During the past three decades, polymerization reactions in supercritical fluids have seen an unprecedented scientific interest due to the necessity to substitute the currently used harmful organic solvents with more benign substances in the quest for more sustainable solutions. One of the most studied reactions is the free-radical dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide (scCO₂). During this polymerization reaction it has been observed that the pressure varies in a peculiar manner resulting in either positive or negative pressure differences during the reaction depending on the conditions.

In this study a set of experiments has been designed and carried out to separate the effect of pressure from other parameters affecting the outcome, such as the monomer-to-solvent ratio or the monomer-to-stabilizer ratio. The range of the conditions examined, as far as the formed dispersion is concerned, vary between marginally stable dispersions to well dispersed systems. The previously established technique of reaction calorimetry for supercritical fluids is used to monitor the heat released by the reaction, thus its evolution and combine it with the pressure variations.

At first, an observed reaction deceleration is investigated, which appears under less stable dispersion conditions. Experimental results show that very small pressure changes have dramatic effects on the reaction evolution. Using the reaction calorimetry technique, the coagulative nucleation stage is clearly identified and its heat release rate is quantified for the first time. Additionally, based on the experimental data and previously reported modeling results, a theoretical explanation is constructed that gives an engineering insight of the observed reaction deceleration.

This theoretical approach helps also explain the observed pressure variation during the reactions under different conditions. Although in this polymerization the net volume contraction is approximately 20% the pressure does not always decrease. In fact, in most of the cases the final pressure is higher than the initial one. Two important parameters that explain this behavior are identified and are the negative nonideal mixing volume of MMA with CO₂ and of PMMA with CO₂ and the monomer partitioning between the two phase, namely the CO₂-rich phase and the polymer-rich phase.

Last but not least, the heat transfer efficiency under varying reaction conditions is monitored. Results show that even for significantly higher pressures the released reaction heat rate remains in a relatively constant level, whereas the overall heat transfer coefficient increases. As a result the reaction conditions become more safe.