Solvent effects on mechanism of tetramethylene polymerization

The polymerization mechanism of tetramethylenes was reinvestigated under inclusion of solvent effects. The approach of a methanol molecule to a borderline diradical, a typical diradical, and a typical zwitterion was studied by a valence, charge, and dipole moment analysis of SINDO 1 calculations. Whereas the solvent molecule has no effect on the character of the zwitterion, the borderline diradical was found to switch to a zwitterion at the approach of the methanol molecule if the distance between the donor carbon and the methanol oxygen is below 2 Å. A similar switch of character was observed for the typical diradical at CO distances below 1.5 Å. From energy considerations it is concluded that borderline diradicals can follow a zwitterionic polymerization mechanism in polar solvents, whereas typical diradicals are much less likely to do so.