Ring-Opening Polymerization of ε-Caprolactone and Styrene Oxide–CO(2) Coupling Reactions Catalyzed by Chelated Dehydroacetic Acid–Imine Aluminum Complexes

A series of chelated dehydroacetic acid–imine-based ligands L(1)H~L(4)H was synthesized by reacting dehydroacetic acid with 2-t-butylaniline, (S)-1-phenyl-ethylamine, 4-methoxylbenzylamine, and 2-(aminoethyl)pyridine, respectively, in moderate yields. Ligands L(1)H~L(4)H reacted with AlMe(3) in toluene to afford corresponding compounds AlMe(2)L(1) (1), AlMe(2)L(2) (2), AlMe(2)L(3) (3), and AlMe(2)L(4) (4). All the ligands and aluminum compounds were characterized by IR spectra, (1)H and (13)C NMR spectroscopy. Additionally, the ligands L(1)H~L(4)H and corresponding aluminum derivatives 1, 3, and 4 were characterized by single-crystal X-ray diffractometry. The catalytic activities using these aluminum compounds as catalysts for the ε-caprolactone ring-opening polymerization (ROP) and styrene oxide-CO(2) coupling reactions were studied. The results show that increases in the reaction temperature and selective solvent intensify the conversions of ε-caprolactone to polycaprolactone. Regarding the coupling reactions of styrene oxide and CO(2), the conversion rate is over 90% for a period of 12 h at 90 °C. This strategy dispenses the origination of cyclic styrene carbonates, which is an appealing concern because of the transformation of CO(2) into an inexpensive, renewable and easy excess carbon feedstock.