Free radical polymerisation of methyl methacrylate in supercritical CO2 requires addition of a surfactant (amphipathic macromolecule) to produce poly(methyl methacrylate) (PMMA) in high yield and molar mass and with well-defined particle sizes. This paper describes the use of poly(dimethylsiloxane)monomethacrylate macromonomers in stabilising the free radical polymerisation of methyl methacrylate in supercritical CO2. In particular, the effects of surfactant molar mass (2-10 kD) and concentration on the nature of the product PMMA are examined, It is found that the 2 kD surfactant is the most effective at low (wt./wt.) concentrations and may be a more viable option for industrial production than the higher molar mass surfactants, although the particle sizes of the PMMA products are different.