Computational investigation of mechanisms for ring-opening polymerization of epsilon-caprolactone: Evidence for bifunctional catalysis by alcohols

Stepwise addition/elimination and concerted mechanisms for the methanolysis of epsilon-caprolactone, as a model for the initiation and propagation of ring-opening polymerization (ROP), have been investigated computationally using the B3LYP/6-31G* density functional method, with assistance from one or two ancillary methanol molecules. The effects of specific solvation by these extra methanols in cyclic hydrogen-bonded clusters are very significant, with barrier height reductions of about 50 kJ mol(-1). However, the effects of bulk solvation as treated by the polarized continuum model are almost negligible. Increasing the ring size lowers the barriers for both the addition and elimination steps of the stepwise mechanism but does not do so for the concerted mechanism; a stepwise mechanism is preferred for methanol-assisted ROP. The essential catalytic role of solvent molecules in this reaction is to avoid the unfavorable accumulation or separation of charges.