Polyphosphazene helical structure

Aspects of phosphazene research " and of the history of phosphazenes have been examined. The prototrope equilibrium (Scheme 13) between the polyaminophosphines (NH form) and the polyiminophosphines (polyphosphazene or PH form) has been studied by Quantum Mechanical ab initio calculations. When R = H, the energy difference is in favour of the NH tautomer, but if R = NH2 the more stable is the polyphosphazene form. In fact, the preference for the later increases with the electronegativity of the R substituent, and in agreement with the experimental facts, the calculations showed that the polymerization of the monophosphazanes (1) (see Section 1) should be favourable when the electronegativity of R is about 3. The polyhydrido phosphazene has an helical structure with small bond alternation. The calculations also support the mechanisms for the formation of the polyhydridophosphazene (3) from tris(amino)phosphine P(NH2)3 (la) via its phosphazene (NH2)2HP=NH (2a) tautomer. ... 

Yashima, E. Maeda, K. Helicity induction on optically inactive polyacetylenes and polyphosphazenes. In Synthetic Macromolecules with Higher Structural Order, Kahn, T. I. M., Ed. ACS Symposium Series 812 American Chemical Society Washington, DC, 2002 pp 41-53. 

---