Aluminium salophen and salen initiators in the ring-opening polymerisation of rac-lactide and rac-β-butyrolactone: Electronic effects on stereoselectivity and polymerisation rates

• Four new aluminium salen and salophen complexes.
• Electronic variation of ligand backbone.
• Changes in polymerisation rates and tacticity in lactide and β-butyrolactone polymerisation.

Three aluminium salophen and two aluminium salen complexes were synthesised, characterised and screened in the ring-opening polymerisation (ROP) of rac-lactide and rac-β-butyrolactone. The focus was on controlling the apparent polymerisation rate (kp) and stereoselectivity of poly(lactic acid) and poly(3-hydroxybutyrate) by modulating the electron density at the aluminium centre or by switching from an alkyl backbone (salen complex) to an aryl backbone (salophen complex). The salen complexes generally showed higher kp as well as isoselectivity compared to the salophen complexes. For instance, salophen and salen complexes biased the microstructure of poly(3-hydroxybutyrate) towards syndiotacticity and isotacticity, respectively. Electron-withdrawing or electron-donating backbones on a salophen complex tuned kp, with electron-donating backbones offering faster kp.

Four new aluminium alkyl complexes supported by salen and salophen ligands are prepared, characterised and tested for activity in the ring-opening polymerisation of cyclic esters. Variation of the ligand electronics accessed differences in the stereoselectivity of β-butyrolactone and rac-lactide polymerisation as well as changes to polymerisation rates in these.Figure optionsDownload as PowerPoint slide