Polymerization mechanism of emulsion

The basic constituents of all commercial emulsion polymerization recipes are monomers, emulsifiers, and polymerization initiators. Other common components are modifiers, inorganic salts and free alkaH, and shortstops. The function of these different components and the mechanism of emulsion polymerization have been described (43,44). 

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

Therefore, the polymerization progresses within the micelle structure by following the traditional mechanism of emulsion polymerization. 

Features 2 to 4 are attributed to the aqueous medium. Emulsion polymerization forms submicrometer-sized particles, so-called latex particles. The particles are stabilized with ionic and/or noionic emulsifiers. The process to form submicrometer particles is very complicate because of the contribution of two phases, aqueous and oil, to particle. The mechanism of emulsion polymerization is described in the next section. 

Harkins proposed tile mechanism of emulsion polymerization in 1943 (2). His qualitative theory needs a few corrections but its basis has been and will be accepted. The theory is explained using Figure 11.1.2. 

The mechanism of emulsion polymerization ensures that the polymer concentration at the polymerization locus is semidilute or concentrated, which results in a greater probability of branch chain formation [299, 300]. This effect produces unique average branching densities and unique distributions of branching densities, that are significantly different from corresponding bulk polymerization [266,301]. 

Gardon, J.L. Mechanism of emulsion polymerization. Rubber Chem. Technol. 1970, 43, 74. 

The mechanisms of emulsion polymerization using classical emulsifiers are complex. The use of macromolecular emulsifiers complicates even more the picture of phenomena occurring in emulsion polymerization for the following reasons ... 

Polymerizations performed with an emulsifier above its critical micelle concentration with all the monomer solubilized within the micelles and without any monomer present as emulsion droplets may be described as micellar polymerizations [62]. Although such systems can never produce a high yield of polymer per unit volume they are advantageous if it is desired to use photochemical initiation, these being transparent whereas emulsions are opaque. Micellar polymerizations can help to elucidate the mechanism of emulsion polymerizations. They are useful practically when it is desired to copolymerize hydrophilic and hydrophobic monomers to synthesize associative thickeners [63,64]. 

Besides being an interesting method fw industrial processes, seeded polymerization of VC has provided valuable insight into kinetics and mechanisms of emulsion polymerization. 

It is obvious from the foregoing facts that the mechanism of emulsion polymerization involves far more than the mere bulk polymerization of monomer in a finely divided state. In fact, the very small particle size of the latex, relative to that of the original monomer emulsion, indicates the presence of a special mechanism for the formation of such polymer particles. 

Emulsion polymerization involves the emulsification of monomers in an aqueous phase, and stabilization of the droplets by a surfactant. Usually, a water-soluble initiator is used to start the free-radical polymerization. The final product is a dispersion of submicrometer polymer particles, which is called latex. The locus of polymerization is the micelle. Typical applications are paints, coatings, adhesives, paper coatings and carpet backings. The latex particles can have different structures (see Fig. 2). Excellent text books on the applications and structure-property relationships exist [11-15]. Besides a full description of the kinetics and mechanism of emulsion polymerization [16], a textbook adapted for use as material for people entering the field is also available [17]. 

The full mechanism of emulsion polymerization is still not completely worked out. It is not yet clear why a simultaneous increase in the polymerization rate and in the molecular weight of the product are often observed. Also, in emulsion polymerization, at the outset of the reaction the monomer is in the form of finely dispersed droplets. These droplets are about I fi in diameter. Yet, during the process of a typical polymerization, they are converted into polymer particles that are submicroscopic, e.g., 1000 A in diameter. 

Some Peculiarities of the Kinetics and Mechanism of Emulsion Polymerization [24-36]... 

Tauer, K., Hernandez, H., Kozempel, S., Lazareva, O., and Nazaran, P. (2008) Towards a consistent mechanism of emulsion polymerization - new experimental details. CoUoid Polym. Sci., 286, 499. 

The first hypothesis of the mechanism of emulsion polymerization was formulated by Harkins [305]. According to this hypothesis, the water-soluble initiator decomposes in the aqueous phase. 

SOME PECULIARITIES OF THE KINETICS AND MECHANISM OF EMULSION POLYMERIZATION... 

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