Using glycidyl methacrylate-linked poly(dimethylsiloxane), methyl methacrylate was polymerized in supercritical $CO_2$. The effects of $CO_2$ pressure, reaction time, and mixing on the yield, molecular weight, and molecular weight distribution (MWD) of the poly(methyl methacrylate) (PMMA) products were investigated. The shape, number average particle diameter, and particle size distribution (PSD) of the PMMA were characterized. Between 69 and 483 bar, the yield and molar mass of the PMMA products showed a trend of increasing with increasing $CO_2$ pressure. However, the yield leveled off at around 345 bar and the particle diameter of the PMMA increased until the pressure reached 345 bar and decreased thereafter. With increasing pressure, MWD became more uniform while PSD was unaffected. As the reaction time was extended at 207 bar, the particle diameter of PMMA decreased at $0.48{pm}0.03%$ AIBN, but increased at 0.25% AIBN. Mixing the reactant mixture increased the PMMA yield by 18.6% and 9.3% at 138 and 207 bar, respectively.