N-heterocyclic ligand optimization for aluminum complexes in ε-caprolactone and L-Lactide polymerization

Abstract A series of aluminum complexes bearing heterocyclic amine ligands were synthesized, and their application in e-caprolactone (CL) and L-Lactide (LA) polymerization and copolymerization were studied. The CL polymerization results revealed that ligands containing the higher electronegativity atom increased the polymerization rate of CL polymerization. Among these Al complexes, NON-AlMe3 bearing benzofurazan ligand was the best catalyst in CL polymerization ([NON-AlMe3] = 10 mM, conversion = 94% after 4 min at room temperature). In LA polymerization, the catalytic trend of Al complexes was random, and NCN-AlMe3 bearing 1-benzylbenzimidazole ligand was the best catalyst ([NCN-AlMe3] = 10 mM, conversion = 90% after 45 min at 80 °C). The selectivity of the production of a PLA-gradual-PCL copolymer could be controlled by different Al complexes. Our study revealed that the four-coordinated trimethyl Al complexes bearing heterocyclic amine ligands were easily synthesized and constitute an ideal catalyst design in CL polymerization.