Coordinating effect in ring-opening polymerization of ε-caprolactone using aluminum complexes bearing bisphenolate as catalysts

Abstract

A series of Al complexes bearing diphenolate ligands was synthesized and their application for the ring-opening polymerization of ε-caprolactone was studied. Positional variation of the substituent on the aryl ring of the RC*H(4,6-di-t-butylphenol)₂ ligand was shown to have a considerable influence on the catalysis result. Complexes with a ortho-substituent showed greater catalytic activity than those with a para-substituent. Substitutions of an aryl moiety by H or methyl groups resulted in the catalytic activity falling between that of the ortho-substitution Al complexes and that of the para ones. Our results demonstrate that the coordinated functional group in the ortho-position of the phenyl ring could increase the catalytic activity. Moreover, X-rays of the structure and DFT analysis revealed that the coordinated functional group in the ortho-position could bridge two Al centers resulting in the transformation of a dinuclear Al complex with bridging benzyl alkoxide into a complex with terminal benzyl alkoxide, further promoting the efficacy of the initiator.