Ring-Opening Polymerization of ε-Caprolactone by Using Aluminum Complexes Bearing Aryl Thioether Phenolates: Labile Thioether Chelation

Abstract

In this study, aluminum complexes bearing ferrocene-based and arylthiomethylphenolate ligands were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. The catalytic activity of the reduced form of Al complexes was higher than that of the oxidized form. The CL polymerization rate of the reduced form <b>fcO</b>_<b>2</b><b>AlMe</b> (75 min, conversion = 100%) was higher than that of the oxidized form <b>fc</b>^<b>ox</b><b>O</b>_<b>2</b><b>AlMe</b> (4320 min, conversion = 45%), and the CL polymerization rate of <b>fc(OAlMe</b>_<b>2</b><b>)</b>_<b>2</b> (40 min, conversion = 100%) was higher than that of <b>fc</b>^<b>ox</b><b>(OAlMe</b>_<b>2</b><b>)</b>_<b>2</b> (60 min, conversion = 97%). Electron deficiency substituents on phenolate decreased the catalytic activity of Al complexes bearing arylthiomethylphenolate ligands. Density functional theory calculations revealed that thioether coordination stabilized the transition state (<b>TS1</b>) and that the oxidized form <b>fc</b>^<b>ox</b><b>(OAlMe</b>_<b>2</b><b>)</b>_<b>2</b> exhibited weaker thioether coordination and higher activation energy in <b>TS1</b> compared with those of the reduced form <b>fcO</b>_<b>2</b><b>AlMe</b>. In addition, our study determined that the thioether group is a suitable chelating group for Al catalysts in CL polymerization due to its labile nature.