Bimetallic constrained aluminum alkoxide complexes for ring-opening polymerization of cyclic esters: activity enhancement via cationic activation

Abstract

Dinuclear aluminum complexes bearing a constrained ‘indanimine’ ligand based on a short hydrazine bridge were synthesized. Single-crystal X-ray crystallography reveals bimetallic penta-coordinated aluminum centers having a distorted trigonal bipyramidal geometry. A short Al–Al distance of 4.1 Å was observed due to a highly rigid framework and a short hydrazine bridging moiety. The bimetallic aluminum isopropoxide complex reacted with N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMFB) giving an activated aluminum species via a proton abstraction by isopropoxide. The activated complex was shown to be significantly more active in the ring-opening polymerization (ROP) of ε-caprolactone (CL) and δ-valerolactone (VL) compared to the original non-activated aluminum alkoxide complex. On the other hand, the ROP of L-lactide (LA) by the activated complex completely shut down the polymerization possibly due to a strong coordination of the adjacent ester group to the cationic aluminum center. This unique behavior of LA was then applied as an inhibitor in the ROP of CL. Therefore, the ongoing ROP of CL was shown to switch off immediately when LA was added (OFF mode). The polymerization may resume only when the temperature is increased to 90 °C (ON mode) giving an example of ON/OFF switchable ring-opening polymerization.