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JOIJRNAL OF POLYMER SCIENCE VOL. 62, PAGES 197-209 (1962)

Polymer NMR Spectroscopy. VI. MethylMethacrylate-Styrene and Methyl

Methacrylate-a-Methylstyrene Copolymers*

F. A. BOVEY, Central Research Department, Minnesota Mining and

Manufacturing Company, St. Pa ul, Minnesota

The theory of copolymerization of vinyl and diene monomers wasdeveloped many years ago1 and has been tested and st>rongly upported

by a large number of subsequent experimental investigations. However,

nearly all of these studies have dealt with the overall copolymer com-

position, and only a few attempts have been made to verify the predictions

of the theory with regard to the microstructure of the polymer chains,

i.e., the distribution of monomer sequence lengths. Since the equations

that predict these sequence lengths are based on the same assumptions as

those that predict the copolymer composition, without any additional

hypotheses, there is perhaps no very serious question of the correctness of

the predicted structural details. Nevertheless, the absence of experi-mental verification constitutes a definite lacuna in an otherwise satisfactory

picture. As an example of the efforts made to supply this information, the

work of Marvel2 and Alfrey3 and their coworkers may be cited (see ref. 1,

pp. 137-145). In their studies the rate and extent of zinc dechlorination

of vinyl chloride copolymers was measured; however, the zinc apparently

did not confine itself to the expected removal of chlorine atoms in 1,spositions along the chain, and so the results were inconclusive. The infra-

red spectra of copolymers often deviate from a mere summation of the

corresponding homopolymer ~ p e c t r a . ~ uch deviations are marked

enough to be useful when the immediate environments of the backbone

atoms (most commonly hydrogen) are altered in the copolymer, as, for

example, in copolymers of hydrocarbon monomers with fluorine-substituted

ethylenes. Side-chain absorptions are in any case affected only to a small

degree, if at all. At best, infrared spectra supply only qualitative indica-

tions that the substance concerned is indeed a copolymer, and do not pro-

vide a basis for the calculation of sequence lengths.

For block and graft copolymers, to which the classic copolymerization

equations cannot be applied, there frequently is no theoretical or experi-

mental basis for a prediction of the detailed polymer structure. In such

cases, new analytical methods clearly are needed.

and Manufacturing Company, St. Paul, Minnesota.

* Contribution No. 219 from the Central Research Depnrtment, Minnesota Mining

197



198 F. A. BOVEY

Finally, nothing at all can be stated or predicted at present concerning

the stereochemical conjiguration of the monomer units in copolymers of anytype, even of the random or alternating types to which the copolymeriza-

tion equations apply and whose structure might therefore be thought wellunderstood in other respects. It is true tha t from the practical viewpointsuch information is of less interest than in the case of homopolymers,

because it appears less likely that the properties of copolymers will be

profoundly affected by variations in the stereochemistry of the monomer

units. Nevertheless, some effect on both physical and chemical properties

is certainly to be expected, and in any case the question is of theoreticalinterest.

I n this paper are reported preliminary studies of methyl methacrylate

copolymers which indicate that high-resolution N M R spectroscopy of

solutions of copolymers can yield valuable information concerning bothsequence lengths and stereochemical configurations of the monomer units.

At the present stage of this work, it is not possible to disentangle sequenceand configurational effects satisfactorily and therefore the conclusions are

necessarily somewhat tentative and incomplete. Further investigations

are in progress, but it was thought best to publish the present data so t,hat

others may be encouraged to make use of this potentially useful technique.

Experimental

Polymer Preparation

All polymers were prepared in bulk in Pyrex ampules which were sweptwith prepurified nitrogen before being sealed. Monomers were vacuum-

distilled before use. In the methyl methacrylate-styrene copolymeriza-

tions, the total weight of monomer in each ampule was 20 g., the monomer

weights being adjusted to give methyl methacrylate/styrene mole ratios of

10:90,25:75,50:50,75:25, and 90:10. In each ampule, 5 mg. of benzoyl

peroxide was dissolved. Polymerization was carried out by immersingthe ampule in a boiling-water bath until the first slight increase of viscosity,

indicative of incipient polymerization, was observed. The ampule was

then quickly cooled in ice water. Monomer conversions ranged from 4.5

to 8.1 . The polymers were precipitated in a large volume of met,hanol

and dried for 16 hr. in vacuo at 75'. Methyl methacrylate-a-methyl-styrene copolymers were prepared similarly. The methyl methacrylate/ a-

methylstyrene mole ratios were 12:88, 28:72, 54:46, 78:22, and 91.5:8.5.The first two systems were very slow t,o polymerize, and therefore were

initiated by Co60yrad iation; 17.5 Mrad over a 233-hr. period at approxi-

mately 25' gave 2.8 monomer conversion in each. The system highest

in methyl methacrylate was polymerized with 20 mg. of benzoyl peroxide;

1.6 conversion was reached in approximately 30 min. at 100'. The54 :46 and 78 :22 systems contained 20 mg. of benzoyl peroxideas sensitizer

and were exposed to ultraviolet radiation (275-w.G.E. sun lamp) for 15 hr.;

they gave monomer conversions of 5.7 and8.8 , respectively. Copolymer-



POLYMER NMR SPECTROSCOPY. VI 199

ization reactivity ratios are somewhat affected by temperature, but the

effect is small and was not considered significant in the present study.

Sample PreparationPolymer solutions for N M R spectral measurements were prepared by

dissolving 0.100 g. of polymer in 0 .5 ml. of solvent containing 29i’, of tetra-

methylsilane as internal reference ~tandard.~or the styrene and a-

methylstyrene homopolymers and for the copolymers made from the

systems having the two highest styrene and a-methylstyrene contents,

carbon tetrachloride was employed as solvent. For the other polymers,

chloroform was the better solvent (i.e., gave spectra of better resolution)

but solutions in carbon tetrachloride were also employed in order to permit

measurement of the aromatic peak positions, which are obscured by the

chloroform resonance. For the methyl methacrylate homopolymer,

chloroform was used. The polymer solutions were transferred to 5 mm.O.D. Pyrex NMR tubes which were evacuat,ed and then sealed under

about 400 mm. of nitrogen pressure in order to prevent boiling at the

temperature of observation, 90°.

Spectral Measurements

A Varian V-4300-2 40 Mcycle/sec. spectrometer, equipped with a Varian

heated probe, Varian field homogeneity control unit, Hewlett Packard

522-B frequency counter, and Varian recorder, was employed. In thespectra, the peak for the methyl group of the tetramethylsilane reference

appears at the extreme right and its position is taken as lO .OOO ppm.Peak positions on this scale are termed 7-values, as previously describedJ5s6

and are determined by linear interpolation in these spectra.

Experimental Results and Interpretation

In Figure 1 the spectra of the methyl methacrylate-styrene copolymers

are shown, together with those of the homopolymers. The spectra of the

homopolymers and of the copolymers from the 1O:OO and 25:75 methyl

methacrylate/styrene feed ratios are shown as recorded. For the other

copolymers, the left-hand portion of the spectrum with the aromatic peaksis shown as recorded in carbon tetrachloride solvent; the remainder of the

spectrum is shown as recorded in chloroform solvent. The peaks, fromleft to right, correspond to the following polymer protons. At the left

(3.0-3.5 7 is the resonance of the phenyl protons of the styrene units,

except in Spectrum g, where the chloroform proton resonance appears in

this position. I n the polymers from 90 and 75 mole-yo styrene feeds, the

ortho ring protons can be distinguished as a smaller peak or shoulder at

higher field than that of the meta and para; a t lower styrene feed ratios,only a single peak can be seen. Similar behavior has been observed for

styrene-butadiene copolymer^.^ In the region 6.4-7.2 7 are the peaks for

the methoxyl protons of the methyl methacrylate units. There are further



200 F. A. BOVEY

Fig. 1. N M R spectra of solutions of methyl methacrylatestyrene copolymers and

homopolymers, 0.10 g. in 0.5 ml. of CHCL or CCl, (see text) with 2 tetramethylsilane

as internal reference 7 10.00): a) polystyrene; b ) 10:90 (methyl methacrylate-

styrene mole ratio in monomer feed); (c ) 25:75; ( d ) 50:50; ( e ) 75:25; (f) 9O:lO;

9 )polymethyl methacrylate.

peaks, less clearly defined, in the 7.5-7.9 r region. For the present it

appears best to interpret these also as corresponding to methoxyl groups(see Discussion below), but such an interpretationis tentative, particularly

as corresponding peaks can be seen much Iess clearly, if a t alI, in the a-

methylstyrene copolymers. The region 8.1-8.5 corresponds to the CH,

protons of the methyl methacrylate units and the CHCHz protons of the

styrene units. I n three of the spectra (d, el and f) , these can be distin-guished, the styrene protons being a t somewhat higher field. Finally, a t

highest field (8.8-9.5 r) are the a-methyl groups of methyl methacrylate

units. The peaks corresponding to isotactic, heterotactic, and syndio-



POLYMER NMR SPECTROSCOPY. V I 201

Fig. 2. NMR peak positions for methyl methacry lates tyrene copolymers and homo-

polymers in CHCla or CCl, (see text) as a function of f i , the mole fraction of methyl

methacrylate in the monomer feed: 0 ) tyrene protons; 0 )methyl methacrylate protons;

a) ot yet assigned with certainty but interpreted in the text as methyl methacrylate

methoxyl protons.

tactic triads6 cannot be distinguished in any of the copolymers, although

the peak shapes (particularly in Spectra e and f ) indicate an unresolved

multiplicity. I n Figure 2, peak positions are plotted as a function of the

mole fraction of methyl methacrylate jl)n the feed. Most of the peaks

move “upfield” as the proportion of styrene in the copolymers is increased.

This is particularly marked in the case of the a-methyl peaks, and the

methoxyl peak at lowest field. As in the spectra of butadiene-styrene

copolymers,’ this shielding effect is due to the magnetic anisotropy of the

styrene phenyl groups,8-11 and becomes particularly marked when most of

the methyl methacrylate units are isolated between styrene units.

The spectra of the methyl methacrylate-a-methylstyrene copolymers are

similar to those of the methyl methacrylate-styrene copolymers, and are

not shown. The backbone methylene regions (8.1-8.6 7 ) of the two

comonomer units cannot be distinguished from each other. On the otherhand, the methyl methacrylate a-methyl regions are clearly multiplets

except a t the two lowest values of fi. In the spectrum of the 46 :54 methyl

methacrylate-a-mcthylstyrene copolymer the shape of the methyl meth-



202 F. A. BOVEY

3 0.1 0 .2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fig. 3. NMR peak positions for methyl methacrylate-a-methylstyrene copolymers

and homopolymers in CHCl, or CCI, (see text) as a function of fi the mole fraction of

methyl methacrylate in the monomer feed: 0 ) -methylstyrene protons; 0 )methyl

methacrylate protons.

acrylate a-methyl resonance is markedly altered from that observed at

higher f i ; the peak at lower field (8.83 T , which corresponds to isotactic

triads in the homopolymers and is normally very small in free-radical

polymers, appears to be the most prominent. The a-methyl protons of the

a-methylstyrene units appear at highest field, 9.6-9.8 7 . Peak positions

for the a-methylstyrene copolymers are plotted in Figure 3. It can be seen

that the phenyl groups of a-methylstyrene are somewhat less effective than

styrene units in shielding neighboring protons, for most of the peak posi-

tions show a smaller “upfield” slope with increasing a-methylstyrene

content.

In both series of copolymers, the methoxyl resonance is surprisingly

complex and appears to offer the best promise of yielding fundamentalinformation concerning the microstructure of the chains. I n methyl

methacrylate homopolymers the methoxyl protons appearas a single narrow

peak a t 6.40 T regardless of the stereochemical configuration of the chains.



POLYMER N M R SPECTROSCOPY. VI 203

I n the copolymers, the methoxyl resonance is evidently very sensitive to

the configuration and proximity of the styrene units, there being in all

spectra at least two, and sometimes three or more, peaks clearly visible.

I n the next section, we shall at tempt an explanation of this result.

Discussion

We shall assume that only nearest neighboring styrene units produce

sufficiently marked shielding effects on the methoxyl protons to require

detailed consideration. That this is not strictly correct is shown by the

apparently continuous “upfield” shift of the methoxyl resonance with

increasing styrene content; but a t least there is no reason to believe that

more distant styrene units cause the appearance of separate methoxyl

peaks. It is evident that, considered without regard to stereochemicalconfiguration, a methyl methacrylate unit ml in a copolymer may exist in

any of three situations with respect to the nearest neighboring units,

mlmlml, mlmlm2, and m2m1m7, hat is, only like units as neighbors, onecomonomer unit m2 as neighbor, or surrounded by m2 units. The proba-

bilities of occurrence of these triads may be readily calculated if the re-

activity ratios r1 and r2 for the copolymerization system are known. The

probability that a monomer sequence picked at random will contain nl

and only n1 units nlbeing any arbitrary number, and the subscript re-

ferring to monomer 1, which, in the present systems, is methyl meth-

acrylate), is given by the following (see ref. 1, p. 134 et seq.).

For monomer 1 :

For monomer 2:

In the above equations,

Pll and P z 2are, respectively, the probability that monomer 1 will add to agrowing chain ending in ml and the probability that monomer 2 will add

to a chain ending in m2; fl and f2 are, of course, the mole fractions ofmonomer 1 and monomer 2 in the feed. In order to interpret the spectra,

we require the mole fractions of monomer units involved in specifiedsequences rather than the probabilities of occurrence of those sequences.

Of the total ml units in a very long (“semi-infinite”) chain, the mole

fraction occurring in sequences having a length exactlyn1will be given by:



204 F. A. ROVEY

5 )

The denominator can he expressed as:

(1 Pii)(1 + 2Pii + 3Pii' + 4P1l3+ . . . ) (1 Pll)-'

We therefore have:

F m l , n 1 ) = n ~ 1 1 ( ~ l - ~ ) ( 1 1 1 ~

F m 2 , n r )= n ~ ~ 2 2 ( ~ * - ) ( 12 2 ) '

(6)

( 7)

We may now calculate the mole fraction of ml units having oiily mI neigh-

bors (it should be noted that this is quite different from the mole fractioii ofml units in sequences for which n l is 3).

and correspondingly for monomer 2:

This quantity will be given by:

FIII 1 4 I i . ( m l . n l = 8 ) + 4 F ( m l , n l = 4 ) + //6 F(rnI,n1=6) +

n = m

(1 PI,)' c n1 )P11 (nl -1 )

P11* 1 P11)?(1+ 2P1, + 3P,,2 + . . . )

= PllZ (9)

The mole fraction of ml units having one ml and one m2 neighbor is givenby :

n1=3

F211 (or F112) F(rnl .n1=2) + I 3 F ( m I , n l = s ) + I d F ( m l . n t = 4 ) +2

. . . + ( l n l , n l ) + . . 10)n l

nl = -n l = 2

2 1 Pll 2

cP l l ( n l - l )

2P1,(1 P I ]

Finally, the mole fraction of ml units having oiily m2neighhors is the frac-tion of ml units in sequences for which n1 = 1, i.e. :

F Z ~ Z F ( m l , n , = l ) 1 Pll)' (12)

(Obviously, if any two of these quantit ies are known the third is known,

since F111+ F2 + F2 2 = 1.) Exactly analogous relationships hold for

m2 units.The relationships of Flll, F211,nd FZ l 2,o Pl1 ppear to be closely analo-

gous-in fact, are identical in form-to the relationships of Pi, Ph, and P,

(the probnlditics of orcurreiicc of isotnctic, heterotartic, and syndiotactic



206 F. A. BOVEY

When we examine Spectrum b in Figure 1 (fl 0.10) in the light of

Figure 4, it is at once evident that the copolymer spectra cannot be ex-

plained in terms of sequence probabilities alone. For fl 0.10, Fzlz is

approximately 0.91; t h a t is, over 90 of the methyl methacrylate unitsare isolated between styrene units. We see, however, three methoxyl

peaks, at 6.89, 7.17, and (rather poorly defined) at about 7.53 7; their

relative areas are impossible to estimate with accuracy in the present

spectra but appear to be approximately 35 :45 :20. We assume that these

resonances correspond to the three configurations possible for an m2mlmz

sequence, that is, dzdldz (or 121112 , dzd 2 (or 12dld2, lzlldl, and dzlllz , and lzdzlz

(or d, l ldz . These configurations might be thought of as corresponding

to the isotactic, heterotactic, and syndiotactic triads in a homopolymer

chain16 ut it must be kept in mind that there is a certain arbitrariness in

deciding which copolymer triad is to be called isotactic (or “co-isotactic”)and which is to be called syndiotactic (or ‘(co-syndiotactic”), since one

must decide which substituents to align along the planar zigzag. Despite

this semantic difficulty, however, the stereochemical differences are very

real, and could easily account for the shielding values observed. It is

noteworthy that, whereas the a-methyl group resonance is markedly

shifted by configurational differences in the neighboring carbomethoxy

groups and not by configurational differences in the neighboring styrene

units, just the reverse is true of the carbomethoxy groups, if the above

interpretation is accepted.

For mzmlml or mlmlm2) sequences, it is assumed that only two of the

four possible stereochemical configurations are magnetically distinguish-

able; that is, / Id1 (or dzllll cannot be distinguished from lzdlll (or dzlldl ,

and dzdlll (or 1211dl cannot be distinguished from dzdidl or 121111 , but 12dldl

and/or lzdlll can be distinguished from dzdlll and/or dzdld,. It is further

assumed that one of these (it is not possible to say which at present) has its

resonance in the 6.4-6.8 r region and that the other appears at 7.0-7.2 r

and that they occur in the approximate ratio of 0.55:0.45 for those

methoxyl protons giving the lower field resonance to those giving the

higher. Thus, the four types of mzmlml triads, which are distinguishable

in principle, actually appear as only two peaks in most of the spectra. I n

Spectrum d, Figure 1, which corresponds to a 50:50 molar feed ratio, two

peaks can be distinctly seen in the 6.4-6.8 region, although they are not

well resolved.

For the mlmlml sequences it is known, from studies of the homopoly-

mers16 hat the stereochemical configuration of the neighboring units does

not affect the methoxyl peak position of the central units, and so these

units in such sequences are assumed to have their resonance near 6.40 r

except as influenced by the presence of styrene units as next nearest and

more distant neighbors.If we assume that these same distributions of configurations occur in the

m2m1m2 nd mlmlmzsequences wherever they occur in the chain (and this

must be so if the usual assumption is correct, that only the end unit

This is discussed below.



P OL YM E R N M R SPECTROSCOPY. V I 207

of the growing chain influences the configuration of the monomer unit to be

added), then all of the relative peak areas and positions in the methoxyl

region can be accounted for, except for the occasional appearance of an ill-

defined multiplicity in the 7.6-7.9 T region. For example, in the 50:50spectrum (Fig. 1, d ) the two closely spaced peaks in the 6.5-6.8 r region

would correspond to m2mlm2 (0.17 of the total methoxyl resonance),m2mlml (0.23), and mlmlml (0.10). As f1 is increased, the resonance

narrows rapidly and, of the two peaks in this region, that a t higher field

(6.72 r apparently dwindles rapidly. This probably then corresponds to

m2mlm2. I n the f l 0.75 spectrum (Fig. 1, e ) t,he ratio of methoxyl peakareas does not correspond a t all to the sequence ratios of Figure 4, but can

be consistently interpreted if the above configurational ratios are also

taken into account.

The spectra of the methyl methacrylate-a-methylstyrene copolymerscan be explained qualitatively along similar lines, but here the smalldifferences in sequence probabilities shown in Figure 4 do not suffice to

account for the differences between these spectra and those of the nearest

corresponding styrene copolymers. For instance, in the 54 :46 methyl

methacrylate-a-methylstyrene spectrum, the methoxyl resonance atlower field is considerably larger than that a t higher field; such is not true

of the 50:50 styrene copolymer. Again, the resonance a t 7.6-7.9 r doesnot appear clearly, if a t all, in the a-methylstyrene copolymer spectra. At

present., a consistent quantitative interpretation of these spectra cannot be

offered, bu t it will probably he found that the same general treatment willhold.

To complete the formalism, we note that to define both the sequencesand configurations of the monomer units in a two-component copolymer,assuming that a ropagation” (see ref. 6) has occurred and that some

convention has been agreed on, as to what the term ((co-isotactic” (and

therefore “co-syndiotactic” and “co-heterotactic”) shall signify, we needto define the four monomer placement probabilities ull, u12,uzl, and u22 ;

ull is the probability that monomer 1will add in the isotactic configurationto a growing polymer chain ending in an ml unit, u12 is the probability that

monomer 2 will add in a ‘(co-isotactic” manner to a growing chain ending

in an ml unit, etc. We then have the following relationships (in which it is

of course to be understood that all d’s may be replaced by1’s and vice versa;that is, dld ld l 111111, ldlll dllld7, etc.):

All the other spectra can be similarly explained.

where :



208 F. A. BOVEY

F2ll (or F112) = Fdzd ld l or Fd l d l d z ) + Fl zd l d l or Fdld ld l I z )

f Fd2 d l i I (or F z l d l d 2 ) f F i z d l i l O r F z l d 1 i 2 ) (17)

where:

Fd l d l d l 2ull(fl21+ Ulz)P11(1 Pll)

F i 2 d l d l 2U11[(1 an) + (1 U I Z ) ] P U ~ p11

Fd2d , t l = 2(1 11)(Uz1 + U l z ) p 1 1 ( 1 PII)

(18)

(19)

21)

Rz 2 Fd2 d l d 2 f ~~ r l ~ d ~Or F d z d l i z ) + F i 2 d l i l (22)

(23)

(24)

2 5 )

F i 2 d l i l 2(1 ail) [ ( I Uzi) + (1 uiz) IPii(1 Pii) (20)

where:

Fd2d l d z azl~lz(1 '11)~

F l l f l d z O r Fdzd112) [u12(1 UZI) + ffzl(1 U I Z ) ] ( ~ P11)

u12 UlZUZl + UZl ) ( l Pld'

F12dl iI (1 u21)(1 ulZ)(l Pll)'

A corresponding set of equations will hold for triad sequences with central

m2 units. These can be written down by replacing all 1's by 2's and viceversa in the above equations; ull and uZ2 an be obtained from studies of

homopolymers; u11is known already.6 One way of evaluating ulz and uz1

for the copolymer systems considered in this paper would be to preparemodel compounds corresponding to central methyl methacrylate units

flanked by styrene units, the styrene units both having the same stereo-chemical configuration, the relationship of which to that of the central

unit must be known. The peaks in the methoxyl region of Spectrum b ,

Figure 1 , could then be assigned, and from their areas and any two of the

eqs. (23), (24), and (25) the ~ 1 2nd aZ would be known and the micro-

structure of the copolymer chain could be regarded as fully defined.

Clearly, this will be no easy task.

The author is indebted to Dr. G. V. D. Tiers for many stimulating and helpful dis-

cussions, to Mr. R. B. Calkins for the careful operation of the NMR spectrometer, to

Mrs. Kathryn LaCroix for the preparation of polymer samples and tubes, and to Dr.

A. R. Shults for reviewing the mathematical development.

References

1. Alfrey, T., Jr., J. J. Bohrer, and H. F. Mark, Copolymerization, Interscience,

2. Marvel, C. S., G. D. Jones, T. W. Mastin, and G. L. Schertz, J. A m . Chem. Soc.,

3. Alfrey, T., Jr., H. C. Haas, and C. W. Lewis, J . Am. Chem. Soc., 73,2851 (1951).

4. Meiklejohn, R. A., (Central Research Dept., Minnesota Mining Mfg. Co.)

5. Tiers, G . V. D., J . Phys. Chein. , 62, 1151 (1958).

6. Bovey, F. A., and G. V. D. Tiers, J . Polyiner Sci., 44, Ii3 (1960).

7. Bovey, F. A., G. V. D. Tiers, and G . Filipovich,J . Polymer Sci., 38,73 (1959).

New York-London, 1952.

64,2358 (1942).

private communication.



POLYMER NMli SPECTROSCOPY. V I 209

8. Waugh, J. S., and R. W. Fessenden, J . A m . Chem. SOC., 9.846 (1957).

9. Johnson, C. E., Jr., and F. A. Bovey, J . Chem. Phys ., 29,1012 (1958).

10. Pople, J. A., W. G. Schneider, and H. J. Bernstein, High Resolution Nuclear Mag-

11. Jackman, L. M., Applications of Nuclear Magnetic Resonance Spectroscopy in

12. Walling, C., E. R. Briggs, K. B. Wolfstirn, and F. R. Mayo, J . Am. Chem. SOC.,

netic Resonance, McGraw-Hill, New York, 1959, p. 180.

Organic Chem istry, Pergamon, 1959, pp. 112-113, 125-129.

70,1537 (1948).

Synopsis

The NMR spectra of methyl methacrylatestyrene and methyl niethacrylate-a-

methylstyrene copolymers exhibit, among other features of interest, an unexpected

multiplicity in the 6.4-7.8 7 region, characteristic of methoxyl protons. This multi-

plicity is believed due to magnetic shielding by styrene units. By comparing the results

of a statistical analysis of monomer sequence probabilities with the relative areas of the

methoxyl peaks, i t is shown that the degree of shielding of the methoxyl protons is

dependent not only upon the presence of styrene units as nearest neighbors along the

chain but also upon their stereochemical configuration with respect to the methyl metha-

crylate units. By means of reasonable assumptions it is possible to explain the copoly-

mer spectra and to give a partial description of the stereochemical configuration of the

methyl methacrylatestyrene copolymers. The NMR method should be valuable in

the analysis of other copolymers, particularly block and graft copolymers.

R6sum6

Le spectre de resonance magnetique nucleaire des copolymkres de methacrylate de

rnethyle-styrkne et de methacrylate de rn6thyle-a-rn6thylstyrkne montre parmi

d’autres propri6t6s intbressantes une multiplicite inattendue dans la region de 6.4-7.87,

caracteristique des protons m6thoxyles. On croit que cette multiplicite est due l’bcranmagnetique crE6 par les unit& du styrkne. En comparant les resu ltah de l’analyse

statistique des probabilites de sequence de monomkre avec les surfaces relatives des pics

m6thoxyl6, on montre que le degre de recouvrement des protons rnethoxyles depend non

seulement de la presence des unites styreniques au voisinage imniediat de la chaine, mais

aussi de leur configuration stereochimique par rapport aux unites de methacrylate de

methyle. En faisant des suppositions raisonables il est possible d’expliquer le spectre du

polymhre et de donner une description partielle de la configuration sterbochimique du

copolymkre methacrylate de methyle-styrkne. La mbthode de la r6sonance magnetique

nucleaire serait precieux d m s l’analyse des autres copolymkres, sur tout tlans le cas des

copolymkres bloc et greffes.

ZusammenfassungDie NMR-Spektrcn von Methylmethacrylat-Styrol und Methylmethacrylat-a-

Methylstyrolcopolymeren zeigen, neben anderen interessanten Ziigen, eine unerwartete

Multiplizitat im 6,4-7,8 7- Bereich, der fur Methoxylprotonen charakteristisch ist.

Diese Multiplizitat wird auf eine magnetische Abschirmung durch Styrolbausteine

zuruckgefuhrt. Durch Vergleich der Ergebnisse einer statistischen Analyse der Wahr-

scheinlichkeit von Monomersequenzen mit dem relativen Flacheninhalt der Methoxyl-

maxima wird gezeigt, dass der Abschirmungsgrad der Methoxylprotonen nicht nur von

der Gegenwart von Styrolbausteinen als niichste Nachbarn Ian@ der Kette sondern auch

von ihrer stereochemischen Konfiguration in bezug auf die Methylmethacrylatbausteine

abhangt. Auf Grund plausibler Annahmen konnen die Copolymerspektren gedeutet

und eine teilweise Beschreibung der stereochemischen Konfiguration der Methyl-methacrylat7Styrolcopolymeren gegeben werden. Die NMR-Methode sollte bei der

Analyse anderer Copoly mcrer, besonders Block und Pfropfcopolymerer, wertvoll sein.

Received October 27, 1061

the nmr paper

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