Supercritical fluids have come to the fore as useful potential alternatives to volatile organic solvents. They have the clear advantage over VOCs of not being flammable nor toxic and of being isolable from product simply through release of the pressure so that they revert to the gas phase.

UK chemists have now used 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in supercritical CO₂ to carry out a ring-opening polymerisation of DL-lactide to polylactic acid. This “green” process avoids toxic solvent and expensive metal catalysts and
occurs at a lower temperature than industrial polymerization.

• Controlled polymerization of lactide using an organo-catalyst insupercritical carbon dioxide,
  Idriss Blakey, Anguang Yu, Steven M. Howdle, Andrew K. Whittaker, Kristofer J. Thurecht
  Green Chem. 2011.
  DOI: 10.1039/C1GC15344G