B(C6F5)3-catalysed hydrosilation, heterodehydrocoupling, and demethanative coupling reactions of the Si-H bonds in poly(phenylsilane) allow the introduction of 10-40% new sidechains in this polymer. The resulting new polymers contain an unusually wide variety of functionalities including Si-C, Si-O, Si-N, and Si-S bonds, whose presence is confirmed by NMR and IR spectroscopies. Gel permeation chromatography (GPC) and thermogravimetric analysis (TGA) are consistent with conservation of the all-silicon backbones in these modified polymers, a result of the high chemoselectivity of the borane-catalysed reactions for Si-H versus Si-Si bonds. UV-visible spectroscopy is sensitive to the presence of new functional groups in the modified polysilanes, although the high proportion of residual Si-H groups attenuates the changes in the σ-delocalized chromophores. Limitations in substrate scope, arising from issues of borane-substrate complexation or competing catalytic over-reduction chemistry, have been identified, and the potential for achieving greater degrees of sidechain substitution at higher reaction temperatures has been demonstrated for the hydrosilation of 1-hexene.