Desorption of free radicals

These results explain the findings of Blackley and Haynes who also showed that the molecular weight of the polymer formed in the presence of ethyl benzene was lower than that in its absence. Calculation from their experimental data shows that their n varied from 0.005 to 0.039 radicals per particle, well into Case 1. Thus, their explanation on the basis of the Trommsdorff "gel" effect cannot be correct since this requires the mutual termination of two macroradicals in a particle, which obtains only under Case 3 kinetics. Similar experiments on the effect of the diluents on "insitu" (unseeded) and seeded emulsion polymerization indicates that n decreases due to desorption of free radicals from the particles (27). 

A. Radical desorption. Data for a number of experimental studies have been modeled by a kinetic scheme that includes desorption of free radicals. Presumably, radical desorption follows a radical transfer reaction. The mobile free radical could possibly cross the particle-water Interface into the water phase. Nomura (3 has published a recent review paper on radical desorption. 

Other recent contributions to this aspect of the subject include those of Brooks and Qureski and Brooks. The former of these papers is concerned with the distribution and desorption of free radicals during the emulsion polymerization of styrene. The latter gives a simplified treatment of the type of problem which has been dealt with by Birtwistle and Blackley, and the Napper group in essence, the Brooks treatment involves truncation of the infinite series of Smith-Ewart differential difference equations. 

Based on the above reaction scheme and the assumptions that (a) a monomer-swollen micelle can be successfully converted into a particle nucleus via the capture of a free radical in the continuous aqueous phase, (b) the volumetric growth rate for particle nuclei (p = dvpidt, where Vp is the volume of a particle nucleus) is constant, (c) desorption of free radicals out of the particle does not occur, and (d) the amount of surfactant molecules dissolved in the continuous aqueous phase and adsorbed on the monomer droplet surfaces is insignificant, the rate of formation of particle nuclei is then equal to the rate of generation of free radicals in the continuous aqueous phase. 

Taking into account the effect of desorption of free radicals out of the particle nuclei, the following equation was developed to predict the rate of particle nucleation in the emulsion polymerization of relatively hydrophilic monomers. 

Based on the Smith-Ewart theory [1], Stockmayer [3] derived the following equations to calculate the steady value of n when desorption of free radicals out of the latex particles is insignificant (i.e., kdes = 0). 

At low values of a, the emulsion polymerization system is characterized by (a) the very slow overall rate of absorption of free radicals by the latex particles and/or the very large population of latex particles (i.e., pJNp ) and (b) the very large rate constant for desorption of free radicals out of the latex particles and/or the very large ratio of the surface area to volume of a single latex particle (i.e., /CdesV p ) tliis regime, desorption of free radicals out of the latex particles plays an important role in the emulsion polymerization kinetics. The value of n is smaller than 0.5 [Smith-Ewart Case 1 kinetics, Eq. (4.6)] and n increases rapidly with increasing a. At medium values of a, the emulsion polymerization system is characterized by (a) the very small rate... 

It should be noted that the rate of absorption of free radicals by the latex particles from the continuous aqueous phase (p or a ) is not equal to the rate of generation of free radicals in the continuous aqueous phase (p, or a) when desorption of free radicals out of the latex particles (m) and/or the bimolecular termination of free radicals in the continuous aqueous phase (Y) cannot be neglected in the emulsion polymerization system. In addition to the particle nucleation mechanisms discussed in Chapter 3, to gain a fundamental understanding of transport of free radicals in the heterogeneous reaction system (e.g., absorption of free radicals by the latex particles, desorption of free radicals out of the latex particles and reabsorption of the desorbed free radicals by the latex particles) is thus required to predict the emulsion polymerization... 

Desorption of Free Radicals in Emulsion Homopolymerization Systems... 

The pseudo-homopolymerization approach [30, 52] can be used to calculate the average rate coefficient (kjes) for desorption of free radicals out of the latex particles in emulsion copolymerization systems. For a binary polymerization system comprising comonomers A and B, k es can be expressed as... 

Effect of Interfacial Properties on Desorption of Free Radicals... 

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

Strictly speaking, any model based on the time-independent thermodynamics cannot be used to adequately predict the concentration of monomer in latex particles during Smith-Ewart Interval II. This is because the free radical polymerization of monomer in the discrete latex particles is governed by the simultaneous kinetic events such as the generation of free radicals in the continuous aqueous phase, the absorption of free radicals by the particles, the propagation of free radicals with monomer molecules in the particles, the bimolecular termination of free radicals in the particles, and the desorption of free radicals out of the particles. The equilibrium (or saturation) concentration of monomer in the growing latex particles may not be achieved if the rate of consumption of monomer in the major reaction loci is much faster than that of diffusion of monomer molecules from the monomer droplets to the reaction loci. Therefore, the equilibrium concentration of monomer in the latex particles represents an upper limit that is ultimately attainable in the course of polymerization. Nevertheless, the general... 

Chern [42] developed a mechanistic model based on diffusion-controlled reaction mechanisms to predict the kinetics of the semibatch emulsion polymerization of styrene. Reasonable agreement between the model predictions and experimental data available in the literature was achieved. Computer simulation results showed that the polymerization system approaches Smith-Ewart Case 2 kinetics (n = 0.5) when the concentration of monomer in the latex particles is close to the saturation value. By contrast, the polymerization system under the monomer-starved condition is characterized by the diffusion-con-trolled reaction mechanisms (n > 0.5). The author also developed a model to predict the effect of desorption of free radicals out of the latex particles on the kinetics of the semibatch emulsion polymerization of methyl acrylate [43]. The validity of the kinetic model was confirmed by the experimental data for a wide range of monomer feed rates. The desorption rate constant for methyl acrylate at 50°C was determined to be 4 x 10 cm s ... 

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