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162 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 Thus, the success of a green, CO2-based chemical pro- cess can depend as much on mechanical design as on chemical design. 3.6. Carbon dioxide as a monomer It has been known since 1969 that carbon diox- ide can be copolymerized with oxiranes to form poly(ether-carbonates) [189]. Production of a poly- carbonate using CO2 instead of phosgene (the usual route) is indeed a green process, in that not only is a harmful chemical replaced with a benign alternative, but the production of substantial quantities of salt (the usual byproduct in polycarbonate production) is avoided. Poly(ether-carbonates) formed from oxiranes and CO2 could be applied as degradable surfactants (using ethylene oxide) or low energy alternatives to polyesters polyols in polyurethane manufacture (us- ing propylene oxide). They have also been found to be the most CO2-philic, non-fluorinated materi- als yet identified [147] and hence they themselves could enhance the wider use of CO2 as a benign sol- vent. There are, however, some key technical hurdles that have substantially prevented the commercial- ization of a CO2-based route to a polycarbonate to date: 1) Most of the catalysts developed to date have not demonstrated particularly high activity when used with either ethylene oxide or propylene oxide, the comonomers most likely needed to produce economically viable copolymers [190]. On the other hand, a number of catalyst systems have been shown to be highly effective in the copoly- merization of CO2 with cyclohexene oxide [191], although this copolymer has not attracted any sig- nificant industrial interest owing to monomer cost versus polymer properties. 2) Those catalysts that have shown high activity in CO2 /propylene oxide copolymerizations have not permitted significant incorporation of CO2 into the copolymer (typically <10% carbonate) [192]. 3) Catalysts developed to date tend to produce sub- stantial amounts of low molecular weight, cyclic carbonate when used with either ethylene oxide or propylene oxide. In many cases, over 80% cyclic material is produced. The low molecular weight cyclic cannot be polymerized, and hence current catalysts could not be employed economically. Early work (1970s–1980s) focused on the assess- ment of zinc catalysts for the copolymerization of oxiranes and CO2 [190]. These catalysts typically employed a reaction between a dialkyl zinc and a multi-hydroxyl containing compound to create the active catalyst. Polymerization times were relatively long, significant amounts of cyclic carbonate were produced, yet alternating copolymer (100% carbonate) could be generated. Molecular weight distributions in these polymerizations could be very broad, often >5.0. Nevertheless, a zinc system was eventually used to synthesize an ethylene oxide–CO2 alternating copolymer that was applied commercially (PC Corp., Wilmington, DE) as a ceramic binder (this copolymer degrades cleanly to gaseous byproducts at tempera- tures >470 K). Recent work in this area has focused on the devel- opment of ‘single-site’ style catalysts to allow better control over molecular weight [191]. However, while these new catalysts have proven to be very effective in the copolymerization of cyclohexene oxide and CO2, none have been able to solve the problems observed during copolymerizations of CO2 and either ethylene oxide or propylene oxide. In general, in copolymer- izations of CO2 and propylene oxide, catalysts derived from aluminum exhibit high activity and produce predominantly copolymer with a narrow molecular weight distribution, yet allow little CO2 incorporation into the copolymer [192]. Zinc catalysts allow for high levels of CO2 in the copolymer, yet produce pre- dominantly low molecular weight alkylene carbonate. Indeed, the generation of copolymers of CO2 and either propylene or ethylene oxide would represent green chemistry, as these materials would have ready markets and alternative routes to their production (via phosgene) are highly problematic from a sustainable viewpoint. Until the technical hurdles to efficient copolymerization (see above) can be overcome, a CO2-based route to aliphatic polycarbonates, and in- deed, aliphatic polycarbonates in general, will not enjoy widespread use. Whereas a variety of other polymers have also been generated from CO2 [193], either the properties of these new materials (vis-à-vis their cost) have not been promising or the efficiency of the polymerization low and hence, they are techni-PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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