Incorporating Sodium to Boost the Activity of Aluminium TrenSal Complexes towards rac‐Lactide Polymerisation

Four novel homo- and heterometallic sodium and/or aluminium complexes based on the TrenSal ligand, [LH3], have been synthesised and fully characterised, including by single-crystal X-ray diffraction experiments. While [LAl] was completely inactive towards rac-lactide ring-opening polymerisation, incorporating sodium to form heterometallic [LNaAlMe] changes the aluminium geometry from octahedral to tetrahedral, leading to good catalytic activity in the presence of co-initiator BnOH (kobs=3.19×10−2 min−1; room temperature, toluene solvent) and good polymerisation control. Under identical conditions, homometallic sodium complexes showed higher activities in rac-lactide polymerisation than [LNaAlMe], with [LNa3] being extremely active (kobs=1.21 min−1) but displaying unusual second-order monomer dependency and poor polymerisation control. 1H NMR spectroscopic studies suggest that polymerisation with [LNaAlMe] or [LH2Na]/BnOH follows an activated monomer mechanism, whereas [LNa3] operates via simultaneous coordination-insertion and activated monomer mechanisms. Overall, heterometallic [LNaAlMe] provides a balance of good activity and control compared to the homometallic analogues.