Experimental and Theoretical
Study of the Living Polymerization
of <i>N</i>-Silylphosphoranimines. Synthesis of New Block
Copolyphosphazenes
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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, BrMe<sub>2</sub>PN–SiMe<sub>3</sub>, [Cl<sub>3</sub>PNPCl<sub>3</sub>][X] (X = PCl<sub>6</sub><sup>–</sup>, Cl<sup>–</sup>), Cl<sub>3</sub>PN–SiMe<sub>3</sub>, ClMe<sub>2</sub>PN–SiMe<sub>3</sub>, and [Me<sub>3</sub>PNPMe<sub>2</sub>Cl]<sup>+</sup> were used as representative model reagents.
Density functional theory (DFT) calculations in the gas phase adjusted
for solvent effects on ClMe<sub>2</sub>PN–SiMe<sub>3</sub>, [Cl<sub>3</sub>PNPCl<sub>3</sub>]<sup>+</sup>, Cl<sub>3</sub>PN–SiMe<sub>3</sub>, and ClMe<sub>2</sub>PN–SiMe<sub>3</sub> 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. XR<sub>2</sub>PN–SiMe<sub>3</sub>, react better with electron-deficient
cationic end groups such as N–PCl<sub>3</sub><sup>+</sup>, 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 [NP(Ph)(Me)]<sub><i>n</i></sub>-<i>b</i>-[NP(OCH<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>]<sub><i>m</i></sub> and [NP(Ph)(Me)]<sub><i>n</i></sub>-<i>b</i>-[NP(O<sub>2</sub>C<sub>12</sub>H<sub>8</sub>)]<sub><i>m</i></sub>, which
are excellent candidates for micellation studies