A new, ambient-temperature method for the direct synthesis of organo-substituted polyphosphazenes is described. It involves the initiation of a series of organophosphoranimines R(R’)-XP = NSiMe(3) (3, R = Ph, R’ = X = Cl; 5, R = R’ = Ph, X = Cl; 7, R = Me, R’ = Et, X = Cl; 9, R = R’ = CF3CH2O, X = Br) with catalytic amounts of PCl5 in CH2Cl2 to yield (after treatment with NaOCH2CF3 in the case of 3) the corresponding polyphosphazene species (N = PRR’)(n) (4, R = Ph, R’ = OCH2CF3; 6, R = R’ = Ph; 8, R = Me, R’ = Et; 10, R = R’ = OCH2CF3) with narrow polydispersities. The molecular weights of the polyphosphazenes were controlled by altering the ratio of monomer to initiator. The polymer chains were found to be active after chain propagation since further addition of monomer resulted in the formation of higher molecular weight polymer. For monomers 7 and 9 optimum polymerization behavior was found to occur at 35 degrees C in the absence of solvent in the presence of catalytic quantities of PCl5. These reactions proceeded to 100% completion, while maintaining molecular weight control and narrow polydispersites. In the case of polymers 4, 8, and 10, which were synthesized with a 10:1 monomer to initiator ratio in CH2Cl2, the resultant polymers were analyzed by gel permeation chromatography (GPC) (4, M(n) = 2.9 x 10(3), polydispersity (PDI = M(w)/M(n)) = 1.07); 8, M(n) = 2.2 x 10(3), PDI = 1.31; 10, M(n) = 7.8 x 10(3), PDI = 1.23). Poly(diphenylphosphazene), (N = PPh(2))(n) (6), was insoluble in common organic solvents and was characterized by magic angle spinning (MAS) solid state P-31 NMR spectroscopy. The effects of side group steric bulk, electron-withdrawing or -donating properties, and leaving group types on the ambient-temperature cationic induced polymerizations are discussed.