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, [LH 3 ] , 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 ( k obs =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 [LNa 3 ] being extremely active ( k obs =1.21 min −1 ) but displaying unusual second‐order monomer dependency and poor polymerisation control. 1 H NMR spectroscopic studies suggest that polymerisation with [LNaAlMe] or [LH 2 Na] /BnOH follows an activated monomer mechanism, whereas [LNa 3 ] 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.