Free-radical polymerization chains

For a chain process we have initiation, propagation, and termination to consider. We will frame our discussion in terms of a radical polymerization, but the analysis applies to any chain reaction. We begin with initiation. 

We will describe the initiator as X2, recognizing that many initiators give rise to two radicals, each of which can larmch a polymerization. We will symbolize the monomer as M. We consider the initiation process to be the combination of the two reactions in Eq. 13.13. The subscript 1 on the M indicates it is the first monomer of the chain. Typically the first step, decomposition of the initiator, is rate-determining. In addition, not every radical formed will successfully iniHate a polymerization, so we introduce a factor/equal to the proportion of 

Propagation involves the addition of monomer to the growing polymer chain and leads to the expression for Vp shown in Eq. 13.15, where M represents a polymer chain that terminates in a radical. This analysis implicitly assumes that the propagation rate is independent of chain length, as discussed above. 

Both of the major termination reactions, disproportionation and recombination, involve two radicals. The termination rate, r can thus be written as in Eq. 13.16, where both the exponent and the factor of two reflect this feature of termination. 

In order to obtain the rate of polymerization, Vp, apply the steady state approximation to M . That is, assume the rate of formation of M equals the rate of destruction, or r, = r,. This produces Eq. 13.17. Inserting this result back into the equation for Tp (Eq. 13.15) gives Eq. 13.18 for the rate of polymerization. This is an important result. It tells us that the rate of polymerization is proportional to the concentration of monomer, but it will also be proportional to the square root of the concentration of initiator. The initiator concentration is changing constantly du ring the polymerization, but it is a first-order decomposition. Thus, we can write the first-order rate behavior as Eq. 13.19 (recall Eq. 7.34) and substitute into Eq. 13.18 to get Eq 13.20. 

1 A bottle of styrene left untouched for long periods will be found to have polymerized even though the inhibitor has not been removed monomer from which the inhibitor has been removed has an even shorter shelf-life. For this reason, it is suggested that styrene is disposed of after 12-18 months and that with the inhibitor removed used immediately. 

Although the presence of water is generally not an issue in free-radical chain polymerization (indeed water may be a suitable medium for polymerization as in Protocols 5-7) unlike, for example, chain-growth polymerization initiated by anionic species, it is always advisable to use solvents of the highest purity and this will generally include some element of predrying. In general, solvents should be distilled, particularly as a number of suitable solvents for polymerization reactions contain stabilizers which usually serve to mop up free radicals and therefore inhibit the polymerization 

Highly flammable, may form peroxides, irritating to eyes and respiratory system Extremely flammable, may form peroxides 

Caution Cancer suspect agent, highly flammable, toxic by inhalation Flammable, harmful by inhalation 

Chloroform can be simply purified by passing through a column of basic alumina to remove the ethanol, which is added as a stabilizer. Chloroform must be stored in the dark to avoid the photochemical generation of phosgene 

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In this section, the important concepts related to the formation of hydrogels by free radical copolymerization/cross-linking are examined. Greater depth beyond the scope of this chapter can be obtained from textbooks on polymer chemistry and the papers cited herein. As stated earlier, almost all gels produced from monomers for pharmaceutical applications are synthesized by free radical chain polymerizations. 

Commonly used monomers for UV curing include acrylates (7), styrene/unsaturated polyesters (8,9), and thiol-ene compositions (10-12). Currently, acrylate-functional systems constitute a major share of the UV curable polymers market, mainly due to their rapid curing via free radical chain polymerization. 

Free Radical Chain Polymerization (Addition Polymerization) 173... 

Energies of Activation for Propagation (fp) and Termination (ft) in Free Radical Chain Polymerization... 

Chain Transfer Constants of Solvent to Styrene in Free Radical Chain Polymerization at 60°C... 

Name three widely used thermoplastics produced by free radical chain polymerization ... 

The most widely used graft copolymer is the styrene-unsaturated polyester copolymer (Equation 7.35). This copolymer, which is usually reinforced by fibrous glass, is prepared by the free radical chain polymerization of a styrene solution of unsaturated polyester. 

POLYVINYLIDENE CHLORIDE. [CAS 9002-86-2J. A stereoregular, thermoplastic polymer is produced by the free-radical chain polymerization of vinylidene chloride (H>C=CCIi) using suspension or emulsion techniques. The monomer lias a bp of 31.6°C and was first synthesized in 1838 by Regnault. who dehydrochlorinated 1,1.2-trichloroethane which he obtained by the chlorination of ethylene. The copolymer product has been produced under various names, including Saran. As shown by the following equation, the product, in production since the late 1930s, is produced by a reaction similar to that used by Regnault nearly a century earlier ... 

POLVINYLIDENE FLUORIDE. This product is made by the free-radical chain polymerization of vinylidene fluoride (H2C=CF2). This odorless gas which has a boiling point of —82°C is produced by the thermal dehydrochlorination of 1,1,1-chlorodifluoroethane or by the dechlorination of 1,2-dichloro-l.l-difluoro-ethane. As shown by the following equations, 1,1,1-chlorodifluoroethane may be obtained by the bydroflnorination and... 

Maleic anhydride [R1 equals -CH=CH- in Eq. (2.7) cis isomer] is reacted with aliphatic diols to form low molar mass unsaturated polyesters, UP. For molar masses higher than 1000 g/mol, products are diluted with a liquid vinyl monomer, most often styrene. This reactive mixture, generally called unsaturated polyester, UP resin, can be transformed into crosslinked polymers through a free-radical chain polymerization (see Sec. 2.3). 

Such oligomers are often used in ultraviolet (UV) cure coatings (free-radical chain polymerization, Sec. 2.3). 

Free Radical Chain Polymerization Going One Step Better ... 

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. 

Comparison of the Two Reactions Step-Growth Polymerization in More Detail Making PET in the Melt Interfacial Poly condensation Chain-Growth Polymerization in More Detail Free Radical Chain Polymerization Going One Step Better Emulsion Polymerization Copolymerization Ionic Chain Polymerization It Lives ... 

Recendy, photopolymer systems have aroused increased interest because of their manifold applications in several high technologies [1-3]. Among such systems, those derived from photoinduced polymerization play an important role. The fundamental principles of these systems are based on the production of species X by photoreactions, which then initiates thermal reactions of low-molecular products leading to polymer or network formation see Eq. (1). In general, these thermal reactions are associated with low activation energies (about 60 kJ mol 1 for free radical chain polymerization). Therefore, such processes can also occur suffidentiy fast at room temperature. 

Hjerten et al. [124] introduced the monolithic stationary phases based on acrylamides in the late 1980s. The cross-linked polyacrylamide can be directly synthesized within the mold by a one step free-radical chain polymerization. Acrylamide, methacrylamide, or piperazine diacrylamide are often employed as monomers, while V,A -methylene-bis-acrylamide is used as a cross-linker. 2-acrylamido-2-methylpropane sulphonic acid, vinylsulphonic acid, butyl methacrylate, or stearoyl methacrylate are usually added to the polymerization mixture to provide charge and functional groups [114]. 

The free-radical chain polymerization of maleimides and the iV-substitnted derivatives has been extensively and both homo- and copolymerization... 

Table 3 Chain transfer constants (Ctr) of solvents to styrene in free-radical chain polymerization at 60 C...

Problem 6.8 Consider the following scheme of reactions for free-radical chain polymerization initiated by thermal homolysis of initiator with cage effect [6] ... 

Some monomers undergo direct photoinitiation and free-radical chain polymerization when exposed to ultraviolet or visible light. For other monomers, a photosensitizer must be added to the system. Photosensitizers are compounds that absorb ultraviolet or visible li t and then dissociate into free radicals or transfer energy directly to the monomer. 

The second step of initiation [Eq. (8.83)], being slower than the first [Eq. (8.82)], is rate-determining for initiation (unlike in the case of free-radical chain polymerization) and so though the amide ion produced upon chain transfer to ammonia can initiate polymerization it is but only at a rate controlled by the rate constant, ki, for initiation. Therefore, this chain transfer reaction may be considered as a true kinetic-chain termination step and the application of steady-state condition gives Eq. (8.90). 

Free-Radical Chain Polymerization. In contrast to the typically slow stepwise polymerizations, chain reaction polymerizations are usually rapid with the initiated species rapidly propagating until termination. A kinetic chain reaction usually consists of at least three steps, namely, initiation, propagation, and termination. The initiator may be an anion, cation, free radical, or coordination catalyst. 

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