Radical entry in emulsion polymerization: Propagation at latex particles/water interfaces

In emulsion polymerization, complete entry of an initiator-derived, surface-active radical may involve its adsorption onto latex particles/water interfaces and subsequently its propagation with one more monomer molecule therein. However, all publications to date have defined this propagation step as a three-dimensional bulk reaction between a surface-active entry radical and a monomer molecule. This is incorrect conceptually. It is proposed that the rate of the propagation of surface-active entry radicals with monomer at latex particles/water interfaces be expressed as −d[MZ⋅]w/l/dt=kIA[M]P[MZ⋅]aq. In this equation, A is the interfacial area between water and latex particles; [M]P and [MZ⋅]aq are the mean concentrations of monomer in the particle phase and entry radicals in the aqueous phase, respectively; kI is the radical propagation constant at the interfaces, and may be estimated via transition state theory. For seeded styrene polymerization by Hawkett et al. (J. Chem. Soc. Faraday Trans. 1 76 (1980) 1323), kI≈∼4.2×10−9kp(mol−1dm4s−1) is estimated. Here kp is the propagation rate coefficient in bulk polymerization. This alternative approach should be useful for one to simulate radical entry rate in emulsion polymerization where the propagation step may be rate-determining, such as under monomer-starved conditions.