The room temperature addition of stoichiometric amounts of trimethyl phosphite, P(OMe)(3), to N-silyl(halogeno)organophosphoranimines BrRR'P=NSiMe3 in chlorinated solvents led to the direct formation of high molecular weight polyphosphazenes [RR'P=N](n). The majority of polymerizations were complete within 18 h. The polymers prepared include poly(dialkylphosphazenes) (e.g., [nBu(2)P=N] 1b), poly(alkylarylphosphazenes) [PhMeP=N] 1d), new materials featuring unsaturated substituents (e.g., nHex{H2C=C(H)CH2}P=N](n) ln), and random copolymers (e.g., ([PhMeP=N](x)-[PhnBuP=N](y); where = 2:1, 7a). The precursor silylaminophosphincs RR'P N(SiMe3)(2) (5a-q) and bromo(silylamino)phosphoranimines BrRR'P=NSiMe3 (4a-q) were synthesized and fully characterized prior to polymerization studies. The presence of alkoxy, carboranyl, and/or phenyl substituents on the N-silylbromophosphoranimines, as found in [BrEt-[CF3CH2O]P=NSiMe3 (4g), Br(2-Me)-o-C2B10H10)Et P=NSiMe3 (4h), or BrPh2P=NSiMe3 (4i), respectively, was found either to severely retard or to preclude polymerization altogether. The mild reaction conditions enabled the preparation of polyphosphazenes that are substituted with reactive alkyne groups (e.g., R = Et, R' = CH2C CSiMe3 1p), materials that have not been accessible using high-temperature thermal routes. These moieties undergo further convenient chemical transformations as illustrated by deprotection of 1p by TBAF center dot 3H(2)O (TBAF = tetra(n-butyl)ammonium fluoride) as well as chemical cross-linking with the disiloxane HMe2SiOSi-Me2H in the presence of Karstedt's catalyst. The polyphosphazene materials were characterized by a variety of techniques including H-1, C-13, and P-31 NM R spectroscopy and GPC and, in selected cases, by IR, DES, TGA, DSC, and WAXS.