Emulsion Polymerization in Carbon Dioxide

Beckman, E.). (2005) Inverse emulsion polymerization in carbon dioxide, In Supercritical Carbon Dioxide, (eds M.F. Kemmere and T. Meyer) Polymer Reaction Engineering, Wiley-VCH Verlag GmbH, Weinheim, Germany, pp 139-156. 

Inexpensive, efTective surfactant design is probably the key issue that must be dealt with in order to render emulsion polymerization in carbon dioxide a viable process, and hence advances in the understanding of the solvent behavior of CO2 must continue to drive this application forward. 

Figure 5 Templating of porous polymers in carbon dioxide-in-water emulsions. The polymerization takes place in the continuous phase about CO2 droplets. (From Ref. 52.)...

Regarding choice of continuous phase, in theory we might operate an inverse emulsion polymerization in any fluid with a relatively low critical temperature (for example, approximately 373 K and below), including alkanes and alkenes, CFCs, HFCs, CO2, N2O, Xe, and dimethyl ether. However, practical considerations greatly reduce this list the flammability of alkanes, the danger inherent in the use of N2O (it is a strong oxidant), the cost of HFCs and Xe, and the ozone-depleting capacity of CFCs have meant that only CO2 is seriously considered as a replacement for conventional oil-based continuous phases. We consequently focus our attention on carbon dioxide. 

Restricting our continuous phase to carbon dioxide means that the monomers in question must be relatively C02-phobic (in other words hydrophilic or at least very polar). Note that this greatly reduces the number of monomers that would be viable candidates for inverse emulsion polymerization in CO2 (or in another supercritical continuous phase) indeed even some water-soluble monomers such as acrylic acid are miscible with CO2 under relatively mild conditions. We will examine the issues surrounding monomer selection and surfactant design as they relate to the phase behavior of the system in the next section. 

Sun F, Pan Y, Wang J, Wang Z, Hu C, Dong Q. Synthesis of conducting polyanihne-montmorillonite nanocomposites via inverse emulsion polymerization in supercritical carbon dioxide. Polym Composite 2011 31 163-172. 

Adamsky, F. A. Beckman, E. J. Inverse Emulsion Polymerization of Acrylamide in Supercritical Carbon Dioxide. Macromolecules 1994, 27, 312. 

In this special volume on polymer particles, recent trends and developments in the synthesis of nano- to micron-sized polymer particles by radical polymerization of vinyl monomers in environmentally friendly heterogeneous aqueous and supercritical carbon dioxide fluid media are reviewed by prominent worldwide researchers. Polymer particles are prepared extensively as synthetic emulsions and latexes, which are applied as binders in the industrial fields of paint, paper and inks, and films such as adhesives and coating materials. Considerable attention has recently been directed towards aqueous dispersed systems due to the increased awareness of environmental issues. Moreover, such polymer particles have already been applied to more advanced fields such as bio-, information, and electronic technologies. In addition to the obvious commercial importance of these techniques, it is of fundamental scientific interest to completely elucidate the mechanistic details of macromolecule synthesis in the microreactors that the polymer particles in these heterogeneous systems constitute. 

Inverse emulsion polymerization of acrylamide in supercritical carbon dioxide. Macromolecules, 27 (1),... 

Use diluents In a gas system, inerts such as nitrogen, carbon dioxide, or steam can be used to mitigate the reaction exotherm. In a liquid system, a solvent can be used. Another possibility is to introduce a second liquid phase that has the function of absorbing and transferring heat, that is, water in an emulsion or suspension polymerization. Adding an extraneous material will increase cost, but the increase may be acceptable if it allows scaleup. Solvents have a deservedly bad name in open. 

Many volatile low-molecular-weight organics are completely miscible with carbon dioxide at relatively modest temperatures and pressures. However, nonvolatile compounds or those with higher molecular weights, especially polymers, are often insoluble. Insoluble liquid compounds may be dispersed into CO2 with the aid of appropriate surfactants to form a kinetically stable o/c emulsion [10,11]. Stable emulsions are important in separation processes, heterogeneous reactions and materials formation processes, such as precipitation with a compressed fluid antisolvent [40]. These emulsions are the precursors to solid latex particles in dispersion polymerization. Stabilization of o/c emulsions has been studied in-situ to understand surfactant design for polymerization [10,11]. 

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