Well-defined polyurethane-polydimethylsiloxane particles of tunable diameter in the range of 0.5-20 mu m were synthesized in "one-shot" by step-growth polymerization using supercritical carbon dioxide (scCO(2)) as a dispersant medium. Polymerizations were carried out at 60 degrees C and above 25 MPa, after the solubility of each reactant in scCO(2) has been determined in its typical reaction concentration. The synthesis of such copolymers was achieved by polyaddition between short aliphatic diols, that is, ethylene glycol, 1,4-butanediol (BD) or polyethylene oxide (M-n = 200 g mol(-1)), and tolylene-1,4-di-isocyanate (TDI) in the presence of mono or di-isocyanate-terminated polydimethylsiloxane (PDMS) as reactive stabilizers and dibutyltin dilaurate as a catalyst. The nature of the diol used as well as the functionality of the reactive stabilizer incorporated was found to have a dramatic effect on the molar mass and the morphology of the resulting product. Thus, copolymers obtained from the polyaddition of BD and TDI in the presence of di-isocyanate-terminated PDMS exhibit molar mass up to 90,000 g mol-1. Thermal behaviors of copolymers were also examined by differential scanning calorimetry. All samples exhibited only one glass transition temperature (T-g) and were found to be totally amorphous. A logical decrease of the T-g was observed as the length of the diol incorporated increased, that is, as the density of urethane linkages within the polymer decreased. (c) 2007 Wiley Periodicals, Inc.