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The dissemination of volatile organic compounds, chlorofluorocarbons, and aqueous-waste streams into our environment has prompted considerable worldwide concern, and environmental issues are now of paramount interest to the chemical industry. To survive, the chemical industry must be able to conform to more environmentally sound practices in the manufacture and processing of products. Herein, we describe the use of supercritical CO sub 2 continuous phase. This procedure allows for the synthesis of high molar mass acrylic polymers in he form of micrometer-sized particles with a narrow size distribution.
This polymerization methodology obviates the use of aqueous and organic dispersing media in this classical manufacturing route to many of the polymers made on a large commercial scale such as polystyrene, poly(alkyl acrylate)s, poly(ethylene-co-vinyl acetate), poly(vinyl chloride) (PVC), styrene-butadiene rubber, acrylonitrile-butadiene-styrene terpolymers, poly(acrylic acid), and poly(acrylamide) (1). Typically, these polymers are synthesized with either water (for water-insoluble polymers) or an organic solvent (for water-soluble polymers) used as the dispersing medium. Such heterogeneous polymerizations usually form at least two phases in which the starting monomers or the resulting polymer, or both, are in the form of a fine dispersion in an immiscible liquid phase; particle sizes typically range from 50 nm to several millimeters and can be controlled with surfactants to within a relatively narrow size distribution. There are basically four different heterogeneous polymerization techniques --precipitation, suspension, emulsion, and dispersion processes--which are distinguished by (i) the initial phase behavior of the polymerization mixture, (ii) the polymerization kinetics, and (iii) the mechanism of particle formation (2).
This report focuses on the design of a dispersion polymerization in supercritical CO sub 2 . Typically, a free radical dispersion polymerization starts as a one-phase, homogeneous system such that both the monomer and the polymerization initiator are soluble in the polymerization medium but the resulting polymer is not (3, 4). As a result, the polymerization is initiated homogeneously and the resulting polymer phase separates into primary particles. Once nucleated, these primary particles become stabilized by amphipathic molecules present in the system that prevent particle flocculation and aggregation. Polymer colloids produced by dispersion polymerizations are usually stabilized by a "steric" mechanism as compared with an electrostatic mechanism that is common to colloidal stabilization in aqueous environments (3-7). Steric stabilization of a colloidal dispersion is...