Thermal atropisomerization and photoluminiscence spectra of very high glass transition temperature chiral poly(binaphthoxyphosphazenes) with a secondary helicoidal structure

The optically active ([alpha] = -192degrees) and mesomorphic (2theta = 6degrees, d = 14.7 Angstrom) isotactic poly(2,2'-dioxy-1,1'-binaphthylphosphazene) R-(-)-[NP(O2C20H12)](n) (1-R) (M = 840 000) has a very high glass transition temperature, T-g (329 degreesC), and can be thermally degraded between 100 and 160 degreesC to lower M-w distributions without decomposition. The specific rotation in solution varied significantly with the M-w, with the temperature, and with the wavelength, while the specific rotation of the related random nonisotactic copolymer [NP(O2C20H12)](n) (2-R/S) (M-w = 750 000, [alpha] = -50degrees), having different proportions of the chiral R and S repeating units (70% and 30%, respectively), was independent of the M-w and the temperature. Those observations are consistent with the secondary helicoidal structure for polymer 1-R in solution. The steady-state and lifetime fluorescence measurements in various solvents and temperatures have shown that the lifetime of the excimers formed by the binaphthyl units are too short to evidence the effects of the helicity in the fluorescence spectra. Above 250 degreesC, a slow atropisomerization of the 2,2'-dioxy-1,1'-binaphthylphosphazene units takes place, which, supporting recent predictions on glassy polymer matrices, becomes much faster as the temperature approaches the T-g.