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Polymerizations in Supercritical Carbon Dioxide Chemical Reviews, 1999, Vol. 99, No. 2 551 while decreasing particle size distribution from 1.29 to 1.03. Furthermore, the small percentage of helium in the reaction mixture decreases the solvent strength of the medium, as determined by solvatochromic investigations. Other scientists have noted the in- crease in retention times in SCF chromatography85-87 and the reduced extraction rate in SCF extractions88 when helium is present in the CO2. An acoustic technique, used by Kordikowski and co-workers, measured the composition of helium in CO2 tanks and showed the content to be unpredictable, presum- ably due to common lack of equilibration in the cylinder.89 Therefore, these diverse experiments illustrate that even seemingly “benign” components such as an inert gas can effect the solvent quality of CO2, and the CO2 purity needs to be taken into consideration for process development and scale-up issues if the process, such as dispersion polymeriza- tion, is to be commercialized. Another approach to preparing monodisperse PMMA particles in CO2 has been the use of graft copolymers as stabilizers. While PFOA is an effective steric stabilizer for PMMA, fluoropolymers in general are expensive. The other class of CO2-soluble poly- mers includes PDMS (12). PDMS-based surfactants are cheaper, but are less soluble than PFOA at comparable molecular weights in CO2. The first work in this area by DeSimone involved the use of a monomethacrylate-terminated PDMS macromono- mer (13), which reacts to form the stabilizer in situ.90 However, these studies showed that only a small portion of the added macromonomer actually copo- lymerized; the majority of the unreacted macromono- mer could be removed from the surface of the particles by extraction with hexane or CO2. Polymer- izations were conducted at both liquid and super- critical conditions, and by varying the reaction con- ditions, particles with a narrow particle size distribution were obtained in sizes ranging from 1.1 to 5.8 μm. In contrast, PDMS homopolymer gave low yield and unstable latexes, presumably due to its inability to anchor and its lack of affinity for the PMMA particles. The dispersion polymerization of MMA stabilized by PDMS-monomethacrylate has recently been investigated in detail by O’Neill and co-workers, who studied the particle formation91 and growth92 for this system using turbidimetry. In the particle formation regime, mechanisms of coagulative nucleation and controlled coagulation were identified. To prevent uncontrolled coagulation (precipitation), a threshold pressure (207 bar) and stabilizer concen- tration (2 wt % based on monomer) were required. Latex stability was limited by the low solubility of surfactant in the absence of monomer in CO2. The chronology of yield, particle size, morphology, particle number density, and molecular weight was studied. The results agreed with the experiments by Shaffer90 and with predictions from the model of Paine93 for conventional dispersion polymerizations. A study by Lepilleur and Beckman employed a preformed graft copolymer dispersant which had an anchoring backbone of poly(MMA-co-hydroxyethyl methacrylate) and CO2-soluble poly(perfluoropro- pylene oxide) side chains (3).94 Variations in the molecular weight and architecture of the stabilizer were explored by examining the effect of factors such as graft chain length and graft density on the resulting PMMA polymer colloid. At a given backbone length, as the graft chain length was changed, the trends in results varied, depending on the length of the backbone. For a given graft chain length, increas- ing the graft density (number of grafts on the backbone) resulted in better stabilization. At a con- stant backbone length, increasing the graft density resulted in reduced particle size and distribution, as did more, shorter grafts compared to fewer, longer grafts. As the backbone length was increased, the particle size and distribution decreased. It was shown that the backbone length must be long enough to anchor to the particles, but there must be enough soluble component to maintain the overall solubility of the surfactant in CO2 for a successful dispersion polymerization. Indeed, the dispersion polymerization of MMA in compressed CO2 has been investigated in detail. Homopolymers, random copolymers, macromono- mers, and graft copolymers have all been effectively employed as stabilizers to prevent coagulation in the PMMA/CO2 system. The dispersion polymerization of styrene in super- critical CO2 using amphiphilic diblock copolymers has also been studied in detail. The surfactants which were investigated contained a PS anchoring block and either a PFOA95 or PDMS (14)96 soluble block. These reactions yield spherical PS particles which were isolated in the form of a dry, free-flowing powder. The resulting high yield (>90%) of PS was obtained in the form of a stable polymer colloid comprised of submicron-sized particles. For the block copolymeric stabilizers, the anchor-to-soluble balance (ASB), or ratio of the two block lengths, of the stabilizer was found to be a crucial factor affecting both the stability of the resulting latex in CO2 and the particle morphology. The affinity of these am- phiphilic diblock copolymers for the PS particle surface was confirmed by interfacial tension mea- surements in a CO2 continuous phase.97 In the studies of PFOA-based surfactants, the effects of surfactant block chain lengths, stabilizer concentration, and anchor block length on PS product were studied.95 A series of PS-b-PFOA copolymers were studied in which the length of both blocks was increased. Increasing the anchor and soluble block lengths resulted in decreased particle size (from 0.40 μm to 0.24 μm) and particle size distribution (from 8.3 to 1.1), presumably due to more effective stabi- lization of PS particles. With a constant anchor length of 3.7 × 103 g/mol PS, varying the PFOA block from 1.4 × 104 to 2.5 × 104 g/mol did not produce any trends in PS molecular weight or particle size. In this series, the anchor block may have been too short for strong adsorption to the particles, reducing the efficiency of stabilization. The effect of varying the anchor length at constant soluble block length was not studied with PS-b-PFOA. The PFOA homo- polymer was not an efficient stabilizer for PS, result- ing in low polymer yields of 44%. In contrast, PFOA homopolymer was an efficient stabilizer for PMMA,PDF Image | Polymerizations in Supercritical Carbon Dioxide
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