Highly Isoselective and Active Zinc Catalysts for rac-Lactide Polymerization: Effect of Pendant Groups of Aminophenolate Ligands

Highly isoselective and active ring-opening polymerization of rac-lactide to obtain stereocomplexed polylactide is a long-standing challenge. In this contribution, zinc catalysts with good isoselectivities and high activities (Pm = 0.87, TOF = 3312 h–1 at 25 °C; Pm = 0.92, TOF = 117 h–1 at −20 °C, toluene) for rac-lactide polymerization are reported. These catalysts are coordinated by a well-designed chiral oxazolinyl or achiral benzoxazolyl aminophenolate ligand. Both types of zinc complexes afforded multiblock isotactic PLAs, proving to be formed via a chain-end control mechanism regardless of the existence of a chiral group or not. Preliminary studies suggested that the dihedral angle between the phenoxy and oxazolinyl/benzoxazolyl planes in these complexes reflects the steric bulkiness of the ligand and has a close relation with the isoselectivity of the complex, which might serve as a guide in obtaining new isoselective catalysts of similar types. Furthermore, unprecedented combination of excellent activity (TOF up to 44 000 h–1) and sufficient isoselectivity (Pm = 0.80) could be achieved under solvent-free immortal conditions with catalyst loadings as low as 0.005 mol % (vs monomer), which shows the potential of industrial application.