Thermal Degradation of Polymers.II. Synthesis and Characterization of a Compound Which Models the Head-to-Head Linkage in Poly(Methyl Methacrylate)

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As models for a poly(methyl methacrylate ) chain which contains a head-to-head unit, the (�)- and meso -isomers of tetramethyl 2,4,5,7-tetramethyloctane-2,4,5,7-tetracarboxylate have been synthesized by oxidative dimerization of a trimethylsilyl ketene acetal. The isomers were characterized by 1H and 13C n.m.r. spectroscopy, and the molecular structure of the meso -isomer was determined by X-ray crystallography. Molecular mechanics (MM2) calculations showed that three conformations of each isomer, about its central bond, had comparable steric energies. Calculated energy barriers to interconversion of the conformers were low.