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Experimental results of a turbidimetric in situ investigation of the heterogeneous radical polymerization (dispersion polymerization) of methyl methacrylate and polydimethylsiloxane-monomethacrylate in supercritical carbon dioxide are presented. The experiments were carried out at 60°C and 330 bar. Turbidity spectra were measured directly in the autoclave to determine the average particle diameter s t and the particle density N AV /V during the first five minutes of polymerization. The results show that the particle density increased until a maximum was achieved, whereas the particle diameter increased during the first stage. Moreover, a comparison with kinetic data suggests that the particles nucleate through coagulation of polymer that is formed in the homogeneous phase.