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158 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 Stabilizers for dispersion polymerization in conven- tional systems require a soluble component and an an- choring component—DeSimone’s group prepared the first successful stabilization system from homo- and co-polymers of fluoroacrylate monomers [162]. Small amounts of these copolymers permitted the rapid poly- merization of methyl methacrylate (MMA) in CO2 in the form of monodisperse particles ≈1 micron in size. Johnston et al. later showed that stabilization of the particles was due in large part to effective solvation of the CO2-philic, fluorinated blocks of the copoly- mer [163]. If conditions (temperature and pressure) were such that the fluorinated chains would collapse, flocculation of the particles would take place. Beck- man and Lepilleur [164] also examined the dispersion of MMA in CO2; here comb-type copolymers (acry- late backbone and fluoroether side chains) were em- ployed. Once the backbone was above a certain chain length, monodisperse, micron size particles could be rapidly formed. Finally, Howdle et al. [165] found that one could create a very simple but effective sta- bilizer for MMA polymerization—a fluoroether car- boxylic acid. Hydrogen bonding between the acid and MMA’s carbonyl provided anchoring sufficient to sta- bilize the dispersion and hence form small PMMA particles. As in the case for emulsion polymerization, practi- cal dispersion polymerization in CO2 will ultimately require a stabilizer that is both sustainable and inex- pensive and hence the fluorinated materials investi- gated heavily during the 1990s are not likely to be applied industrially. A reactive silicone (polydimethyl siloxane, acrylate terminated) has been applied as a stabilizer in MMA polymerization [166], but its per- formance was far less satisfying than the various flu- orinated stabilizers that have been evaluated. As in the case of emulsion polymerization, use of an initi- ating system that operates at low temperature (versus the typical thermally triggered azo- and peroxide com- pounds) would lower process temperature (and hence pressure) substantially. Finally, although micron-size particles of MMA (and other monomers) were readily formed, latex stability was relatively poor, with mate- rial settling out in a matter of hours (versus the desired days and weeks). This is not entirely surprising, as the low viscosity of CO2 (1/10 that of water) produces a relatively high terminal settling velocity. If the cost of the stabilizer could be lowered and the stability of the latex improved, a CO2-based dispersion could form the basis of a low VOC coating system. A potentially sustainable CO2-based (and hence solvent-free) coating formulation might be devel- oped even if the rapid settling of the latex cannot be corrected. If polymer particles, produced either in water or in CO2 then recovered and dried, could subsequently be re-dispersed in CO2, then one could ship the dry particles from manufacturer to remote customer and still employ a non-VOC (CO2-based) spray coating system. Use of such a system would save the large amount of energy needed to transport essentially solvent (CO2 or water) long distances. Johnston et al. have investigated the mechanics of particle re-dispersal and also the design of surfactants that would allow such polymerization and re-dispersal [167]. Their initial results are promising. Although not entirely similar, the commercial UniCarb process [40] was an early attempt to address the stability ver- sus sustainability balance in spray coatings. The con- ventional coatings process employed polymer beads dispersed in a mixture of a good solvent and a poor yet volatile solvent. The UniCarb process replaced the poor solvent with CO2 (also a poor-yet-volatile solvent) while retaining the good solvent to main- tain the stability of the dispersion. Replacement of the poor solvent with CO2 reduced VOC emissions by 60%. One area where CO2 would exhibit advantages over both water and organic solvents would be dispersion polymerization of hydrolytically sensitive monomers. In such a case, water would be green but technically infeasible, while apolar organics would be technically feasible yet not sustainable. DeSimone and Shiho have illustrated this using a glycidyl methacrylate monomer [168]. Again, if an effective yet inexpensive surfactant could be identified, use of CO2 in such an application would be both green and technically efficient. 3.4.2.3. Suspension polymerization in CO2 . In sus- pension polymerization, neither the monomer nor the polymer are soluble in the continuous phase, but the stabilizer structure and concentration are such that only droplets are formed (no micelles) and hence the kinetics of the polymerization resemble that of bulk polymerization. Suspension polymerization is typically applied to hydrophobic vinyl monomers in water, a process that is itself relatively greenPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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