Reappraisal of the origins of the polymodal molecular mass distributions in the formation of poly(methylphenylsilylene) by the Wurtz reductive-coupling reaction

Variations of yields and molecular weight parameters of poly(methylphenylsilylene) formed through sodium-mediated Wurtz reductive coupling of dichloromethylphenylsilane in toluene, xylene, diethyl ether, tetrahydrofuran, and diphenyl ether both in the presence and in the absence of 15-crown-5 and at different temperatures are described. The effects of various terminating reagents are also considered. The crown ether when it is present in the reaction mixture is shown to act as a phase transfer agent for the sodium and this is interpreted as having two effects. The first is to activate the alkali metal for reductive coupling, which at lower temperatures is a necessary condition for polymer formation. The other is to transport the alkali metal to points on the high molecular weight polymer chains where, as a prelude to backbiting, it induces a highly selective chain scission; this is explained by it occurring only at gauche defects in the otherwise rodlike all-trans sequences within the polymer in solution. The polymodal molecular weight distributions that are commonplace for polysilylenes are rationalized in terms of a competition between such activation and degradation processes.