Polymerization of Lactides and Lactones by Metal-Free Initiators

Abstract

New metal-free and thermally stable initiators as hexaalkylguanidinium salt (HAGs) derivatives have been investigated for the polymerization of lactones. Polymerization occurs smoothly at low temperature (120 °C) in bulk and has been determined to be dependent on the nature of the anionic counterions. Among the various HAG salts investigated, the bromide derivative offers the highest activity (of the same order of magnitude as that of the tin(II) octanoate catalyst). By mixing HAG bromide (HAG,Br) and sodium carboxylate salts, the catalytic activity increased, likely because of improved solubilization of the catalyst in the polymeric medium. Although a weak molecular weight was obtained, the polymerization process was determined to be efficient, in term of the consumption of any lactone monomers, making the process promising. For example, (3S)-l-lactide polymerizes smoothly in the presence of a mixture of HAG,Br and sodium carboxylate with 65% enantioselective excess.