Emulsion polymerization radical absorption

Hayashi et al., 1989], involving the addition of monomer and initiator to a previously prepared emulsion of polymer particles, is especially useful for this purpose since it allows the variation of certain reaction parameters while holding N constant. Thus, h in seeded styrene polymerization drops from 0.5 to 0.2 when the initiator concentration decreases from 10-2 to 1CT5 M. At sufficiently low Ru the rate of radical absorption is not sufficiently high to counterbalance the rate of desorption. One also observes that above a particular initiation rate ([I] = lO-2 M in this case), the system maintains case 2 behavior with h constant at 0.5 and Rp independent of Ri. A change in Ri simply results in an increased rate of alternation of activity and inactivity in each polymer particle. Similar experiments show that h drops below 0.5 for styrene when the particle size becomes sufficiently small. The extent of radical desorption increases with decreasing particle size since the travel distance for radical diffusion from a particle decreases. 

Unzueta et al. [18] derived a kinetic model for the emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA) employing both the micellar and homogeneous nucleation mechanisms and introducing the radical absorption efficiency factor for micelles, F, and that for particles, Fp. They compared experimental results with model predictions, where they employed the values of Fp=10 and Fn,=10", respectively, as adjustable parameters. However, they did not explain the reason why the value of Fp, is an order of magnitude smaller than the value of Fp. Sayer et al. [19] proposed a kinetic model for continuous vinyl acetate (VAc) emulsion polymerization in a pulsed... 

Usually particle formation by initiation in the monomer droplets droplet nucleation) is not considered important in conventional emulsion polymerization. This is because of the low absorption rate of radicals into the monomer droplets, relative to the other particle formation rates. However, when the monomer... 

The influence of the emulsifier (SHS) concentration on Np is more pronounced in the conventional emulsion polymerization system (Rp°c[SHS]y, y= 0.68) than in mini-emulsion polymerization (y=0.25). This result is caused by the different particle formation mechanism. While homogeneous nucleation is predominant in the conventional emulsion polymerization, monomer droplets become the main locus of particle nucleation in mini-emulsion polymerization. In the latter polymerization system, most of the emulsifier molecules are adsorbed on the monomer droplet surface and, consequently, a dense droplet surface structure forms. The probability of absorption of oligomeric radicals generated in the continuous phase by the emulsifier-saturated surface of minidroplets is low as is also the particle formation rate. 

Clearly, the dynamics of radical transfer (radical absorption or entry and desorption or exit) has a great influence on the kinetics of emulsion polymerization. Thus, a better understanding of these processes is crucial for both process and product control. 

There is a close relationship between particle nucleation and radical absorption by the particles or droplets of the segregated phase. Both processes depend on reactions in the continuous phase, and are influenced mutually. The idea that a growing radical enters a monomer-swoUen micelle along this direction is an oversimpHfication that has existed for almost 70 years [52]. At the time, a lack of any relevant experimental technique meant that direct studies of the nucleation step during emulsion polymerization were not possible. 

Figure 3.33 lists a recipe for emulsion polymerization of polystyrene in a water dispersion of monomer droplets and soap micelles [20]. The reaction is started by light-sensitive, water-soluble initiators, such as benzoyl peroxide. If one compares the sizes of the dispersed droplets, one notices that the small soap micelles that contain also styrene in their interior are most likely to occasionally initiate a polymerization of the monomer on absorption of a free radical. Once initiated, the reaction continues until a second free radical molecule enters the micelle. Then the reaction is terminated, until a third radical starts another molecule. Monomers continuously add to the micelles, so that the polymerization continues. Keeping the free radical generation constant, a relatively narrow molar mass distribution can be obtained. 

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... 

Unzueta and Forcada [31] studied the emulsion copolymerization of methyl methacrylate and n-butyl acrylate. It was assumed that both micellar nucle-ation and homogeneous nucleation are operative in this emulsion polymerization system. Based on the experimental data and computer simulation results, the values of the free radical capture efficiency factors for monomer-swollen micelles (f ) and polymer particles (Fj) that serve as adjustable parameters in the kinetic modeling work are approximately 1(T and 10, respectively. The reason for such a difference in the free radical capture efficiency factors is not available yet. Table 4.2 summarizes some representative data regarding the absorption of free radicals by the monomer-swollen micelles and polymer particles obtained from the literature. 

As mentioned above, the two most popular reaction mechanisms involved in the absorption of free radicals by the monomer-swoUen micelles and polymer particles are the diffusion- and propagation-controlled models. Nevertheless, liotta et al. [39] were inclined to support the colUsion-controlled model. A dynamic competitive particle growth model was developed to study the emulsion polymerization of styrene in the presence of two distinct populations of latex particles (i.e., bimodal particle size distribution). Comparing the on-line density and particle size data with model predictions suggests that absorption of free radicals by the latex particles follows the collision-controlled mechanism. 

Reabsorption of the desorbed free radicals by the latex particles may contribute to the emulsion polymerization kinetics, as proposed by Ugelstad and Hansen [19], According to Gilbert [55], the overall rate of absorption of free radicals by the latex particles (pa) can be written as... 

In emulsion polymerization, X , in the absence of chain transfer reactions, is equal to the rate of growth of a polymeric radical divided by the rate of absorption of oligomeric radicals by the latex particle [105] ... 

The application of these comprehensive models to the prediction of the emulsion polymer molecular weight distribution requires a fundamental understanding of the very conaplex reaction mechanisms and knowledge of various kinetic parameters (e.g., the rate coefficients for the absorption of free radicals by the latex particles, the desorption of radicals out of the particles, and the bimolecular termination reaction). However, these mathematical models in combination with the polymer molecular weight distribution data may serve as a useful tool for estimating the values of the kinetic parameters involved in emulsion polymerization. 

---