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of mixing). However, because enthalpic interactions between CO2 and methylene groups are not favor- able, increasing the side chain length beyond a certain point led to decreased miscibility. Johnston recently reported that polymers that exhibit low interfacial tensions (and hence low cohesive energy densities) tended to also exhibit low miscibility pressures in carbon dioxide [74]. Clearly, the phase behavior of polymers in CO2 is tied to CO2’s low cohesive energy density, but its Lewis acid character will also play a significant role if the polymer contains Lewis base groups. For exam- ple, Beckman found that polybutadiene, a very low co- hesive energy density polymer, is more ‘CO2-philic’ than other vinyl polymers of higher cohesive energy density [144]. However, both polypropylene oxide and polyvinyl acetate exhibit lower miscibility pressures than polybutadiene, likely owing to the presence of Lewis base groups in each of the latter polymers de- spite exhibiting higher cohesive energy densities than polybutadiene. Topology also plays a role in determining phase behavior. Beckman and Lepilleur [145] found that increases to polymer chain branching generally low- ers miscibility pressure in CO2. This result confirms earlier results on branched polyolefins in alkanes [146]. Finally, McHugh found that topology can play an extraordinary role in determining the phase be- havior of polymers in CO2. The miscibility pressures of polyvinyl acetate, for example, lie at pressures hundreds to thousands of bar lower than those for polymethyl acrylate (an isomer of PVAc) [143]. The underlying mechanism for this behavior is entirely unknown. In the late 1990s, Beckman’s group [147] proposed a hypothesis for design of CO2-philic polymers that incorporated the earlier conclusions reached by both McHugh and Johnston. Beckman et al. proposed that CO2-philic polymers should incorporate monomers (or functional groups) that contain several features: high flexibility (and thus low Tg), low cohesive energy density and also Lewis base groups to provide loci for specific interactions between the polymer and CO2. They demonstrated the effectiveness of the hypothe- sis by designing highly CO2 -soluble ether-carbonate copolymers. Modified polydimethyl siloxane (PDMS) was also examined [148]—experimental work by Ki- ran [149] had shown that PDMS exhibits UCST type phase behavior at room temperature, suggesting that the enthalpic interaction between PDMS and CO2 is non-optimal. Fink et al. then showed that addition of Lewis base groups (in side chains) to PDMS lowered miscibility pressures in CO2 by hundreds of bar. Fi- nally, Wallen [150] has proposed that CO2 can exhibit specific interactions other than simple Lewis acid-base type. Wallen has found, via both simulation work and experiment, that an aldehyde will exhibit interactions between the carbonyl oxygen and the carbon atom in CO2 as well as a weak hydrogen bonding interaction between the aldehyde H and the oxygen in CO2. In summary, we have made great strides in our understanding of CO2 –polymer phase behavior since the days when ‘CO2 is like hexane’ was conventional wisdom. However, as shown by recent work from McHugh, Beckman, and Johnston, a fundamental understanding of CO2 –polymer thermodynamic be- havior is still lacking. Poly(fluoroacrylates) are the most CO2-philic polymers known, but their high cost renders their application problematic. If one could, from first principles, design a non-fluorinated, truly CO2-philic polymer, this would greatly enhance the potential for industrial application of CO2, both in polymer science and general chemical processing. 3.4. Chain polymerization and CO2 In chain polymerizations, an initiating species is formed which then contacts a monomer, creating the beginning of an active chain. This chain then grows rapidly to form the polymer molecule. Finally, a chain-terminating event may take place (or monomer may be depleted), ending growth of the chain in question. The various chain polymerization types are then further subdivided based on the type of initiating species and also the relative rates of initiation and growth [151]. 3.4.1. Free radical solution polymerization In free radical chain polymerization, an initiator (through thermal, chemical or photochemical stimu- lation) forms an active radical that contacts a vinyl monomer, forming the growing chain. Termination takes place either through chain coupling or dispro- portionation. Molecular weight distributions can be broad (>2.0) and average molecular weight rises rapidly with conversion, leveling off as long chains E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 155PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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