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

摘要

In this study, the ring-opening polymerization (ROP) ofcyclic trimer N_3P_3Cl_6 catalyzed by [Et_3Si(N_3P_3Cl_6)][CHB_{11}H_5Br_6] (3) obtained from the reaction ofN_3P_3Cl_6 (1) and Et_3Si(CHB_{11}H_5Br_6) (2) at room temperature wasinvestigated. This provided a unique opportunity to explore thepolymerization mechanism. The coordinatively unsaturated cation[N_3P_3Cl_5]^+ is responsible for the ROP of N_3P_3Cl_6, and is formedby the intramolecular elimination of Et_3SiCl from 3. The detectionof Et_3SiCl by ^1H-NMR as the catalysis proceeded offers new evidencefor the formation of [N_3P_3Cl_5]^+. The progress of the catalysiswas followed using ^{31} P-NMR and revealed the build-up of a polymer.Cyclic phosphazenes with rings larger than that of N_3P_3Cl_6[(Cl_2P=N)_n (n = 4-7)] were observed fairly early in the course ofthe ROP. The propagation of that polymerization at room temperatureproceeded via a living cationic mechanism.