Abstract: Compositions and methods for controlled polymerization and/or oligomerization of silane (and optionally cyclosilane) compounds, including those of the general formulae SinH2n and SinH2n+2, as well as halosilanes and arylsilanes, to produce soluble polysilanes, polygermanes and/or polysilagermanes having low levels of carbon and metal contaminants, high molecular weights, low volatility, high purity, high solubility and/or high viscosity. The polysilanes, polygermanes and/or polysilagermanes are useful as a precursor to silicon- and/or germanium-containing conductor, semiconductor and dielectric films.