Emulsion polymerization fundamentals

Acrylonitrile and its comonomers can be polymerized by any of the weU-known free-radical methods. Bulk polymerization is the most fundamental of these, but its commercial use is limited by its autocatalytic nature. Aqueous dispersion polymerization is the most common commercial method, whereas solution polymerization is used ia cases where the spinning dope can be prepared directly from the polymerization reaction product. Emulsion polymerization is used primarily for modacryhc compositions where a high level of a water-iasoluble monomer is used or where the monomer mixture is relatively slow reacting. 

The verification of EPM on the well characterized styrene and styrene-MMA polymerizations has allowed us to use the same model structure to obtain fundamental insights into emulsion polymerizations involving other monomers of significant importance to Du Pont. 

The emulsion polymerization system fundamentally includes hydrophobic monomer, water-soluble initiator, emulsifier, and water. Therefore, the initial system of emulsion polymerization is truly an emulsion. 

The foundations of emulsion polymerization were described originally by Harkins [39]. The first theoretical treatment was proposed by Smith and Ewart [40]. The theory was later modified to some extent by O Toole [41] and more fundamentally by Garden [42], who proposed an unsteady-state mechanism for the concentration of free radicals in the emulsion particles. Tauer [43], Gilbert [44] and Lovell and El-Aasser [45] have produced recent reviews. 

As discussed in Chapter 4, emulsion polymerization received a significant boost in the United States during the Second World War. When Japan overran countries that supplied natural rubber to the West, a crash program to manufacture synthetic rubber was initiated in the United States and Canada. The product was called Government Rubber-Styrene (GR-S), and was produced by the emulsion polymerization of butadiene and styrene. The fundamental recipe for GR-S is still used as a teaching tool for those learning the art and science of emulsion polymerization. 

These kinds of advances in control implementation in turn bring new life to efforts to develop detailed models of other polymerization reactions, since it is no longer such a major leap to on-line implementation. Efforts to model emulsion polymerization and heterogeneous Ziegler-Natta polymerization are among the most ambitious [26-28], Fundamental modeling efforts on... 

In addition to the practical interest, the process presents challenges encouraging further fundamental exploration. A thorough study not reported here, has been performed on the mechanism and kinetics of the polymerization of acrylamide in AOT/water/toluene microemulsions (Carver, M.T.r Dreyer, U. Knoesel, R. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press. Carver, M.T. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press). The termination reaction of the polymerization was found to be first order in radical concentration, i.e. a monoradical reaction instead of the classical biradical reaction. Another major conclusion was that the nucleation of particles is continuous all throughout the polymerization in contrast to conventional emulsion polymerization where particle nucleation only occurs in the very early stages of polymerization. These studies deserve further investigations and should be extended to other systems in order to confirm the unique character of the process. 

The theory also has relevance to the so-called seeded " emulsion polymerization reactioas- In these reactions, polymerization is initial in the presence of a seed latex under conditions such that new particles are unlikely to form. The loci for the compartmentalized free-radical polymerization that occurs are therefore provided principally by the particles of the initial seed latex. Such reactions are of interest for the preparation of latices whose particles have, for instance, a core-shell" structure. They are also of great interest for investigating the fondamentals of compartmentalized free-radical polymerization processes. In this latter connection it is important to note that, in principle, measurements of conversion as a function of time during nonsteady-state polymerizations in seeded systems offer the possibility of access to certain fundamental properties of reaction systems not otherwise available. As in the case of free-radical polymerization reactions that occur in homogeneous media, investigation of the reaction during the nonsteady state can provide information of a fundamental nature not available through measurements made on the same reaction system in the steady state. 

From a fundamental point of view it is interesting to speculate on tbe differences that could exist between the kinetics of emulsion polymerization initiated by y radiation and those of a conventional chemical initiator with, for example, potassium persulfate. At dose rates jiving a free radical flux comparable to those achieved with chemical initiation any differences... 

Another problem involves the classification of these metal-based heterogeneous systems into suspension, dispersion, and emulsion polymerizations similarly to conventional systems. This is due to not only a lack of detailed analysis of reaction mechanisms and particle sizes but also fundamental differences in several aspects such as the locus of initiation and the molecular weight of polymers in comparison with the conventional counterparts. The terms suspension and emulsion will be used in the following sections for simple classification but are not based on the strict definition for conventional free radical systems. 

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 heterogenous nature of emulsion polymerization processes, and the stepwise mechanisms involved add complications to the understanding of the kinetics. Despite the investigations conducted in emulsion polymerization over the last few decades, the scientific representation of these complex processes remains incomplete. With recent advances in fundamental chemistry and colloid science, model uncertainties have diminished. 

Summary. All of the phases and the physical and chemical mechanisms discussed in this section are important during the course of an emulsion polymerization reaction. They influence the reaction kinetics and the properties of the latex produced. Not all of the phenomena that can occur are understood in a quantitative manner. Nevertheless, considerable advances have been made in the fundamental understanding and the commercial exploitation of emulsion polymerization processes. The remainder of this chapter will focus on reactor types and reaction kinetics. 

In spite of its widespread technical use, the fundamental principles and the mechanism of this type of polymerization are still not completely understood. This is due partly to the complexity of the heterogeneous system of an emulsion itself, which only in recent years has been sufficiently cleared up to allow an investigation of polymerizations taking place in such systems. Although a considerable number of patents pertain to empirical rlil s and conditions of emulsion polymerization and date back as far as 1912, it was only around 1936 that an explanation of the mechanism of the reaction was attempted. 

Research sponsored by the government and private corporations has contributed much to the understanding of fundamentals that govern the complex process of emulsion polymerization. Emulsifiers play the triple role of emulsifsdng the monomers, furnishing micelles which are the sites of polymer initiation, and stabilizing the latex during the polymerization and afterward. To obtain satisfactory rates of polymerization at low tem-... 

Polymerization reactions at the liquidjliquid interface are of fundamental importance in emulsion polymerization, which has been used on the industrial scale [46]. Electrochemical polymerization at the liquid liquid interface was first reported by Cunnane and Evans [47] for the oxidation of 1-methylpyrrole and 1-phenylpyrrole dissolved in DCE with Fe in... 

The basis for the kinetic mechanisms of inverse emulsion polymerization can be found in the seminal woik of Vanderhoff et al. published in 1962 which dealt with the polymerization of sodium p-vinylbenzene sulfonate in xylene [26]. There was then a gap of almost two decades before new fundamental studies were reported, mainly motivated by the novel applications developed in the field of oil recovery processes. However, the results are in many respects contradictory and... 

Although batch emulsion polymerization is not frequently used, it will be discussed first because it is easier to imderstand as the fundamental processes occur in a sequential way, whereas in the semicontinuous and continuous modes the processes occur simultaneously. 

---