Emulsion and Suspension Polymerization

4 Polymer-LDH Nanocomposites Prepared by Emulsion and Suspension Polymerization 

Both emulsion and suspension polymerization are heterogeneous polymerization techniques, in which the polymerization occurs in the monomer-swollen latexes and monomer droplets dispersed in a continuous liquid phase, usually water, respectively. The emulsion polymerization technique involves a water-soluble initiator, a water-insoluble monomer and a micelle-forming surfactant. The monomer is emulsified and stabilized in a continuous aqueous phase by the 

Similarly, the suspension system consists of an initiator, monomer and stabilizer in the liquid medium. However, in suspension polymerization, the initiator is oil soluble and the stabilizer is a non-micelle-forming stabilizing agent such as poly(vinyl alcohol) or polyvinylpyrroKdone. The locus of polymerization, in contrast to emulsion polymerization, is in the monomer droplets. The final particles have a size between 20 pm and 2 mm. Therefore, suspension polymerization differs from emulsion polymerization in the larger size of the particles, the applied stabilizing agent and the kind of initiator employed. 

Many advantages are shown by emulsion and suspension polymerization. First, the continuous water phase can act as an excellent heat conductor and allow the heat to be removed from the system. This is an effective way to increase the polymerization rate of many reactions. Second, the viscosity remains close to that of water and is not dependent on molecular weight since the polymer molecules are contained within the particles. Third, emulsion polymerization is unique in the sense that an increase in molar mass can be achieved without reducing the rate of polymerization. Therefore, high molecular weight polymers can be obtained at fast polymerization rates. By contrast, there is a tradeoff between molecular weight and polymerization rate in bulk and solution free-radical polymerization. 

2 Preparation of Polymer-LDH Nanocomposites via Emulsion and Suspension Polymerization 

In bulk- and solution-phase free-radical polymerization, there is a tradeoff between molecular weight and polymerization rate. This is especially true for controlled/living radical polymerization. In emulsion polymerization, however, high molecular weight polymers can be made at fast polymerization rates. Emulsion polymerization is a type of radical polymerization that is frequently used for making polymers of high molecular weight. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified with surfactants in a continuous phase of water. 

Suspension polymerization is a similar method. In this process a monomer, or a mixture of monomers, is dispersed by mechanical agitation 

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A new process, from Norway, has filled the size gap between emulsion and suspension polymerization techniques [7,8]. This novel polymerization method, the so-called swollen emulsion polymerization has been developed by Ugelstad for producing uniform polymeric particles in the size range of 2-100 /nm. This process comprises successive swelling steps and repolymerizations for increasing the particle size of seed polymer particles by keeping the monodispersity of the seed latex. 

Free-radical polymerization of alkenes has been carried out in aqueous conditions.115 Aqueous emulsion and suspension polymerization is carried out today on a large scale by free-radical routes. Polymer latexes can be obtained as products (i.e., stable aqueous dispersions... 

Dispersion polymerization involves an initially homogeneous system of monomer, organic solvent, initiator, and particle stabilizer (usually uncharged polymers such as poly(A-vinyl-pyrrolidinone) and hydroxypropyl cellulose). The system becomes heterogeneous on polymerization because the polymer is insoluble in the solvent. Polymer particles are stabilized by adsorption of the particle stabilizer [Yasuda et al., 2001], Polymerization proceeds in the polymer particles as they absorb monomer from the continuous phase. Dispersion polymerization usually yields polymer particles with sizes in between those obtained by emulsion and suspension polymerizations—about 1-10 pm in diameter. For the larger particle sizes, the reaction characteristics are the same as in suspension polymerization. For the smallest particle sizes, suspension polymerization may exhibit the compartmentalized kinetics of emulsion polymerization. 

Although solution polymerizations are typically employed, ROMP can also be carried out by bulk, emulsion, and suspension polymerization, and in a number of reaction media. Reaction efficiencies, molecular weights, and PDI values for ROMP of NBE and COE in supercritical CO2 using 10b, 22b, and 33a were generally similar to those... 

PAA can be prepared using bulk polymerization, aqueous polymerization, nonaqueous polymerization, inverse phase emulsion and suspension polymerization. The precise structure of the resulting PAA chain is dependent upon many factors including the polymerization process and conditions. The tacticity of... 

Uses. The azobisnitriles have been used for bulk, solution, emulsion, and suspension polymerization of all of the common vinyl monomers, including ethylene, styrene vinyl chloride, vinyl acetate, acylonitrile, and methyl methacrylate. The polymerizations of unsaturated polyesters and copolymerizations of vinyl compounds also have been initiated by these compounds. 

The most common methods of producing homopolymers and copolymers of vinylidene fluoride are emulsion and suspension polymerizations, although other methods are also used.55... 

In conclusion we may say that a conversion near 100% can only be reached within reasonable time if the polymerization temperature is above the glass transition point of the polymer. This conclusion holds for bulk, emulsion, and suspension polymerization but, of course, not for solution polymerization where the solvent-polymer mixture usually has a glass transition point which is well below ordinary polymerization temperatures. 

These equations are perfectly general and therefore are applicable to solution, bulk, suspension, and emulsion polymerization systems. In the case of solution and bulk systems there is, in effect, only one particle with a volume of that of the whole reaction mixture. In emulsion and suspension polymerizations, the reaction mixture is subdivided into a large number of small particles and the influence of the state of subdivision is expressed by the factor z. 

As a reaction medium for transition rnetal-catalyzed polymerizations, water will, most likely, not be the first choice. The extreme water sensitivity of Ziegler or Phillips catalysts is well known. However, carrying out polymerization reactions in aqueous systems offers unique advantages. Thus, traditional free-radical emulsion and suspension polymerization are carried out on a large scale industrially. A brief review of these established reactions demonstrates some specific properties of polymerizations in aqueous systems. 

In dispersion polymerization, by contrast to emulsion or suspension polymerization, a monomer which is soluble in the reaction medium is polymerized. In analogy to fhe aforementioned types of polymerization, an insoluble polymer is obtained. The reaction is carried out in the presence of non-ionic surfactants or soluble polymers, which can stabilize the polymer particles generated to form a stable latex. Wifh particle sizes of ca. 1 to 15 pm, dispersion polymerization can cover the particle size range between emulsion and suspension polymerization. 

Figure 9.8 Schematic diagram of emulsion and suspension polymerizations. Each droplet is a type of microbatch reactor having a high surface-to-volume ratio...

In emulsion and suspension polymerizations, the continuous water phase serves as an excellent conductor of heat and allows the heat to be removed from the system (Figure 9.8). Therefore, the polymerization can be conducted at a high rate. This is a kind of flash chemistry without using micro devices. Instead, the reaction is carried out in small size droplets. The high surface-to-volume ratio of droplets is responsible for the fast heat transfer. 

By comparison with the intensively investigated syntheses of low molecular weight compounds by biphasic catalysis, catalytic polymerization in aqueous systems has received less attention. This is somewhat surprising, as polymerization in aqueous systems offers unique advantages, as illustrated by the large-scale applications of free-radical emulsion and suspension polymerization. 

Polymerization of vinyl monomers by free-radical mechanisms is perhaps the most widely encountered and best understood mode of vinyl polymerization. The popularity of free-radical polymerization is due in substantial part to the many advantages that this route to polymers offers to industry. The polymerization process is noteworthy for its ease, convenience, and relative insensitivity to impurities, such as water and oxygen, that plague ionic polymerizations. Indeed, it is common to carry out free-radical polymerizations in water as a suspending medium, as in emulsion and suspension polymerization. Another advantage of free-radical polymerization is that it offers a convenient approach toward the design and synthesis of myriad specialty polymers for use in almost every area. 

Fig, 3. Uncertainty in the nomenclature of various macroemulsion polymerization subdomains as a function of the nucleation mechanism and the average number of macroradicals per particle. The off-diagonal elements, designated with question marks, have properties of both emulsion and suspension polymerizations... 

The comparative conversion speed of a bulk, emulsion, and suspension polymerization and the average molecular weights of the polymers obtained for similar reaction conditions are presented in Table 15-6,... 

Table 15-6. Comparison of Bulk, Emulsion, and Suspension Polymerization of Styrene...

The cationic palladium a-diimine complexes are remarkably functional-group tolerant. Ethylene polymerizations can be carried out in the presence of ethers, organic esters, and acids, but nitriles tend to inhibit polymerizations. In addition, polymerizations have been carried out in the presence of air and in the presence of an aqueous phase.Aqueous emulsion and suspension polymerizations using these catalysts have been developed as a route to microspheres of polymer for adhesives as well as for other applications.2 ° 2 Preparation of elastomers is often complicated by difficult solvent removal, so polymerizations in supercritical CO2 have been investigated. It is also possible to combine the activity of the palladium catalysts with other polymerization techniques such as living-free-radical polymerizations. One interesting observation is that the... 

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