Abstract In radical polymerization using a batch reactor, material concentrations and hot spots often appear in the reactor, which may have a bad influence on the polymer properties, and control of a reaction may become impossible, depending on the degree of the hot spots. Recently, supercritical carbon dioxide (SC- CO 2 ) has been attracting attention as a solvent for various reactions, not just as a conventional extraction solvent. The advantage of SC- CO 2 in polymerization is that the polymer can be readily separated from the reaction mixture as drops or particles in SC- CO 2 . Therefore, it can be expected that the viscosity of the reaction mixture is low and that mixing is promoted. In this study, the following research was done on polybutylacrylate polymerization, in which it is known that heat generation and viscosity are high. At first, the polymerization rate and the molecular weight distribution of the polymer were investigated under varying conditions. In order to clarify the reaction mechanism, the relation between the phase separation state in the reactor and the polymerization was also investigated. Furthermore a step polymerization process was proposed based on this knowledge, and preliminary tests on polybutylacrylate were performed.