A density functional theory study of the mechanisms of scandium-alkoxide initiated coordination-insertion ring-opening polymerization of cyclic esters

The generally accepted coordination-insertion mechanisms of scandium-alkoxide initiated ring-opening polymerizations of epsilon-caprolactone (CL), trimethylene carbonate (TMC) and (S,S)-lactide (LLA) were investigated using density functional theory calculations. The geometries and corresponding energies of four key proposed intermediates were evaluated and found consistent with these structures. The transition states corresponding to monomer addition indicate a penta-coordinate scandium and have nearly the same energies for all monomers. Thus CL, TMC and LLA undergo similar processes with relatively small differences in the energies of Sc-monomer coordination but the pre-transition state complex of TMC is much lower in energy than that of the other monomers. The dipole moments of the intermediates close to the transition states and the transition states themselves in all cases are lower than those of the reactants predicting faster reaction rates in low polarity solvents, consistent with experimental results. This study may be applicable to ring-opening polymerizations mediated by yttrium and other rate earth complexes. (C) 2009 Elsevier Ltd. All rights reserved.