rac-Lactide polymerization using aluminum complexes bearing tetradentate phenoxy-amine ligands

A family of aluminum-methyl complexes supported by tetradentate phenoxy-amine ligands has been prepared and employed in the ring-opening polymerization of rac-lactide; the ligands include N,N-bis(3,5-dimethyl-2-hydroxybenyl)-N′,N′-dimethyl-1,2-diaminoethane (L¹), N,N-bis(3,5-diisopropyl-2-hydroxybenyl)-N′,N′-dimethyl-1,2-diaminoethane (L²) and N,N-bis(3,5-dichloro-2-hydroxybenyl)-N′,N′-dimethyl-1,2-diaminoethane (L³). Polymerizations of rac-lactide were carried out by treatment of the aluminum-methyl complexes with PhCH₂OH and rac-lactide at 70 °C, affording well-controlled formation of polylactide (PLA) and a moderate isotactic bias for initiators bearing L¹ and L²; the chloro-substituted ligand L³ afforded largely atactic PLA.