The PCl5-induced polymerization of the fluorophosphoranimine PhF2P=NSiMe3 at ambient temperatures (35 degrees C) yields the poly(fluorophosphazene) [N=PF(Ph)](n). This THF-soluble product was treated with an excess of sodium trifluoroethoxide in refluxing dioxane to replace the fluorine atoms by trifluoroethoxy groups and generate the known hydrolytically stable polymer [N=PPh(OCH2CF3)](n). The molecular weights in this system can be controlled by variation of the monomer to initiator ratios, and polydispersities for [N=PPh(OCH2CF3)](n) are in the range of 1.05-1.32. In order to explore the synthetic utility of [N=PF(Ph)](n), the polymer was treated with Grignard or alkyllithium reagents such as MeLi, n-BuLi, or p-tolylmagnesium bromide to produce the poly(organophosphazenes) [N=PPh(R)](n) (R = Me, n-Bu, p-tolyl). Complete replacement of the fluorine atoms using alkyl/aryllithium reagents induced a significant decline in molecular weights. However, treatment of [N=PF(Ph)](n) with a 0.5 molar equiv of MeLi at -78 degrees C, followed by replacement of the remaining fluorine atoms with NaOCH2CF3 in refluxing dioxane, produced the mixed-substituent polyphosphazene {[N=PPh(OCH2CF3)(0.5)(Me)(0.5)](y)} with M-n = 2.4 x 10(4) (PDI = 1.10). The phosphoranimine PhF2P=NSiMe3 also reacts at 35 degrees C with "living" poly(dichlorophosphazene), (N=PCl2)(n), to produce the block copolymer {[N=PCl2](n)[N=PPh(F)](m)}. Halogen replacement by NaOCH2CF3 in refluxing dioxane produced the known hydrolytically stable block copolymer {[N=P(OCH2CF3)(2)](n)[N=PPh(OCH2CF3)](m)} with M-n = 4.5 x 10(4) (PDI = 1.30).