Vibrational spectra and rotational isomerism of poly(dialkoxyphosphazenes)

The IR spectra of poly(dialkoxyphosphazenes) with a general formula [-P(OC m H 2m+1 ) 2 =N-] n , where m = 1-9, were studied in the temperature interval from -100 to +100°C. The Raman spectra of methoxy, ethoxy, butoxy, and octaoxy derivatives were measured at room temperature. The spectra of poly(dialkoxyphosphazenes) are interpreted using the results of calculations of the frequency branches for poly(dimethoxyphosphazene). The spectral regions sensitive to conformational changes in the side branches of poly(dialkoxyphosphazenes) are established on the basis of model calculations for diethoxyphosphazenes with different conformations (trans or gauche) relative to the P-O and C-O bonds. It is shown that the backbone conformations of the poly(dialkoxyphosphazenes) studied in this work are the same and do not vary with the temperature. The bands of the symmetric chain vibrations V s PN in the region of 600 cm -1 are observed in the Raman spectra and are not manifested in the IR spectra. On the contrary, the antisymmetric vibrations V as PN at 1240 and 1325 cm -1 are present only in the IR spectra. This behavior agrees with the assumption of a helical structure of the polymer backbone. The temperature-induced changes in the vibrational spectra of poly(dialkoxyphosphazenes) are caused by conformational transitions in the side chains: as the temperature decreases, the proportion of trans isomers increases and that of the gauche isomers decreases.