Chain-growth free-radical polymerization

The free-radical chain mechanism described earlier for polyethylene (Sec. 3.16) is typical of chain-growth polymers. The overall reaction is 

Free-radical chain-growth polymerization requires a radical initiator, of which benzoyl peroxide is an example. It decomposes at about 80°C to give benzoyloxy radicals. These radicals may initiate chains or may lose carbon dioxide to give phenyl radicals that can also initiate chains.  

Initiator radicals add to the carbon-carbon double bond of the vinyl monomer to produce a carbon radical. 

Experience shows that the initiator usually adds to the least substituted carbon of the monomer that is, to the CH2 group. This gives a carbon radical adjacent to the substituent. There are two reasons for this preference First, the terminal vinylic carbon is less hindered and therefore more easily attacked, and second, the substituent L usually can stabilize an adjacent radical. 

The carbon radical formed in the initiation step then adds to another monomer molecule, and the adduct adds to another, and so on. 

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Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

Free radical chain growth polymerization takes place through three distinct chemical steps. These are shown in Fig. 1. 

Fig. I. Mechanism for free radical chain growth polymerization.

Ethylene is also polymerized by free-radical chain-growth polymerization. With ethylene, the free-radical intermediates are less stable, so stronger reaction conditions are required. Ethylene is commonly polymerized by free-radical initiators at pressures around 3000 atm and temperatures of about 200 °C. The product, called low-density polyethylene, is the material commonly used in polyethylene bags. 

Free-radical polymerization is the most widely used process for polymer synthesis. It is much less sensitive to the effects of adventitious impurities than ionic chain-growth reactions. Free-radical polymerizations are usually much faster than those in step-growth syntheses, which use diFFereiit monomers in any case. Chapter 7 covers emulsion polymerization, which is a special technique of free-radical chain-growth polymerizations. Copolymerizalions are considered separately in Chapter 8. This chapter focuses on the polymerization reactions in which only one monomer is involved. 

Reactions (1) and (4) are essentially the same as the addition of reactive species to the monomer, which is the same as the initiation and propagation reactions in the free radical chain growth polymerization. However, the kinetic chain length in vacuum is very short, and in a practical sense these reactions can be considered to be stepwise reactions. Cycle I consists of reactions of reactive species with a single reactive site, and cycle II is based on divalent reactive species. Reaction (3) is a cross-cycle reaction from cycle II to cycle I. The growth via cycle I requires the reactivation of the product species, whereas cycle II can proceed without reactivation as long as divalent reactive species or monomers with double bond or triple bond exist. 

The term free radical is often used in the context of a reactive intermediate, as in the case of polymerization of vinyl monomers, but the same structure (unpaired electron) can and does exist in a kind of immobilized environment. For example, a bulk-polymerized (monomer and initiator only in the polymerization system) poly(methyl methacrylate) (PMMA) contains an appreciable number of free radicals that can be detected by electron spin resonance (ESR) [1]. When the polymerization system becomes highly viscous toward the end of the bulk polymerization, gel formation occurs and immobilizes the growing end of free radical chain growth polymerization, preventing recombination of two free radical ends of growing chains. 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

The material formation in the luminous gas phase (plasma polymerization) is less specific to the chemical structure of molecules. Benzene, which is a nonpolymer-izable solvent in the free radical chain growth polymerization, polymerizes readily in the luminous gas phase. Benzene not only polymerizes, but its rate of deposition is nearly equivalent to that of acetylene, i.e., a benzene molecule is equivalent to three molecules of acetylene in the luminous gas phase. 

The effect of pulsed discharge on plasma polymerization may be viewed as the analogue of the rotating sector in photoinitiated free-radical chain growth polymerization. The ratio r of off time I2 to on time li, r = (t2/h), is expected to influence the polymerization rate depending on the relative time scale of I2 to the lifetime of free radicals in free-radical addition polymerization of a monomer. This method was used to estimate the lifetime of free radicals in conventional photon-initiated free radical polymerization. 

In both the polymerizations, free radicals are the species that are responsible for the formation of bonds in the depositing materials. The growth mechanism, however, is not by the conventional chain-growth free-radical polymerization. In a conventional free-radical chain-growth polymerization, two free radicals and 10,000 monomer molecules yield a polymer with degree of polymerization 10,000, which does not contain free radicals. In contrast to this situation, in plasma polymerization and Parylene polymerization, 10,000 species with free radical(s) recombine to yield a polymer matrix that has an equivalent degree of polymerization, and contains numbers of unreacted free radicals (dangling bonds). 

Free-Radical Chain-Growth Polymerization Process... 

What kind of peroxides are available for initiations of free-radical chain-growth polymerizations List and draw structures of various types. 

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