Thermal-Degradation of Polymers and Polymer Models. V. A Model for Copoly(styrene-methyl methacrylate)

The synthesis of (�)- and meso-dimethyl 2,3-dimethyl-2,3-bis(2′-methyl-2′-phenylpropyl)butane-dioate from methyl 2,4-dimethyl-4-phenylpentanoate , via a silyl enol acetal, is described. The X-ray structure of the meso isomer shows serious distortions from normal bond lengths and angles, most notably a length of 162.7(4) pm for the crowded central C-C bond. The compounds were synthesized as models for head-to-head units which might form by chain termination through dimerization of macroradicals during copolymerization of styrene and methyl methacrylate . Thermal degradation of the diphenyl -substituted diesters at 210° gave products which may have formed by plausible routes from radicals resulting from homolytic cleavage of the crowded C-C bond. Inclusion of the phenyl groups makes the molecules more crowded than the corresponding tetraesters which were reported earlier, and a comparative rate study shows that the aromatic compounds are less stable to heat as a result of the penultimate effects of greater crowding at carbons which are β to the thermally vulnerable bond.