Batch free radical polymerization of 1-vinyl-2-pyrrolidone (VP) in super-critical carbon dioxide (scCO(2)) was studied in the presence of a reactive polysiloxane surfactant (PDMS-mMA). Phase behavior investigation showed that when the initial concentration of the surface active macromonomer was higher than 2.5% w/w with respect to the monomer, the reaction mixture, in the absence of efficient stirring, was initially opaque to the visible light, and it slowly turned to an orange tint. Polymerization experiments carried out with surfactant concentration higher than the aforementioned value proceeded with a fast kinetics, and led to the formation of spherical nanoparticles with almost quantitative yields (higher than 98% with a reaction time lower than 70 min). The effect of the concentration of the surface active macromonomer, the initiator and the monomer, and of the density of the fluid phase on the kinetics of the process and on the morphology of the particles was investigated. A marked decrease of the number-average diameter of the polymer particles with the surfactant concentration was obtained without any particle agglomeration. A dependence on [Initiator] (-0.16) of the particle diameter was observed. Such scaling law exhibits an exponent higher than any previously reported for dispersion processes and rather close to those foreseeable on the basis of Smith-Harkins kinetics for emulsion polymerization. Collected experimental results strongly suggest that the polymerization of VP in the presence of PDMS-mMA could proceed with a nucleation mechanism different from that postulated in pure dispersion polymerization stabilized by graft-forming surfactants. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 173-185, 2004.