Controlled polymerisation of lactide using an organo-catalyst in supercritical carbon dioxide

The controlled, ring-opening polymerisation of DL-lactide in supercritical carbon dioxide (scCO(2)) using benzyl alcohol as an initiator and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an organo-catalyst, is reported. Despite reports of DBU being efficiently converted to a carbonate salt in the presence of a proton source, it was found that DBU was still an efficient catalyst for the ring opening polymerisation of DL-lactide. Matrix-assisted laser desorption ionisation time of flight mass spectroscopy and H-1 nuclear magnetic resonance analysis demonstrated that end-group fidelity was maintained on the resulting polymer and significant transesterification was not observed under anhydrous conditions. We report a truly `green' process for the synthesis of polylactic acid (PLA) with the total absence of potentially toxic organic solvents and inorganic catalysts. In addition, the reaction in scCO(2) is conducted at temperatures much lower than that required for bulk polymerisation of LA.