Herein, we report the emulsion polymerization of a water-soluble vinylic monomer in supercritical carbon dioxide. The water-soluble/CO2-insoluble monomer, N-ethylacrylamide, was effectively emulsified in CO2 continuous phase using an amphiphilic diblock copolymer consisting of a D-glucose-containing glycopolymer and a fluorinated block. The resulting high yield (> 90%) of water-soluble poly(N-ethylacrylamide) was obtained in the form of a stable polymer colloid comprised of submicrometer-sized particles. The particle diameter and the distribution of sizes were shown to be influenced by various reaction parameters. Higher temperature and higher surfactant and initiator concentrations resulted in particles with smaller sizes. Two initiators, 2,2'-azobisisobutyronitrile (AIBN) and a fluorinated azo-derivative, bis-[2-(F-octyl)ethyl]-4,4'-azobis-4-cyanopentanoate, were used, and their thermal decomposition behaviors in supercritical CO2 were investigated. Compared with the AIBN-initiated system, the emulsion polymerizations initiated by the fluorinated azo-initiator showed no polymerized monomer droplets, and the resultant particles were more uniform.