We describe the radical dispersion polymerisation of methyl methacrylate in supercritical carbon dioxide using a poly(dimethylsiloxane) monomethacrylate as the stabiliser. Initial observations showed that in the absence of efficient stirring, acceptable yields, molecular weight and particle morphology for PMMA were observed. However, efficient stirring of the reaction mixture was found to substantially reduce yield and quality of the resulting PMMA. We speculate that this is caused by radical quenching occurring at the metal wall of the reaction vessel during the reaction. By lowering of the initiator concentration it was found that a coating of high molecular weight PMMA was formed on the exposed internal metal surface of the cell. Subsequent experimental studies showed that the coating acts as a barrier to inhibit radical termination and facilitate the polymerisation of MMA with stirring, We show that consideration must be given to the composition of the metal surface of the autoclave, and that by careful choice of reaction conditions, a durable, and impermeable PMMA coating may be deposited within the autoclave.