Exploring Steric Effects of Zinc Complexes Bearing Achiral Benzoxazolyl Aminophenolate Ligands in Isoselective Polymerization of rac-Lactide

A series of tridentate achiral benzoxazolyl-based aminophenolate zinc complexes, LZnN­(SiMe3)2 (L = 2-{[benzoxazoly-CH2N­(R3)-]­CH2}-6-R1-4-R2-C6H2O, R1 = R2 = Cl, R3 = Bn (1); R1 = R2 = t Bu, R3 = Bn (2); R1 = trityl, R2 = Me: R3 = Bn (3); R3 = phenethyl (4); R3 = 3-methylbutyl (7); R3 = n-hexyl (8); R3 = cyclopentyl (9); R3 = cyclooctyl (11); R3 = 1-adamantyl (12)), was synthesized via the reactions of Zn­[N­(SiMe3)2]2 and 1 equiv of the corresponding aminophenol proligands. All of the complexes were obtained as racemates, and the X-ray diffraction studies confirmed the monomeric structures of typical complexes 11 and 12, where the metal center is tetra-coordinated by three donors of the aminophenolate ligand and one silylamido group. All of the complexes proved to be efficient initiators for the ring-opening polymerization of rac-lactide (rac-LA) at ambient temperature, and the polymerizations were better controlled in the presence of 2-propanol. The substituents on the ortho-position of the phenoxide unit of the ligand and the skeleton nitrogen atom show significant influences on the stereoselectivity of the corresponding complex toward the polymerization of rac-LA, leading to the production of heterotactic biased polylactide (PLA) by complexes 1 and 2 (P m = 0.40–0.44) and moderately to highly isotactic PLA by complexes 3–12 (P m = 0.74–0.89). Detailed mechanism studies and microstructure analysis of typical PLA samples revealed that these zinc initiators afforded isotactic stereoblock PLAs via a chain-end control mechanism, and there is no obvious polymer exchange process during the polymerization process.