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

Abstract

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. <b>D</b>^<b>Bu</b>_<b>2</b><b>Al</b>_<b>3</b><b>Me</b>_<b>5</b> exhibited higher catalytic activity than did the dinuclear aluminum complex <b>L</b>^<b>Bu</b>_<b>2</b><b>Al</b>_<b>2</b><b>Me</b>_<b>4</b> (16 times as high for CL polymerization; [CL]:[<b>D</b>^<b>Bu</b>_<b>2</b><b>Al</b>_<b>3</b><b>Me</b>_<b>5</b>]:[BnOH] = 100:0.5:5, [<b>D</b>^<b>Bu</b>_<b>2</b><b>Al</b>_<b>3</b><b>Me</b>_<b>5</b>] = 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 <i>tert</i>-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.