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

Abstract

The controlled, ring-opening polymerisation of DL-lactide in supercritical carbon dioxide (scCO₂) 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 ¹H 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 scCO₂ is conducted at temperatures much lower than that required for bulk polymerisation of LA.