In this study, the ring-opening polymerization (ROP) of cyclic trimer N(3)P(3)Cl(6) catalyzed by [Et(3) Si(N(3)P(3)Cl(6))][CHB(11)H(5)Br(6)] (3) obtained from the reaction of N(3)P(3)Cl(6) (1) and Et(3)Si(CHB(11)H(5)Br(6)) (2) at room temperature was investigated. This provided a unique opportunity to explore the polymerization mechanism. The coordinatively unsaturated cation [N(3)P(3)Cl(5)](+) is responsible for the ROP of N(3)P(3)Cl(6), and is formed by the intramolecular elimination of Et(3)SiCl from 3. The detection of Et(3)SiCl by (1)H-NMR as the catalysis proceeded offers new evidence for the formation of [N(3)P(3)Cl(5)](+). The progress of the catalysis was followed using (31)P-NMR and revealed the build-up of a polymer. Cyclic phosphazenes with rings larger than that of N(3)P(3)Cl(6) [(Cl(2)P=N)(n) (n = 4-7)] were observed fairly early in the course of the ROP. The propagation of that polymerization at room temperature proceeded via a living cationic mechanism.