SYNOPSISThe exit or desorption of free radicals from latex particles is an important kinetic process in an emulsion polymerization. This article unites a successful theory of radical absorption (i.e., initiator efficiency), based on propagation in the aqueous phase being the rate determining step for entry of charged free radicals, with a detailed model of radical desorption. The result is a kinetic scheme applicable to true "zero-one" systems (i.e., where entry of a radical into a latex particle already containing a radical results in instantaneous termination), which is still, with a number of generally applicable assumptions, relatively simple. Indeed, in many physically reasonable limits, the kinetic representation reduces to a single rate equation. Specific experimental techniques of particular significance and methods of analysis of kinetic data are detailed and discussed. A methodology for both assessing the applicability of the model and its more probable limits, via use of known rate coefficients and theoretical predictions, is outlined and then applied to the representative monomers, styrene and methyl methacrylate. A detailed application of the theory and illustration of the methodology of model discrimination via experiment is contained in the second article of this series.
SYNOPSISIn emulsion polymerizations, desorption (exit) from latex particles of monomeric radical species that arise from transfer can be a n important determinant of the overall kinetics. An examination of various methodologies for the testing of postulated free radical exit mechanisms is made. These utilize the model descriptions for the exit process presented in the accompanying article of Casey et al., employing data consisting of conversion as a function of time for the approach to steady state 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-radical concentration for a series of styrene polymerizations a t 50°C, where the average number of free radicals per particle ( i i ) never exceeds 0.5. It is demonstrated for these systems that while the conversion/ time dependence from a single run, under conditions sensitive to exit, is insensitive to mechanistic assumptions as to the fate of desorbed free radicals, the variation of the exit rate coefficient with particle size so obtained suggests a second order dependence on 6, implying complete re-entry of desorbed free radicals under all conditions studied. Once the monomeric radicals have re-entered, they are more likely to remain inside the particle where they will either propagate or undergo termination rather than re-escape. The article also presents a n estimate for the rate coefficient at 50°C of the first propagation step of the monomeric radical subsequent to transfer. The conclusions drawn here for seeded systems should prove useful for study of particle nucleation mechanisms, when exit is particularly likely in small, newly formed, particles.
Bimolecular termination processes in the seeded emulsion polymerization of styrene have been studied by using large seed particles and -ray initiation. The observed relaxation kinetics differed significantly from those previously found for methyl methacrylate in that the results could not be explained simply in terms of propagation-driven ("residual" or "reaction-diffusion") termination. The data have been successfully interpreted, however, by a "short-long" model that differentiates between free radicals attached to short chains, whose mobility is not significantly diminished by entanglements, and those attached to long, entangled chains. The latter are formed by the rapid growth to entanglement length of the former, whereas the short chains arise both from free-radical entry into the particles and from chain transfer to monomer by long chains. Bimolecular termination between short and long chains is the predominant radical-annihilation event. The dependence upon weight fraction of polymer of the rate coefficient obtained experimentally for short-long termination is in good agreement with that calculated from the rate of diffusion of short species through the latex particles. The relaxation-derived termination rate coefficients were also found to predict the experimentally observed Trommsdorff-Norrish gel effect in chemically initiated seeded emulsion polymerization systems.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.