A New Method for Determining Propagation Rate Coefficients at High Fraction of Polymer

Most industrial free-radical polymerizations are carried out in (or pass into) a region where the weight fraction (w,) of polymer is high. Under these conditions, the propagation rate coefficient ($1 is diffusion-controlled: the viscosity of the monomer/polymer mixture is so high that the rate-limiting step is the diffusion of monomer to the macroradical; k, decreases from its low-conversion value (k,'), and the polymerization rate decreases. Hitherto, the only reliable means of obtaining k, at high wp has been through e.s.r.'y2 [values of $(w,) inferred from global modelling of observed kinetic data are model-dependent and can be in gross error1]. We give here a new method for obtaining k,(wp) at high wp using rate data for a monodisperse seeded emulsion polymerization. In a seeded emulsion polymerization in 'Interval III ' ,~ the time dependence of the fractional conversion x is given by4