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Polymerizations in Supercritical Carbon Dioxide Chemical Reviews, 1999, Vol. 99, No. 2 547 typically ranging in size from 100 nm to 10 μm. The history and theory of dispersion polymerization in organic media have been summarized in a book by Barrett44 and a recent review by Sudol.50 Due to the good solubility of many small organic molecules in CO2, dispersion polymerization constitutes the best heterogeneous method that has been developed thus far for producing high molecular weight, CO2- insoluble, industrially important hydrocarbon poly- mers. In contrast to dispersion polymerization, the reac- tion mixture in an emulsion polymerization is initially heterogeneous due to the low solubility of the monomer in the continuous phase. Emulsion polymerization is a very active area of research, and the reader is referred to several recent books and reviews.20,49,51-53 For a reaction to take advantage of the desirable Smith-Ewart/Harkins kinetics,54 the monomer and initiator must be segregated with the initiator preferentially dissolved in the continuous phase and not in the monomer phase. Traditionally, emulsion polymerizations employ oil-soluble mono- mers such as acrylics or styrenics dispersed in an aqueous medium containing a water-soluble initiator such as sodium persulfate, while “inverse” emulsion polymerizations employ water-soluble monomers, such as acrylamide, dispersed in an organic medium containing an oil-soluble initiator such as an organic azo or peroxide species. In either case, the insoluble polymer is stabilized as colloidal particles. The repulsive forces, which result from the surface charges imparted by an ionic initiating species, an added small molecule ionic surfactant, an added amphiphilic macromolecular stabilizer, or a combina- tion of these to provide a charged surface, prevent the coagulation of the growing particles and lead to a stabilized colloid. As a result of the kinetics of an emulsion polymerization, high molecular weight poly- mer can be produced at high rates, with the rate of polymerization dependent upon the number of poly- mer particles. The polymer which results from an emulsion polymerization exists as spherical particles typically smaller than 1 μm in diameter. Due to the high solubility of most vinyl monomers in CO2, emulsion polymerization in CO2 probably will not be a very useful process for the majority of commercially important monomers, although there are exceptions. For example, inverse emulsion polymerization of acrylamide in a water/CO2 system has been re- ported.56 1. Homogeneous Polymerizations Amorphous or low-melting fluoropolymers can be synthesized homogeneously in CO2 by either free- radical or cationic methods. Poor solubility in most common organic solvents represents an inherent problem in the synthesis and processing of many high molar mass amorphous fluoropolymers. In fact, chlorofluorocarbons (CFCs) and CO2 are the best solvents for these types of polymers.4 However, due to the environmental problems associated with CFCs, they are no longer a viable solvent option. Carbon dioxide presents an ideal inert solvent for the polym- erization of these types of highly fluorinated mono- mers without the environmental concerns. Using supercritical CO2 as the solvent, DeSimone used free-radical initiators to effect the synthesis of high molar mass amorphous fluoropolymers.4,11,57-59 Due to the high solubility of the polymers in the CO2 continuous phase, the polymerizations remained homogeneous throughout the course of the reactions. Several fluorinated acrylate monomers, such as 1,1- dihydroperfluorooctyl acrylate (FOA), have been polymerized using this methodology to give high yields of high molecular weight polymer. Styrenes with a perfluoroalkyl side chain in the para position, such as p-perfluoroethyleneoxymethylstyrene, con- stitute a second type of monomer polymerized via homogeneous solution polymerization in supercritical CO2. The product obtained from this polymerization in CO2 was identical to the product of a solution polymerization in 1,1,2-trichloro-1,2,2-trifluoroethane (Freon-113), indicating that CO2 does indeed repre- sent an exceptionally good replacement solvent for this type of reaction. Moreover, this technique has proven to be valuable in the synthesis of statistical copolymers of the appropriate fluorinated monomers and other hydrocarbon monomers such as methyl methacrylate (MMA), butyl acrylate, ethylene, and styrene. Homogeneous solution polymerization has also been employed by DeSimone to prepare CO2- soluble polymeric amines by copolymerizing FOA with 2-(dimethylamino)ethylacrylate or 4-vinylpyri- dine.60 The initiation and propagation kinetics of free- radical polymerizations in supercritical CO2 have been studied. The earliest work in this area used ultraviolet spectroscopy to explore the thermal de- composition rate and initiator efficiency of 2,2′-azobis- (isobutyronitrile) (AIBN) in CO2.11 By comparing the decomposition rate, kd, and the initiator efficiency, f, in CO2 to that reported in the literature for other solvents, the authors observed higher initiator ef- ficiencies, relative to higher viscosity solvents, as a result of the decreased solvent cage effect in the supercritical phase. More recent work has evaluated the free-radical propagation kinetics of styrene and methyl methacrylate in CO2 using the pulsed laser polymerization (PLP) technique.61 Since the conver- sions were kept very low (<5%), the polymer chains remained in solution during these experiments as styrene monomer is a good (co)solvent for PS. The propagation rate, kp, was found to be very close to the bulk values for both monomers, indicating that the presence of CO2 does not interfere with the chain- growth process. Another study compared the kinetics of the homogeneous polymerization of 1H,1H,5H- perfluoropentyl acrylate in supercritical CO2 to those in conventional, liquid solvents such as Freon-113.62 Indeed, these kinetic studies have verified that CO2 is an excellent medium in which to conduct free- radical reactions. 2. Precipitation Polymerizations A topic of many early studies in CO2 was the free- radical polymerization of industrially important vinyl monomers.63 Although many common vinyl mono- mers exhibit high solubility in CO2,7 most of the corresponding polymers exhibit exceedingly poorPDF Image | Polymerizations in Supercritical Carbon Dioxide
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