A simple polymerization of trichlorophosphoranimine (Cl3P = N- SiMe3) mediated by functionalized triphenylphosphines is presented. In situ initiator formation and the subsequent polymerization progress are investigated by 31 P NMR spectroscopy, demonstrating a living cationic polymerization mechanism. The polymer chain lengths and molecular weights of the resulting substituted poly(organo) phosphazenes are further studied by 1 H NMR spectroscopy and size exclusion chromatography. This strategy facilitates the preparation of polyphosphazenes with controlled molecular weights and specific functional groups at the alpha-chain end. Such well-defined, mono-end-functionalized polymers have great potential use in bioconjugation, surface modification, and as building blocks for complex macromolecular constructs.