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3.4.2.1. Emulsion polymerization in CO2 . In emul- sion polymerization, neither the monomer nor the polymer is soluble (to any appreciable extent, there is always some measurable monomer solubility) in the continuous phase and sufficient surfactant is present to form micelles (the locus of the polymerization) and to stabilize the large droplets of monomer that are also present (the latter form monomer reservoirs). The kinetics of the emulsion polymerization are such that (unlike in bulk or solution free radical polymer- ization) both high rate and high molecular weight are possible. Carbon dioxide, while not a powerful sol- vent, is miscible with a large variety of volatile, low molecular weight vinyl monomers [157]. As such, identifying a suitable candidate for emulsion polymer- ization is problematic, as one must find a monomer that exhibits a sizeable phase envelope under the conditions of interest, yet under conditions where the surfactant to be employed is miscible (in CO2, the converse is much simpler to identify—a mixture where the monomer is miscible and the surfactant is not!). This has proven to be difficult and to date only acrylamide, acrylic acid and N-vinyl formamide have been investigated in any detail [158]. The case for acrylamide is further complicated by the fact that it is a solid at temperatures below 353 K and hence has been employed as an aqueous solution—the presence of the water renders subsequent polymer particle size analysis difficult. Emulsion polymerization of water soluble monomers in CO2 is a viable target in the context of green chemistry, in that the commercial route employs an organic continuous phase and also requires significant energy input to separate product from emulsion following polymerization. The key issue in emulsion polymerization is the design of the surfactant—it must be soluble in CO2 at moderate pressures, effective and relatively low cost. Early work employed fluorinated surfactants (nonionic and anionic), as these were known to be CO2-philic [158]. Results showed that one could in- deed generate high polymer at high rates, but the surfactants employed were more valuable (even at 1% loading and below) than the polymers being generated and recycle is difficult to achieve economically. Al- though silicone-functional surfactants have also been evaluated [159] in emulsion polymerization, their per- formance is not as good as their fluorinated cousins, and their cost can be quite high (for siloxane-based materials generated from the cyclic tetramer (D4), cost is approximately five to ten times as high as tradi- tional hydrocarbon surfactants. For mono-functional materials created from the D3 cyclic trimer, the cost approaches that of fluorinated materials.) The prac- ticality of the process would be greatly enhanced by discovery of an effective yet low cost surfactant. In work to date, AIBN (azo bis(isobutyrnitrile)) was usually employed as the initiator and hence process temperatures were set at 330–340 K to achieve rea- sonable polymerization rates (AIBN half-life at 343 K is ≈4 h). As such, process pressures were relatively high (>200 bar). Clearly, use of an initiating system that operates at lower temperatures (photochemical or redox [151]) would lower the required process pressure and hence also render emulsion polymer- ization in CO2 more practical (see, for example Ref. [160]). It should be noted that such an initiator system would be more expensive than that currently em- ployed, an added cost that must be factored into the total. 3.4.2.2. Dispersion polymerization in CO2 . Disper- sion polymerization [161], where the monomer is sol- uble in the continuous phase (here CO2) while the polymer is not, has seen extensive research activity over the past decade. Because most, if not all vinyl monomers are miscible with CO2 at relatively mod- est pressures (complete miscibility below 100 bar at 313 K in many cases), while high polymers are no- toriously insoluble, dispersion polymerization seems well suited to adaptation to carbon dioxide. If one were to conduct a dispersion polymerization in a con- ventional liquid, a low molecular weight alcohol or alkane would be the preferred continuous phase and thus CO2 could replace a significant volume of or- ganic solvent. Separation of the product polymer from the continuous phase in a CO2 system would not re- quire drying/devolatilization, a potentially significant energy savings. Because many vinyl monomers lend themselves to dispersion polymerization in CO2, the key requirement to successful demonstration was find- ing a suitable stabilizer. Finally, because a successful dispersion polymerization produces a stable latex that can then form the basis for a coating formulation, it was hoped that the analogous process in CO2 would produce a coating formulation that could be sprayed without VOC release. E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 157PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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