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ranec—developed rules of thumb for designing copolymers that would be highly soluble in C0 2 . According to these rules, one monomer should provide a polymer that is highly flexible and interacts only weakly with other polymer chains. The second monomer should contain functional groups, such as Lewis bases, that want to associate 2 with C0 2 , thus making the polymer more soluble in C02. Ideally, Beckman says, the two monomers should want to avoid each other, encouraging the copolymer to dissolve in a solvent that ordinarily would not dissolve either of the ompressed carbon dioxide could homopolymers. replace noxious organic solvents The researchers designed a model in a wide range of industrial-scale C02-phile by incorporating carbonyl chemical processes if the work of Eric J. groups (Lewis bases) into a polyether deBeckman fulfills its promise. rived from propylene oxide. To make Carbon dioxide—especially in its suthese ether-carbonate copolymers, Beckpercritical state—has attracted much inPoly(ether-carbonate) copolymers man explains, "we had to come up with terest as an environmentally benign solproduced from propylene oxide and C0 2 some new sterically hindered aluminum vent. But it's "a very feeble solvent," catalysts that would allow us says Beckman, a chemical to efficiently copolymerize engineering professor at C02 and cyclic ethers." the University of PittsThe composition of the burgh. Special C02-miscicopolymers generated using ble additives called "C02these catalysts can be varied philes" and high C0 2 presfrom that of a homopolymer sures are required to of propylene oxide (100% dissolve otherwise insolupolyether) to that of an alterble or poorly soluble matenating copolymer of prorials such as polar, ionic, pylene oxide and C02 (100% organometallic, or high polycarbonate). By optimalmolecular mass comly balancing the proportion pounds. The most effective of the two monomers, BeckC02-philes have been exman's group has produced pensive fluorocarbons Styranec (from left), Beckman, and Sarbu with the apparatus they ether-carbonate copolymers such as fluorinated poly- use to study the solubility of their copolymers In pressurized C02. that are soluble in C0 2 at ethers. The high cost of these solubilizers, however, has limited Georgia Institute of Technology. "What lower pressures than either the 100% the commercialization of otherwise Eric has done is to take boutique chem- polyether or 100% polycarbonate. promising C02-based processes. Beckman was hoping that these coistry and make it affordable. I think it's Now, Beckman's group has designed neat science, neat engineering, and neat polymers would perform at least as well as the more expensive fluorinated polyand synthesized a series of economical economics." copolymers that contain no fluorine and In a Nature commentary, chemist ethers. But they actually perform "a lot readily dissolve in C0 2 at low pressures Walter Leitner of the Max Planck Insti- better," he was surprised to find. And [Nature, 405,165 (2000)]. The copoly- tute for Coal Research in Mulheim, Ger- their performance hasn't even been opmers—poly (ether-carbonate) s—are many, notes that supercritical C0 2 may timized yet. prepared from C0 2 and inexpensive oxi- find use in areas as diverse as the dyeThe Pittsburgh chemical engineers ranes such as propylene oxide or ethyl- ing and cleaning of fibers and textiles, also have created block copolymers that ene oxide. polymerization and polymer process- function as surfactants. When added to The principles used to design these ing, purification and crystallization of a pressurized, stirred mixture of water copolymers could guide the design of a pharmaceuticals, and chemical synthe- and C02, the copolymer stabilizes the wide range of nonfluorinated C02-philes sis. Beckman's "welcome and long- resulting emulsion for hours. Beckman hopes to use these new from low-cost raw materials, Beckman awaited" innovation, Leitner writes, expoints out. This would make a variety of pands "both our practical opportunities C02-soluble copolymers to make chelatC02-based processes economically fa- and our understanding of how to design ing agents, catalyst ligands, and other compounds that will expand the uses of vorable, particularly those in which the C02-philic materials." solubilizers are difficult to recycle. Drawing on recent studies of the sol- supercritical C02. "It remains to be seen "I think this is a tremendous ad- ubility of polymers in C02, Beckman what we can do" with these materials, vance," comments Charles A. Eckert, a and his coworkers—postdocTraian Sar- he says. Ron Dagani professor of chemical engineering at bu and graduate student Thomas J. Sty-
BETTER, CHEAPER 'GREEN' SOLVENT
Low-cost,fluorine-free,C0 -soluble copolymers promise to expand industrial uses of supercritical carbon dioxide
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MAY 15,2000 C&EN
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