Entry of free radicals into latex particles in emulsion polymerization

Abstract

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.

Surface Patterning with Colloidal Monolayers

“…[31,195] For styrene, this length was determined to be 5 monomer units. [195][196] The fate of the aqueous oligoradical and therefore of the course of reaction is determined by whether or not 94 Particle synthesis and characterization it enters a seed particle before reaching the critical chain length. This point is marked as A in the scheme.…”

The Effect of Allylic Sulfide-Mediated IrreversibleAddition-Fragment Chain Transfer on the EmulsionPolymerization Kinetics of Styrene

“…Here, ρ i is the rate of radical production in the aqueous phase, and f is the initiator efficiency. The Maxwell-Morrison Model predicts f well for styrene emulsions, with the efficiency term f " r p2k d rIsk t,aq q 0.5 pk p,aq C w q`1 s p1´zq [26], where k t,aq (= 4ˆ10 9 L/M/s) and k p,aq (estimated as 4k p ) are the termination and propagation rate coefficients for aqueous phase radicals, respectively, C w is the monomer concentration in the aqueous phase (taken as 4.3ˆ10´3 M ) [27], and z is the critical degree of polymerization for irreversible entry of a radical into a particle (z = 2 fits most data well) [28]. The parameter k d is the rate constant of initiator decomposition, rIs is the initiator concentration, V p is the Polymers 2015, 7, volume of a polymer particle, k t,p is the termination rate constant in the polymer particles, and k dM is the rate constant for monomeric radical desorption from the polymer particles, given by Equation (9):…”

An Advanced Approach on the Study of Emulsion Polymerization: Effect of the Initial Dispersion State of the System on the Reaction Mechanism, Polymerization Rate, and Size Distribution of Polymer-Monomer Particles

“…These models are the "collision process" [123,124], "diffusion process" [27,29], "diffusion/propagational" model (Maxwell et al [125]), "collision/empirical" model (Dougherty [126] and Penlidis et al [127]), "surface coverage" model (Yeliseyeva and Zuikov [128]), and "colloidal" model (Penboss et al [129]). …”