Tetracoordinate aluminum complexes bearing phenoxy-based ligands as catalysts for epoxide/anhydride copolymerization: Some mechanistic insights

A series of dimethyl aluminum complexes of the type [E,O]AlMe2 stabilized by phenoxy-imine, phenoxy-thioether and phenoxy-amine ligands have been tested as catalysts for the ring-opening copolymerization of epoxides and anhydrides. All complexes revealed to be active in the copolymerization of cyclohexene oxide with succinic anhydride showing activities higher than pentacoordinate salen aluminum complexes. Interestingly, these complexes revealed to be active in the copolymerization of limonene oxide with phthalic anhydride. A moderate influence of the ligand structure on the efficiency and selectivity of the catalyst was also observed. Some mechanistic insights have been gained through NMR studies allowing the identification of a zwitterionic species formed in the initiating step of the polymerization reaction.