The vinyloxycyclophosphazene derivatives, N3P3Cl5OCH=CH2 (1) and N4P4Cl7OCH=CH2 (6), undergo radical polymerization to produce the highly functionalized homopolymers [CH(ON3P3Cl5)-CH2](n) (7) and [CH(ON4P4Cl7)CH2](n) (8). These materials undergo a remarkable two-step thermal decomposition process. The first step is an exothermic cross-linking reaction involving the elimination of HCl at modest temperatures. The activation energy for the first step is lower for 7 than for 8. The second step is an endothermic elimination of the ore-bridged cyclophosphazene dimer, (N3P3Cl5)(2)O. Copolymers have been obtained from mixtures of 1 and 6. The calculated reactivity ratios show a preference for incorporation of 1 over 6 into the copolymers. Attempted copolymerization of 1 with styrene leads to mixtures of the respective homopolymers. Attempts to polymerize N3P3X5OCH=CH2 [X = F(2), OCH3 (3), OCH2CF3 (4), N(CH3)(2) (5)] lead to various outcomes including insoluble materials, oligomers, or recovery of unreacted monomer. The sensitivity of the polymerization to the nature of the phosphazene substituents has been related to monomer electronic structure and reactivity of the exocyclic groups.