TABLE OF CONTENTS Polymerization in aqueous emulsions, which has been widely developed technologically, represents a special case of free radical chain polymerization in a heterogeneous system [52 58]. Most emulsion polymerization systems comprise a water-insoluble monomer in water with a surfactant and a free radical initiator. Although it might be thought that polymerization of water-insoluble monomers in an emulsified state simply involves the direct transformation of a dispersion of monomer into a dispersion of polymer, this is not really the case, as evidenced by the following features of a true emulsion polymerization:
The polymer emulsion (or latex) has a much smaller particle size than the emulsified monomer, by several orders of magnitude.
The polymerization rate is much faster than that of the undiluted monomer, by one or two orders of magnitude.
The molecular weight of the emulsion polymer is much greater than that obtained from bulk polymerization, by one or two orders of magnitude.
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.
The mechanism of emulsion polymerization, as originally proposed by Harkins [59], can best be understood by examining the components of this system, as depicted in Fig. 2, for a...
