A series of cyclic thionylphosphazenes were prepared and density functional theory (DFT) calculations initiated to gain insight into their susceptibility to ambient ring-opening polymerization with Lewis acid catalysts (GaCl3, AlCl3). DFT analyses determined that PBE1PBE-GD3BJ provided the optimal calculated structures for 1, 5, 5’ and 8. The gauge-independent atomic orbital (GIAO) method (M05-2X-GD3//PBE1PBE-GD3BJ) provided a good approximation for the 31P NMR resonances. Energy calculations indicate that thionylphosphazene phosphorus substitution promotes heterolytic cleavage of the thionyl S–Cl bond, as does adduct formation with GaCl3 or AlCl3, suggesting that 4 has potential for GaCl3-catalyzed ROP reactions at ambient temperatures. The generation of thionylphosphazene cation from 1 with catalytic quantities of Lewis acids (10:1 GaCl3, AlCl3) initiates the ambient temperature ROP to poly(thionylphosphazene) (2) in a minimum amount of solvent. Polymerization/depolymerization studies of 1 and 2 in solution was found to be fully reversible in the presence of GaCl3.