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550 Chemical Reviews, 1999, Vol. 99, No. 2 Kendall et al. Table 2. Summary of Dispersion and Emulsion Polymerization Studies in Liquid and Supercritical Carbon Dioxide monomer(s) stabilizers ref(s) acrylamide 2,6-dimethylphenylene oxide divinylbenzene and ethylvinylbenzene methyl methacrylate styrene vinyl acetate vinyl acetate and ethylene amide end-capped poly(hexafluoropropylene oxide) 56 PFOA homopolymer, PFOA-based random copolymers, 60 and PS-PFOA block copolymer fluorinated methacrylate-PMMA block copolymer 55 PFOA homopolymer 34,81,84 PDMS-based macromonomer 90-92 fluorinated graft or block copolymers 94,176 PFOA homopolymer; PS-PFOA block copolymers 95 PDMS homopolymer; PS-PDMS block copolymers 96,177 FVE-MVE block copolymer 135 PFOA and PDMS homopolymers, PDMS macromonomer, 99,100 PVAc-PDMS and PVAc-PFOA block copolymers PDMS homopolymer; PVAc-PFOA block copolymer 100 the particle surface and must not be in a collapsed state at the reaction density. The majority of the work in dispersion polymeriza- tions in supercritical CO2 has focused on methyl methacrylate (MMA). However, several other mono- mers have also been investigated as shown in Table 2. In 1994, DeSimone reported the dispersion polym- erization of MMA in supercritical CO2.81 This work represents the first successful dispersion polymeri- zation of a lipophilic monomer in a supercritical fluid continuous phase. In these experiments, the authors took advantage of the amphiphilic nature of the homopolymer PFOA (Mn ) 1.1 × 104 or 2.0 × 105 g/mol) to effect the polymerization of MMA to high conversions (>90%) and high degrees of polymeriza- tion (>3000) in supercritical CO2. These polymeriza- tions were conducted in CO2 at 65 °C and 207 bar, and AIBN or a fluorinated derivative of AIBN was employed as the initiator. The spherical polymer particles that resulted from these dispersion poly- merizations were isolated following venting of CO2 from the reaction mixture. Scanning electron micros- copy showed that the product consisted of spheres in the 1-3 μm size range with a narrow particle size distribution (see Figure 3). In contrast, reactions that were performed in the absence of PFOA resulted in relatively low conversion (<40%). Moreover, the polymer that resulted from these precipitation poly- merizations had an unstructured, nonspherical mor- phology that contrasted sharply with the spherical polymer particles produced in the dispersion poly- merizations. Without a doubt, the amphiphilic PFOA macromolecule played a vital role in the stabilization of the growing PMMA colloidal particles. Recent work in this area has revealed that very low amounts (0.24 wt % based on MMA) of PFOA (Mw ) 1.0 × 106 g/mol) are needed to prepare a stable dispersion of PMMA latex particles.34,82 Again, the PMMA particles were in the 1-3 μm size range. In addition, a large percentage (up to 83%) of the stabilizer could be subsequently removed from the PMMA product by extraction with CO2. As a result of the relatively high cost of the stabilizer and the possible effects that residual stabilizer may have on product performance, the ability to remove and recycle the PFOA constitutes an important aspect of this system. The effects of the reaction time and pressure on the resulting conversion, molar masses, Figure 3. Scanning electron micrograph of PMMA par- ticles produced by dispersion polymerization in CO2 using PFOA as the stabilizer. and particle size of the polymer products were investigated. A gel effect occurs within the PMMA particles between 1 and 2 h of reaction time. This result parallels the gel effect within the polymer particles which is normally observed between 20 and 80% conversion in a typical dispersion polymerization in liquid organic media.44 More importantly, the ability of CO2 to plasticize PMMA facilitates the diffusion of monomer into the growing polymer particles, allowing the reaction to proceed to high conversion. To complement the use of fluorinated acrylates as stabilizers, the phase behavior of PFOA in CO2 was thoroughly investigated. These cloud point experiments indicated lower critical solution temperature (LCST) phase behavior. Further study on MMA dispersion polymerizations with PFOA stabilization investigated the influence of helium concentration in CO2 on the resulting PMMA particle sizes and distribution.83,84 This study was important since many tanks of CO2 are sold with a helium head pressure. It was found that the presence of 2.4 mol % helium in CO2 increased the PMMA average particle diameters from 1.9 to 2.7 μm,PDF Image | Polymerizations in Supercritical Carbon Dioxide
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