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552 Chemical Reviews, 1999, Vol. 99, No. 2 Kendall et al. as already illustrated. The specific affinity that PFOA has for PMMA is not extended to PS. However, PFOA homopolymer with unstable thiuram endgroups was shown to stabilize micron-sized PS particles via chemical grafting.98 PS-b-PDMS polymeric stabilizers were studied to determine the effects of anchor block length and soluble block length, CO2 pressure, and concentra- tions of surfactant, monomer, and initiator.96 In comparing two stabilizers with a PDMS block length of 2.5 × 104 g/mol, the longer PS anchor block length yielded particles with a larger diameter (0.22 vs 0.46 μm) with a narrower particle size distribution (1.31 vs 1.08). The stabilizers with a longer PDMS of 6.5 × 104 g/mol resulted in collapsed latexes and coagu- lated particles. The authors speculate that this may be due to the lower ratio of anchor-to-soluble block or due to lower solubility of the surfactant in CO2. A kinetic study of the dispersion polymerization showed that the molecular weight and the conversion in- crease as a function of time. As expected, both the concentration of monomer and the concentration of stabilizer affected the morphology of the resulting PS particles. Additionally, the temperature and pressure of the reaction mixture were found to effect results, such as average particle diameter and molecular weight, of the PS product. The importance of polymer collection procedures was shown by the aggregation of PS particles collected from the reactor after complete depressurization. In contrast, the polymer particles released from the reactor during the depres- surization did not show any signs of flocculation in the SEM images. It was noted that none of the PS- b-PDMS copolymers were soluble in CO2 in the absence of monomer. PDMS homopolymer was not capable of stabilizing styrene dispersion polymeriza- tions in CO2, as yield and molecular weights of PS were comparable to reactions in which no surfactant was used. The preparation of stable dispersions of poly(vinyl acetate) (PVAc) and ethylene-vinyl acetate copoly- mers in liquid and supercritical CO2 has recently been investigated.99,100 Both fluorinated and siloxane- based stabilizers including homopolymers, block co- polymers, and reactive macromonomers were em- ployed. The influence of the concentration of stabilizer, stabilizer anchor-soluble balance, and pressure on the resulting colloidal product was explored. In addition, turbidimetry was used successfully to monitor dis- persed phase volume fractions, particle sizes, and number densities during the polymerization. The vinyl acetate polymerizations stabilized by PDMS homopolymer, vinyl-terminated PDMS macromono- mer (15), or PVAc-b-PDMS (16) all produced col- lapsed latexes of high yield and high molecular weight polymer, whereas the polymerizations stabi- lized by PVAc-b-PFOA (4) remained stable latexes. Turbidity showed that PFOA and PVAc-b-PFOA with a short anchoring group (PVAc Mn ) 4 × 103 g/mol) had inefficient anchoring to the PVAc particles. The PVAc-b-PFOA with the longest blocks (PVAc Mn ) 3.1 × 104 g/mol; PFOA Mn ) 5.4 × 104 g/mol) produced the smallest diameter polymer particles. Cooper and coauthors recently reported the first synthesis of well-defined cross-linked polymer micro- spheres in supercritical CO2.55 Divinylbenzene and ethylvinylbenzene were copolymerized at 65 °C and 310 ( 15 bar using AIBN as an initiator. The reaction was performed both in the presence and in the absence of a surfactant. Without surfactant, 1.5-5 μm particles were isolated. When the polymerization employed a 3 wt % (based on monomer) of a block copolymer surfactant containing PMMA and a fluori- nated methacrylate (5), much smaller particles (e0.41 μm) with a relatively narrow particle size distribution were observed. Yields greater than 90% were ob- tained both with and without surfactant. This study demonstrated the formation of various sizes of cross- linked microspheres in supercritical CO2. Adamsky and Beckman have explored the possibil- ity of carrying out an inverse emulsion polymeriza- tion of acrylamide in supercritical CO2.56,101 In these reactions, acrylamide was polymerized in the pres- ence of water, a cosolvent for the monomer, in a CO2 continuous phase at 345 bar and 60 °C with AIBN initiation. Reactions were conducted both with and without the stabilizer, an amide end-capped poly- (hexafluoropropylene oxide) (6). In the absence of the stabilizer, the precipitation polymerization of acryl- amide resulted in a high conversion of high molecular weight polymer which formed a single solid mass in the bottom of the reaction vessel. In the presence of stabilizer, the reaction solution was reported to have a milky-white appearance that was indicative of latex formation; however, the conversion and molecular weights were comparable to those obtained in the unstabilized reactions. More studies in the field of inverse emulsion polymerizations in CO2 are expected with the recent discoveries of surfactants which have the ability to form microemulsions of water and water soluble molecules in CO2. In 1993, DeSimone reported the use of the macromonomer technique to synthesize and characterize an amphiphilic graft copolymer with a CO2-philic PFOA backbone and hydrophilic poly- (ethylene oxide) (PEO) grafts (8).102 In this study, solvatochromic characterization was employed to demonstrate that the PEO grafts enabled the solubil- ity of the hydrophilic, CO2-insoluble dye methyl orange in supercritical CO2. This graft copolymer was further characterized by small-angle X-ray scattering (SAXS) and shown to form spherical micelles in the presence of water in a CO2 continuous phase.103 This work represented the first direct confirmation that micelles can form in a CO2 continuous phase. Beck- man and co-workers have studied the phase behavior of both silicone-based and fluoro ether-functional amphiphiles in supercritical CO2.12,104 A fluoro ether- functional amphiphile (7) was shown to permit the extraction of thymol blue from aqueous solution into CO2. The effect of pressure on the emulsion behavior of fluoro ethers terminated with sorbitol esters, sulfates, and sulfonates in mixtures of CO2 and water were investigated.105 Another advance in this area was made when Johnston and co-workers demon- strated the formation of a one-phase microemulsion consisting of the hybrid fluorocarbon/hydrocarbonPDF Image | Polymerizations in Supercritical Carbon Dioxide
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