Zinc and magnesium complexes of tetradentate mono-anionic ligands of the {ONNS}-type as catalysts for ring opening polymerization of rac-lactide

During the last three decades the interest in biodegradable plastics has increased. One of the most commonly used biodegradable plastics is polylactic acid (PLA), which is produced by ring opening polymerization (ROP) of lactide (LA), the cyclic dilactone of lactic acid. Typical ROP catalysts are homogeneous metal complexes, preferably based on environmentally-benign and biocompatible metals. Our group has recently introduced zinc- and magnesium-based ROP catalysts based on tetradentate monoanionic {ONNN}-type ligands. These catalysts exhibited a broad range of lactide ROP activities, including a living polymerization character, isoselectivity, and high activity. Herein, we describe our efforts to develop a related family of zinc- and magnesium-based ROP catalysts which include a related sequential {ONNS} ligand wherein the thio-donor is a part of a thiophene ring. The ligands were prepared by a two-step synthesis, including a reductive-amination and nucleophilic substitution. The metal complexes, {ONNS}Zn-Et, {ONNS}Zn(HMDS) and {ONNS}Mg(HMDS) were obtained easily and were characterized by NMR and crystallographic studies, that demonstrated that the thiophene group was not bond to the metal. Lactide ROP structure-activity relationship studies were conducted, and revealed that some of the catalysts were highly activity in polymerization of rac-lactide, and the polymers obtained had narrow molecular weight distributions. Further studies of this interesting group of ROP catalysts are in progress.