Molecular weight evolution in catalytic chain transfer polymerization of methyl methacrylate expanded with dense CO2 is reported. Experimental molecular weight and polydispersity index data are presented at 50 degrees C in the range of conversion from 1 to 25%, and at pressures of 5 and 6 MPa. A cobaloxime complex is used as the chain transfer catalyst. Both molecular weight and polydispersity increase in the range of conversion achieved at conditions below the homogeneous expansion limit. Predici simulations suggest that either irreversible catalyst deactivation or cobalt-carbon bond formation, between the catalyst and the propagating radicals, is the most likely mechanism for the increase in molecular weight with conversion. At conditions above the homogeneous expansion limit, a bimodal molecular weight distribution is observed, indicating two zones of polymerization. These conditions produce relatively high molecular weight macromonomers with broad molecular weight distributions.