On page 848,X1(63.4 ppm): -MMA-MMA-MMA-should read Xl(63.6 ppm): -MMA-MMA-MMA--, and Xz(63.6 ppm): -S T E M M A E S t -should read X3(63.4 ppm): -St E M M A Z S t -.
NMR chemical shifts of the methoxyl and a-methyl protons in the alternating methyl methacrylate-styrene copolymer are calculated by taking into account the contributions of the diamagnetic shielding and the magnetic anisotropy effect of the benzene rings in styrene units. The conformational probabilities of the copolymer chain are calculated according to Flory's matrix method. It is deduced from the calculation that NMR should occur at higher field for the methoxyl and at lower field for the a-methyl protons of the methyl methacrylate unit in meso configuration with an adjacent styrene unit (regarding the methoxycarbonyl and phenyl groups) than for the corresponding protons in the unit of racemic configuration. Further, the differences between the chemical shifts for these protons in meso and racemic dyads are nearly equivalent to the differences in chemical shifts between two adjacent peaks of the observed triplet spectra. This result indicates that the spectral assignment based on a theoretical calculation is quite consistent with the one made by a conventional simulation method: from the lower magnetic field, peaks of cosyndiotactic, coheterotactic, and coisotactic'triads appear in this order for the methoxyl protons and in the opposite order for the a-methyl protons. Magnetic anisotropy effect contributes to these tacticity-dependent chemical shifts to a far greater extent than diamagnetic shielding. The variation of the chemical shifts due to measuring temperature, probability of isotactic addition, and interactions between groups more than five bonds apart is also discussed.
SynopsisBy the use of various metal halides methyl methacrylate and styrene were copolymerized to produce equimolar alternating sequences and different cotacticities. The 13C-NMR spectra of these copolymers were simple in comparison to those of random copolymers because of the fixed monomer sequence which yielded sharply split triplets for carbonyl, methoxy, and quaternary carbons. The relative intensities in these split peaks varied according to the metal halide used. A comparison of the intensities made it possible to obtain clear-cut and quantitative information on the methyl methacrylate-centered triad cotacticity of the copolymers. The spectral assignment with respect to the methoxy carbon was definitely justified by the combined use of partly relaxed Fourier transform and selective decoupling techniques. The spectrum of aromatic C1 carbon in styrene units also split into three main peaks. From their relative intensities the splitting was attributed to styrene-centered triad cotacticity. The assignment of this carbon was compared with two other assignments made for random copolymers of methyl methacrylate with styrene; they were contradictory, however. Furthermore, an apparent discrepancy was observed between methyl methacrylateand styrene-centered tactic triads of these alternating copolymers. The origin of this discrepancy suggests a close relationship with the copolymerization mechanism.
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