Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ϵ-caprolactone (CL) polymerization was investigated. DBu2Al3Me5 exhibited higher catalytic activity than did the dinuclear aluminum complex LBu2Al2Me4 (16 times as high for CL polymerization; [CL]:[DBu2Al3Me5]:[BnOH] = 100:0.5:5, [DBu2Al3Me5] = 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.
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Chang, Y. (Contributor), Lai, Y. (Contributor), Lo, Y. (Contributor), Wu, K. (Contributor), Chen, H. (Contributor), Li, C. (Contributor), Kosuru, S. R. (Contributor), Lai, F. (Contributor) & Ding, S. (Contributor), Cambridge Crystallographic Data Centre, 2021