High-yield controlled syntheses of polysilanes by the Wurtz-type reductive coupling reaction

The factors that determine the Wurtz-type reductive coupling of dichloroorganosilanes by alkali metals are reviewed. A strong recommendation is advanced for carrying out all such polysilane syntheses in tetrahydrofuran at ambient temperature, conditions under which the reaction gives far higher yields and generally much narrower product molecular weight distributions. The higher yields are attributed to the ability of the solvent to sequester the sodium ion and thereby stabilize the anionic chain carriers. The bimodal molecular weight distributions of the product polymers, which are ubiquitous when such reactions are carried out in high-boiling-point aromatic solvents under reflux, as is common practice, are attributed to it being a defect-diffusion-controlled polymerization at the alkali metal surface. The narrower molecular weight distributions that result from syntheses in tetrahydrofuran are attributed to the suppression of defect diffusion rates at lower temperatures.