Ring-opening polymerization (ROP) in supercritical carbon dioxide (scCO(2)) has been the subject of much recent interest, although few publications describe the development of stabilizers to produce biodegradable particles of poly(L-lactide) (PLLA) and polyglycolide (PGA). Here we describe the synthesis of a series of novel fluorinated diblock copolymers by the acid-catalyzed esterification of well-defined blocks of polycaprolactone (PCL) with Krytox 157FSL, a carboxylic acid terminated perfluoropolyether. These diblock copolymers were then tested as stabilizers in the ROP of glycolide and L-lactide, or a mixture of the two, in scCO(2), and this resulted in the corresponding homopolymers or random copolymers. In the absence of stabilizers, only aggregated solids were formed. When the reaction was repeated with a stabilizer, PGA and PLLA were obtained as discrete microparticles. The stabilizer efficiency increased as the length of the polymer-philic PCL block increased. One optimized stabilizer worked at loadings as low as 3% (w/w) with respect to the monomer, demonstrating these to be extremely effective stabilizers. It was found that to produce microparticles with this process, the product polymers must be semicrystalline; amorphous polymers, such as poly(lactide-co-glycolide), are plasticized by scCO(2) and yield only aggregated solids rather than discrete particles. (c) 2005 Wiley Periodicals, Inc.