Polymerization Kinetics in Nonuniform Latex Particles

Araki et al. [38] studied the feasibility of applying resonant soft X-ray scattering to chemically heterogeneous soft condensed matter nanomaterials. Two structured styrene-acrylic emulsion polymer particles with average particle diameters close to 230 nm were examined. This technique can be used to obtain the effective radii corresponding to the two polymer phases within the latex particles, and it can serve as a powerful complementary tool to neutron and hard X-ray scattering techniques for the characterization of structured soft condensed matter nanomaterials. 

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The Smith and Ewart-Stockmayer-O Toole treatments [48-50] (see Chapter 4) that are widely used to calculate the average number of free radicals per particle (n) are based on the assumption that the various components of the monomer-swollen latex particles (e.g., monomer, polymer, free radicals, chain transfer agent, etc.) are uniformly distributed within the particle volume. A latex particle in emulsion homopolymerization of styrene involves uniform distribution of monomer and polymer within the particle volume except perhaps for a very thin layer near the particle surface. In the case of free radicals, this uniform distribution would only hold in a stochastic sense. However, as illustrated in Eq. (8.1), free radicals are not distributed uniformly in the latex particles when water-soluble initiators are used to initiate the free radical polymerization. The assumption of uniform distribution of free radicals in the latex particles would be valid only if the particles are very small or chain transfer reactions are the dominate mechanism for producing free radicals. If such a nonuniform free radical distribution hypothesis is accepted, the very basis of the Smith and Ewart-Stockmayer-O Toole methods might be questioned. Despite this potential problem, the Stockmayer-O Toole solutions for the average number of free radicals per particle have been used for kinetic studies of many emulsion polymerization systems. The theories seem to work reasonably well and have been tested extensively with monomers such as styrene. 

Chem and Poehlein [52] developed a kinetic model based on the nonuniform free radical distribution function to predict the grafting efficiency of the emulsion emulsion polymerization of styrene in the presence of polybutadiene seed latex particles. The predominant grafting reaction appears to be the attack of growing polystyrene chains on the allyl hydrogen atoms of... 

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