The Ring-opening polymerization (ROP) of D-lactide (D-LA) catalyzed by bifunctional thiourea catalyst has been studied by performing density functional theory calculations at the B3LYP/6-31G(d) level. Two pathways (A and B) for the catalyzed ROP of D-LA have been characterized in detail. It has been found that path A is much more favorable in energy than path B. The calculated results indicate that the catalyst promotes the ROP of D-lactide by bifunctional activation of the carbonyl of LA monomer and of the initiating/propagating alcohol via hydrogen bonding to the thiourea group and to the Bronsted basic (tertiary amino) group, respectively. The calculated results provide a general model that explains the mechanism of the title reaction.

Key words: bifunctional thiourea, ring-opening polymerization, lactide, reaction mechanism, density functional theory