The wall effect: how metal/radical inter-actions can affect polymerisations in supercritical carbon dioxide

We describe the radical dispersion polymerisation of methyl methacrylate in supercritical carbon dioxide using a poly(dimethylsiloxane) monomethacryla te as the stabiliser. Initial observations showed that in the absence of ef ficient stirring, acceptable yields, molecular weight and particle morpholo gy for PMMA were observed. However, efficient stirring of the reaction mixt ure was found to substantially reduce yield and quality of the resulting PM MA. 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 i nitiator concentration it was found that a coating of high molecular weight PMMA was formed on the exposed internal metal surface of the cell. Subsequ ent experimental studies showed that the coating acts as a barrier to inhib it radical termination and facilitate the polymerisation of MMA with stirri ng, We show that consideration must be given to the composition of the meta l surface of the autoclave, and that by careful choice of reaction conditio ns, a durable, and impermeable PMMA coating may be deposited within the aut oclave. (C) 1999 Elsevier Science Ltd. All rights reserved.