Free radical polymerization methods

Figure 15.18 Steps in the free-radical polymerization of ethylene. In this polymerization method, free radicals initiate, propagate, and terminate the formation of an addition polymer. An initiator (Y—O—O—Y) is split to form two moleoules of a free radioal (Y—O ). The free radical attacks the tt bond of a monomer and oreates another free radioal (Y—O—CH2—CH2 ). The process continues, and the ohain grows (propagates) until an inhibitor is added (not shown) or two free radioals oombine.

In their polymerization, many individual alkene molecules combine to give a high molecular weight product Among the methods for alkene polymerization cationic polymerization coordination polymerization and free radical polymerization are the most important An example of cationic polymerization is... 

This technique is based in the fact that when cellulose is oxidized by ceric salts such as ceric ammonium nitrate Ce(NH4)2(N03)6 free radicals capable of initiating vinyl polymerization are formed on the cellulose. However, the possibility remains that the radical formed is an oxygen radical or that the radical is formed on the C-2 or C-3 instead of the C-6 carbon atom. Another mechanism, proposed by Livshits and coworkers [13], involves the oxidation of the glycolic portion of the an-hydroglucose unit. Several workers [14,15], however, have found evidence for the formation of some homopolymer. In the ceric ion method free radicals are first generated and are then capable of initiating the grafting process [16-18]. 

Polymers derived from triazoles can be formed by three fundamental methods. In the simplest way 3,5-disubstituted triazoles are treated with acetylene to obtain 1-vinyl derivatives which are then polymerized with free radical catalysts. The resulting polymers do not appear to be used on their own but mixed with other polymerized alkenes they facilitate the dyeing of synthetic fabrics. 

Styrene is one of the few monomers that may be polymerized by free-radical, anionic, cationic, or coordination (Ziegler-Natta) methods. This property, common to styrene and most of its derivatives, is the consequence of the availability of a benzylic position in these monomers, which is capable of stabilizing a radical, carbanionic, or carbocationic center, as well as possessing a polarizability amenable to the charge distributions required by coordination methods of polymerization. 

Controlled free-radical polymerization (CFRP) has been used successfully to produce block, graft, and other controlled architecture copolymers within the last decade for a variety of free radically polymerizable monomers. The main techniques include reversible addition fragmentation and transfer (RAFT) polymerization, stable free-radical polymerization (SFRP) mediated by nitroxide/alkoxyamine based radicals, atom transfer radical polymerization (ATRP), diphenyl ethylene (DPE) mediated polymerization, and novel precipitation/emulsion polymerization based methods like free-radical retrograde precipitation polymerization (FRRPP). ... 

Free-radical polymerization is an important process for the synthesis of polymers because free radicals are compatible with a wide variety of functional groups that are not compatible with ionic and metal-catalyzed polymerization. Therefore, free-radical polymerization is the most widely used method to produce polymers in industry. 

Direct emulsification can be used to produce latexes from polymers that cannot be polymerized by free-radical mechanisms and from natural polymers or their derivatives. Three methods or a combination of methods can be applied in direct emulsification... 

Professor DeSimone s group at North Carolina has focused on carrying out polymerization reactions in supercritical fluids (7representative example is shown in Figure 4(b), where a fluorinated monomer which is soluble in CO2 is polymerized via free radical reactions. Control of molecular weight distribution has been achieved and the rates are comparable to other synthesis methods. As a direct result, there is considerable commercial interest in using supercritical CO2 as solvent replacement for fluorinated polymer synthesis. 

Styrene-butadiene rubber (SBR) for automobile tires is a random copolymer. It is prepared by two methods, free-radical and anionic polymerization, both of which are carried out on a mixture of styrene and 1,3-butadiene. Free-radical initiation is essentially nonselective and gives the random copolymer. Anionic initiation is carried out under conditions designed to equalize the reactivity of the two monomers so as to ensure randomness. 

Free-radical polymerization n. A reaction initiated by a free radical derived from a polymerization catalyst. Polymerization proceeds by the chain-reaction addition of monomer molecules to the free-radical ends of growing chain molecules. Major polymerization methods such as bulk, suspension, emulsion, and solution polymerization involve free radicals. The free-radical mechanism is also useful in copolymerization, in which alternating monomeric units are promoted by the presence of free radicals. Lenz RW (1967) Organic chemistry of high polymers. Interscience Publishers, New York. Odian G (2004) Principles of polymerization, 4th edn. Wiley-Interscience, New York. 

Polymerization by free-radical methods affords polymers that have considerable branching. 

Among many other synthetic methods free-radical polymerization has almost exclusively been used for surface grafting. To initiate the radical polymerization from a substrate surface, free radicals or peroxides should be generated on the surface. For... 

Poly(A/-VinylpyrrolidinonG). A/-Vinylpyrrolidinone (NVP) monomer 6 polymerizes under free-radical conditions via bulk, solution, and suspension methods (Fig. 32). 

In this section we present a short overview of the polymerization of VC. The most common method is polymerization by free radicals [305]. According to the ease of homolytic splitting of the n bond in the monomer, radical polymerization takes place in the presence of suitable initiation systems. In general, there are three methods for producing radicals available for the polymerization of VC (A) thermal cleavage of azo or peroxo compounds, (B) oxidation-reduction processes, and (C) metal alkyls in connection with oxygen. After the initiation step, chain growth takes place rapidly ... 

A large number of compounds with carbon-carbon double bonds have been polymerized to yield materials with useful properties. Some of the more familiar ones are listed in Table 6.2. Not all are effectively polymerized under free-radical conditions, and much research has been carried out to develop alternative methods. The most notable of these, coordination polymerization employs transition-metal catalysts and is used to prepare polypropylene. Aspects of coordination polymerization are described in Sections 7.16 and 14.14. Chapter 27 is devoted entirely to synthetic polymers. 

The biggest factor that accounts for the wide range of photoinitiation rates in Figs. 9 and 10, is the method of polymerization of the acrylic copolymer, namely, the type and amount of initiator and the solvent used in the polymerization. Acrylic polymers are prepared by free-radical polymerization. Typical free-radical initiators... 

Ethylene, the simplest olefin monomer, can be polymerized using free radical initiation or coordination polymerization. The polymerization can be performed via solution, bulk, slurry, and gas phase methods. PE is the most commonly used synthetic polymer worldwide. The industrial production and scientific research of PE still continues to expand. This continued growth and reasonable profitability exists because of the variety of applications for which PE is suited. Specific attributes include high chemical resistance, a large range of mechanical properties, low specific gravity, low production cost, and facile processability. ... 

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