MNDO MODEL STRUCTURE FOR POLY(DIPHENOXYPHOSPHAZENE) DERIVED FROM CLUSTERS OF SEVERAL RELATED PHOSPHAZENES

We have determined that the backbone of poly(diphenoxyphosphazene) is not planar. Instead, significant evidence of a twisted T-ribbon like helix conformation is seen. The degree of twist is crudely estimated as 9 degrees (40 monomer units per revolution) making this compound one of the largest for which an approximate structure is first elucidated with the MNDO technique. The helix radius, neglecting extension of side groups, is 0.82 A. The phenyl groups remain approximately perpendicular to their nearest NPN backbone fragment. Several series of finite phosphazene clusters, including rings and oligomers, are characterized. It is found that many structural features are transferable to the polymer. The remaining features necessary to fill out the MNDO model of poly(diphenoxyphosphazene) are estimated by extrapolation of data originating from diphenoxyphosphazene oligomers (up to six monomer units).