Exit Of Free Radicals Research Articles

Effect of a Mixed Anionic−Nonionic System of Surfactants on the Entry and Exit of Free Radicals into Polystyrene Particles

Seeded emulsion polymerizations of styrene were conducted with various ratios of the anionic surfactant sodium lauryl sulfate and the nonionic surfactant Triton X-405 adsorbed on the polystyrene latex particles. The polymerization kinetics were monitored utilizing the Mettler RC1 reaction calorimeter and subsequently analyzed to determine the effects of the adsorbed surfactants on the entry of free radicals into the particles and the desorption of radicals from the particles. All the reactions were run in interval III at 50 °C, and “zero−one” conditions were determined to exist in order to analyze the data by the “slope−intercept” method. Using the RC1, the start of the polymerization could be determined within 4 s. No significant effect of the surfactant ratio (40% surface coverage) was noted on the pseudo-first-order entry rate coefficients within experimental error. Larger experimental errors were noted in the determination of the pseudo-first-order exit coefficients with no discernible effect of the s...

Free radical exit in emulsion polymerization. I. Theoretical model

AbstractThe 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. © 1994 John Wiley & Sons, Inc.

The kinetics and compartmentalization of seeded emulsion polymerization of styrene with a series of seeds of different size

The Interval II kinetics and compartmentalization effect of emulsion polymerization of styrene with the seeds of five radii (r=40.2, 51.3, 76.5, 99.7 and 252.0 nm) have been studied. The kinetic parameters, i.e., the rate coefficients ρ, K and C which refer, respectively, to the entry of free radicals into latex particles, the exit of free radicals from the particles and the bimolecular termination of free radicals within the particles; nss and nss(thermal), the average number of free radicals per particle in the steady state and in the thermally induced background polymerization, respectively, are obtained. The propagating rate coefficient kp, the termination rate coefficient kt, as far as possible, are calculated in the runs. From this work, it can be concluded that the kinetic behavior, the kinetic parameters and the compartmentalization effect of emulsion polymerization are greatly influenced by the latex particle size.

The effect of aqueous-phase solubility on free-radical exit from latex particles

The technique of γ-radiolysis relaxation is used to study the rate coefficient for exit (desorption) of free radicals from styrene latex particles at 30 °C in the presence of methanol (which serves to increase the solubility of monomer in the aqueous phase). The observed exit rate coefficient is directly proportional to the solubility of monomer in the aqueous phase, until the latter becomes so high that the transfer-dominated limit is reached. These results are in quantitative agreement with the Nomura transfer/diffusion theory for exit.

Effects of hydrocarbon diluents on the kinetics of the seeded emulsion polymerization of styrene

Kinetic relaxation studies (using γ-rays from a 60Co source as initiator) of the seeded emulsion polymerization of styrene have shown that the presence of hydrocarbon diluents, such as ethylbenzene, toluene and cyclohexane, can significantly increase the rate coefficient for the exit of free radicals from the latex particles. This increase was evident even when the chain-transfer constant for the diluent was less than that for styrene. Such an increase may arise from the low reactivity of the diluent free radicals with the monomer molecules. This allows the diluent free radicals additional time, compared with the monomer free radical, to undergo diffusional exit from the particles before reaction with the monomer confines the species to the latex particles. The free-radical exit rate coefficient is thus determined inter alia by both the chain-transfer constant for the diluent and the reactivity of the diluent free radicals, as demanded theoretically. Large increases in the exit rate coefficient are predicted to occur with certain diluents at high replacements of the styrene by diluent.Seeded emulsion polymerizations of styrene initiated by potassium peroxydisulphate confirmed that certain diluents may increase the exit rate coefficient significantly. Surprisingly, it was found that the entry rate coefficient may also be increased significantly. This was attributed to the presence of the diluent promoting the stabilization of those free-radical species that are capable of entering the latex particles at lower degrees of aggregation. This reduces the rate of bimolecular termination of the free-radical species located outside the latex particles. At higher monomer replacement levels, however, it is possible that the rate of entry of free radicals into the latex particles may be reduced as a consequence of the depletion of the monomer concentration in the aqueous phase. Thus the entry rate coefficient in the presence of diluents is determined inter alia by two processes whose effects tend to be opposed: on the one hand, enhanced stabilization of the coagulating colloidal free-radical species and, on the other, depletion of the monomer concentration in the aqueous phase.

Emulsion polymerization kinetics. General solutions for Smith–Ewart cases I and II

Matrix methods are used to obtain general solutions for the Smith-Ewart case II mechanism for emulsion polymerizations; this relates to systems for which bimolecular termination is considered to be more important than free radical exit from the particles. These solutions are compared with simplified forms of the general solutions derived from case I, wherein exit is assumed to be more significant than termination. In neither case is resort made to the steady state approximation. There appears to be general agreement between theory and the results for the seeded emulsion polymerization of vinyl acetate, although discrepancies were apparent when very stringent comparisons were made.