A new approach to the synthesis of poly(dichlorophosphazene) and poly( organophosphazenes) has been developed via the cationic condensation polymerization of phosphoranimines. The effects of solvent, temperature, concentration, and initiator on the cationic condensation polymerization of Cl3P=NSiMe3 and PhCl2P=NSiMe3 are described. The ambient temperature polymerization of PhCl2P=NSiMe3 is faster in toluene, benzene, and dioxane than in methylene chloride. or chloroform. The polymerizations of Cl3P=NSiMe3 and PhCl2LP=NSiMe3 were monitored by both NMR (P-31 and H-1) and GPC methods. The initial polymerization rates are slow, presumably because of the precipitation of phosphazene short chain salts, RCl2P=N-[PR(CI)=N](n)-PCl3+PCl6- (where R = Cl or Ph; n = 0, 1, 2, ...). After the chains redissolve (similar to 15-60 min), polymerization proceeds, with the propagation rates following pseudo-first-order kinetics for both monomers. The reactions in toluene, benzene, or dioxane yielded polymers with controlled molecular weights in the range of 10(5), with narrow polydispersities (<1.3). The usefulness of this approach for the synthesis of a biomedically important polyphosphazene has also been demonstrated.