Vinyl monomers, chain-growth polymerization

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network... 

Commodity Chain-Growth Polymers. Two of the largest commodity wafer-soluble polymers are poly(vinyl alcohol) (PVA) and polyacrylamide (PAM). They are prepared by the free-radical initiation of vinyl monomers, a chain-growth polymerization technique. 

The fundamental chemistry of the structural adhesives described here can change very little. Vinyl and acrylic monomers polymerize by chain growth polymerization initiated by free radicals or ions. Isocyanate and epoxy compounds react with compounds containing active hydrogen in step growth polymeriza-... 

Cationic polymerizations of vinyl monomers differ from other chain-growth polymerizations particularly as follows ... 

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. 

Chain-growth polymers, also called addition polymers, are prepared by chain reactions. These compounds are formed by adding monomers to the growing end of a polymer chain. The conversion of vinyl chloride to poly(vinyl chloride) is an example of chain-growth polymerization. These reactions were introduced in Section 15.14. 

Chain-growth polymerization involves the sequential step-wise addition of monomer to a growing chain. Usually, the monomer is unsaturated, almost always a derivative of ethene, and most commonly vinylic, that is, a monosubstituted ethane, 1 particularly where the growing chain is a free radical. For such monomers, the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain, namely anionic, cationic, or free radical polymerization by coordination catalyst is generally considered separately as the nature of the growing chain-end may be less clear and coordination may bring about a substantial level of control not possible with other methods. ... 

Scheme 9.2 Initiation and propagation processes of chain-growth polymerization (addition polymerization of vinyl monomers)...

Addition polymerization of vinyl monomers is one of the most popular classes of chain-growth polymerization. Depending on the nature of the active species, the polymerizations are categorized as cationic, radical, or anionic. These polymerizations are usually very fast and highly exothermic. 

In the previous chapter, the synthesis of polymers by step polymerization and the kinetics of the process were considered. We turn our attention now to chain-growth polymerizations. The reader should recall that the features.that distinguish step-growth and chain-growth polymerizations are summarized in Table 5.1. A large number of different class of unsaturated monomers, such as ethylene (CH2=CH2, the simplest olefin), a-olefins (CH2=CHR, where R is an alkyl group), vinyl compounds (CH2=CHX, where X = Cl, Br, I, alkoxy, CN, COOH, COOR, CeHs, etc., atoms or groups), and... 

Chain growth polymerization has the characteristic of having an intermediate within the process that cannot be isolated [5], The intermediate can be a metal complex, a free radical, or an ion. These intermediates are transient to the process. The terms vinyl, olefin, and addition polymerization have been associated with this process [13], Monomer units add to a chain very rapidly once it has been initiated. Initiation is the creation of an active center such as a free radical or carbanion [13], An example is the thermal decomposition of benzoyl peroxide shown in Figure 3.4. To propagate the chain, an additional monomer is added at a very rapid rate as monomer concentration is reduced. Figure 3.5 shows the propagation of polystyrene. 

The monomers used most commonly in chain-growth polymerization are ethylene (ethene) and substituted ethylenes. In the chemical industry, monosubstituted ethylenes are known as alpha olefins. Polymers formed from ethylene or substituted ethylenes are called vinyl polymers. Some of the many vinyl polymers synthesized by chain-growth polymerization are listed in Table 28.1. 

Homopolymerization. Nonionic Monomers. The chain-growth polymerization of vinyl monomers occurs via a three-part mechanism initiation, propagation, and termination. Under steady-state conditions, the rate of polymerization (R ) is defined (21-24) by equation 1 ... 

In chain-growth polymerization, monomers can onlyjoin active chains. Monomers contain carbon-carbon double bonds (e.g., ethylene, propylene, styrene, vinyl chloride, butadiene, esters of (meth)acrylic acid). The activity of the chain is generated by either a catal) t or an initiator. Several classes of chain-growth polymerizations can be distinguished according to the type of active center ... 

Ionic polymerization uses anionic or cationic catalysts. Monomers with electron-accepting groups at the double bonds such as vinyls are catalyzed by anionic catalysts such as organometallic compounds [16, 17]. Monomers with electron-donating groups at the double bonds are catalyzed by cationic catalysts such as Lewis acids, Ziegler catalysts, and Friedel Crafts catalysts [16, 17]. Polyacetals are linear polymers that can be produced by anionic or cationic chain-growth polymerization of formaldehyde [2, 14]. 

Major commercial synthetic specialty polymers are made by chain-growth polymerization of functionalized vinyl monomers, carbonyl monomers, or strained ring compounds. Depending on monomer structure, the polymerization may be initiated free radically, anionically, or cationically. Copolymers or terpolymers with random, alternating, block, or graft sequences can be prepared under appropriate reaction conditions. There are numerous mediods used to prepare specialty polymers in the research laboratory. However, only a few are of commercial interest. Of particular commercial interest is synthesis of specialty polymers in solutions, dispersions, suspensions, or emulsions. 

---