Precipitation polymerization of methyl methacrylate (MMA) in tetrahydrofuran (THF) with compressed CO2 as an antisolvent was carried out at 335 K up to 8.5 MPa, and the decomposition rate of the initiator, 2,2'-azobisisobutyronitrile (AIBN), was studied with UV/vis. Gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) were used to determine the average molecular weights and glass transition temperature (T-g), and scanning electron micrography (SEM) was used to observe the morphologies of the product. The results showed that the average molecular weights of the polymer vary considerably with the volume expansion of the solution induced by compressed CO2. (C) 2003 Wiley Periodicals, Inc.