Monomer droplets in emulsion polymerizations

The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production if, is typically of the order of 1013 radicals L-1 s-1. (The symbol p is often used instead of Rj 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 verified. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insignificant 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. 

Vieira et al. [176] also showed that the NIR spectra of polymer latices were sensitive to the presence of monomer droplets in the reaction medium. This is in accordance with the results previously reported by Santos et al. [55] for polymer suspensions, as monomer droplets in emulsion polymerizations have sizes typically in the same range as observed in suspension polymerizations. This could be used to avoid the formation of monomer droplets during starved polymerizations and to detect abnormal process operation conditions, considerably improving the process operation. 

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