The effect of pressure ranging from ambient atmosphere to 28.5 MPa on the free radical polymerizations of methyl methacrylate (MMA) in carbon dioxide (CO2) was investigated and discussed. The poly (methyl methacrylate) (PMMA) with high molecular weight was synthesized at quite high conversion of MMA in the polymerization at or below 9.2 MPa, as compared to those polymerized under 11.8-28.5 MPa. A phase transition behavior of MMA-CO2 binary mixture from homogeneous state to vapor-liquid equilibrium (VLE) state was observed below 10.51 MPa. In such a VLE system, almost all MMA was found to exist in the liquid phase with higher concentration than that in homogenous system. Thus, the fast polymerization rate of MMA and high molecular weight of PMMA could be related to the VLE state of MMA/CO2 under low pressure. Similar phenomena were also observed in the polymerization systems of styrene and vinyl acetate in CO2, respectively. (C) 2008 Wiley Periodicals, Inc.