Zinc(II) complexes of (benzoimidazolyl)amine ligands, N-(1H-benzoimidazol-2-ylmethyl)aniline (L1) and N-(1H-benzoimidazol-2-ylmethyl)-2-bromoaniline (L2), [Zn2(L1)2(OBz)4] (1), [Zn2(L2)2(OBz)4] (2) and [Zn(L1)2(OAc)2] (3) have been employed in the ring-opening polymerization of lactides. The complexes were found to form active catalysts in the polymerization of the D,L-lactide (D,L-LA) and L-lactide (L-LA) monomers. The polymerization kinetics proceeded via pseudo-first order mechanism with respect to the monomer. Low molecular weight polymers were obtained with relatively narrow molecular weight distribution of 1.30-1.50. The ligand architecture, the nature of the carboxylate anion and solvent system influenced the rate of lactide polymerization reactions. Polymerization reactions performed in methanol exhibited higher rates of reaction than those in toluene. The activation parameters measured for lactide polymerization using 3 were obtained as ΔH°=38.4±0.7 kJmol-1, ΔS°=-167.2±1.8 JK-1mol-1 and are consistent with a highly ordered transition state. The polymers produced were mainly atactic and isotactic.

Keywords: ring-opening polymerization; lactides; kinetics; tacticity