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 (scCO(2)) is reported for the first time. The key aim is to exploit scCO(2) 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 [((PrO)-Pr-i)Zr(OPh(Bu-t)(2)-CH2)(3)N] to yield highly heterotactic poly(lactide) (PLA) homopolymer (P-r = 0.74-0.84) from rac-LA, demonstrating control of the PLA microstructure in scCO(2). 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 degrees C, almost 100 degrees C lower than the conventional melt process (typically performed at 130-180 degrees C), representing a very significant potential energy saving.