Aluminium complexes supported by a thioether-bridged salen ligand: synthesis, characterization and application in ε-caprolactone homopolymerization and copolymerization with L-lactide

Abstract

Several aluminium alkyl and alkyloxide complexes supported by a thioether-bridged salen ligand were synthesized by the protonolysis reaction. Reactions of N,N’-bis(3,5-di-tert-butylsalicylidene)-2,2’-diaminophenylsulfide (H2L) with one or two equivalents of AlMe3 in toluene at 100 °C afforded the corresponding mono- and binuclear aluminium methyl complexes LAlMe (1) and L(AlMe2)2 (2). Treatment of the in situ generated complex 1 with one equivalent of ROH (R = PhCH2, iPr, CH3OCH2CH2) produced the desired aluminium alkyloxide complexes LAlOCH2Ph (3), LAl(OiPr) (4) and LAlO(CH2)2OCH3 (5). All the molecular structures were established by single crystal X-ray diffraction studies. It was found that aluminium alkyloxide complexes 3-5 showed higher catalytic activity for the homopolymerization of ε-caprolactone (ε-CL) than corresponding aluminium methyl complexes 1 and 2. Furthermore, these alkyloxide complexes were also efficient initiators for the copolymerization of L-lactide and ε-CL to give practically random copolymers.