Preparation and Characterization of Aluminum Alkoxides Coordinated on salen-Type Ligands: Highly Stereoselective Ring-Opening Polymerization of rac-Lactide

A series of salen-type ligands (L 1 H 2 –L 6 H 2 ) with sterically bulky cumyl groups have been synthesized. Reaction of these ligands with AlMe 3 yields the mononuclear aluminum complexes [LAlMe] ( 1 – 3 ) or dinuclear species [L 2 Al 2 Me 4 ] ( 4 – 6 ), respectively. Further reaction of [LAlMe] ( 1 – 3 ) with benzyl alcohol produces [LAl(OBn)] ( 1a – 3a ), respectively. Solid-state structural studies reveal that complexes 1a and 2a are mononuclear; however, complex 6 is a dinuclear species. Aluminum alkoxides 1a – 3a are highly stereoselective in the ROP of rac -lactide, producing polylactide (PLA) with 94–97% enantiomeric selectivity ( P m ) at high conversion. Their high enantioselectivity leads to PLA with high T m (205 °C). The polymerization of l -lactide by these complexes also shows good living features with narrow PDI values ( M w / M n = 1.06–1.25) signaling less or no transesterification, which can be further verified by MALDI-TOF mass spectrometric analysis.