Dispersion and Emulsion Polymerizations

This chapter considers copolymerizations. There are two basic types of heterogeneous polymerizations: The initial reaction medium comprises several phases and polymerization occurs in a heterogeneous system, as in emulsion and suspension reactions, and the reaction medium is initially homogeneous and the polymer forms a separate phase as the polymerization proceeds. Examples are dispersion reactions, described below, and polymerization of acrylonitrile. The chapter focuses on dispersion and emulsion polymerizations. In dispersion polymerization process, the monomer and initiator are soluble in the continuous phase and the polymer particles, which precipitate as they are produced, are stabilized against coagulation by dispersants that comprise different segments that are respectively soluble and insoluble in the continuous phase. Dispersion polymerizations have been used successfully as an alternative to solution polymerization of vinyl polymers for application as surface coatings. An emulsion consists of a discontinuous liquid phase dispersed throughout a different, continuous liquid phase. Milk and the sap of the rubber tree are examples of naturally occurring emulsions. The term latex is used also to denote aqueous dispersions of polymers. Most emulsion polymerizations are free-radical reactions. The main difference from alternative free-radical polymerizations, such as those in bulk, solution, and suspension systems, is that the propagating macroradicals in emulsion reactions are isolated from each other. Encounters between macroradicals are hindered as a consequence, and termination reactions are less frequent than in comparable systems in which the reaction mixture is not subdivided. Emulsion polymerizations thus often yield high-molecular-weight products at fast rates when suspension or bulk reactions of the same monomers are inefficient.