Insights into the interaction between bis(aryloxide)alkylaluminum and N-heterocyclic carbene: from an abnormal Lewis adduct to a frustrated Lewis pair for efficient polymerizations of biomass-derived acrylic monomers

Abstract

This contribution presents the development of a general Lewis pair (LP) catalyst for efficient and/or controlled polymerizations of inert biomass-derived acrylic monomers, including methyl crotonate (MC), (E,E)-methyl sorbate (MS), and β-angelica (β-AL). Through a comprehensive study on the interaction between bis(aryloxide)alkylaluminum Lewis acids (LAs) and N-heterocyclic carbene (NHC) Lewis bases (LBs), a new frustrated Lewis pair (FLP) has been constructed which comprises MeAl(BHT)₂ (BHT: 2,6-di-tert-butyl-4-methylphenoxide) and 1,3-di-tert-butyl-4,5-dimethylimidazol-2-ylidene (Me-I^tBu). Such a FLP can mediate efficient polymerizations of MS, MC, and β-AL regardless of the addition sequence, on account of its stability without the formation of abnormal Lewis adducts and the noninteracting FLP feature that enables sufficiently “free” LAs and LBs with suitable steric hindrance for catalysis. Moreover, a high degree of control over the polymerization of MS has also been achieved using the MeAl(BHT)₂/Me-I^tBu FLP, affording PMSs with high M_n up to 600.3 kg mol⁻¹. An exclusive initiation via a basic mechanism in the polymerization of MS has been revealed, leading to the formation of linear PMSs with a unique conjugated diene chain end that brought about significantly enhanced thermal stability of the resultant PMSs.