Experimental and Theoretical Study of the Living Polymerization of N-Silylphosphoranimines. Synthesis of New Block Copolyphosphazenes

The sequential living polymerization of N-silylphosphoranimines for the synthesis of polyphosphazene-b-polyphosphazene diblock copolymers (PP-b-PP) has been studied both experimentally and theoretically. For the experiments, BrMe2P=N-SiMe3, [Cl3P=N=PCl3][X] (X = PCl6-, Cl-), Cl3P=N-SiMe3, ClMe2P=N-SiMe3, and [Me3P=N=PMe2Cl](+) were used as representative model reagents. Density functional theory (DFT) calculations in the gas phase adjusted for solvent effects on ClMe2P=N-SiMe3, [Cl3P=N=PCl3](+), Cl3P=N-SiMe3, and ClMe2P=N-SiMe3 confirmed the experimental observations. The results have shown the necessity of starting with mono-end-capped initiators to avoid the formation of triblock chains. It was also demonstrated that the nature of the nucleophilic N-silylphosphoranimines and the electrophilic cationic end groups of the living polyphosphazenes strongly affects the polymerization reaction, imposing limits to its synthetic potential. Thus, good electron donor N-silylphosphoranimines, i.e. XR2P=N-SiMe3, react better with electron-deficient cationic end groups such as =N-PCl3+, probably by molecular orbital (MO) control. The results led to the designed synthesis of well-defined PP-b-PP block copolymers with narrow molecular weight distributions of formula [N=P(Ph)(Me)(n)-b-[N=P(OCH2CF3)(2)](m) and [N=P(Ph)(Me)](n)-b-[N=P(O2C12H8)](m), which are excellent candidates for micellation studies.