Emulsion polymerization scope

Emulsion polymerizations most often involve the use of water-soluble initiators (e.g. persulfate see 33.2.6.1) and polymer chains are initiated in the aqueous phase. A number of mechanisms for particle formation and entry have been described, however, a full discussion of these is beyond the scope of this book. Readers are referred to recent texts on emulsion polymerization by Gilbert4 and Lovell and El-Aasser43 for a more comprehensive treatment. 

Therefore, the "micellar size effect" offers, at least, an explanation for the variations observed in the x-values in emulsion polymerization with various monomer-surfactant combinations. Verification of this argument in detail is under investigation, however, and discussion of these results is beyond the scope of the present paper. 

There is clearly scope for further investigations of the behavior of nonionic emulsifiers in emulsion polymerizations. 

The scope of this introductory chtq>ter is to describe some of the basic features of emulsion polymerization and latexes and to identify some of the outstanding issues which are still unresolved. 

Only types (l)-(4) fall within the scope of this chapter. No further reference will be made to emulsion-polymerized prolybutadiene rubbers, because they are now of little industrial significance relative to the styrene-butadiene rubbers. Poly(vinyl chloride) is discussed elsewhere in this book. Brief reference will also be made in this chapter (Section 15.5) to the production and properties of carboxylated variants of styrene-butadiene rubber latexes. It may also be noted that latexes of rubbery terpolymers of styrene, vinyl pyridine and butadiene, produced by emulsion polymerization, have long been of considerable industrial importance for the specialized application of treating textile fibres (e.g., tyre cords) in order to improve adhesion between the fibres and a matrix of vulcanized rubber in which they are subsequently to be embedded. 

Recently, a number of groups have also reported the Pickering stabilization of monomer-containing lipid droplets by clays, and their subsequent free radical polymerization [291]. As these articles fall outside the scope of this review, they will not be discussed further. Although not strictly speaking in the scope of the present review, it is also worth mentioning the recent work of Voorn et al. on the first surfactant-free inverse emulsion polymerization stabilized with hydrophobic MMT platelets [292],... 

To create particles with a hydrophobic core and a hydrophilic shell, preformed hydrophobic polymer particles can be used as substrate for the covalent grafting of hydrophilic polymer chains. Aqueous emulsion polymerization is thus the method of choice for synthesis of core particles with submicrometric diameter (due to the scope of this review article, discussion will be restricted to aqueous emulsion polymerization, although some examples of particle substrates created via suspension polymerization exist however, with much larger diameters). The grafting methods are very close to those used for the surface modification of inorganic materials, such as for instance silicon wafers or colloidal silica particles [46,48-52], Various strategies can be employed such as ... 

We mainly focus on the use of emulsion polymerization and miniemulsion polymerization techniques to prepare hybrid latex particles containing both organic and inorganic materials, because the scope of these kind of materials in applications is much broader than that of, for example, the alkyd resin containing latex particles. 

In industrial processes stirred reactors with munerous technical variations are mainly employed. Besides jacket cooling, the heat of polymerization can be removed by various techniques as, for instance, hot cooling or internal fittings inside the reactor. Reactors for commercial processes have a size between 10 and 200 m and are pressure-proof up to 200 MPa (2000 atm) if necessary (39). The vessels normally have a cylindrical shape with dished tops and bottoms. The mixing system depends on the particular heterophase polsrmerization technique, as it has to fulfill different requests. A detailed discussion is beyond the scope of this article and the reader is referred to chemical engineering text books or reviews dealing with special heterophase polsrmerization techniques such as for emulsion polymerization (153,154) and for bulk and suspension polymerization (155,156). 

The kinetics of emulsion polymerization are determined to a considerable extent by the mechanisms of particle formation. This is a complicated matter, that lies outside the scope of this book (see, e.g., Hamielec and MacGregor, 1982). 

In emulsion polymerization, the molecular weights strongly depend on the average number of radicals per particle. Detailed mathematical models for the calculation of linear [23] and nonlinear [24-34] polymers for any value of n are available. A detailed discussion of this issue is outside the scope of this chapter. Instead, particular solutions for the limiting cases of Smith-Ewart [7] are presented in Table 4.1 where for Case 3, it was considered that the main chain growth termination event was bimolecular termination. 

The emulsion polymerization kinetics have been the subject of extensive research since the 1940s, but there are still many aspects of emulsion polymerization that are not completely understood. The discussion of emulsion polymerization kinetics is beyond the scope of this chapter and there are many excellent references on emulsion polymerization kinetics [4,5],... 

If the oil phase is replaced in an oil in water (o/w) emulsion by a hydrophobic monomer, or the water phase in a water in oil (w/o) emulsion by a hydrophilic monomer, an emulsion is obtained that can be employed as a precursor for the preparation of polymer latexes [10, 11]. Similarly, if both phases are replaced, the oil phase by a hydrophobic phase containing a monomer and the water phase by a hydrophilic phase containing a monomer, the generated emulsion could be employed as a precursor in the preparation of polymer composites [12]. However, concentrated emulsions that are generated and stable at room temperature may become unstable at the polymerization temperature. To be suitable for the preparation of polymers and polymer composites, the concentrated emulsion must first form and, subsequently, it must remain stable at the temperature at which polymerization takes place. The scope of the present section is to investigate the factors that influence the formation and stability of concentrated emulsions at the preparation and polymerization temperatures in order to identify the physico-chemical conditions that ensure their stability [9],... 

Chlorine radicals that are kinetically free are only fonned to a veiy limited extent Although a detailed discussion of the merits of the various mechanisms of chain transfer are outside the scope of this ctuqiter, the question of whether the radical resulting from chain transfer is a chlorine radical or the mudi less wato -soluble radical (4) is important for the discussion of the desorption of monomer radicals from particles to the aqueous phase. Another important consequence of the pronounced chain transfer is that the same molar mass is obtained with both emulsion and suspension routes of polymerization when carried out at the same temperature. 

In this chapter, emulsion, miniemulsion, microemulsion, suspension, and dispersion polymerization are discussed. Polyolefins are outside the scope of the chapter. 

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