Free-Radical Chain Growth

Copolymer Feed ratio Incorporation Intrinsic viscosity (dL/g) 

We have also studied the kinetics of free radical initiation in CO2 using azobis(isobutyronitrile) (AIBN) as an initiator [35]. These experiments were accomplished using high pressure UV spectroscopy, and illustrated that AIBN decomposes more slowly in CO2 than in traditional hydrocarbon solvents, yet the initiator efficiency is much greater in CO2 due to the reduced solvent cage effect in the low viscosity supercritical medium. The main conclusion drawn from this work was that CO2 can therefore be employed effectively as a solvent for free radical polymerizations and remains an inert solvent even in the presence of highly electrophilic hydrocarbon radicals. 

A significant advantage of conducting polymerization and oligomerization of fluoroalkanes in carbon dioxide rather than other solvents is the absence of chain transfer to CO2. Radicals generated from fluoroalkene monomers such as tetrafluoroethylene (TFE) are quite electrophilic, and will undergo facile chain transfer to virtually any hydrocarbon that is present in the system. Moreover, highly reactive monomers such as TFE can be handled more safely as 

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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. 

We begin our discussion of copolymers by considering the free-radical polymerization of a mixture of two monomers. Mi and M2. This is already a narrow view of the entire field of copolymers, since more than two repeat units can be present in copolymers and, in addition, mechanisms other than free-radical chain growth can be responsible for copolymer formation. The essential features of the problem are introduced by this simpler special case, so we shall restrict our attention to this system. 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

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.

The rate of propagation in the micelles is similar to that described for other free radical chain growth, but since the free radical concentration is equal to the number of active micelles, the value of N/1 is used instead of [M ]. Thus, the rate of propagation is dependent on the number of micelles present. 

Chain-growth, or addition, polymers are made by adding one monomer unit at a time to the growing polymer chain. The reaction requires initiation to produce some sort of reactive intermediate, which may be a free radical, a cation, or an anion. The intermediate adds to the monomer, giving a new intermediate, and the process continues until the chain is terminated in some way. Polystyrene is a typical free-radical chain-growth polymer. 

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. 

We turn our attention now to chain-growth polymerizations. The reader should recall that the Features which distinguish chain-growth and step-growth polymerizations were summarized in Section 5.2. The present chapter is devoted to the basic principles of chain polymerizations in which the active centers are free radicals. Chain-growth reactions with active centers having ionic character are reviewed in Chapter 9. 

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. 

Both the monomer and polymer are soluble in the solvent in these reactions. Fairly high polymer concentrations can be obtained by judicious choice of solvent. Solution processes are used in the production of c(5-polybutadiene with butyl lithium catalyst in hexane solvent (Section 9.2.7). The cationic polymerization of isobutene in methyl chloride (Section 9.4.4) is initiated as a homogeneous reaction, but the polymer precipitates as it is formed. Diluents are necessary in these reactions to control the ionic polymerizations. Their use is avoided where possible in free-radical chain growth or in step-growth polymerizations because of the added costs involved in handling and recovering the solvents. 

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 Precipitation Polymerizations... 

The copolymerization of hydrophobically modified monomers should exhibit similar behavior, but opposite in direction to ionogenic monomers. Increasing hydrophobe size and solubility diflPerences with the nonionic monomer should make it difficult to obtain significant spacings between hydro-phobic monomers in chain-growth copolymerizations. In view of what has been delineated in surfactant-micelle (40) and ionogenic monomer behaviors, a multitude of unique structural possibilities could be obtained in hydrophobically modified copolymers synthesized by free-radical chain-growth processes. 

FIGURE 3.2 Schematic representation of the development of molar mass, for example, M, with monomer conversion for chain growth (free radical), chain growth (living) and step growth mechanisms for the polymer buildup reaction. 

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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