Block copolymers based on siloxanes and fluorinated block copolymers were synthesized and tested as surfactants for nitroxide mediated polymerizations (NMP) in supercritical carbon dioxide. Styrene was polymerized in the presence of TEMPO at 130 °C using various surfactants. Siloxane-based surfactants were synthesized by a new route based on NMP; the perfluorinated surfactants were also synthesized by NMP. The dispersion polymerizations of styrene in scCO2 reveal conversions of up to 35% in 4 hours at 19.1 MPa. The resulting polydispersity indices lie in the range 1.2-1.4. The best results are obtained using a perfluorinated surfactant. Preliminary copolymerization experiments of the polystyrene macroinitiator in scCO 2 have shown successful chain-extension with butyl acrylate (BuA) resulting in the formation of a poly (St-b-BuA) block copolymer.