Entry rate coefficients in emulsion polymerization systems

Abstract

A detailed theory is presented for the rate coefficient for entry of free radicals into latex particles in emulsion polymerization systems; this yields an expression for the radical capture efficiency. The first-order rate coefficient for entry into a latex particle (ρ) can be accurately represented as ρ=ρ_A+αkn, where ρ_A is the rate coefficient for entry of free radicals arising directly from initiator, α is a ‘fate parameter’ giving the relative importance of re-entry and heterotermination of desorbed free radicals, k is the exit rate coefficient and n is the average number of free radicals per latex particle. Expressions for p_A and α in terms of the rate coefficients for the component aqueous-phase mechanisms are also presented, including the dependence on the concentration of initiator and number density of latex particles. This theory is applied to extensive data on the seeded emulsion polymerization of styrene: this is a typical non-polar monomer, for which capture efficiencies are commonly low. The data fitting includes an extensive sensitivity analysis. An excellent fit to the data is obtained, whence rate coefficients for the individual aqueous phase processes may be deduced. It is concluded for this system either that entering free radicals are colloidal in nature and/or that the entering oligomeric free radical must displace a surfactant molecule from the surface of the particle. Expressions enabling one to calculate ρ(and thus the capture efficiency) for use in modelling studies of non-polar monomers are presented.