Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. D Bu 2 Al 3 Me 5 exhibited higher catalytic activity than did the dinuclear aluminum complex L Bu 2 Al 2 Me 4 (16 times as high for CL polymerization; [CL]:[ D Bu 2 Al 3 Me 5 ]:[BnOH] = 100:0.5:5, [ D Bu 2 Al 3 Me 5 ] = 10 mM, conversion 93% after 18 min at room temperature). Density functional theory calculations revealed a polymerization mechanism in which CL first approached the central Al atom and then moved to an external Al. The coordinated CL ring was opened because the repulsion of two tert -butyl groups on the ligands pushed an alkoxide initiator on an external Al to initiate CL. In these trinuclear Al catalysts, the central Al plays a role in monomer capture and then collaborates with the external Al to activate CL, accelerating polymerization.