Polymerization, radical dispersion type

Emulsion polymerization refers to a unique process employed for some radical chain polymerizations. It involves the polymerization of monomers in the form of emulsions (i.e., colloidal dispersions). The process bears a superficial resemblance to suspension polymerization (Sec. 3-13c) but is quite different in mechanism and reaction characteristics. Emulsion polymerization differs from suspension polymerization in the type and smaller size of the particles in which polymerization occurs, in the kind of initiator employed, and in the dependence of polymer molecular weight on reaction parameters. 

The radical polymerization in disperse systems may be divided into several types according to the nature of continuous phase and the polymerization loci the dispersion, emulsion, miniemulsion, microemulsion, suspension, etc. 

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... 

Fully exfoliated PS/clay nanocomposites were prepared via free radical polymerization in dispersion [58, 59]. Thermally stable organoclay was obtained by modifying MMT with 3-(trimethoxysilyl)propyl methacrylate (MPTMS) (Table 3.4, Figure 3.14). Analyses by XRD and TEM revealed that nanocomposites with low clay loadings exhibited exfoliated structures, whereas intercalated structures were obtained at higher clay loadings. Another silane-type organoclay was prepared by modification of vermiculite with the... 

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

In Figure 11.2.1A the process of dispersion polymerization is shown focusing on the formation process of particles. Dispersion polymerization starts from a homogeneous solution, and when oligomeric radicals and polymer, formed in the monomer solution, do not have affinity for the medium, they become insoluble and precipitate. The precipitate is unstable and homoaggregates to become primary particles. Primary particles homoaggregate further until they become stable secondary particles. The mechanism to keep the particles stable depends on what type of stabilizer is used. The propagation processes from nuclei to primaiy particles and from primary to secondary ones does not have to be considered as discontinuous steps. However, it... 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

Controlled/ living radical polymerization (CLRP) processes are well-established synthetic routes for the production of well-defined, low-molecular weight-dispersity polymers [99]. The types of CLRP processes (initiator-transfer agent-terminator (INIFERTER), atom transfer radical polymerization (ATRP), nitroxide-mediated radical (NMRP) polymerization, reversible addition-fragmentation transfer (RAFT)) and their characteristics are described in Section 3.8 of Chapter 3 and in Section 14.8 of Chapter 14. 

The upsurge of interest in this type of colloidal system followed the development in the 1950s of the technique known as dispersion polymerization [3.54]. This process provides a means of preparing nonaqueous polymer dispersions in a controlled manner. A wide range of such dispersions have been made, mainly by free-radical addition polymerization. 

This broad definition allows fw a variety of polymerization mechanisms, and it is formally possible for dispersion polymerizations to be anionically or cation-ically initiated, or to be stepwise rather than chain polymerizations. These types of dispersion polymerizatitm are less commcn and are outside the scope of this chapter, which is devoted largely to radically initiated systems. A recent review [2] covers polycondensations by dispersion methods. 

The chemical structure of studied polymers is shown in Fig. 2. Three different types of functionalized polymers have been investigated. In the first case (DRGMMA) the copolymer has been got by free radical polymerization of a 1 1 molar ratio mixture of methyl methacrylate and 4 -(N-ethyl-N-(methacryloxyethyl)amino)-4-nitroazobenzene obtained by esterification with methacrylic acid of commercially available Disperse Redttl. It corresponds to... 

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