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(although water remediation and energy use for dry- ing represent targets for improvement). CO2 has been used in the suspension polymerization of acrylic acid in CO2 in the hope of replacing the conventional hydrocarbon continuous phase. Polyacrylic acid is a very low-cost commodity material, and hence such a process must produce dry, free-flowing powder at rel- atively low pressure and with an inexpensive stabilizer [169]. 3.4.2.4. CO2 as non-solvent in heterogeneous poly- merizations. Cooper et al. [170] have explored a novel application of CO2 in heterogeneous polymer- ization. Here, CO2 is used as the porogen in the sus- pension polymerization of styrene/divinyl benzene, where the resulting porous beads form the basis for ion exchange resins. Typically a hydrocarbon porogen is employed and hence must be separated from the product and disposed after use. A good porogen must be miscible with the monomer (as is the case with CO2 and styrene) yet immiscible with the polymer (as in CO2/polystyrene). Generally, one alters the pore size and total surface area of the beads through alterations to porogen composition; Cooper showed that one could achieve the same tunability through pressure alterations to CO2. 3.4.3. Other chain polymerizations in CO2 Carbon dioxide has been employed as a solvent for cationic and metal-catalyzed ring-opening poly- merization of various monomers in CO2. Biddulph and Plesch first examined cationic chain polymeriza- tion of isobutylene in CO2 in 1960 [171]; Kennedy later also examined this reaction [172]. This work demonstrated that cationic polymerization is indeed viable but that the premature precipitation of the poly- mer lessens any advantages one might have derived from use of a green solvent. DeSimone later applied knowledge of CO2-philic compounds to greater ad- vantage by examining the homogeneous cationic poly- merization of fluorinated monomers (both vinyl and functional oxetane) in CO2 [173]. As the DeSimone group demonstrated earlier, polymerization of fluori- nated monomers in CO2 is a very effective technique for polymer production without the use of hydro flu- orocarbon solvents. Metathesis polymerization is also viable in CO2, yet the hydrocarbon monomers employed produce poly- mers that rapidly precipitate upon attaining even mod- est chain length [174]. The same is true for oxidative polymerizations of either pyrrole or dimethyl phenol. It has been shown that one can prevent the seemingly inevitable precipitation through use of fluorinated sta- bilizers (and hence formation of a dispersion), but the high cost of the stabilizers has inhibited further con- sideration of such routes. Not surprisingly, anionic polymerization in CO2 produces at best carboxy-terminated oligomers, as the terminal anion reacts quickly with CO2 to pro- duce the less reactive carboxylate. Carbon dioxide is also an efficient chain terminator in Ziegler-Natta and metallocene type catalyst systems—as such, CO2 cannot currently be used as a solvent in controlled olefin polymerizations, the largest volume polymer- izations currently. Because these polymerizations tend to be low pressure gas-phase reactions of ethylene and propylene, it is not clear what role carbon diox- ide could play even if the catalysts could tolerate its presence. 3.4.4. Industrial activity: chain polymerizations in CO2 DuPont has filed a number of patents [175] describ- ing the use of CO2 as a solvent for chain polymeriza- tion of fluorinated monomers. This technology, plus patents filed by coworkers at the University of North Carolina [154], formed the basis for the construction of a semi-works facility in North Carolina with an annual capacity of over 1000 tons of fluoropolymer (there are plans to expand this capacity significantly by 2006). 3M and Xerox have also obtained recent patents in this area [176], although their supercritical CO2 research efforts appear to have been discontinued several years ago. The EU funded (1.5 million Euros, 12/97–12/00) a multi-year study (Superpol project) linking four universities with polymer manufacturers Solvay, Goldschmidt and DSM to explore the use of su- percritical fluids in polymer production. While the consortium includes both prestigious universities and well-known companies, the results to date [177] have not significantly added to the information described above. Solvay has recently acquired the fluoropoly- mers business of Ausimont, and hence may invest in CO2 -based fluoropolymer polymerization technology in the future. E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 159PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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