FREE-RADICAL EXIT IN EMULSION POLYMERIZATION .2. MODEL DISCRIMINATIONVIA EXPERIMENT

In emulsion polymerizations, desorption (exit) from latex particles of monomeric radical species that arise from transfer can be an importan t determinant of the overall kinetics. An examination of various metho dologies for the testing of postulated free radical exit mechanisms is made. These utilize the model descriptions for the exit process prese nted in the accompanying article of Casey et al., employing data consi sting of conversion as a function of time for the approach to steady s tate polymerization conditions. Experimental data are presented on the exit rate coefficients as a function of such experimental parameters as: particle size, monomer concentration, and aqueous-phase free-radic al concentration for a series of styrene polymerizations at 50-degrees -C, where the average number of free radicals per particle (nBAR) neve r exceeds 0.5. It is demonstrated for these systems that while the con version/time dependence from a single run, under conditions sensitive to exit, is insensitive to mechanistic assumptions as to the fate of d esorbed free radicals, the variation of the exit rate coefficient with particle size so obtained suggests a second order dependence on nBAR, implying complete re-entry of desorbed free radicals under all condit ions studied. Once the monomeric radicals have re-entered, they are mo re likely to remain inside the particle where they will either propaga te or undergo termination rather than re-escape. The article also pres ents an estimate for the rate coefficient at 50-degrees-C of the first propagation step of the monomeric radical subsequent to transfer. The conclusions drawn here for seeded systems should prove useful for stu dy of particle nucleation mechanisms, when exit is particularly likely in small, newly formed, particles. (C) 1994 John Wiley & Sons, Inc.