The catalytic chain transfer polymerisation of methyl methacrylate expanded with dense CO2 is reported. Experimental values of the chain transfer constant (C-s) are presented at 50 degrees C and in the range of pressure from 0.1 to 6 MPa, using a cobaloxime complex as the chain transfer catalyst. The effect of small quantities of poly(methyl methacrylate) on the volumetric expansion of the monomer with CO2 is considered. Experimental data are presented on the pressure limit for homogeneous expansion as a function of the molecular weight of the polymer. It is shown that the values of C-s in CO2-expanded methyl methacrylate are significantly higher than that in bulk monomer. This effect is mainly attributed to the enhancement of the chain transfer rate coefficient, arising from the reduction in the viscosity of the medium, and is consistent with a diffusion-controlled rate-determining step in the chain transfer process. (c) 2006 Elsevier B.V. All fights reserved.