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(SiMe) (L = 2-{[benzoxazoly-CHN(R)-]CH}-6-R-4-R-CHO, R = R = Cl, R = Bn (1); R = R = Bu, R = Bn (2); R = trityl, R = Me: R = Bn (3); R = phenethyl (4); R = 3-methylbutyl (7); R = n-hexyl (8); R = cyclopentyl (9); R = cyclooctyl (11); R = 1-adamantyl (12)), was synthesized via the reactions of Zn[N(SiMe)] 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 = 0.40-0.44) and moderately to highly isotactic PLA by complexes 3-12 ( P = 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.