Electron rich salen-AlCl catalysts as efficient initiators for the ring-opening polymerisation of rac-lactide

The ring-opening polymerisation of lactide is a useful means to prepare biodegradable materials with well controlled polymer architectures and bespoke material properties. While homogeneous ligandAl-OR complexes have shown great success in this field, initiation from ligand-AlCl complexes has lagged behind. Here, we report four new salen-AlCl complexes featuring NEt2-substituents, which display high catalyst activities towards rac-lactide ring-opening polymerisation (kobs < 1.9 x 10-3 s-1) via in situ activation with a single equivalent of propylene oxide. Incorporating Lewis basic NEt2 groups into the ligand scaffold not only improves the initiation efficiency but also avoids the need for a Lewis basic co-catalyst and excess epoxide. Notably, studies of our amino-substituted catalysts reveal that the formation of a hexacoordinate aluminate species may hinder rather than enhance catalytic activity.