelaxation times than with 13C T, measure-ments. However, this is not correct since protonand carbon-13 T, values are the complement ofone another and are not always identical.Accounts of nuclear magnetic relaxation and thetheories of polymer chain motions can be foundin a number of reviews. The last among them is(46).

Spevacek and Schneider (47) showed withthe aid of a T, 1 H study that PMMA formedstereocompexes in CC16, CD3CN, toluene andbenzene solution. The smallest syndiotacticsequence length in complexes is 8 (in benzenesolution) or 3 (CC14, CD3 CN solution) monomerblocks. The relationship between iso- and synd-iosequences in a stereocomplex is 1:1.5. Thestereocomplexes between iso- and syndiotacticPMMA have been formed by means of exchangeinteraction between the ester groups. In dilutesolutions of s-PMMA a considerable portion(76%) of polymer segments are intramolecularlyassociated. The motion of associated segmentsappears as isotropic with an effective correlationfrequency of 10' -107 Hz.

1.0

o

8 0.5 •

o

MCCCs

- y^

cccc\

MCCM^ /

• i l l

jy

I IA/I

0 0.5Mole fraction vinylidene chloride

1.0

Figure 10. Measured tetrad distributions in bulkprepared MAA-vinylidene chloride copolymerscompared with distributions calculated from r1= 0.40 and r2 = 2.5 (solid lines) (44).

Hatada and coworkers (48) have shown thatthe tacticities of poly(alkyl methacrylates) can beworked out in detail by using the large differencein spin-lattice relaxation times of protons in a-Meand ester groups to eliminate the ester groupresonance overlap with the a-Me signal whichnormally obscures splittings due to tacticity.Data were given for several C, -C5 -alkyl metha-crylate homopolymers. In other work (49)Hatada demonstrated that T1 values for isotacticsequences were longer than for syndiotactic.Table 2 shows the correlation times for Me anda-Me groups which were calculated from x H and1 3 C T1 -values for iso- and syndiotactic PMMA.

With the aid of proton spin-lattice relaxationmeasurements at 100 and 250 MHz, the seg-mental motion of poly(4-vinyl-pyridinium brom-ide) in methanol was studied (50). The necessarygeometrical parameters were received from theconformational calculation of hexads rrmrrassuming two models with and without Br" ionnear pyridinium. For these two models of thecharge distribution, the potential barriers of therm triad mobility have been calculated. The bestagreement between experimental data andtemperature curves of T, was achieved byAHRT = 6 kcal/mol, ( T R ) 0 = 10"13 s for the

Vol. 7, No. 1 37

Table 2. XH and 13C Correla t ion Times for I so- andSyndiotactic PMMA ( T C X 1 0 1 1 S) (57).

Group

™. 338

i

• 7.8

X3C

3-18.it

1 H

7-716

s

1 3C

7-122

aliphatic chain and AHgf = 2 kcal/mol, (T )0 =1.4X10" 1X sfor pyridinium ion. In the temper-ature range of 250-350 K the vibrational ampli-tude of the chain increases from 40° to 85° .

III. INVESTIGATION OF CHAINMICROSTRUCTURE BY13C NMR

SPECTROSCOPY

A. Advantage ofX3C NMR compared with1H NMR in Microstructure Analysis

Since the advent of commercial pulsed Four-ier transform 1 3C NMR instrumentation, greatadvances have been made in the elucidation ofpolymer microstructure (51, 52). Firstly, thetwenty-fold increase in chemical shift range over1 H NMR allows much better resolution of smallstructural differences. Secondly, the relaxationtimes of *3 C nuclei in CHn groups (n > 0) aredominated by dipolar interaction with theattached protons. Since the C-H bond lengthremains constant from one polymer to another,1 3 C relaxation times are a reliable probe ofmolecular mobility.

Figure 1 la, the proton NMR spectrum for anisoprene-acrylonitrile copolymer, shows charac-teristic broad peaks and yields little structuralinformation. Figure l ib , the proton decoupled1 3 C NMR spectrum for the same sample, givessharp peaks for each type of carbon atom, and isused with the coupled spectrum to assign thepeaks (53). The peaks corresponding to CN-car-bon atoms are still not singlets in the decoupledspectrum. This is because of the microstructureeffect which may be observed for other carbonatoms. Table 3 presents the structure composi-tion of poly(isoprene-acrylonitrile)'s (54).

Matsuzaki's poly(2-vinylpyridine) investiga-tion (55) may be cited as another example of theadvantage of 13 C NMR. The * H NMR spectrum

of the poly(2-vinylpyridine-0,3-d2) in D2SO4(Figure 12a) shows three peaks of methine pro-tons, which are assigned to i, s and h triads.Since the absorption peaks of hetero- and syndi-otactic triads of methine protons overlap those ofmethylene protons in nondeuterated polymers,only isotactic triad intensities can be obtainedfrom 1H NMR spectra of poly(2-vinylpyridine).The x 3 C signals (Figure 12b) split into a numberof peaks. This splitting may be due to pentadtacticity. The results (Table 4) show thatpoly(2-vinylpyridine) obtained by radical poly-merization (with AIBN as initiator) is an atacticpolymer with Bernoullian statistics. The pentadtacticities of the isotactic polymer (prepared withPhMgBr as initiator) were then calculated on thebasis of a first-order Markovian process.

Finally one must note that 1 3 C NMR spec-troscopy allows one to obtain microstructureinformation inaccessible by other means.

B. Nuclear Relaxation and the NuclearOverhauser Effect

Noise decoupling in 1 3C NMR spectroscopyaids assignment by collapsing multiplets to sin-glets, and in addition selectively enhances thesignals through the nuclear Overhauserenhancement (NOE). It has been found that theintensities of carbons of similar hybridization andnumber of attached protons are directly corre-lated (46, 51). Carbons of different type areusually correlated by a single empirical NOEfactor measured directly from the spectra (49,56-59). It has been found (46, 53, 56, 59, 60)that the NOE factor for the carbon-13 nucleus ina main chain or near it is the same for a numberof polymers in solution. This is proven by theagreement of the xH and 13C microstructuraldata. Recently a number of authors makes use ofparamagnetic additions (nitroxil radicals (56), or


120 s80ppm

Figure 11. *H NMR at 80 MHz and x 3 C{1H} at20 MHz (b) spectra of isoprene-acrylonitrilecopolymer (A content is 38 mol %) dissolved inCC14 and CDC13 (53).

Table 3. Structure Composition ofPoly(isoprene-acrylonitrile)'s (62).

AlAl

Sequence

*-tai1-to-tai1-head-to-head

III ( I DIAIIAAAAA

0.0.0.0.0.0.

LAJ,18

i*58125M785U1010i»5

mass*

000000

.638

.190

.172

.761

.177

.062

* l-isoprene,

B-2

2 ppm 158 156 ppm

acetylacetonate of Cr (60-62) and of Fe(III) (63)in order to decrease the NOE effect.

The influence of stereochemistry on relaxa-tion has been investigated for a few polymers.Isotactic PMMA is appreciably more mobile thansyndiotactic, the T, values being in the ratio1:1.5 (see Table 2). Inoue et al. (64, 65, 66)report a Tn-iso/T -syndio ratio of 2 for C6D6solutions at 80 C. For polystyrene andpoly(a-methylstyrene) (59, 65, 66) on the otherhand, the isotactic form is slightly less mobile.

Figure 12. *H at 100 MHz (2) and ^ C ^ H } at25.1 MHz (b) NMR spectra ofpoly(2-vinylpyridine) observed in D2SO4 at60° C. (Samples B-l and B-2 were prepared withAIBN and PhMgBr as initiator) (55).

Vol. 7, No. 1 39

Table 4. Pentad Tacticity ofPoly (2-vinylpyr idine) (63).

Pentad

mmmmmnrnirrmmrmmrmmtnrrrmrmmrrmrmrrrrrmrrrr

Compos iObserved

0.040.100.070.16

0.32

0.110.160.04

tionCalculated

0.050.120.060.120.12*0.12*0.06*0.140.140.07

* Total mmrr+rmrm+mrrm: 0.30

The T1 activation energies are independent ofconfiguration. Randall (67) and Asakura (67a)have measured the 13C relaxation times ofnumerous stereochemical sequences in the CH,,CH2 and CH3 regions of an atactic polypropoly-lene sample. The carbons from isotacticsequences tended to exhibit the longest T, val-ues, but the largest differences between iso- andsyndiotactic units was 32% for CH carbons (Fig-ure 13). The activation energies for all T, val-ues were independent of configuration, as forpolystyrene. The origin of the small stereochemi-cal dependence of T, in polystyrene and polypro-pylene is probably connected therefore withslightly different values of the force constants(46).

Gronski et al. have studied the dependence ofn C T, values on sequence distribution in styr-ene-butadiene (68) and 1,4-1,2-butadiene (69,70) copolymers. In the styrene-butadiene system,the T, values for the para-phenyl carbon for twosamples with average block lengths of 1 and 6are 0.56 and 0.33 s respectively in CHC13 at53° C and 60 wt%. The comparable value forpolystyrene is 0.11 s. The factor of 3 increaseshown by the sample with < n g > = 6 is indica-tive of segmental motions involving the coopera-tion of perhaps three or four monomer units.Similar effects are observed in the 1,4-1,2 buta-diene copolymer. For example, the T1 value forthe CH of a 1,2-butadiene unit is 0.80 s when itsneighbors are also 1,2-units, but 1.65 s when itsneighbors are cis-l,4-units.

403.0

2.0 h

1.0

3.0 251/T°K *103

Figure 13. Arrhenius plot of syndiotactic (A andisotactic (0) polypropylene methyl relaxation(67).

It may be pointed out that the carbonrelaxation study acquired greater significancethan l H because of their simpler interpretationand of a possibility of evaluating polymer seg-mental mobility in solids (71).

C. Microstructure Analysis of Macromolec-ules with the Aid ofX3CNMR Spectroscopy

1. Polyolefins1 3 C NMR has proven to be an informative

technique for measuring stereochemical sequencedistributions in vinyl polymers. Chemical shiftsensitivities to tetrad, pentad and hexad place-ments have been reported for 13C NMR spectraof branched polyethylenes, polypropylene (PP),polyvinylchloride (PVC), polyvinylalcohol (PVA),and polystyrene (PS).

Pulsed FT 1 3 C NMR studies clearly demon-strated the presence of ethyl, butyl and longerchain branches in low density polyethylene(72-75). The concentrations of ethyl, n-butyl, andlonger chain branches were determined as 3-4,


10-13, and 8-21 per 1000 carbon atoms accord-ingly. The methyl carbon of the ethyl branchwas seen as three resonances. These were asso-ciated with isolated butene units (11.2 ppm) andadjacent butene-1 units as m and r dyads (10.8and 10.4 ppm respectively). The same study wasmade for PVC (73, 73a) (2-4 branches per 1000carbons).

Randall (63, 67) made PP i 3 C resonanceassignments with the aid of T, 's and the modelcompound study of Zambelli et al. (76) whereonly nine resonances were observed. Figure 14shows *3 C NMR spectra of PP with the peakassignments. r i i

Tonelli (77) demonstrated that the stereose-quence-dependent 1 3 C NMR chemical shiftsobserved in hydrocarbon polymers can be under-stood on the basis of the interaction betweencarbons separated by three bonds.

A chemical inversion in PP chain was con-sidered in papers (78-81). The sequence distri-butions of inverted propylene units were attrib-uted to Bernoullian (79) or in an opposite view,to first-order Markovian (80) statistics. IsotacticPP was prepared in the presence of organome-tallic cocatalysts bearing * 3 C.-enriched methylsubstituents (81). The enriched methyl carbon isdetected, in stereoregular placement, on the endgroups and never undergoes transformation tomethylene. Therefore it is unlikely that interme-diates are involved in the polymerization mecha-nism. In addition, since neither a chiral carbonnor a spiralized chain participates in the twoaddition steps, the steric control arises, unequi-vocally, from the chirality of the catalytic center.

The effects of the tacticity on the 13C NMRspectra of PVC were calculated and observed in(82-84). Keller and coworkers showed in theirinvestigations that 13C NMR spectroscopyallowed immediate determination of CC12 groupsin chlorinated polyethylene, PP (85), and PVC(86). By combination with proton resonanceinvestigations the quantitative analysis of chlori-nated polymers with respect to the constitution,i.e., CH2, CHC1, and CC12 group content provedpossible. The constitution curves obtained deviateslightly from those calculated for the chlorinationof CH2 groups by Bernoullian statistics. Thedeviations can be sufficiently described by sub-stitution statistics proposed by Frensdorff andEkiner (87) for parameters X = 0.6 for chlori-nated polyethylene and 0.9 or 1.6 for PVC, andare discussed with respect to the chlorinationmodel of Kolinski and coworkers (88).

By using the appropriate experimental con-ditions (in DMSO solution) Wu (89) resolved themethine carbon signal into a triplet of triplets in

29.2 27.2

47.7 45.7

Figure 14. 13 COH} NMR spectra of methyl (a),methine (b), and methylene (c) carbons in PP at120° C (67).

PVA spectra at 67.9 MHz which was readilyassignable to pentad tacticity. Quantitative anal-ysis of this spectra proved that stereoregularityof radical-initiated polymerization of vinyl ace-tate was almost atactic. The stereochemicalsequence distribution in the isopoly(vinyl alcohol)

Vol. 7, No. 1 41

derived from cationic polymerization conforms tofirst-order Markovian statistics. The conforma-tional aspects of poly(vinyl acetate) have beendiscussed in (90).

Randall (91) has made an assignment of sig-nals in x 3 C NMR spectra of amorphous polys-tyrene (PS) with the aid of model compounds (92)and Paul-Grant calculations of the chemicalshifts (71). It has been found that ring currentsof neighboring phenyls influenced the methylene

carbon chemical shifts. The stereochemicalsequence distribution in PS is in accord withBernoullian statistics (92a). By using the inducedcurrents approach the increments for the chemi-cal shift of a quarternary carbon due to diamag-netic screening by the neighboring aromatic sub-stituents for atactic (55, 93, 94) and regularconformations (95) of the iso- and syndiofrag-ments of PS, poly-2-vinylpyridine and

Table 5. Assignment of 1 3 C NMR Signals of 1,4-PI (115)-

CarbonChemical Shift (ppm from TMS)

trans-trans trans-cis cis-trans cis-cis

CO)C(2)

3913426

.67• 38.69

39

26

• 91• 55•55

3213426

.01

.68

.45

32.134.26.

258536

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

SampleFractions of Dyad Sequence

trans-trans trans-cis cis-trans cis-cis

Chicle1somer ipercha1somer iPI

zed

zed

qutta

ci s-

66.160.1(61.5)*24.9(25.0)*

012.4(16.9)*25-1(25-0)

015-3(16.9)24.6(25.0)

33.96.3(4.7)25.4(25.0)

* The values in parantheses are calculated from Bernoullian statistics.


poly-4-vinylpyridine were calculated. The temp-erature dependence of chemical shifts of thetriads of quarternary carbon of atacticpoly-2-vinylpyridine was studied from -20° to50° C. On the basis of the theoretical and experi-mental data, a model of an atactic chain waspresented for polymers with a different ampli-tude of torsional oscillations for different struc-tures in the absence of free rotation. Conclusionsconcerning a conformational set of the irregularchain of macromolecules were made: the isofrag-ments were predominantly from the right-handand left-hand spirals of the 3t type; the syndiof-ragments contained equal parts of trans-confor-mation and spiral structures 21 (95).

2. Polydienes

Recently several papers were published con-cerning the sequence distribution study in poly-butadiene (36, 52, 96-105) (PB) and polyisoprene(36, 106-109) (PI) by 1 3 C NMR spectroscopy.The * * C peak assignments were made with theaid of model compound spectra (96-98) and of theGrant-Paul additivity coefficient calculations.

Each of the olefinic-carbon signals of thecis-1,4 and trans-1,4 units in PB were reported(36, 99) to split into two peaks which were ten-tatively assigned to the olefinic carbons of thecentral monomer unit in the triad sequences ofcis-1,4 (C) and trans-1,4 (T) units (Figure 15).The ultrasonic irradiation of the polymer solutioncaused an enhancement of the resolution in *3 CNMR spectra as well as in the decoupled *Hspectrum (Figure 15c). The observed dyad frac-tions fitted well to the theoretical curves calcu-lated by assuming BernouUian statistics (Figure16). It is in good agreement with those obtainedby *H NMR and IR measurements. The distri-bution of cis and trans configurations inl,4-poly(2,3-dimethyl-l,3-butadiene) follows Ber-nouUian statistics as well (100).

The * 3 C NMR spectra of chickle PI and cis-trans isomerized 1,4-PI's were studied in the C,,C2, and C4 carbon* signals of the isomerizedPi's. The new signals were assigned to the car-bon atoms in cis-trans linkages (Table 5). Table6 shows the fractions of the dyad sequences. Itwas found that the cis-1,4 and trans-1,4 unitswere randomly distributed in the isomerized Pi's.

Randall has shown (101) that the sequencedistribution of 1,2- and 1,4-units in hydrogenatedPB's conforms to the first-order Markovian

a

ABCD

132 1305,ppm

128

Figure 15. ^CC1!!} NMR spectra of a mixtureof cis-1,4 and trans-1,4 PB's (a), isomerized PB(b) and (c) ultrasonic-irradiated product of (b)(36).

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

Vol. 7, No. 1 43

0.5trans - U -fraction

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

statistics. It may be explained by the stericdependence of a terminal 1,2-unit upon polymer-ization. Similar results were obtained from x 3 CNMR spectra of poly(2-phenyl-l,3-butadiene)(102). The consideration of position distributionsof 1,2-units in the PB chain makes it possible toassign 64 various triads (103, 103a). The triadassignment of PB aliphatic carbons was made in(104, 105, 105a).

Gronski and coworkers (107) and Beebe(108) published the 1 3 C NMR microstructureresults of a binary PI with 3,4-cis-l,4 structuralunits and of a ternary PI with 3,4 and cis/trans-1,4 units. It has been shown that for allsignals, the best agreements between predictedand experimental intensities is found for theMarkov model.

Coleman (110) has studied polychloropreneat 67.91 MHz. The dyad and triad microstruc-ture was characterized. The back turning oftrans-1,4 and cis-1,4, and isomerized 1,2- and3,4 units was determined.

1 3 C NMR spectroscopic data obtained formodel compounds imitating regular and irregularaddition of monomer units in linear PI werecompared with the chemical shifts calculatedusing the empirical regularities found for thebranched alkanes and alkenes and a good corre-lation was established (109). The validity of the

results obtained was confirmed by investigationof the carbon spectra of hydrogenated and deu-terated Pi's which contain chain fragments withirregular addition of units. Samples of hydro-genated Pi's shown in Figure 17 give resonancelines that correspond to the methylene carbons at

30ppm

20 10

Figure 17. Aliphatic part of the 13C NMR spec-tra at 67.88 MHz of Pi's (109).

34.62 and 27.61 ppm, respectively. For thedeuterated Pi's, the methylene carbon reso-nances of trans- and cis-units in head-to-headaddition was found at 38.6 and 31.4 ppm, withthose in the tail-to-tail addition of both isomers at28.4-28.8 ppm. The latter findings offer a prac-tical means of characterizing irregularities in PI.


3. Olefinic Copolymers

Ethylene-propylene copolymers (EPC) havebeen well studied (111-114) with the aid of 1 3CNMR spectra of model alkanes (112, 113). Car-man and Elgert (111, 112) have developed amathematical model of EPC polymerizationwhich accurately accounts for the intensity ofeach peak in any spectrum of EPC. This is a ter-polymer model in which the propylene is addedby either primary or secondary insertion. Thuspropylene inversion is determined from the ratioof contiguous to isolated propylene sequences.The stereochemical environment of the isolatedethylene units, and the arrangement of theneighboring propylene units in EPC, prepared inthe presence of syndiotactic- and isotactic-specificcatalysts were investigated (113, 115) by com-paring 1 3 C NMR spectra of selectively1 3 C-enriched copolymers. The implications ofcopolymer structure on polymerization mecha-nisms are considered. In the presence of homo-geneous syndiotactic specific catalyst systems,both the regiospecificity and stereospecificity arecontrolled by the last unit of the growing chainend. Stereoregulation is transmitted throughachiral ethylene units, but not in isotactic poly-merization. The meaning of these facts is that

the isotactic regulation arises from the asymme-tric spatial arrangement of the ligands in thecatalytic centers, whereas the syndiotactic regu-lation arises from the asymmetry of the last unitof the growing chain end; syndiotactic regulationis therefore last whenever the last unit is achi-ral.

The dyad-tetrad sequence distribution in pro-pylene-butene-1 copolymers was determined in(114-116). The monomer distribution is in goodagreement with Bernoullian statistics (115). Theanalysis of methine triads and tetrads of back-bone methylene carbons have been verified usingfirst-order Markovian theory (116). Coisotacticshift contributions also account for the reverseorder of the propylene-centered from that pre-dicted by the Grant-Paul equation.

Quite a number of authors (51, 89, 117, 118)investigated ethylene-vinyl acetate copolymers.The intensities of the methine and methylenepeaks were related to the triad populations. Theplots of triad population variations withmonomer ratios are given in Figure 18. Thesimilar triad splitting of the quarternary carbon,CN or CO groups was obtained in 1 3 C spectra ofrandom styrene copolymers with acrylonitrile(119, 120), acrylic acid (121) and MMA (51) andalternating styrene-MMA copolymers (122).

ino

80

o 603Q.OQ- 40O> 20

\ vvv\ /

a

EVVrVVE -/

v EVE "

EEV

• VEV>

b

+ VEE

v;

EEE A

80 60 40 20 80Vinyl acetate, mole %

60 40 20

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

Vol. 7, No. 1 45

AmKfcnArNrAT ArNrArNrA*r m r m | | m m r m

llCH,

r m rr

r — NmAmN

m r

U6 145 144 128 127 126 125 124 123 350, ppti

30 25 20

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 at67.88 MHz of alternating methacryloni-trile-o:-methyls tyrene copolymer with those of thesyndiotactic homopolymers showed that thecopolymer had the random configuration withdominantly syndiotactic enchainment of monom-ers (123), in contrast to 1 H NMR results. Fig-ure 19 shows the triad and pentad peak assign-ments. The relative configurational enchainmentof a-methylstyrene (A) and metha-acrylonitrile(N) in eyrthro-diisotactic structure is m, whereasin threo-diisotactic is r. Slight deviation fromexact alternating copolymerization was shown bythe presence of NNA triad or its correspondingpentads.

The radical copolymers of MMA with MAAand a-methacrylophenone were studied (124,125). In this case Bernoullian statistics describesthe chain growth too. The steric factors and highpolarizability of aromatic keto-groups caused thelarge values of P m = 0.40-0.43 (125).

4.Diene Copolymers

The first peak assignments for alternatingbutadiene-propene, -acrylonitrile, isoprene-pro-pene, -acrylonitrile and some polyalkenylenesand polypentadienes has been obtained by Gattiand Carbonaro (126) with the aid of

off-resonance experiments and of calculations bythe Grant and Paul scheme.

The 13 C NMR spectrum of butadiene-styrenecopolymer has 30 peaks at 25-46 ppm and 19peaks at 114-146 ppm assigned to 152 possibletriads of 6 units: cis-1,4; trans-1,4; "head-to-tail" and "head-to-head"-l,2-butadiene (B);"head-to-tail" and "head-to-head" styrene (S)(51, 127-129). In general, one would expect allthe styrene in samples to be in BSB triads andwould therefore expect a pattern of absorptions(BS and SB) very similar to that of the vinylunits (Bv and vB), also expected to be randomlydistributed (51, 127). Styrene average blocklengths were found to vary greatly (1.2—5.9units) while vinyl butadiene units showed notendency to block together, cis units only a smalltendency (1.0—1.7) and trans units a moderatetendency (1.2—3.4). Styrene units display a ten-dency to block with trans units whereas vinyland cis units generally prefer to block with transunits (127, 129).

The butadiene-vinylchloride copolymersobtained by radiation copolymerization in chan-nel complexes of urea have randomly distributedunits. Vinylchloride forms predominantly syndio-tactic sequences (130).

Emulsion processed butadiene-acrylonitrile(51) and isoprene-acrylonitrile (53, 54)


copolymers were investigated. The vinyl and CNpeaks were the most sensitive to environmentand splitting into triad components. The struc-ture of these copolymers is highly alternating(see Table 3). At 28% acrylonitrile it is essen-tially block butadiene with short runs (1—3units) of alternating A and B increasing to 7—8unit lengths at 40% (51). With increasing con-version the content of block-triads increases. Thedata prove that the theories of Medvedev andSmith-Ewart apply to emulsion polymerization(54).

We also studied the microstructure of copo-lymers of or^-unsaturated ketones (K) with iso-prene (I) by 13C NMR spectroscopy (60). Figure20 shows the spectra of copolymers of isoprenewith alkylvinylketones CH2 =CH-COR (R =

g.Jflho(MfOoo

•8 855COo

c _L _«»—

J L1

220 140 100 40 20 0fr.ppm

Figure 20. 1 3C NMR spectra of copolymers iso-prene with methyl- (a) isopropyl- (b) and tert-bu-tylvinylketone (c) in CDC13 (60).

CH3, CH(CH3)2 or C(CH8),). The splitting of

Vol. 7, No. 1

signals into two components corresponding to KIIand KIK, IKI and IKK triads demonstrates thetendency of these copolymers to alternation atradical polymerization. The statistical treatmentof the data obtained shows (Table 7) that thecharacter of polymer chain propagation followsfirst-order Markovian statistics, and the averagelength of alternated sections depends on the con-formation of the alkyl substituent ina^-unsaturated ketones (S-cis or S-trans). Simi-lar alternation was discovered for radical poly-merized butadiene-methacrylate copolymers(131).

Table 7. The Values of ConditionalProbabilities, Average Lengths of

Block <nKK(l l)> andAlternating Unit <r>KI(IK)>>

and Isomeric Composition of Isoprenewith Alkylvinylketones Copolymers (68)

Ps f Titnf* t f* ITI u! dlllG L v£ I

[K]PKK (1 I ) / K IPKI (IK)/KKPKI ( IK) /KIPKK(I O/KK< nKK(l l ) >< n KI (IK)>

t rans-1 , kc i s -1 ,4 13 , * I

(I(1(1(I

1

K)1)K)1)

-CH3 -CH

0.48010.1030.89700.829-670.695O.2M0.062

R

(CH3)2

0.48510.1670.83301.005-990.5100.3530.138

0.5*510.0530.9*701.00

19.00.6730.3270

Microstructure of chloroprene-2,3-dichloro-butadiene copolymers prepared in free-radical-initiated systems have been studied (132). Theassignments were given in dyad form as combi-nations of tail-to-head, head-to-tail, or cis-chloro-prene components. The calculated monomerreactivity ratio product, r, »r2 > 1, showed thatthe copolymers had a slight tendency towardblockiness. The monomer composition influencedmicroblockiness.

To analyze the effect of monomer composi-tion on a microstructure of copolymers of pipe-rylene with acrylonitrile obtained by the poly-merization in DMSO, the l 3 C NMR at 67.88MHz was used (61). The peak assignments in1 3 C spectra (Figure 21) were made with the aid

47

of chemical shifts calculated by the additivescheme of Lindeman and Adams.The data on a triad composition in copolymer

chain show (Table 8) that the character of chainpropagation accord with the first-order Marko-vian statistics.

CM CO

o o oo o

WO 20

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


Table 8. The Values of Conditional Probabilities, Average Lengths ofBlock < " A A (PP) "* anc' Alternating Unit <nAP(PA)>» anc* IsomericComposition of Piperyiene in Copoiymers with Acrylonitrile (69).

Parameter Acrylonitrile Content, mol

51.5 65.1 75-6 89.5

PAA(PP) /AP (PA)PAP(PA)/AA(PP)PAP(PA)/AP(PA)PAA (PP) /AA (PP)<nAA(PP)><nAP(PA)>trans-1,4 Pcis-3,4 P

c-c-c-c(c)-

1.000.2610.73901.003.830.8730.105

0.6800.4290.5710.3201.472.33O.8760.096

0.3970.5030.4970.6032.521.990.8880.064

0.1040.5520.448O.8969.651.810.9250

0.022 0.028 0.048 0.075

At more than 67 mol.% content acrylonitrile inthe copolymer, the chain is transformed fromsyndiolike into isolike. The increase of acryloni-trile content also increases the possibility of1,4-trans-addition and decreases the possibilityof 3,4-cis and cyclic addition of piperylene. Incopolymer with 73.8 mol.% of acrylonitrileobtained by emulsion polymerization, cyclicstructures are absent.

The quantity of the above examples is largeenough so as to be convinced of the considerableachievements of 1 3 C NMR in microstructureanalysis of macromolecules. However, 1 3 C NMRhas the same difficulties as that of 1 H NMR:limited precision of sequence analysis throughline superposition and complexity of well-foundedline assignments.

IV. NEW METHODS OFMICROSTRUCTURE ANALYSIS

A. Use of Shift Reagents for ChainMicrostructure Analysis

Recently, paramagnetic salts containing lan-thanides such as europium or praseodymiumhave been effectively used for the investigationof polymer and copolymer microstructure andchain conformation. The first applications ofparamagnetic shift reagents to a number ofpolymers containing a basic lone-pair

functionality in the monomer unit were made forspectral simplification (133-143). It has beenreported that the use of Eu(dpm)3, Eu(fod)3 andPr(fod)3 improved the resolution in 100 and 220MHz spectra of PMMA, poly(vinyl methyl ether),poly(vinyl acetate), poly(propylene oxide), polysi-loxanes (64, 133, 134), polyethers (135, 136),polyoles (137-139), polylactones (140), etc. Guil-Iet et al. (134) found that the order of shifts forthe various peaks in o-dichlorobenzene as solventwas s C-CH3 > i C-CH3 > h C-CH 3 > iOCH3 > s OCH3 > h OCH3 for the triadsequence peaks of the methyl and methoxycar-bonyl signals. It had been found that in benzenesolution at room temperature, the order of shiftsobtained was i C-CH 3 > h C-CH 3 > sC-CH3 > i OCH3 > h OCH3 > s 0CH3. Theexplanation of this is primarily a reflection of thedependence of polymer conformation on tacticity(133-135). Figure 22 shows the XH NMR spec-trum of a sample of poly(vinyl acetate) in CDC13,and the effect of the addition of small quantitiesof Eu(fod), and Pr(fod), to the solution. It canbe seen that with both reagents the methoxycar-bonyl protons are readily separated into theabsorptions for iso-, hetero- and syndiotactictriads. The slopes of the lines in the diagramgive a clear indication of the degree of shift andit is noted that the Pr(fod)3 gives larger shiftsthan Eu(fod)3 but in the former the broadening isa bit greater.

Vol. 7, No. 1 49

ppm

Figure 22. Effect on the XH NMR spectrum ofpoly(vinyl acetate) of adding Eu(fod)3 andPr(fod)3 (133).

Slonim and coworkers (137) have shown thatthe values of paramagnetic shifts depended onthe europium distribution between different coor-dinating centers of polyethylene glycol and poly-formaldehyde chains. To determine the contentof ordered trans-gauche-trans (TGT) conforma-tion in polyethylene glycol x H NMR spectra weremeasured. The singlet peak at the lowest fieldwas assigned to the TGT conformation of theCOCCOC sequence (138). The stereospecific con-tact interactions in the NMR spectra of polyol-lanthanide (La3 + , Pr3*, Nd3*, Eu3 \ Tb3 + ,Yb3*) complexes were investigated (139). It hasbeen shown that the contact increment in par-amagnetic shifts is greatest if the chain is planarzig-zag. In other cases the isotropic proton shiftis pseudocontact predominantly.

Inoue and Konno (64) established the possi-ble conformations in solution of iso- and syndio-tactic PMMA, by comparing the observed valuesof pseudocontact shift with the values of the geo-metric factor (1 — 3cos26)/r3, in the McConnell-Robertson equation, calculated for any glideplane or heliocoidal chain conformations. Figure23 shows the paramagnetically induced protonshifts A<5 of the three groups, «-CH3, CH2, andOCH3 of iso- and syndiotactic PMMA in CDC13and CeD6 solutions with increasing Eu(dpm)3

concentration. The values can be related to therelative distance of protons from the coordinatingsite and they are dependent on solvent for syndi-otactic PMMA. In general the flexible polymerchain in solution cannot always take the spe-cially fixed conformation. However, it has beenshown that isotactic PMMA has the right-handed(10/1) helix conformation. The trans zig-zagconformation suggested for synditactic PMMA isa special case of glide-plane or heliocoidal confor-mations.

The effect of paramagnetic shift reagentsEu(dpm)3 and Pr(dpm)3 on the 'HNMR spectraof atactic poly-4-vinylpyridine (P-4-VP) wasstudied in CDC13 solution in the temperaturerange 28-100° C (140). From the analysis of J H(360 MHz) and 1 3 C (22.63 MHz) spectra themicrotacticity of radical P-4-VP was determinedand was well described by Bernoullian statisticswith P m = 0.52. It was shown also that *Hshifts of signals induced by Eu(dpm)3 werepseudocontact by nature. The values of the geo-metrical factor were calculated for the fragmentsof regular conformations of iso- and syndiochainsof a macromolecule. Based on a comparativeanalysis (according to a principle of maximumprobability) of the experimental and calculatedvalues of shifts induced by Eu(dpm)3, the con-formational composition of the polymer wasdetermined. It has been shown that the struc-tural contents were [2,] = 47 mol.% (TTGG),[Zs] = 28 mol.% (trans-zig-zag) and [3, ] = 25mol. % (iso) which were in agreement with themodel of P-4-VP microtacticity (55, 95). Fromthe temperature relationship of the * H shifts ofsignals a conclusion was made about the varia-tion of P-4-VP conformational set to the directionof the increase of the syndiotrans-form content.

We suggested an analytical method of inves-tigating microstructure of copolymers with theuse of shift reagents. The possibility of practicalapplication of this method has been shown, theanalysis of butadiene or isoprene and acryloni-trile, MMA, or alkylvinylketone copolymersbeing examples (54, 141-143). Figure 24 showsthe resulting spectral effect of Eu(fod)3 additionto the isoprene-acrylonitrile (38 mol % A) copo-lymer solution. The values of paramagneticshifts of methyl, methylene, and vinylidene triadsignals decrease in AAA, AAI, LAI, and AIA,All, III succession. Quantitative data were cal-culated from the content of triads in the copo-lymers obtained under emulsion copolymerizationon sodium alkylsulfonate (ASS) and potassiumabietate (AP) micelles, in mass or solution withdifferent monomer content and conversion. It


4.02D

EOQ.Q.

.1.6<0.8

0

b

yt

^T*7. .

1.20.S0.4

0

0.40.20

20 20 40

1.6

0.80

0.80.4

0

/ ^

0 10 20

0.8

0.40

0.2

n0 10 20

Figure 23. Variation of the paramagnetically induced proton shift with the amount of Eu(dpm)3 for iso-tactic (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).

C71Egi.00>

f

U. ppm

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

was found that a blocking level of the copolymersincreased with an increase of local concentrationand a decrease of polar monomer molecule diffu-sion velocity in the micelle matrix (54, 143). Thestability analysis of aqueous mixed ASS-AP

micelles and micelles of fatty acids (144, 145)with the relaxation probe Mn2 * allowed us tomake a conclusion about the principal influenceof detergent association in the micelle matrixupon microstructure (54, 145). The isoprene with

Vol. 7, No. 1 51

acrylonitrile or MMA copolymers obtained onmatrices of different ASS-AP compositions havea maximum blocking level with a maximum sta-bility at 66.7 mol. % AP. Figure 25 shows the

0.4

NT

- io -£o

. 6

- 2

20 60 100lAA) , mol %

Figure 25. Relationship between triad contentsand Ai> % (CH2)n in 1 H NMR spectra of ASS-APmicelles and AP contents in them (145).

relationship between the matrix stability cri-terion (Avu of the (CH2 ) n signal in the spectrumof mixed micelles) and triad content. The chain-growth statistics of emulsion copolymers arefirst- or second-order Markovian (145). The sameresults were obtained for styrene-MMA copo-lymers investigated without shift reagents (146).

The x H NMR spectra of ethylene- and vinylchloride-vinyl acetate copolymers with low vinylacetate (VA) content were measured by use ofEu(fod)3. The shifted signals of the acetate

methyl and methine protons were tentativelyassigned to the triad sequences with a VA unitas a center, and their evaluated concentrationswere compared with calculated concentrations bymeans of the random copolymerization theory(147). Similar investigations were made forchloroprene-MMA copolymer with aid ofEu(dpm)3 (148).

The investigation of microstructure of theunsaturated polymers and copolymers withoutpolar groups is performed by the use of the com-posite complex: double bond—AgNO3—Eu(fod)3(148a).

The examples show that use of shift reagentsfor microstructure analysis and for the study ofthe polymerization mechanism of polymers hav-ing unresolved monomer unit spectra is particu-larly effective.

B. Magic Angle Spinning and High Resolu-tion NMR Spectroscopy in Solid Polymers

Two sorts of line-narrowing techniques areemployed to obtained high-resolution, naturalabundance 13C NMR spectra of solid polymers.Dipolar broadening of the 13 C lines by protons isremoved by strong resonant decoupling (referredto as dipolar decoupling) by using XH decouplingrf fields of about 10 G (149). These decouplingfields are comparable to the proton linewidth. Inmost cases the resulting 13C NMR spectra arestill severely complicated by chemical shift ani-sotropies, so that, in general, only a few broadlines are observed. A dramatic improvement ofthe resolution can be achieved by additionalmechanical spinning of polymer samples at theso-called "magic angle" with a frequency some-what greater than the dispersion of chemicalshifts (149, 150).

Even with the resolution achieved by a com-bination of dipolar decoupling and magic anglespinning, a FT experiment on a solid polymerstill has a serious limitation. Namely, a delaytime of several 13C spin-lattice relaxation times(T1) must be tolerated before data sampling canbe repeated. These repetitions are necessary toprovide a suitably strong signal by a time-aver-aging process. Since some *3 C T, 's for solidpolymers are on the order of tens of seconds(149), the time-averaging process becomes tedi-ous. These delays can be avoided, however, byperforming a matched spin-lock (or Hartmann-Hahn) cross-polarization (CP) experiment (151).With this technique, polarization of the carbonsis achieved by a polarization transfer fromnearby protons, spin-locked in their own rf field,via static dipolar interactions in a time TQJJ(SL).


This polarization transfer is a spin-spin orT2-type process and generally requires no morethan 100 MS (149). Most important, the CPtransfer can be repeated and more data accumu-lated after allowing the protons to repolarize inthe static field. For glassy polymers near roomtemperature this is more efficient than 13 Crepolarization by spin-lattice processes and gen-erally occurs in less than a half second (152).

Schaefer (149, 153) has discovered that CPand magic angle spinning are compatible. Asshown in Figure 26, high resolution in the dipo-lar-decoupled 1 3C spectrum of solid PMMA isachieved with quite low spinning frequencies, ofthe order of 500 Hz. This is true despite the factthat the chemical shift dispersion of the low-fieldresonance arising from the carbonyl carbon isabout 3 KHz (154).

With separate lines resolved for individualcarbons, a variety of relaxation experiments canbe performed and interpreted in terms of themotions of the polymers in the solid state. It iswell known that a *H rotating-frame relaxationtime (Tjn) is sensitive to motions associated withfrequencies in the 10-100 KHz range.

Figure 27 shows the results of 1 3 C T l pexperiments for solid polycarbonate, both withand without magic angle spinning, at 3 KHz(149). Five lines are well resolved. The lowestfield line arises from the overlap of the carboxylcarbon resonance with that of the nonprotonatedaromatic carbons. This line has the longest T^p.The two lines just to higher field arise from theprotonated aromatic carbons. These lines havethe same relaxation behavior as one another andare characterized by a short T ^ . The two linesat high field are due to the quarternary andmethyl carbons. The methyl carbon T±p is inter-mediate between that of the low field combina-tion line, and that of the protonated aromatic-carbon line. This behavior simply reflects theweaker coupling of nonprotonated carbons tomore distant protons, as determined by theinverse sixth power dependence on the internu-clear separation common to all dipolar interac-tions.

The 1 3 C T l p and TCH(SL) have been meas-ured also for poly(phenylene oxide), polystyrene,polysulfone, poly(ether sulfone), PVC (149, 155),polybutadiene and poly(ethylene oxide) (156).

The 1 3 C T l p at 32 KHz is dominated byspin-lattice processes rather than spin-spin pro-cesses. This means that the T ^ ' s contain infor-mation about the motions of the polymers in the10—50 KHz region, while the TQJJ'S containinformation about the near static interactions.The T, 's and NOE factors contain information

0.56 kHz

0.77

1.05

1.25

1.55

2.10

3.20

Figure 26. 10 G dipolar-decoupled 1 3 C NMRspectra of PMMA magic angle rotor, as a func-tion of spinning frequency. The CP was per-formed with spin-temperature alternation toremove artifacts (154).

about the motions in the 5—30 MHz regions.Interpretation of the T l p ' s of these polymersemphasises the dynamic heterogeneity of theglassy state. Details of the relaxation processesestablish the short-range nature of certain lowfrequency side-group motions, while clearlydefining the long-range cooperative nature orsome of the main-chain motions, the latter notconsistent with a local-mode interpretation ofmotion. For instance, polystyrene the TQJJ andT l p values are 1.2 and 3.5 ms for aromaticrings, 0.5 and 3.6—4.1 ms for main chain car-bons (155). These motions involve cooperative

Vol. 7, No. 1 53

torsional oscillations within conformations ratherthan another (149, 153). The ratio of T Q H toTjp for protonated carbons in the main chain of

100 ppm

without spinning

with spinning

rotating frame 0.025time, msec

12

Figure 27. CP 1 3 C NMR spectra of polycarbo-nate with and without magic angle spinning, as afunction of the time the carbon magnetizationwas held in the rotating frame without CP con-tact (149).

each polymer is found to have a direct correla-tion with the toughness or impact strength for allstudied polymers (149). This empirical correla-tion was rationalized in terms of energy dissipa-tion for chains in the amorphous state in whichlow-frequency cooperative motions were deter-mined by the same inter- and intrachain stericinteractions.

Thus one-hundred Hertz resolution has beenachieved in the 1 3 C NMR spectra of solid poly-mers by a combination of dipolar decoupling andmagic angle spinning. The high resolution per-mits individual resonance lines to be assigned tospecific carbons and monomer unit sequences inthe polymer.

V. CONCLUSIONS

Finishing on this consideration of microstruc-ture analysis of polymer chains with the aid of*H and 1 3 C NMR spectroscopy, one must notethat this field of polymer physical chemistry setthe powerful arsenal of varied investigationmethods against extremely intricate problems.

The use of spectrometers with superconduct-ing magnets, shift reagents and magic anglerotation represents the newest directions in

NMR. However, actually all of these methodswill not be able to secure the complete resolutionof signals in the 1 H and 1 3 C NMR spectra ofpolymers. Therefore the extraction of spectralinformation about microtacticity and conforma-tion of the chain with the aid of computers iswidely practiced.

We may expect increased understanding ofthe microstructures of mainly linear polymersand copolymers as experimental techniquesimprove. In turn this should lead to betterunderstanding and hence control of the chaingrowth mechanism through variation of the con-ditions of synthesis. Great possibilities then openup for the design of polymers of specific physicalcharacteristics for industrial application leadingto increased cost effectiveness and qualityimprovement. It is hoped that this review willhave shown that this subject offers considerableintellectual challenge together with its potentialeconomic importance.

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analysis of chain microstructure by h and c nmr … · analysis of chain microstructure by 1h and 1...

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