Free radical exit in emulsion polymerization. I. Theoretical model

Abstract

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.

Cross-Chain Transfer Rate Constants of Styrene-Terminated Radicals to Methyl Acrylate and Methyl Methacrylate

“…2,3,5 It has been argued that the only intraparticle termination event operating in zero-one systems is instantaneous termination between a long radical and a monomeric radical (formed by chain transfer). 1 This monomeric radical can only have entered the particle already containing the long radical, after having escaped from another particle first (exit). In that case the rate of chain stopping by termination is equal to the rate of reentry of monomeric radicals (F re ) and not to the total rate of entry (F) as suggested by Clay et al 18 This also means that the contribution of termination is very small and that the error introduced by not extrapolating to a zero initiator concentration is negligible, because for a system to be zero-one, the rate of chain transfer to a small molecule should be much higher than the rate of entry F I 1,2 (ignoring the effect of thermal entry).…”

“…In emulsion polymerization systems with a low average number of radicals per particle (<0.5), the escape of radicals from the particles (exit) is rate-determining, and the exit rate is determined by the rate of transfer to monomer 1 or other small molecules capable of diffusing out of the particles. 2 In emulsion polymerization systems with a relatively high average number of radicals per particle and, in general, in homogeneous free-radical polymerizations, the rate of polymerization is determined by termination, the rate of which is enhanced considerably by transfer to monomer or other small molecules.…”

Polymer chain extension in semibatch emulsion polymerization with RAFT‐based transfer agent: The influence of reaction conditions on polymerization rate and product properties

“…Thus, a particle has either zero or one radical. Theory 32 and experiments 33 confirm that the zero-one mechanism is valid when the particles are small ($ 100 nm) and are saturated with monomer such that glassy transition is not reached. Polymerization within a relatively large particle is governed by pseudo-bulk kinetics.…”

Modeling particle size distribution in emulsion polymerization reactors

“…Another possibility, more frequently used [8,19,21,22], consists in further dividing particles having one radical in two populations, according to whether the radical is monomeric or polymeric. Since only monomeric radicals are assumed to desorb, this facilitates the description of radical desorption [23][24][25]. For an ideal STR, the corresponding PBEs are, …”

“…Theoretical and experimental tests to check if a given system obeys zero-one kinetics have been developed by Gilbert and co-workers [15,24,27,28]. For example, a necessary but not sufficient condition is that 5 .…”