Abstract: A mechanism for the dehydrogenative polymerization of silanes to polysilanes is proposed, based on studies with zirconocene and hafnocene catalysts. This is a new polymerization mechanism, based on metal-mediated step-growth of the polymer. Key features of the mechanism are: 1) Coordinatively unsaturated metal hydride species appear to be the active catalysts. 2) Other intermediates are polysilyl metal complexes M(SiHR).sub.n H and free polysilanes. 3) The proposed mechanism involves two types of .sigma.-bond methathesis reactions that proceed via four-center transition states. This proposal of a mechanism is based on model reactions and observation of intermediates. Chemistry relevant to the polymerization process suggests that silyl groups readily participate in .sigma.-bond methathesis reactions with d.sup.0 metal centers via four-center transition states.