Heterogeneous polymerization emulsion

References cited provide details of polymerization of the monomers indicated. Heterogeneous polymerizations (emulsion, miniemulsion) are indicated by the monomer being in italics. Monomer/RAF agent combinations that are relatively ineffective are indicated by the monomer being in parentheses. DMAM, A/,W-dimethylacrylamide EHA, 2-ethylhexyl acrylate HPMAM, /V-(2-hydroxypropyl) methacrylamide MAM, methacrylamide NIPAM, W-isopropyl acrylamide tBA, ferf-butyl acrylate VBz, vinyl benzoate. 

Emulsion Polymerization. Emulsion and suspension reactions are doubly heterogeneous the polymer is insoluble in the monomer and both are insoluble in water. Suspension reactions are similar in behavior to slurry reactors. Oil-soluble initiators are used, so the monomer—polymer droplet is like a small mass reaction. Emulsion polymerizations are more complex. Because the monomer is insoluble in the polymer particle, the simple Smith-Ewart theory does not apply (34). 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

Heterogeneous polymerization is used extensively to control the thermal and viscosity problems. There are three types of heterogeneous polymerization precipitation, suspension, and emulsion. Emulsion polymerization is discussed in Chap. 4. Precipitation polymerizations begin... 

ATRP is a very potent method for preparing block copolymers by sequential monomer addition as well as star polymers using multifunctional initators. Furthermore, it can be applied also in heterogenous polymerization systems, e.g., emulsion or dispersion polymerization. In Example 3-15 the ATRP of MMA in miniemulsion (see also Sect. 2.2A.2) is described. 

Homopolymerization. The free-radical polymerization of VDC has been carried out by solution, slurry, suspension, and emulsion methods. Slurry polymerizations are usually used only in the laboratory. The heterogeneity of the reaction makes stirring and heat transfer difficult consequently, these reactions cannot be easily controlled on a large scale. Aqueous emulsion or suspension reactions are preferred for large-scale operations. The spontaneous polymerization of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen, fery pure monomer does not polymerize under these conditions. Heterogeneous polymerization is characteristic of a number of monomers, including vinyl chloride and acrylonitrile. 

Another interesting heterogeneous polymerization using macromonomers is a microemulsion copolymerization to produce particles 10-100 nm in diameter. Gan and coworkers [150] have prepared transparent nanostructured polymeric materials by direct polymerization of bicontinuous micro emulsions consisting... 

The preceding discussion has led us to the conclusion that the surface is the only locus of polymerization which needs to be considered in the heterogeneous polymerization of acrylonitrile. Radicals arrive at the surface at a rate determined by the decomposition of the initiator and efficiency of initiation. Propagation occurs on the surface at a rate determined by the activity of monomer at the surface. By analogy with emulsion polymerization, where monomer diffuses into the particles rapidly enough to maintain near equilibrium activity (14), we assume that the activity of the monomer adsorbed on the particle surface is approximately equal to the mole fraction in solution. The propagation rate constant is presumably influenced somewhat by the presence of the solid surface. 

Heterogeneous polymerizations proceed in two or more phases. Heterogeneity may be caused by the presence of a solid or of a gaseous phase or else the liquid monomer may be dispersed in another liquid with which it does not dissolve. Very important are the systems (a) with a solid initiator and (b) of two practically immiscible liquids. The former is useful for producing stereospecific polymers which are usually formed by a coordination mechanism. The latter makes possible an elegant and efficient removal of the heat of polymerization and it is applied technically with radical polymerizations in suspension or emulsion. 

There are four main types of liquid-phase heterogeneous free-radical polymerization microemulsion polymerization, emulsion polymerization, miniemulsion polymerization and dispersion polymerization, all of which can produce nano- to micron-sized polymeric particles. Emulsion polymerization is sometimes called macroemulsion polymerization. In recent years, these heterophase polymerization reactions have become more and more important... 

The modeling of heterogeneous polymerization systems is generally more complicated than that of the homogenous systems because mass and heat transfer effects between two or more immiscible phases must be considered. Industrially important heterogeneous polymerization reactions include emulsion polymerization, suspension polymerization, precipitation polymerization, and solid-catalyzed olefin polymerization. The general polymerization rate equation is represented simply as... 

Product type Small volume specialty polymers heterogeneous polymerization system (e.g., emulsion, suspension, precipitation) Small volume specialty polymers copolymers Large volume commodity polymers engineering polymers... 

Mathematical modeling is a powerful tool not only for the development of process understanding but also for that of the advanced reactor controls in polymerization processes. The modeling techniques for polymerization processes are reasonably well developed and several commercial simulation packages are available. The modeling of heterogeneous polymerizations such as precipitation polymerization and emulsion polymerization remains a challenge. In the past decade, excellent... 

Among the large variety of systems applied to carry out free-radical polymerization in heterogeneous media, emulsion polymerization is the most... 

Heterogeneous polymerization can be further divided into emulsion polymerization and suspension polymerization. In both types of polymerization, the monomers are dissolved in the dispersed phase. In suspension polymerization, the initiator is dissolved in the dispersed phase as well and nucleation and growth of the beads take place in the droplets. In emulsion polymerization, on the other hand, the initiator is dissolved in the continuous phase, leading to nucleation and bead growth from the continuous phase. 

Smith and Ewart (13a. 13b) quantified the Harkins theory by the equation R = k MpN/2 where Rp is the rate of propagation, kp is the rate constant for propagation, M is the monomer concentration in growing chain particles, and N the number of polymer particles per unit volume. If M is the constant, this equation is reduced to R = k N. Thus, the rate of emulsion polymerization should solely be a function of the number of polymer particles. In actuality, the reaction rate increases up to 20-25% conversion because of the increase in the number of growing radical chains then the rate steadies as does the number of polymer particles up to 70-80% conversion. Beyond this point, the rate drops off because of low monomer concentration. Thus, as Talamini (13c. 13d) has noted, available evidence indicates that emulsion polymerization of vinyl chloride does not resemble true emulsion polymerization as described by Smith and Ewart, but shows the general behavior of heterogeneous polymerization. 

Emulsions, suspensions, and dispersions are examples of colloidal systems. It is important to mention that these terms are not always used consistently in the literature and that this situation may be confusing for students and nonpolymer scientists [24]. From the point of view of polymer science and engineering, these terms refer to heterogeneous polymerizations, particularly polymerizations in aqueous/alcoholic dispersed media. Thus, the aforementioned terms have connotations that have to do with the initiator, monomer, and polymer solubility in each phase as well as with particle size and the main locus of polymerization. These aspects are treated in detail later for the moment, let us assume that there are no chemical reactions and that such terms are used in the context of colloid science. 

There are many unique polymerization processes which share a conunon heritage with emulsion polymerization, but which often are unrecognized as such. It is the purpose of this review to describe some of these emulsion polymerization-like processes and their products. Some further definition is in order unconventional emulsion polymerizations can be described as those processes whereby the product is a polymer latex that physically resembles latex from emulsion polymerization and cannot be grouped into any other recognized form of heterogeneous polymerization. In many cases the reasons why a process is not recognized as an emulsion polymerization is that the polymerization is not via a free-radical process. This review (hscusses four distinct types of polymerization processes, all of which have examples that produce latex particles and in many ways can be described as unconventional emulsion polymerizations. These are free-radical polymerization, ionic polymerization, transition metal catalyzed polymerization and enzyme-catalyzed polymerization. The precise systems discussed in this review are described in Table 23.1. 

The heterogeneity of emulsion polymerization systems offers unique possibilities of structural control of emulsion (co)polymers, on a molecular scale (intermolecular and intramolecular microstructure) as well as on a parficle-size scale (particle morphology). The kinetic and mechanistic features of emulsion (co)polymerization are strongly reflected in molecular size and its distribution, chemical conqxisition and its distribufion, particle moiphology, and product properties. A further fine tuning of polymer properties calls for advanced characterization techniques enable of revealing delicate structural details in emulsion (co)poIymers. 

Various heterogeneous polymerization reactions of hydrophilic or water-soluble monomers in the presence of either difnnctional or multifunctional cross-linkers have been mostly utilized to prepare weU-defined synthetic nanogels. They include precipitation, inverse (mini)emulsion, and inverse micio ulsion polymerization utilizing an uncontrolled free radical polymerization process. 

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