Radical polymerization kinetics particle formation

The emulsion polymerization of vinyl hexanoate has been studied to determine the effect of chain transfer on the polymerization kinetics of a water-insoluble monomer. Both unseeded and seeded runs were made. For unseeded polymerizations, the dependence of particle concentration on soap is much higher than Smith-Ewart predictions, indicating multiple particle formation per radical because of chain transfer. Once the particles have formed, the kinetics are much like those of styrene. The lower water solubility of vinyl hexanoate when compared with styrene apparently negates its increased chain transfer, since the monomer radicals cannot diffuse out of the particles. 

We begin by describing the current understanding of the kinetics of polymerization of classical unsaturated monomers and macromonomers in the disperse systems. In particular, we note the importance of diffusion-controlled reactions of such monomers at high conversions, the nucleation mechanism of particle formation, and the kinetics and kinetic models for radical polymerization in disperse systems. 

Steady-state conversions for VA and MMA polymerizations in a CSTR do not agree with reactor models based on Smith-Ewart Case II kinetics. This is not surprising since such a model does not consider many important phenomena. The particle-formation component of the Smith-Ewart Case II model is based on a simple mathematical relation which assumes that the rate of formation of new particles is proportional to the ratio of free (dissolved or in micelles) surfactant to total surfactant. This equation is based on the earlier concept of particle formation via free radical entry into micelles. 

In the derivation of the kinetic relations it was assumed that free radicals enter the particles one by one the initiation process just described satisfies this condition. This is not the case when radicals are formed by thermal decomposition of an oil-soluble initiator. Such decomposition produces pairs of radicals in the hydrocarbon phase. One would expect a pair of radicals, confined to the extremely small volume of a latex particle, to recombine rapidly. The kinetics of this type of polymerization have been described above. It is recalled here that the subdivision factor, z, and hence rate and degree of polymerization are smaller than 1 and decrease with a. These predictions from kinetic theory are in contradiction to experimental observations. Although some oil-soluble initiators, which are good catalysts in solution systems, are poor initiators in emulsion polymerizations—e.g., benzoyl peroxide—other thermally decomposing peroxides and azo compounds produce polymer in emulsion at rates comparable to those observed in polymerization initiated by water-soluble catalysts, where the radicals enter the particles one by one. Such is the case for cumene hydroperoxide, which at low concentrations yields a rate of polymerization per particle equal to that of a persulfate-initiated reaction. It must therefore be concluded that, although oil-soluble initiators may decompose into radical pairs within the particles, polymer radicals are formed one by one. The following mechanisms are consistent with formation of polymer radicals singly. 

The kinetics of emulsion polymerization is complex, involving a large number of species and at least two phases. The first quantitative approach to emulsion polymerization kinetics led to extensions by many others.The important events to consider are 1) the free-radical reactions of chain formation initiation, propagation, chain transfer, and termination and 2) the phase transfer events that control particle formation radical entry into particles from the aqueous phase, radical exit into the aqueous phase, radical entry into micelles, and the aqueous phase coil-globule transition. In free-radical emulsion polymerization, the fundamental steps are shown schematically in Fig. 1... 

The ultrasonification process is connected with the rapidly increased oil-water interfacial area as well as the significant re-organization of the droplet clusters or droplet surface layer. This may lead to the formation of additional water-oil interface (inverse micelles) and, thereby, decrease the amount of free emulsifier in the reaction medium. This is supposed to be more pronounced in the systems with non-ionic emulsifier. Furthermore, the high-oil solubility of non-ionic emulsifier and the continuous release of non-micellar emulsifier during polymerization influence the particle nucleation and polymerization kinetics by a complex way. For example, the hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier. The hydro-phobic non-ionic emulsifier (at high temperature) can act as hydrophobe. 

It should be noted that to date one more question related to the kinetics of the chain termination reaction in radical polymerization has not yet been conclusively solved. The diffusive nature of termination presupposes a quite definite, non-random distribution of radicals the number of radicals located close to one another must be smaller than that which corresponds to the random law of particle distribution in space. The initiation reaction, i.e., that of the formation of radicals, does not depend on their location in space. This reaction will, therefore, distort the distribution of radicals in space that corresponds to the diffusive termination and will tend to make... 

To represent dispersion polymerization in conventional liquid media, several models have been reported in the literature, mainly focused on the particle formation and growth [33, 34] or on the reaction kinetics. Since our first aim is the reliable description of the reaction kinetics, we focus on the second type of models only. The model developed by Ahmed and Poehlein [35, 36], applied to the dispersion polymerization of styrene in ethanol, was probably the first one from which the polymerization rates in the two reaction loci have been calculated. A more comprehensive model was later reported by Saenz and Asua [37] for the dispersion copolymerization of styrene and butyl acrylate in ethanol-water medium. The particle growth as well as the entire MWD were predicted, once more evaluating the reaction rates in both phases and accounting for an irreversible radical mass transport from the continuous to the dispersed phase. Finally, a further model predicting conversion, particle number, and particle size distribution was proposed by Araujo and Pinto [38] for the dispersion polymerization of styrene in ethanol. 

The kinetics of the free radical emulsion polymerization of a-meth-ylene-y-valerolactone has been investigated (58). Stable polymer latices could be prepared. A homogeneous nucleation is the dominant path for particle formation. Also, the miniemulsion copolymerization with styrene as comonomer has been investigated. Both the reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization and the RAFT bulk polymerization are weU controlled and copolymers with a narrow polydispersity are formed. 

For most emulsion systems, the rate of polymerization is controlled by the rate of entry and exit of free radicals to and from polymer particles, not by the rate of monomer diffusion to the polymerization sites. The entry of radicals into the polymer particles has been treated as a collisional process [14] as well as a diffusional process [15] and a colloidal process [16]. Nomura et al [17] pointed out that radical desorption from the polymer particles and micelles plays an important role in particle formation and numerous examples of deviations from the Smith-Ewart kinetic model have been attributed to radical desorption. 

In the second chapter (Preparation of polymer-based nanomaterials), we summarize and discuss the literature data concerning of polymer and polymer particle preparations. This includes the description of mechanism of the radical polymerization of unsaturated monomers by which polymer (latexes) dispersions are generated. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. The spherical shape of polymer particles and the uniformity of their size distribution are reviewed. The reaction mechanisms of polymer particle preparation in the micellar systems such as emulsion, miniemulsion and microemulsion polymerizations are described. The short section on radical polymerization mechanism is included. Furthermore, the formation of larger sized monodisperse polymer particles by the dispersion polymerization is reviewed as well as the assembling phenomena of polymer nanoparticles. 

---