Conformational stability and force field of polyphosphazenes: MNDO calculations, vibrational spectra and normal coordinate analyses of [NP(R) 2 ] n (R = Cl, OCH 2 CF 3 , OC 6 H 5 )

Raman and Fourier‐transform Raman spectra (1500 − 100 cm −1 ) of (PCl 2 N) n (PDCP) were recorded in the solid phase and in solution at room temperature. Raman (3 500 − 100 cm −1 ) and infrared (4 000 − 200 cm −1 ) spectra of [P(OCH 2 CF 3 ) 2 N] n (PDFP) and [P(OC 6 H 5 ) 2 N] n (PDPP) were recorded in the solid phase and at different temperatures (in the case of Raman spectroscopy). The conformation of the isolated macromolecule PDCP is assumed to be analogous to the geometry of the Cl 2 (O)PN(PCl 2 N) 6 PCl 3 oligomer optimized by the use of MNDO (modified neglect of diatomic overlap) calculations. The optimized cis ‐ trans conformation is in good agreement with the X‐ray experimental data concerning the polymer. The calculated low energy barriers around the PN bond along the chain axis can explain the flexibility of the phosphazene backbone and the elastomeric properties of the polymers. The MNDO calculation of the harmonic force field of Cl 2 (O)PN(PCl 2 N) 6 PCl 3 is in reasonable agreement with the experimental values for (PCl 2 N) n in solution as well as in the amorphous phase. The normal coordinate analyses of (PCl 2 N) n were undertaken according to several structural hypotheses using a force field derived from linear short‐chain molecules. The Raman spectra of PDCP in solution or in the amorphous phase are in reasonable agreement with the vibrational frequencies calculated for a planar cis ‐ trans macromolecule and have a striking resemblance with those of linear short‐chain analogs Cl 2 (O)PN(PCl 2 N) n PCl 3 ( n = 1,2). The Raman and infrared spectra of the substituted polymers PDFP and PDPP are dominated by the characteristic features of the chain substituents.