Free-radical polymerization

The free-radical polymerization of acrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-tail growth of the polymeric free radical by attack on the double bond of the monomer. 

Although the anionic polymerization mechanism is the predominant one for the cyanoacryhc esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend the usable shelf life, free-radical stabilizers such as quinones or hindered phenols are a necessary part of the adhesive formulation. 

In contrast to ionic chain polymerizations, free radical polymerizations offer a facile route to copolymers ([9] p. 459). The ability of monomers to undergo copolymerization is described by the reactivity ratios, which have been tabulated for many monomer systems for a tabulation of reactivity ratios, see Section 11/154 in Brandrup and Immergut [14]. These tabulations must be used with care, however, as reactivity ratios are not always calculated in an optimum manner [15]. Systems in which one reactivity ratio is much greater than one (1) and the other is much less than one indicate poor copolymerization. Such systems form a mixture of homopolymers rather than a copolymer. Uncontrolled phase separation may take place, and mechanical properties can suffer. An important ramification of the ease of forming copolymers will be discussed in Section 3.1. 

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

Manufacture Introduced in 1937 Emulsion polymerization Free radical catalyst 10-40% Acrylonitrile... 

Manufacture Introduced in 1931 Emulsion polymerization Free radical catalysis Mostly trans configuration... 

The polymeric free radical may become saturated by removal of a hydrogen atom from a molecule R"CH2R" which may be a monomer, a polymer, or a solvent molecule... 

Alternately, the polymeric free radical may transfer hydrogen to the monomer. 

The picture of mechanical synthesis is much more complex if the segments of block and graft copolymers can undergo rupture to polymeric free radicals forming multisegment block copolymers, gelled and crosslinked structures. Baramboim in his book (if) describes 14 different possibilities of block ami graft reactions. 

Acrolein is a very reactive monomer with a high tendency to polymerize. Free-radical and cationic polymerizations lead to Insoluble polymers even if the conversions are low. On the other hand, anionic polymerization gives soluble polyacroleins under well-adapted conditions. Because of the difunctional nature of this monomer, the chains of polyacroleins contain different types of units ... 

Preparation of Polymeric Free-Radical Initiators by Anionic Synthesis Polymeric Azo Derivatives... 

The reaction engineering aspects of these polymerizations are similar. Excellent heat transfer makes them suitable for vinyl addition polymerizations. Free radical catalysis is mostly used, but cationic catalysis is used for non-aqueous dispersion polymerization (e.g., of isobutene). High conversions are generally possible, and the resulting polymer, either as a latex or as beads, is directly suitable for some applications (e.g., paints, gel-permeation chromatography beads, expanded polystyrene). Most of these polymerizations are run in the batch mode, but continuous emulsion polymerization is common. 

Yet another method of classification is by their method of polymerization— free radical, ionic, or condensation. 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

Emulsion polymerization—Free-radical polymerization carried out in micelles suspended in water. Ingredients include a surfactant (detergent) to form the micelles, a monomer that is not very soluble in water, and an initiator. The product consists of small particles of polymer suspended in water called a latex. ... 

Addition Homo-Polymerization (Free-Radical Homo-Polymerization)... 

Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz,... 

Tn emulsion polymerization and in some suspension polymerizations, free radicals are generated in a continuous phase and diffuse into a dis-persed-phase particle or droplet where polymerization takes place (5). The molecular weight distributions or, equivalently, the polymer size distributions of these systems depend on the relative rates of radical arrival and termination. Frequently in emulsion polymerization the radicals are terminated so quickly that each particle in the dispersed phase... 

Radical Polymerization. Free radical polymerization consists of three steps, initiation, propagation, and termination or chain transfer. Initiation consists of two steps, decomposition of the initiator to form a radical species, followed by addition of the initiator to the first monomer unit. 

Microgels can be prepared by heterophase polymerization (free radical or controlled radical) of monomers in the presence of a crosslinking agent in aqueous phase. Heterophase polymerization techniques suitable for microgel synthesis are precipitation polymerization and inverse mini- and microemulsion. 

Derive the expression for the rate of a free radical polymerization. Using this expression, account for the Trommsdorff effect and the inability of ethylene to polymerize free radically at ordinary temperatures and pressures. 

In order to gain evidence for interfacial initiation, the redox initiator system was compared with a water-soluble initiator (VA-044) in terms of the emulsion polymerization behavior of butyl acrylate (BA)/[2-(methacryloyoxy)ethyl] trimethyl ammonium chloride (MAETAC). It was found that for the water-soluble initiator system, only homopoly(MAETAC) was formed and BA did not polymerize at all. In the case of VA-044, it was suggested that it may be difficult for polymeric free radicals in the aqueous phase to penetrate the viscous surfactant layer to initiate the polymerization of the BA monomer. On the other hand, it has also been found that BA could be rapidly polymerized under the same conditions if VA-044 is replaced with CHP/TEPA, indicating that radicals are formed in the interface, where they do not need to penetrate through viscous surfactant layer. 

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