MOLAR-MASS CHEMICAL-COMPOSITION DISTRIBUTIONS OF STYRENE METHYL ACRYLATE COPOLYMERS PREPARED BY SOLUTION OR EMULSION POLYMERIZATION

The microstructure of copolymers can be characterized in terms of triad fractions and tacticity parameters (i.e. intramolecular structure), and in terms of a three-dimensional molar-mass chemical-composition distribution (MMCCD) (i.e. intermolecular structure). The microstructure obtained is controlled by the copolymerization conditions, for instance by the choice of the reaction system (homogeneous solution or heterogeneous emulsion), the degree of conversion and the choice of reactants. Computer simulations of emulsion copolymerization (SIEMCO), accounting for the main chemical and physical processes occurring, provide MMCCD predictions of emulsion copolymers. These MMCCDs are calculated by considering the conversion heterogeneity (composition drift) as well as the instantaneous (statistical) composition distribution of the copolymers formed. Cross-fractionation (two-dimensional chromatography) was used to verify the predicted MMCCDs of the copolymer products. The copolymers are separated according to molar mass by means of size exclusion chromatography (s.e.c.), and each s.e.c. fraction is subsequently analysed according to chemical composition by means of gradient elution quantitative thin-layer chromatography/flame ionization detection (t.l.c./f.i.d.). The difference in water solubility of the two monomers (styrene (S) and methyl acrylate (MA)) appears to be one of the major factors determining the microstructure of the copolymers. Depending on conversion, monomer ratio and monomer-to-water ratio, the model predicts either single- or double-peaked MMCCDs, in full agreement with the experimentally obtained distributions.