Zinc complexes of tetradentate mono-anionic ligands as catalysts for ROP of rac-lactide

In recent years the interest in biodegradable plastics has increased significantly. Polylactic acid (PLA) is one of the most commonly used biodegradable plastics, which is obtained by the catalyzed ring opening polymerization (ROP) of lactide (LA) - the cyclic dilactone of lactic acid. Metal complexes are favored over other types of catalysts for this process. Recently, we developed a new family of catalysts based on tetradentate monoanionic {ONNN}-type ligands coordinated to zinc metal. The ligands were obtained by straightforward synthetic steps including condensation and substitution. The zinc complexes were readily obtained and were characterized by spectroscopic and crystallographic studies enabling the investigation of structure–activity relationships in polymerization of lactide. High activity in polymerization of L-lactide and rac-lactide was exhibited in some of these complexes, and the polymers obtained featured narrow molecular weight distributions, consistent with controlled/living polymerization. The PLA obtained from the polymerization of rac-lactide showed a very high isotactic inclination, uncommon for zinc-based complexes. Further studies of this promising family of ROP catalysts are under way.