Ring-opening polymerization (ROP) of (L,L)-lactide (LA) has been initiated by dibutyltin dimethoxide in supercritical carbon dioxide (SC CO2). Polymerization is controlled and proceeds at quasi the same rate as in toluene, which indicates that the reactivity of the propagating species is not impaired by parasitic carbonation reaction. Random copolymerization of LA with P-caprolactone (CL) has also been studied in sc CO2, and the reactivity ratios have been determined as 5.8 +/-0.5 for LA and 0.7 +/- 0.25 for CL. These values have to be compared to 0.7 +/- 0.25 for LA and 0.15 +/- 0.05 for CL in toluene. Good control on ROP of CL and LA in SC CO2 has been confirmed by the successful synthesis of diblock copolymers by sequential polymerization of CL and LA. (c) 2005 Wiley Periodicals, Inc.