A kinetic model is presented that describes the reaction behavior of emulsion copolymerization systems where significant polymerization occurs in both the particle and aqueous phases. Equations for predicting aqueous-phase free-radical concentrations and aqueousphase and particle-phase reaction rates are developed. A method for estimating the radical entry rate coefficient is also presented. The model is applied to two seeded carboxylated emulsion copolymerization systems, acrylic acid-styrene and methacrylic acid-styrene. Both experimental and predicted results reveal that the reaction behavior is greatly affected by the type of acid monomer, partition of monomer between the various phases, and locus of polymerization. The mechanism for the acrylic acid-styrene system is more complicated than that for the methacrylic acid-styrene system. Evidence suggests that the primary reaction locus in the acrylic acid-styrene system shifts from the particles to the aqueous phase after the hydrophobic monomer, styrene, has been consumed.

SYNOPSISPolymerizations of partially neutralized methacrylic acid ( MAA) were performed in both solution and emulsion systems. Polymerizations of MAA in solution were performed at an overall degree of neutralization ranging between 0 and 1. The rate of polymerization of the acid is found to decrease as the degree of neutralization increases due to increased electrostatic repulsion of the dissociated acid species (anions). The degree of neutralization of the unreacted monomer increases as the conversion increases. A kinetic model based on a copolymerization mechanism is used to describe the reaction behavior. Partially neutralized methacrylic acid was also polymerized with styrene in a seeded emulsion system. The reaction rates of both the acid and styrene decrease as the overall degree of neutralization increases. A previously developed emulsion copolymerization kinetic model is extended to account for reaction of the anions and used to investigate the overall "terpolymerization" of the acid, anions, and styrene.

Reaction in the aqueous phase is often significant in emulsion polymerization systems that contain one or more water-soluble monomers. Accurate prediction of the partition of monomers between the aqueous, particle, and droplet phases is necessary for determining reaction rates, copolymer compositions, and surface properties of the latex particles. A model for predicting the partition of monomers between the various phases has been developed for an emulsion copolymer system.-A method for experimentally obtaining thermodynamic interaction parameters that are often not available in the literature has also been developed. This method was applied to two carboxylic acid/styrene systems. The model predictions of the monomer partitioning for these systems were in good agreement with experimental results measured from batch emulsion copolymerization samples. IntroductionThe rate of reaction in an emulsion copolymerization system containing water-soluble monomer(s) depends on the rates in the particle and aqueous phases. The rates of reaction in each phase depend on the local concentrations of monomer. Therefore, it is very important to understand how each monomer is partitioned between the aqueous phase, particles, and droplets.Preliminary experiments involving mixtures of acid monomer (either acrylic acid, AA, or methacrylic acid, MAA, both 100% soluble in water), water, and styrene were conducted to observe the effect of monomer (acid + styrene)/water ratio on the partition of the acid between the aqueous and organic phases. The partition coefficients were measured by mixing specified amounts of the three components, separating the two phases, measuring the aqueous-phase concentration of acid, and then using a mass balance to determine the concentration of acid in the organic phase. No polymer particles were present, and it was assumed that the concentration of styrene in the aqueous phase was negligible. Figure 1 shows that increasing the monomerfwater ratio resulted in a significant increase in the fraction of MAA in the organic phase but produced almost no effect on the fraction of AA distributed in the organic phase. The different partition behavior between the two monomers may be related to differences in hydrophobicity. Due to the extra methyl group, MAA is more hydrophobic than is AA. If the amount of acid is increased relative to the amount of water present, MAA is more likely to associate with the organic phase to a greater extent than does AA due to its more hydrophobic nature or, conversely, the water is more likely to solvate the more hydrophilic AA monomer units than the less hydrophilic MAA monomer units.A more fundamental study of MAA/styrene/water and AA/styrene/water mixtures using the approach of Prausnitz et al. (1986) with a wider range of monomer concentrations led to a better understanding of the partition of acid between styrene and water. These systems are comprised of a solute (acid) that is distributed between two immiscible liquid solvents (styrene, water). Liquid/ liquid equilibrium may be de...

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