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emulsion system are obtained from electron microscopy, light scattering, ultracentrifugation, photon correlation spectroscopy, and other techniques [Debye and Anacker, 1951; Kratohvil, 1964; Munro et al., 1979]. 4-1b-2 Site of Polymerization
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The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production Ri is typically of the order of 1013 radicals L 1 s 1 . (The symbol r is often used instead of Ri in emulsion polymerization terminology.) The locus of polymerization is now of prime concern. The site of polymerization is not the monomer droplets since the initiators employed are insoluble in the organic monomer. Such initiators are referred to as oil-insoluble initiators. This situation distinguishes emulsion polymerization from suspension polymerization. Oil-soluble initiators are used in suspension polymerization and reaction occurs in the monomer droplets. The absence of polymerization in the monomer droplets in emulsion polymerization has been experimentally veri ed. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insigni cant amount (approximately <0.1%) of polymer is found in the monomer droplets in such experiments. Polymerization takes place almost exclusively in the micelles. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. Polymerization of the monomer in solution undoubtedly takes place but does not contribute signi cantly, since the monomer concentration is low and propagating radicals would precipitate out of aqueous solution at very small (oligomeric) size. The micelles act as a meeting place for the organic (oil-soluble) monomer and the water-soluble initiator. The micelles are favored as the reaction site because of their high monomer concentration (similar to bulk monomer concentration) compared to the monomer in solution. As polymerization proceeds, the micelles grow by the addition of monomer from the aqueous solution whose concentration is replenished by dissolution of monomer from the monomer droplets. A simpli ed schematic representation of an emulsion polymerization system is shown in Fig. 4-1. The system consists of three types of particles: monomer droplets, inactive micelles in which
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Fig. 4-1 Simpli ed representation of an emulsion polymerization system.
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polymerization is not occurring, and active micelles in which polymerization is occurring. The latter are no longer considered as micelles but are referred to as polymer particles. An emulsi er molecule is shown as  to indicate one end () is polar or ionic and the other end ( ) nonpolar. Two mechanisms for particle nucleation (i.e., formation of polymer particles) have been discussed. The process described above, called micellar particle nucleation, occurs when radicals from the aqueous phase enter the micelles. (The term heterogeneous particle nucleation has been proposed as an alternative to the term micellar particle nucleation.) The radicals may be primary radicals or, much more likely, oligomeric radicals formed by solution polymerization with degrees of polymerization of $2 5 [De Bruyn et al., 2002; Dong and Sundberg, 2002]. Homogeneous particle nucleation involves solution-polymerized oligomeric radicals becoming insoluble and precipitating onto themselves (or onto dead oligomer in solution) [Fitch et al., 1969; Hansen and Ugelstad, 1978]. The precipitated species become stabilized by absorbing surfactant (from solution, monomer droplets, and micelles) and on subsequent absorption of monomer are the equivalent of polymer particles formed by micellar nucleation. The relative extents of micellar and homogeneous necleation are expected to vary with the surfactant concentration and the solubility of monomer in water. Micellar nucleation is the predominant nucleation process when the surfactant concentration is well above CMC. For styrene and methyl methacrylate, more than 99% of particle nucleation occurs by micellar nucleation [Herrera-Ordonez and Olayo, 2000, 2001; Saldivar et al., 1998]. Since MMA is much more water-soluble than styrene (16 vs. 0.07 g L 1 ), solubility in water is not important in determining the mechanism for nucleation well above CMC. Whether this is the case for vinyl acetate and other monomers more water-soluble than MMA is unclear. Around CMC, micellation nucleation is still the predominant mode of nucleation, but homogeneous nucleation is present more for vinyl acetate and methyl methacrylate and less for styrene. The situation is much different for all monomers when the surfactant concentration is well below CMC. Micelles are absent below CMC and only homogeneous nucleation occurs. In fact, the occurrence of emulsion polymerization in the absence of micelles is evidence for the homogeneous nucleation mechanism. It has been suggested that an important growth process for the rst-formed polymer particles, sometimes called precursor particles, is coagulation with other particles and not polymerization of monomer. This coagulation, called coagulative nucleation, is then considered as part of the overall nucleation process for the formation of mature polymer particles whose subsequent growth occurs entirely by polymerization [Feeney et al., 1984]. Coagulative nucleation is probably important below CMC. The driving force for coagulation of precursor particles is their relative instability compared to larger particles, that is, there is insuf cient surfactant to stabilize a larger number of smaller-sized particles. Once the particles reach a larger size, suf ciently large to be stabilized by the available surfactant, there is no longer a driving force for further coagulation and further growth occurs only by polymerization. Coagulative nucleation is much less important, if at all, above CMC since there is suf cient surfactant to stabilize smaller-sized particles. 4-1b-3 Progress of Polymerization
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A variety of behaviors are observed for the polymerization rate versus conversion depending on the relative rates of initiation, propagation, and termination, which are in turn dependent on the monomer and reaction conditions (Fig. 4-2). Irrespective of the particular behavior observed, three intervals (I, II, III) can be discerned in all emulsion polymerizations based on the particle number N (the concentration of polymer particles in units of number of
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