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Polymerizations in Supercritical Carbon Dioxide Chemical Reviews, 1999, Vol. 99, No. 2 553 surfactant C7F15CH(OSO3-Na+)C7H15 and water in CO2.106 In this work, the water-to-surfactant ratio in a single-phase microemulsion was as high as 32 at 25 °C and 231 bar. It was shown that with 1.9 wt % surfactant, 2 wt % water was solubilized in CO2, which is 10 times the amount of water soluble in pure CO2. In more recent work, Johnston demonstrated the formation of aqueous microemulsion droplets in a CO2 continuous phase using an ammonium car- boxylate perfluoropolyether surfactant, [(OCF2CF- (CF3))n(OCF2)m]OCF2COO-NH4+.107 Several spectro- scopic techniques were employed to investigate the properties of these aqueous microemulsions. These approaches to the formation of microemulsions in nonpolar supercritical fluids has been the focus of two recent reviews.108,109 Since evidence was shown in the studies by DeSimone, Beckman, and Johnston that their surfactants form microemulsions of water and, in some cases water soluble molecules, in CO2, these surfactants could potentially be used to form micro- emulsions of water and a water soluble monomer in CO2 for inverse emulsion polymerizations. The fields of dispersion and emulsion polymeriza- tions have been developed over the past 30 years to become convenient methods for the preparation of spherical polymer particles. Advances in the design and synthesis of amphiphilic block and graft copoly- mers for use as stabilizers in supercritical fluids has allowed tremendous latitude in the composition of the polymeric microspheres. Breakthroughs in the design and synthesis of nonionic surfactants that are inter- facially active in a CO2 medium have been pivotal to the development of the dispersion and emulsion polymerizations technique in this fluid. 4. Polymer Blend Synthesis The plasticization of polymers by supercritical CO2 has been exploited for the synthesis of polymer blends. In general, CO2 is used to swell a CO2- insoluble polymer substrate and to infuse a CO2- soluble monomer and initiator into the substrate. Subsequent polymerization of the monomer gener- ates the polymer blend. Watson and McCarthy used supercritical CO2 to swell solid polymer substrates, including poly(chlorotrifluoroethylene), poly(4-meth- yl-1-pentene), high-density polyethylene (HDPE), nylon-6,6, poly(oxymethylene), and bisphenol A car- bonate, and to infuse styrene monomer and a free- radical initiator into the swollen polymer.35,36,110 The polymerization reaction was run either before de- compression of the CO2 or after decompression and recompression with N2. Mass uptakes of up to 118% based on the original polymer were reported. As expected, diffusion rates in CO2-swollen polymers were dramatically increased over nonswollen poly- mers. Using ethylbenzene as a model for styrene, the equilibrium for monomer uptake was 25 times faster with CO2 under the conditions studied than with out CO2. X-ray analysis showed that the polystyrene existed in phase-segregated regions throughout the matrix polymer, and thermal analysis showed that radical grafting reactions were not significant. In a related study, styrene (with and without cross- linker) was infused into three fluoropolymer sub- strates, polymerized, and subsequently sulfonated in order to provide surface modification to the polymeric substrates.111 Polystyrene was shown to be present throughout the blends and semi-interpenetrating networks of each of the three polymer substrates, PTFE, poly(TFE-co-hexafluoropropylene), and poly- (chlorotrifluoroethylene). The modified surfaces were characterized by X-ray photoelectron spectroscopy and by water contact angle measurements. The wettability of all modified fluoropolymer substrates was increased by the surface modification. In a separate study, the morphology and mechan- ical performance of PS/HDPE composites were iden- tified.112 As in the previous experiments, the polymer substrate, HDPE, was swollen with CO2 in the presence of styrene and initiator. The subsequent polymerization produced the blend, with PS located in the amorphous interlamellar regions of the poly- ethylene spherulite and in the spherulite centers. The PS/HDPE blends exhibited a modulus enhancement, strength improvement with increasing PS content, loss in fracture toughness, and increase in brittle- ness. B. Cationic Polymerizations Cationic polymerizations represent a challenging field in polymer science, and their extension to supercritical fluids has been equally difficult. The high reactivity of carbocations results in fast poly- merization reactions, but also leads to unwanted side reactions such as chain transfer and termination. These side reactions limit the usefulness of cationic polymerizations. The inherent basicity of monomers that are capable of being polymerized cationically and the acidity of the protons to the carbocation on the polymer make proton abstraction by the monomer a built-in side reaction that is difficult to suppress. These side reactions are often reduced by lowering the reaction temperature. Upon cooling, because side reactions typically have higher activation energies, the propagation rate decreases less relative to the secondary reactions, and a higher degree of poly- merization is achieved. Living cationic polymerization methods have been developed to produce well-defined polymers.113 These living methods allow for control of molecular weight, molecular weight distribution, end group functional- ity, polymer microstructure, and reactivity. They also permit the synthesis of block copolymers of precise block length and compositions. Polymerization con- trol is gained in living cationic systems by stabiliza- tion of the active carbocation through nucleophilic interactions.114 This stabilization is generally achieved by association with a suitable nucleophilic counterion or, if the counterion is only weakly nucleophilic, by association with an added Lewis base. Both methods reduce the positive charge on the carbocation and the acidity of the -hydrogens, which results in essentially no chain transfer to monomer. The nu- cleophilic interaction between the counterion and the carbocation is key in stabilizing the polymerization and preventing side reactions. Winstein developed an ion pair spectrum to describe the active site in cationic polymerizations.115 Classical nonliving car-PDF Image | Polymerizations in Supercritical Carbon Dioxide
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