Low-temperature and purification-free stereocontrolled ring-opening polymerisation of lactide in supercritical carbon dioxide

A stereoselective, solvent-free ring-opening polymerisation (ROP) of lactide (LA) in supercritical carbon dioxide (scCO₂) is reported for the first time. The key aim is to exploit scCO₂ to lower the temperature of traditional melt polymerisations, lowering the energy requirement and leading to cleaner polymeric materials. We have utilised a zirconium amine-trisphenolate initiator-stereoselective catalyst [(ⁱPrO)Zr(OPh(^tBu)₂-CH₂)₃N] to yield highly heterotactic poly(lactide) (PLA) homopolymer (P_r = 0.74-0.84) from rac-LA, demonstrating control of the PLA microstructure in scCO₂. In addition, high monomer conversion (86-93%) was achieved in short reaction time (1 h), affording poly(lactide) with a very low degree of transesterification and narrow molecular weight distribution. Most importantly, all the reactions were performed at only 80 °C, almost 100 °C lower than the conventional melt process (typically performed at 130-180 °C), representing a very significant potential energy saving.