the performance of these dispersions. Living polymerization processes that are carried out via emulsion polymerization such as iodine-transfer polymerization and RAFT have been developed. Controlled coagulation techniques can enable better control of the particle size and distribution. Components Monomers Typical monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly soluble in water. Solid monomers are difficult to disperse in water. If monomer solubility is too high, particle formation may not occur and the reaction kinetics reduce to that of solution polymerization. Ethene and other simple olefins must be polymerized at very high pressures (up to 800 bar). Comonomers Copolymerization is common in emulsion polymerization. The same rules and comonomer pairs that exist in radical polymerization operate in emulsion polymerization. However, copolymerization kinetics are greatly influenced by the aqueous solubility of the monomers. Monomers with greater aqueous solubility will tend to partition in the aqueous phase and not in the polymer particle. They will not get incorporated as readily in the polymer chain as monomers with lower aqueous solubility. This can be avoided by a programmed addition of monomer using a semi-batch process. Ethene and other alkenes are used as minor comonomers in emulsion polymerization, notably in vinyl acetate copolymers. Small amounts of acrylic acid or other ionizable monomers are sometimes used to confer colloidal stability to a dispersion. Initiators Both thermal and redox generation of free radicals have been used in emulsion polymerization. Persulfate salts are commonly used in both initiation modes. The persulfate ion readily breaks up into sulfate radical ions above about 50 °C, providing a thermal source of initiation. Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agent such as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50 °C. Although organic peroxides and hydroperoxides are used in emulsion polymerization, initiators are usually water soluble and partition into the water phase. This enables the particle generation behavior described in the theory section. In redox initiation, either the oxidant or the reducing agent (or both) must be water-soluble, but one component can be water-insoluble. Surfactants Selection of the correct surfactant is critical to the development of any emulsion polymerization process. The surfactant must enable a fast