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

Abstract

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 [ M Z ⋅ ] w / l / d t = k I A [ M ] P [ M Z ⋅ ] aq . In this equation, A is the interfacial area between water and latex particles; [ M ] P and [ M Z ⋅ ] aq are the mean concentrations of monomer in the particle phase and entry radicals in the aqueous phase, respectively; k I 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), k I ≈ ∼ 4.2 × 10 −9 k p ( mol −1 dm 4 s −1 ) is estimated. Here k p 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.