Electron rich (salen)AlCl catalysts for lactide polymerisation: Investigation of the influence of regioisomers on the rate and initiation efficiency

Aluminium-alkyl complexes are well known as initiators for lactide ring-opening polymerisation, yet aluminium-chloride complexes remain underexplored despite benefits such as ease of synthesis and improved air-stability. While aluminium-chloride complexes are typically poor initiators, recent studies have shown that electron rich amino-substituted (salen)AlCl complexes can efficiently initiate lactide polymerisation in the presence of an epoxide. Herein, we report eight ether-substituted complexes as efficient initiators for lactide polymerisation, where exchanging strongly electron-donating amino groups for weaker electron-donating methoxy substituents maintains efficient initiation and also improves the propagation rate by a factor of four. Investigation of ortho-, meta-, para- and meta’-methoxy-substituted regioisomers established that the ortho-substituted complex was twice as active as the other regioisomers. Kinetic and spectroscopic studies suggest that the initiation efficiency is influenced by the electronics (ortho and para > meta and meta’), with substituents closer to Al giving improved initiation (ortho > para and meta’ > meta). While electron-donating ortho-substituents often decrease catalyst activity in lactone polymerisation, here we show that ether groups can act as σ-electron-withdrawing groups and π-electron donors, to deliver improved propagation rates, initiation and tacticity control.