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548 Chemical Reviews, 1999, Vol. 99, No. 2 solubilities in CO2. As a result, all of the early studies in this area focused on precipitation polymerizations. In 1968, Hagiwara and co-workers explored the free-radical polymerization of ethylene in CO2 using either γ radiation or AIBN initiation.64-66 In this work, the infrared spectra of the polymers revealed that the presence of the CO2 continuous phase had little effect on the polymer structure. They noted that while the ethylene monomer was initially soluble in the liquid CO2, the polyethylene produced existed in a powder form which could easily be removed from the reactor. Powder products typically result from precipitation polymerizations; the advantage of using CO2 stemmed from the dryness of the resulting polymer. Also in 1968, a French Patent issued to the Sumitomo Chemical Company disclosed the polym- erization of several vinyl monomers in CO2.67 The United States version of this patent was issued in 1970, when Fukui and co-workers published results for the free-radical precipitation polymerization of several hydrocarbon monomers in liquid and super- critical CO2.68 As examples of this methodology, they demonstrated the preparation of the various homopolymers including poly(vinyl chloride) (PVC), PS, poly(acrylonitrile), poly(acrylic acid), and poly- (vinyl acetate) (PVAc). In addition, they prepared the random copolymers poly(styrene-co-methyl meth- acrylate) and poly(vinyl chloride-co-vinyl acetate). Depending upon the monomer and the reaction conditions employed, these reactions resulted in gravimetric yields which varied from 15% to 97% and viscosity average molecular weights (Mv) ranging from 1.2 × 104 to 1.6 × 106 g/mol. More recently, precipitation polymerizations of semicrystalline fluoropolymers in CO2 have been studied by DeSimone. In particular, tetrafluoro- ethylene is a monomer of interest since it was determined that tetrafluoroethylene may be handled more safely as a mixture with CO2.69 Tetrafluoro- ethylene copolymerizations70,71 with perfluoro(propyl vinyl ether) and with hexafluoropropylene or homopolymerizations72 have been performed in CO2, resulting in high yields of high molecular weight (>106 g/mol) polymer. The two major advantages of this process, lack of chain transfer to solvent and absence of undesirable endgroups, have been recently reviewed.63 3. Dispersion and Emulsion Polymerizations As was alluded to earlier, one key to a successful dispersion or emulsion polymerization is the surfac- tant. Its role is to adsorb or chemically attach to the surface of the growing polymeric particle and prevent the particles from aggregating by electrostatic, elec- trosteric, or steric stabilization. Consani and Smith have studied the solubility of over 130 surfactants in CO2 at 50 °C and 100-500 bar.73 They concluded that microemulsions of commercial surfactants form much more readily in other low polarity supercritical fluids such as alkanes and xenon than in CO2. Since traditional surfactants for emulsion and dispersion polymerizations were designed for use in an aqueous or organic continuous phase and are completely Kendall et al. insoluble in CO2, an exciting area of research is the design and synthesis of novel surfactants specifically for CO2. Polymeric surfactants for steric stabilization are effective in solvents with low dielectric constants. This is one reason steric stabilization, rather than electrostatic stabilization, provides the stabilization mechanism of choice for CO2-based systems. The polymeric stabilizer is a macromolecule that prefer- entially exists at the polymer-solvent interface and prevents aggregation of particles by coating the surface of each particle and imparting long-range repulsions between them. These long-range repul- sions must be great enough to compensate for the long-range van der Waals attractions of the par- ticles.44 This complex phenomenon depends not only on the amount and molecular weight of adsorbed stabilizer, but also on its conformation, which is affected by the nature of the solvent. When steric stabilization acts effectively in a heterogeneous sys- tem, the stabilizing molecule attaches to the surface of the polymer particle by either chemical grafting or physical adsorption. Amphiphilic materials such as block and graft copolymers, which have one component that is soluble in the continuous phase and another component, the anchor, that prefers to reside in the polymer phase, offer the highest prob- ability for physical adsorption. The other route to stabilization includes the chemical grafting of the stabilizer to the particle surface, either through chain transfer to stabilizer or by using a functional stabi- lizer that can serve as an initiator, monomer, or terminating agent. In this case, the chemically grafted stabilizer cannot physically desorb from the particle surface, and as a result, grafted stabilizers impart better colloidal stability than physically adsorbed stabilizers. In CO2-based systems, the types of surfactants that have been used for steric stabilization include CO2-philic (CO2-soluble) homo- polymers, copolymers (statistical, block, or graft) containing a CO2-philic component and a CO2-phobic (CO2-insoluble) anchoring component, and CO2-philic reactive macromonomers. Stuctures of the polymeric surfactants for CO2 described in this paper are numbered and located in Figures 1 and 2. Recently, the traditional Napper theory for steric stabilization in colloidal systems has been evaluated for applicability in highly compressible supercritical media. Peck and Johnston have developed a lattice fluid self-consistent field theory to describe a surfac- tant chain at an interface in a compressible fluid, allowing traditional colloidal stabilization theory74 to be extended to supercritical fluid continuous phases.75,76 In their theory, “holes” have been intro- duced into the lattice to account for the decreased concentration of molecules in the less dense super- critical phase. In this way, they are able to account for the compressibility of the continuous phase. The mechanism of steric stabilization was further studied by the Johnston group experimentally by turbidity and tensionmetry measurements of emulsion stabil- ity, critical flocculation density, and reversibility of flocculation.77 The system studied was an emulsion of poly(2-ethylhexyl acrylate) (PEHA) in CO2 in the presence of a surfactant, either poly(1,1-dihydroper-PDF Image | Polymerizations in Supercritical Carbon Dioxide
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