Remarks on Heterophase Polymerization

Concluding Remarks on Heterophase Polymerization.—In order to illustrate the contrasting characteristics of emulsion and bulk phase polymerization, let us compare the rates of polymerization Rp and the degrees of polymerization Xn for styrene in the two systems at corresponding rates (R and p) of primary radical generation. At 60°C the rate constants (see Table XVII, p. 158) are 

Let the monomer concentration [M] be 5 molar both in the bulk phase and in the emulsion particles. The comparison is set forth below  

Since the same propagation rate constant applies to both bulk and emulsion polymerization, comparable rates of polymerization R must obtain when the number of emulsion particles is twice the number of radicals at the steady state in the bulk polymerization. An increase in the bulk rate at the given temperature can only be realized by an increase in the rate of initiation and, thus, an increase in the 

Polymerizations conducted in nonaqueous media in which the polymer is insoluble also display the characteristics of emulsion polymerization. When either vinyl acetate or methyl methacrylate is polymerized in a poor solvent for the polymer, for example, the rate accelerates as the polymerization progresses. This acceleration, which has been called the gel effect,probably is associated with the precipitation of minute droplets of polymer highly swollen with monomer. These droplets may provide polymerization loci in which a single chain radical may be isolated from all others. A similar heterophase polymerization is observed even in the polymerization of the pure monomer in those cases in which the polymer is insoluble in its own monomer. Vinyl chloride, vinylidene chloride, acrylonitrile, and methacryloni-trile polymerize with precipitation of the polymer in a finely divided dispersion as rapidly as it is formed. The reaction rate increases as these polymer particles are generated. In the case of vinyl chloride  

The polymerizations under consideration are characterized by high rates at low temperatures. Frequently the polymerization proceeds so rapidly when the catalyst and monomer are brought together 

---