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

In this study, the ring-opening polymerization (ROP) of cyclic trimer N3P3Cl6 catalyzed by [Et-3 Si(N3P3Cl6)][CHB11H5Br6] (3) obtained from the reaction of N3P3Cl6 (1) and Et3Si(CHB11H5Br6) (2) at room temperature was investigated. This provided a unique opportunity to explore the polymerization mechanism. The coordinatively unsaturated cation [N3P3Cl5](+) is responsible for the ROP of N3P3Cl6, and is formed by the intramolecular elimination of Et3SiCl from 3. The detection of Et3SiCl by H-1-NMR as the catalysis proceeded offers new evidence for the formation of [N3P3Cl5](+). The progress of the catalysis was followed using P-31-NMR and revealed the build-up of a polymer. Cyclic phosphazenes with rings larger than that of N3P3Cl6 [(Cl2P=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.