Twisted Morphologies and Novel Chiral Macroporous Films from the Self-Assembly of Optically Active Helical Polyphosphazene Block Copolymers

A series of optically active helical polyphosphazene block copolymers of general formula R[NP(O2C20H12)]n-b-[NPMePh]m (R-7ac) was synthesized and characterized. The polymers were prepared by sequential living cationic polycondensation of N-silylphosphoranimines using the mono-end-capped initiator [Ph3PNPCl3][PCl6] (5) and exhibit a low polydispersity index (ca.1.3). The temperature dependence of the specific optical activity ([]D) of R-7a,b relative to that for the homopolymers R-[NP(O2C20H12)]n (R-8a) and the R/S analogues (R/S-7a,b), revealed that the binaphthoxyphosphazene segments induce a preferential helical conformation in the [NPMePh] blocks through a sergeant-and-soldiers mechanism, an effect that is unprecedented in polyphosphazenes. The self-assembly of drop-cast thin films of the chiral block copolymer R-7b (bearing a long chiral and rigid R[NP(O2C20H12)] segment) evidenced a transfer of helicity mechanism, leading to the formation of twisted morphologies (twisted pearl necklace), not observed in the nonchiral R/S-7b. The chiral R-7a and the nonchiral R/S-7a, self-assemble by a nondirected morphology reconstruction process into regular-shaped macroporous films with chiral-rich areas close to edge of the pore. This is the first nontemplate self-assembly route to chiral macroporous polymeric films with pore size larger than 50nm. The solvent annealing (THF) of these films leads to the formation of regular spherical nanostructures (ca.50nm), a rare example of nanospheres exclusively formed by synthetic helical polymers.