A model, together with a comparison with previously published experimental data, is presented for initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation. The data had shown that a number of previous models are inapplicable, viz., those assuming that the rate-determining step for free-radical entry into a particle is either diffusional capture, surfactant displacement, or colloidal entry. The data support the supposition that the rate-determining step for freeradical capture by latex particles is aqueous-phase propagation to a critical degree of polymerization, whereupon capture (irreversible adsorption) of the resulting oligomeric free radical by a particle is essentially instantaneous. Mutual aqueous-phase termination of smaller species also occurs. When account is taken of the fact that the rate coefficients for (a) the first aqueous-phase propagation step and (b) aqueous-phase termination are both in the diffusion limit, this model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength. For styrene emulsion polymerization initiated by persulfate, the critical oligomer size for entry was found to be dimeric.

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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.

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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.