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

Abstract

The sequential living polymerization of <i>N</i>-silylphosphoranimines for the synthesis of polyphosphazene-<i>b</i>-polyphosphazene diblock copolymers (PP-<i>b</i>-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 <i>N</i>-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 <i>N</i>-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-<i>b</i>-PP block copolymers with narrow molecular weight distributions of formula [NP­(Ph)­(Me)]_<i>n</i>-<i>b</i>-[NP­(OCH₂CF₃)₂]_<i>m</i> and [NP­(Ph)­(Me)]_<i>n</i>-<i>b</i>-[NP­(O₂C₁₂H₈)]_<i>m</i>, which are excellent candidates for micellation studies