H-1 AND C-13 NMR INVESTIGATION OF THE INTRAMOLECULAR STRUCTURE OF SOLUTION AND EMULSION STYRENE METHYL ACRYLATE COPOLYMERS

The intramolecular structure (triad distribution and tacticity parameter, σSM) of homogenous styrene (S)-methyl acrylate (M) copolymers, obtained by low-conversion solution polymerization, has been studied by1H and13C NMR. With the set of reactivity ratios rS = 0.73 and rM = 0.I9 and classical formulas, based on Alfrey-Mayo (AM) kinetics, it was possible to verify the experimentally observed triad distributions. For the methoxy proton region in the NMR spectra it is difficult to make a correct peak assignment. Two different assignments can fit the observed data on triad level. The well-known Ito-Yamashita (I-Y) assignment leads for the random copolymers to a high degree of coisotacticity (ctms = 0.9), contrary to the value obtained for alternating copolymers (σSM = 0.5). An alternative assignment is possible, which results in σMS = 0.3. In order to discriminate between the two assignments, three 2D NMR COLOC experiments were carried out in an attempt to establish the connectivity over three bonds of the carbonyl C=0 resonances and the protons of the OCH3 resonances. However, the correctness of neither of the two assignments of the methoxy resonances could be rigorously proved by means of the 2D NMR COLOC experiments. The cumulative triad fractions and the molar composition of copolymers, obtained up to very high conversion by means of both solution and emulsion batch processes, could be adequately described by models, on the basis of instantaneous distributions according to AM kinetics. In the case of the emulsion copolymerization it is shown that the model for predicting the sequence distribution should account for the monomer partitioning between the oil and water phases. The agreement found between predicted and observed triad distributions was quite satisfactory for both polymerization processes. It was shown that (statistical) emulsion copolymers have the same degree of coisotacticity as the statistical solution copolymers, regardless of whether or not the I-Y assignment is used for the alternative assignment. © 1990, American Chemical Society. All rights reserved.