New insight into the mechanism of the ambient temperature PCl5-initiated living cationic chain growth polycondensation of the N-silylphosphoranimine Cl3P=NSiMe3 (1) to give poly(dichlorophosphazene), [N=PCl2](n), has been provided by studies of model compound chemistry. Investigations of the reactivity of Cl- salts of the proposed cationic intermediates [Cl3P=N=PCl3](+) ([2](+)) and [Cl3P=N-PCl2=N=PCl3)(+) ([6](+)) toward Ph3P=NSiMe3 (3a) provided evidence that under the usual polymerization conditions that involve a high monomer to initiator ratio, propagation occurs at both chain ends. However, analogous studies of near stoichiometric processes suggested that propagation is faster at one chain end, particularly when the chains are short. In addition, experiments involving [Ph3P=N=PPh3][PCl6] ([9][PCl6]) and the N-silylphosphoranimines R3P=NSiMe3 3a (R = Ph) and 3b (R = p-CF3C6H4), showed that the [PCl6](-) anion, which is formed in the early stages of the polymerization and has hitherto been assumed to be an innocent spectator counteranion, is actually reactive under the reaction conditions and can initiate oligomerization and polymerization. Finally, the absence of reactions between phosphoranimines 3b or 1 with the Cl- salts of the cations [Ph3P=N-PCl2=N=PPh3](+) ([10a](+)), [Ph3P=N-(PCl2=N)(2)=PPh3](+) ([5](+)), and [Ph3P=N-(PCl2=N)(3)=PPh3](+) ([8](+)) with P-Cl bonds located internally but not at the chain ends have shown that chain branching reactions are unlikely to be significant during the polymerization. These results identify key factors that complicate the living PCl5-initiated chain growth polycondensation of 1 and potentially lead to a loss of control over molecular weight and broaden the molecular weight distributions, but also indicate that the polymer formed is essentially linear rather than branched.