The sterically stabilized emulsion polymerization of styrene initiated by a water‐soluble initiator at different temperatures has been investigated. The rate of polymerization (Rp) versus conversion curve shows the two non‐stationary‐rate intervals typical for the polymerization proceeding under non‐stationary‐state conditions. The shape of the Rp versus conversion curve results from two opposite effects—the increased number of particles and the decreased monomer concentration at reaction loci as the polymerization advances. At elevated temperatures the monomer emulsion equilibrates to a two‐phase or three‐phase system. The upper phase is transparent (monomer), and the lower one is blue colored, typical for microemulsion. After stirring such a multiphase system and initiation of polymerization, the initial coarse polymer emulsion was formed. The average size of monomer/polymer particles strongly decreased up to about 40% conversion and then leveled off. The initial large particles are assumed to be highly monomer‐swollen particles formed by the heteroagglomeration of unstable polymer particles and monomer droplets. The size of the “highly monomer” swollen particles continuously decreases with conversion, and they merge with the growing particles at about 40–50% conversion. The monomer droplets and/or large highly monomer‐swollen polymer particles also serve as a reservoir of monomer and emulsifier. The continuous release of nonionic (hydrophobic) emulsifier from the monomer phase increases the colloidal stability of primary particles and the number of polymer particles, that is, the particle nucleation is shifted to the higher conversion region. Variations of the square and cube of the mean droplet radius with aging time indicate that neither the coalescence nor the Ostwald ripening is the main driving force for the droplet instability. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 804–820, 2003

ABSTRACT:The microemulsion polymerization (MEP) of butyl acrylate (BA) stabilized by ionic emulsifier and initiated by oil-soluble dibenzoyl peroxide (DBP) or lauroyl peroxide (LPO) initiators have been studied under conventional (without ultrasound irradiation, CMEP) and ultrasonic (NMEP) conditions. The polymerization rate vs. conversion curve of the microemulsion polymerization of BA initiated by DBP was described by two nonstationary rate intervals. Four nonstationary rate intervals with two rate maxima appear in the microemulsion polymerization of BA initiated by LPO. The maximal rate of polymerization increases with temperature and the increase is much more pronounced under the conventional conditions. The overall activation energy is much larger under the conventional conditions (¼ 84 kJÁmol À1 ) than under the ultrasound (¼ 20 kJÁmol À1 ) conditions. The exit (desorption) rate constants The principle behind the formation of transparent or semitransparent microemulsions (fine emulsion with a droplet size 10-60 nm) is the very low interfacial tension caused by the penetration of coemulsifier into the droplet surface layer. Three-or four-component mixtures containing water, monomer, emulsifier and coemulsifier can form thermodynamically stable micellar solutions (microemulsions). The addition of coemulsifier increases the thermodynamic stability of micelles driving more emulsifier into micellar state, while on the other hand it can decrease the kinetic stability of micelles leading to faster formation and dissolution of micellar aggregates in solution. The thermodynamic stability of micelles is discussed in terms of how emulsifier distributes between the monomeric emulsifier and micellar state. The kinetic stability is discussed in terms of the average time of micellar aggregate. Cosolvents (coemulsifiers) are known to penetrate into micelles. The effect of such penetrating cosolvents can be discussed in terms of two aspects: an increase in distance between emulsifier groups and a decrease in the dielectric constant of micellar layer. This can explain the decrease of CMC as a result of dilution of micellar surface charge. 1 Furthermore, four-component mixtures (called miniemulsions with the droplet size 100-500 nm) containing water, monomer, emulsifier and hydrophobe form mostly kinetically stable emulsions. The principle behind the making of stable miniemulsions is the introduction of a hydrophobic compound into the monomer droplets to retard the diffusion of monomer out of the monomer droplets. In both microemulsion and miniemulsion, the principal locus of particle nucleation is the emulsified monomer droplet. In the former case the dependence of the rate vs. conversion is described by a curve with the maximum at ca. 20-40% conversion and the nucleation proceeds up to the final conversion. In the latter case, the particle nucleation is somewhat shortened up to ca. 30-50% and the dependence of the polymerization rate of styrene vs. conversion can be described by a curve with two maxima (four rate intervals...

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