High molecular weight polyphosphazenes that bear varying ratios of phenoxy and 2,2,2-trifluoroethoxy groups have been synthesized by the reactions of (NPCl2)n with sodium phenoxide, PhO-Na+, and sodium 2,2,2-trifluoroethoxide, CF3CH2O-Na+, using either competitive or sequential exposure to the two nucleophiles. The relative amounts of NP(OCH2CF3)2, NP(OCH2CF3)(OPh), and NP(OPh)2 units in the final polymer are dependent on both the synthesis method and the amounts of CF3CH2O-Na+ and PhO-Na+ used for the synthesis. The sequential reaction of poly(dichlorophosphazene), (NPCl2)n, with CF3CH2O-Na+ followed by PhO-Na+ yields mixtures of polymers that have different compositions depending on the presence or absence of tetra-n-butylammonium bromide or 15-crown-5 ether. The reactions are complicated by the fact that phenoxy groups in NP(OCH2CF3)(OPh) units are replaced by 2,2,2-trifluoroethoxy groups at elevated temperatures or at 25-degrees-C in the presence of either 15-crown-5 ether or tetra-n-butylammonium bromide. The sodium salt of 3-hydroxypropanol replaces both 2,2,2-trifluoroethoxy and phenoxy groups in these polymers at room temperature in the presence of either tetra-n-butylammonium bromide or 15-crown-5 ether. These side-group exchange reactions are affected by steric effects. The mechanisms of these reactions are interpreted from the perspective that the reaction of CF3CH2O-Na+ alone with the small-molecule model compound, (NPCl2)3, yields significant amounts of geminally substituted products in the presence of either tetra-n-butylammonium bromide or 15-crown-5 ether. The T(g) of the mixed-substituent polymers increased as the fraction of phenoxy groups in the polymer increased. For the polymers containing equal amounts of 2,2,2-trifluoroethoxy and phenoxy groups, the polymers with the highest ratio of geminal NP(OCH2CF)2 units had the lowest T(g)’s.