Emulsion polymerization free-radical capture

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. 

Sajjadi [47] developed two mechanistic models for the particle nucleation process involved in the semibatch emulsion polymerization of styrene under the monomer-starved condition. In the first model, Smith-Ewart theory was extended to take into account the particle nucleation under the monomer-starved condition. The number of latex particles per unit volume of water is proportional to the surfactant concentration, the rate of initiator decomposition, and the rate of monomer addition, respectively, to the 1.0,2/3, and -2/3 powers. The second model considers the aqueous phase polymerization kinetics and its effect on the efficiency of free radical capture by the monomer-swollen micelles. This model is capable of predicting some features of the particle nucleation process. 

In the case of more water-soluble monomers and (amphiphilic) macromonomers, the Smith-Ewart [16] expression does not satisfactorily describe the particle nucleation. The HUFT [9,10] theory, however, satisfactorily describes the polymerization behavior or the particle nucleation of such unsaturated hydrophilic and amphiphilic monomers. The HUFT approach implies that primary particles are formed in the aqueous phase by precipitation of oligomer radicals above a critical chain length. The basic principals of the HUFT theory is that formation of primary particles will take place up to a point where the rate of formation of radicals in the aqueous phase is equal to the rate of disappearance of radicals by capture of radicals by particles already formed. Stabilization of primary particles in emulsifier-free emulsion polymerization may be achieved if the monomer (or macromonomer) contains surface active groups. Besides, the charged radical fragments of initiator increases the colloidal stability of the polymer particles. 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

This observation seems to be in line with the Smith-Ewart concepts. The adsorption of surfactants on the surfaces of latex particles influences the capture by the particles of low-molecular-weight polymers formed in the aqueous solution. This in turn affects the reaction kinetics and the formation of new particles. The number of free radicals per particle, which is usually considered to be constant during the major phases of an emulsion polymerization, seems to vary considerably during the polymerization of vinyl acetate [139]. 

In emulsion polymerization, the rate of generation of free radicals is about lO Vm-s while the number of monomer-polymer particles for typical recipes, N, is in the range 10 to 10 particles/ml of the aqueous phase. Consequently, if all the initiator radicals are captured by the monomer-qxilymer particles, each particle will acquire, at the most, a radical every 1 to 100 s. It can be shown that if a particle contains two radicals, mutual annihilation of radical activity will occur within a time span of the order... 

The reactions in the aqueous phase lead initially to a change in the conductivity and subsequently to the formation of latex particles accompanied by the drop in the transmission (cf. Figure 10). Moreover, the shape of the conductivity curve is qualitatively the same as observed for surfactant-free emulsion polymerizations initiated with potassium peroxodisulfate. The bend of the conductivity curves marks the onset of particle nucleation as conducting species are captured in the diffuse electrical double layer of the particles. These results clearly prove that side reactions of carbon radicals in water lead to conducting species. The zeta-potential of the particles is pH-dependent and negative at pH >4. First hints that such radicals can attack water molecules have been obtained by NMR investigations of polymers made by normal emulsion polymerization (i.e. in the presence of surfactants) initiated with azo-initiators.P Ongoing studies try to clarify the reaction mechanisms. 

In emulsion polymerizations, the latex particles once formed swell with monomer, capture free radicals from the aqueous phase and become the primary loci of polymerization. Polymerization thus takes place in the presence of preformed polymer, the weight fraction (Wp) of which may vary from, say, 0.3 to near 1.0. To understand the polymerization in such systems, in which the free radicals are compartmentalized within the latex particles, it is necessary to understand how the preformed polymer influences the course of any subsequent polymerization. Conversely, compartmentalized systems can be regarded as providing a convenient means by which the influence of preformed polymer on the subsequent course of a free radical polymerization may be investigated quantitatively. 

Emulsion polymerization of vinyl chloride is initiated by a water-soluble initiator such as potassium persulfate. Initially in the reactor, monomer droplets are dispersed in the aqueous phase (continuous phase) containing initiator and surfactant (emulsifier). As the reactor content is heated, the initiator decomposes into free radicals. When the surfactant concentration exceeds the critical micelle concentration (CMC), micelles are formed. Free radicals or oligomers formed in the aqueous phase are then captured by these micelles. Vinyl chloride monomer is slightly soluble in water. As the monomer dissolved in water diffuses into micelles containing radicals, polymerization occurs. With an increase in monomer conversion in the polymer particles, separate monomer droplets become smaller and eventually they disappear. The monomer concentration in polymer particles is constant as long as liquid monomer droplets exist. The rate of emulsion polymerization is represented by... 

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