Polyurethane-polydimethylsiloxane particles of tuneable diameter in the range 0.5-5 mum, have been synthesized by step-growth polymerization, both in cyclohexane and in supercritical carbon dioxide (scCO(2)) used as dispersant media. The feasibility of the polyaddition between ethylene glycol and tolylene-2,4-diisocyanate, selected as monomers, was first demonstrated in cyclohexane, in the presence of hydroxy- or isocyanate-terminated polydimethylsiloxane used as a surfmer. The polymerizations were then carried out in scCO(2) at 60 degrees'C and above 200 bar, after the solubility of each reactant has been determined in scCO(2), at various pressures and temperatures. Both in cyclohexane and in scCO(2), the reaction extent was determined by means of FTIR spectroscopy. The structure and the morphology of the resulting powdery polyurethane-polydimethylsiloxane 'core-shell' material were confirmed by NMR, SEC, TEM and SEM techniques. (C) 2004 Elsevier Ltd. All rights reserved.