Aluminum complexes containing salicylbenzoxazole ligands and their application in the ring-opening polymerization of rac-lactide and ε-caprolactone

Abstract

Two series of four-coordinate aluminum (1a–9a) and five-coordinate aluminum (1b–9b) complexes were successfully synthesized via the reactions between the corresponding salicylbenzoxazole ligands and 1 or 0.5 equivalents of AlMe₃, respectively. The synthesized aluminum complexes were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis. The solid-state structures of complexes 7a and 1b were determined using single crystal X-ray diffraction. Upon addition of 1 equivalent of benzyl alcohol, all complexes were efficient initiators for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) and ε-caprolactone (ε-CL). The polymerizations were living with a good control over molecular weights and molecular weight distributions. Under immortal polymerization conditions, all four-coordinate aluminum complexes (1b–9b) exhibited a living polymerization with the obtained molecular weights proportional to the ratio of monomer/benzyl alcohol and the PDIs were narrow. Kinetic studies revealed that both rac-LA and ε-CL polymerizations mediated by all complexes were first-order in monomers. The effects of ligand structure and coordination geometry on the catalytic activity and stereoselectivity were discussed. A good isoselectivity control was achieved for the polymerizations mediated by complexes 4b (P_m = 0.75), 5b (P_m = 0.74), and 9b (P_m = 0.74).