Mechanistic Aspects of Aqueous Heterogeneous Radical Polymerization of Styrene under Compressed CO₂

Abstract

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.

Polymerization‐Induced Self‐Assembly under Compressed CO2: Control of Morphology Using a CO2‐Responsive MacroRAFT Agent

“…Environmentally friendly CO 2 has attracted significant interest over the years as a polymerization medium (supercritical CO 2 in dispersion/precipitation polymerizations) as well as a convenient means of generating miniemulsions . We have previously reported that CO 2 can be used to tune the particle morphology in dispersion PISA of styrene using a 4‐vinylpyridine macroRAFT agent in isopropanol and ethanol/water .…”

“…[18,21,32,33] Environmentally friendly CO 2 has attracted significant interest over the years as a polymerization medium (supercritical CO 2 in dispersion/precipitation polymerizations) [34][35][36] as well as a convenient means of generating miniemulsions. [37][38][39] We have previously reported that CO 2 can be used to tune the particle morphology in dispersion PISA of styrene using a 4-vinylpyridine macroRAFT agent in isopropanol and ethanol/ water. [40] In the presence of CO 2 (8 MPa), the morphologies shifted toward structures of higher interfacial core/corona curvature (i.e., a shift from vesicles to worms to spheres) relative to the case without CO 2 .We have recently reported a convenient strategy for modulating the coronal charge density in aqueous dispersion PISA of 2-hydroxypropyl methacrylate (HPMA) using a statistical copolymer of N,N-diethylaminoethyl methacrylate (DEAEMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA, M n = 475 g mol −1 ) as the stabilizer block.…”

Water and Carbon Dioxide: A Unique Solvent for the Catalytic Polymerization of Ethylene in Miniemulsion

“…Indeed, although the same amount of surfactant has been used for all experiments, the smallest nanoparticles are obtained when the reaction was conducted at P t (Δ P t =0, Table , entries 3, 5, and 8), whereas larger nanoparticles were observed if the pressure was above or below P t (Figure A). As detailed in our previous publications, it is proposed that the smallest particle size is obtained at P t as a result of (i) matching RIs leading to minimization of attractive van der Waals forces between droplets, and (ii) expansion of the dispersed phase with CO 2 causing a reduction in droplet viscosity, which in turn facilitates mechanically induced (stirring) fission of droplets. Based on the presumed miniemulsion‐type mechanism, the number of droplets/particles would ideally remain constant throughout the polymerization.…”

“…Inspired by the pioneering work of Zhang et al, it has been recently reported that applying CO 2 pressure to an oil‐in‐water emulsion can be an efficient and simple means of controlling the droplet size in a miniemulsion . Miniemulsion generation (i.e.…”

“…Inspiredb yt he pioneering work of Zhang et al, [29] it has been recently reported that applying CO 2 pressure to an oil-inwater emulsion can be an efficient and simple means of controlling the droplets ize in am iniemulsion. [30][31][32] Miniemulsion generation (i.e. formation of sub-micron-size droplets) can occur in macroemulsions under compressed CO 2 in the absence of high-energym ixing( e.g.…”