Heterophase polymerizations: A physical and kinetic comparison and categorization

A set of physical and chemical criteria have been identified which provide a basis for the categorization of heterophase free radical polymerization processes, and the development of a systematic nomenclature. These include three quantitative transitional thresholds: a surface tension driving force, the type of stability (kinetic, thermodynamic), and the level of surfactant with respect to the critical micelle concentration. The nucleation mechanism and the average number of macroradicals within the polymer particles provide a secondary criterion for process distinction. Four mutually exclusive domains have been identified: I. Macroemulsion Polymerization, consisting of subdomains where Suspension (Ia) and Emulsion (Ib) behavior dominate, II. Inverse-Macroemulsion Polymerization, a water-in-oil analogy to the preceding encompassing Inverse-Suspension (IIa) and Inverse-Emulsion (IIb), and the thermodynamically stable Microemulsion (III) and Inverse-Microemulsion (IV) polymerizations. Within each domain a unique set of physical and chemical phenomena dominate. A reaction mechanism or model of colloidal behavior must, therefore, be specific for each regime. This is particularly important for water-in-oil polymerizations due to the unique chemistry of various organic phases and interfacial recipes. It is recommended that further development proceed independently for Inverse-Suspension, Inverse-Emulsion and Inverse-Microemulsion polymerizations, as has been the precedent for the more extensively researched oil-in-water polymerizations. A set of criteria is also developed to define and distinguish physically or kinetically identical “emulsion” and “suspension” polymerization processes.