Structural Effects in Heteroatom Systems. I. The Influence of Steric Effects on Oligomer-Polymer Interconversions in the Phosphazene (Phosphonitrile) Series. A Semiempirical Approach

The influence of side-group steric repulsions on the relative thermodynamic stability of polymeric phosphazenes and on the relative polymerizability of cyclic phosphazenes is discussed. Increasing dimensions of the side groups should progressively lower the ceiling temperature until polymer can no longer exist as a stable entity. A simple, semiempirical, graphical method is employed for the estimation of the maximum overlap of van der Waals rotational boundaries for common side groups. The calculated polymerization and depolymerization tendencies were found to be reasonably consistent with the limited, qualitative experimental data at present available. The results also suggest that most polyorganophosphazenes should occupy a cis-trans-planar conformation.