Generation of miniemulsions (nanoemulsions) comprising hydrophobic vinyl monomers and water by use of the compressed gases carbon dioxide and ethylene, respectively, has been investigated in the absence of high energy mixing. It is proposed that transparent/translucent miniemulsions are formed at the transparency pressure (P T ) as a result of the refractive index of the dispersed phase being reduced due to expansion with carbon dioxide/ethylene, resulting in refractive index matched miniemulsions with enhanced stability. Radical polymerization of methyl acrylate at 40 C in miniemulsions induced by carbon dioxide at P T proceeds as a hybrid miniemulsion/emulsion polymerization system generating particles with diameters less than 100 nm. The experimental data are consistent with particle formation occurring mainly via monomer droplet nucleation (miniemulsion polymerization) but also via secondary nucleation in the aqueous phase (emulsion polymerization). Polymerizations at a range of pressures above and below P T revealed limited pressure effects on the polymerization rate and particle size for both carbon dioxide and ethylene.

Radical polymerization of styrene in aqueous emulsions pressurized by CO2 has been investigated with a view to development of low energy miniemulsion polymerization. The general requirement of high energy mixing to generate miniemulsions comprising sub‐micrometer‐size monomer droplets has long been an impediment to wide‐spread industrial application. It is demonstrated by use of online dynamic light scattering and visual observation that CO2 pressurization to the so‐called transparency pressure of an emulsion comprising styrene/hexadecane/Dowfax 8390 (anionic surfactant)/water leads to a decrease in droplet size as well as an increase in emulsion stability. However, mechanistic investigations of radical polymerization under CO2 pressure reveal that these polymerizations do not proceed as true miniemulsion polymerization systems (i.e., via exclusive monomer droplet nucleation), but are also characterized by a significant contribution of particle formation via secondary nucleation as in an ab initio emulsion polymerization.