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cal curiosities rather than potential avenues for green chemistry. Indeed, to achieve the highest impact (with respect to green chemistry), research should be directed at creating catalysts that target the efficient copolymerization of propylene oxide (or perhaps ethylene oxide) and CO2. Generation of an aliphatic polyester from CO2 and an olefin would be a superb example of green chem- istry with a ready market for the material. Aliphatic polyesters, while ‘green’ materials in their own right (they degrade cleanly to non-toxic fragments in the environment), require multiple steps to prepare the monomers and then the polymer, and also significant energy input along the way. A chain polymerization route to aliphatic polyesters starting from olefins and CO2 would be both greener and less expensive than the current method. With the exception of one or two references in the late 1970s [194] and a 1949 patent [195], there has been no published scientific activity on this problem, despite the technical and commercial importance. Calculations performed at the University of Pittsburgh suggest that formation of a lactone (the immediate precursor to a polyester) from CO2 and several olefins should be thermoneutral, and hence the reaction is at least theoretically tractable. 3.7. Industrial activity: condensation polymers and CO2 as monomer As mentioned above, both Crain and Bayer have commercialized the use of CO2 as the blowing agent in continuous polyurethane foam production—20+ plants currently operate using this technology. Further, PC Corp. (DE, USA) sells aliphatic polycarbonate (used as a ceramic binder) generated via the copoly- merization of CO2 and ethylene oxide. Xerox has patented [196] a process where bisphe- nol A polycarbonate is generated from bisphenol A and diphenyl carbonate using CO2 to extract the resid- ual phenol. Further, Akzo-Nobel patented [197] the formation of a degradable surfactant via the copoly- merization of ethylene oxide and CO2, where the polymerization is terminated by a fatty acid. How- ever, it appears that Xerox has ceased their research efforts on polymerization in CO2, while Akzo-Nobel appears to have shut down their research efforts on CO2 /alkylene oxide copolymerizations in early 1998. 3.8. Post-polymerization processing of polymers using CO2 Polymers require far more post-synthesis process- ing than do small molecules, and hence it is not surprising that CO2 plays a role in green post-poly- merization processing of polymers. First, as mentioned previously, CO2 will swell many polymers exten- sively, even those normally considered ‘CO2-phobic’. As shown in the generic phase diagram (Fig. 5), this is because of the asymmetry of the liquid–liquid phase envelope, itself arising from the disparity in size (and hence vapor pressure) of the solvent and solute. Swelling a polymer with CO2 will drop its viscosity significantly (depending upon temperature, by orders of magnitude). This large drop in viscosity allows for a number of CO2-enhanced processes. For exam- ple, Berens and Huvard [198a] demonstrated that the swelling of a polymer by carbon dioxide enhances the rate of infusion of model compounds. Kazarian and Eckert [198b] later exploited this effect in a novel way; they have shown that one can greatly enhance the kinetics of mixing of a CO2-incompatible dye with a polymer. In this work, the dye and polymer are thermodynamically compatible, but the rate of infu- sion of the polymer by the dye is glacially slow. CO2 plasticizes the polymer (while not actually dissolving very much, if any, of the dye), lowering the viscosity and allowing fast blending. The dying of fabric and fibers using CO2 has been extensively examined in Europe and the US [199,200]; here again the dye and polymer are thermodynamically compatible while the dye is sparingly soluble in CO2. Consequently, the dye partitions preferentially into the swollen polymer, where the CO2 diluent enhances the kinetics of the thermodynamically favorable process. It is interesting to note that Johnston [201] outlined the fundamentals for such a process several years ago using a silicone polymer, CO2 and toluene as the model ‘infusant’. The green aspect to this work is a reduction in en- ergy required for mixing, as well as elimination of the aqueous waste stream commonly associated with dying operations. Further, use of CO2 in place of water reduces air emissions and the need for drying of the fibers after dying [202]. It is important to note that here CO2 is being employed as a sustainable alternative to water–water is indeed a green solvent but it can be applied in ways (and in locales) where E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 163PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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