Particle size distributions in electrosterically stabilized emulsion polymerization systems: testing the 'mid-chain-radical' hypothesis

It has been found [Thickett, S. C.; Gaborieau, M.; Gilbert, R. G. Macromolecules 2007 ,40, 4710−4720] that rate and characterization data on the seeded growth of latex particles electrosterically stabilized by a “hairy layer” of anchored poly(acrylic acid) (pAA) can be qualitatively and quantitatively explained by mechanistic events involving mid-chain radicals (MCRs). A considerable range of data was found to be consistent with the supposition that these MCRs are formed by abstraction on the pAA during polymerization, and are slow to propagate but quick to terminate, and thus are a cause of radical loss; they may also undergo β-scission leading to secondary particle formation. Results are reported here for four additional types of experiments on the evolution of the particle size distribution (PSD) in these systems; all these experiments have the potential to refute the mechanistic hypothesis. The experiments comprise (i) evolution of the PSD of small particles, (ii) evolution of the PSD in a bimodal system (competitive growth), (iii) growth rates of separate experiments of small and large particles with the same total amount of pAA, and (iv) the amount of secondary nucleation (formation of small particles) with both pAA and also with poly(ethylene oxide) steric stabilizers. In each case, the data are qualitatively and quantitatively consistent with the predictions of the mid-chain-radical mechanisms and thus support the general applicability of this effect.