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

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.