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156 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 are continuously formed. Low-density polyethylene, polyacrylates, polystyrene, polyvinyl chloride and other materials are formed using free radical initia- tion. Much of the total commercial volume of such polymers is synthesized in the absence of solvent in continuous processes containing only monomer, poly- mer and initiator at temperatures sufficient to create a pumpable polymer melt. As described above, the solubility of most polymers in carbon dioxide is relatively poor, and hence it is not surprising that early work on polymerization in CO2 was relegated to precipitation polymerizations [152]. Although it could be claimed that the plasticizing ef- fect of CO2 on the precipitated polymer might enhance transport of monomer to the growing chain end, no significant advantages (versus the added complication of working at elevated pressure), green or otherwise, were realized from such processes, possibly because the presence of the monomer itself tended to plasticize the polymer. Consequently, one would only expect to observe a significant effect of added CO2 during the later stages of polymerization, when the presence of CO2 might inhibit the well-known Trommsdorf, or autoacceleration effect (the latter occurs when the in- creased viscosity of a polymer melt inhibits chain ter- mination, leading to rapid increases in rate). Because CO2 is a diluent, its presence would also lower the rate in general, a disadvantage [153]. Finally, vinyl polymerizations are exothermic and hence, great care would need to be taken to prevent uncontrolled pres- sure increases. In summary, the disadvantages inher- ent to operating a vinyl polymerization in CO2 have greatly outweighed any advantages to date. In general, it is very hard to justify (from a ‘green’ perspective) adding solvent to a solvent-less process. One exception to this rule is in the surfactant-free precipitation polymerization of fluoromonomers [154], recently scaled up by DuPont to a semi-works size in North Carolina. Typically, fluoropolymers are generated via suspension polymerization in water; the use of carbon dioxide as the solvent provides for a chain-transfer free solvent and eliminates the need for the surfactant (as noted previously, the EPA has re- cently filed a SNUR regarding fluorinated surfactants of the fluorosulfonate variety, possibly restricting their use in future [48]). Interestingly, most fluoromonomer polymerizations are precipitation polymerizations (as shown by McHugh [136], many fluoropolymers are insoluble in CO2). However, addition of CO2 stabi- lizes tetrafluoroethylene, eliminates the need for fluo- rinated solvents and surfactants, and eliminates chain transfer to solvent. Indeed, a recent conversation with a DuPont customer [155] revealed that the fluorinated copolymers produced in CO2 exhibit superior perfor- mance during extrusion, owing to fewer gels and a tighter composition distribution. Hence, in fluoropoly- mer polymerization, CO2 provides green advantages, safety advantages and product advantages. Another possible application for precipitation poly- merization in carbon dioxide involves acrylic acid [156]. Poly(acrylic acid) is currently generated in an emulsion or suspension polymerization in a hydro- carbon continuous phase; removal of the alkane from the product is both energy intensive and waste form- ing. Use of CO2 as the continuous phase allows the generation of dry, free-flowing, granular material. Carbon dioxide has also been proposed as a dilu- ent (reversible plasticizer) for reactions on preformed polymers, reactions that often take place within ex- truders during polymer processing. In theory, the plas- ticizing effect of CO2 will reduce transport limitations of the reactants (in the otherwise highly viscous melt), leading to enhanced rate and thus more complete reac- tion in the same residence time. However, O’Neill and Beckman [153] found that in the case of the polyvinyl acetate-to-butyrate transition (a highly successful in- dustrial process) the presence of the low molecular weight reactants was sufficient to plasticize the melt. Here CO2 acted merely as a diluent, lowering the rate by reducing the concentration of the active species. 3.4.2. Heterogeneous free radical polymerizations Heterogeneous polymerizations are those where the polymer is not soluble in the continuous phase, or solvent [151]. These polymerizations can be further sub-divided based on the thermodynamic affinity of the monomer for the solvent and the nature of the polymer stabilization: 1) Emulsion 2) Dispersion 3) Suspension While simple precipitation can be considered as a form of heterogeneous polymerization, it has been considered separately in the previous section.PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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