Structural effects in heteroatom systems. II. The calculated influence of steric effects on polymerization–depolymerization equilibria in the siloxane and oxymethylene series. Comparisons with the phosphazene system.

The relative stabilities of symmetrically substituted polysiloxanes and polyoxymethylenes are compared in terms of the side-group steric overlap parameters, by means of a semiempirical method described previously. The results provide a qualitative estimate of the relative thermodynamic ease of polymerization of cyclic siloxanes or monomeric methylene oxides or, conversely, of the relative resistance to depolymerization of the polymers. These calculated values are compared with those reported previously for phosphazenes. In general, for polymers of the type, [R₂Si? O]_n, [R₂C? O]_n, and [R₂P?N]_n, if the side group, R, is small (H, halogen or methyl) the inherent steric and bonding characteristics of the chain should cause the polymer stabilities to decrease in the order phosphazenes ≈ siloxanes ? oxymethylenes. If, however, bulkier side groups are present, the order of decreasing stability to depolymerization should be siloxanes > phosphazenes ? oxymethylenes. In all cases, the depolymerization tendency should increase markedly as the side group overlap parameters increase, and polyoxymethylenes should be the most sensitive to such changes. The calculated results are compared with the limited experimental evidence at present available.