Studies on the mechanism of phosphazene ring-opening polymerization (ROP)

In this study, the ring-opening polymerization (ROP) of cyclic trimer N-3 P_3 Cl-6 catalyzed by [Et3 Si(N_3 P_3 Cl_6)][CHB_11H_5 Br_6] (3) obtained from the reaction of N_3P_3 Cl_6 (1) and Et_3 Si(CHB_11H_5 Br_6) (2) at room temperature was investigated. This provided a unique opportunity to explore the polymerization mechanism. The coordinatively unsaturated cation [N_3P_3 Cl_5 ]+ is responsible for the ROP of N_3P_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_3P_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_3P_3 Cl_6 [(Cl_2P=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.