Addition or chain-growth polymers

In these polymerizations, a small molecule reacts with a growing polymer chain to produce a somewhat longer chain. The reaction is 

The overwhelmingly important example of equation (4.1) is the addition polymerization of vinyl monomers such as ethylene, propylene, styrene, vinyl chloride, butadiene, and methyl methacrylate. These polymerizations are characterized by the opening of a double bond, the simplest example being the polymerization of ethylene  

There are many reaction mechanisms for vinyl addition polymerizations. In approximate order of importance they are free radical polymerization, coordination metal catalysis (Ziegler-Natta), anionic polymerization, cationic polymerization, and group transfer polymerization. Regardless of specific mechanism, these polymerizations tend to be fast, essentially irreversible, highly exothermic and approximately first order with respect to monomer concentration. 

The general mechanism of an addition polymerization includes four steps initiation, propagation, chain transfer and termination. Initiation refers to the formation of a growing polymer chain, typically of length n = 1 but occasionally with n = 2 or 3. Propagation refers to the addition of monomer units to the chain as in equation (4.1). Propagation is the predominant means for consuming monomer and is approximately first order with respect to monomer concentration, [A/]. The active chain, R, can be converted to inactive (dead) polymer, P, by chain transfer. Chain transfer to monomer is common to most vinyl polymerization mechanisms  

Chain transfer maintains the number of growing polymer chains. Termination reactions decrease the number. Termination is an intrinsic phenomenon in free radical polymerizations. It occurs by disproportionation 

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In the next group of chapters we shall discuss condensation or step-growth polymers and polymerizations in Chap. 5, addition or chain-growth polymers and polymerizations in Chap. 6, and copolymers and stereoregular polymers in Chap. 7. It should not be inferred from this that these are the only classes of polymers and polymerization reactions. Topics such as ring-opening polymeri-... 

Polymers can be divided into addition (or chain-growth) polymers, formed on simple addition of monomers, or condensation (or step-growth) polymers, formed on the addition of monomers and elimination of a by-product such as water. 

Unlike condensation (step-growth) polymers, which release small molecules, such as water, as they form, the reactions that lead to addition, or chain-growth, polymers incorporate all of the reactants atoms into the final product. Addition polymers are usually made from molecules that have the following general structure ... 

Addition (or chain-growth) polymer A polymer that contains all of the atoms of the original reactant in its structure. This category includes polyethylene, polypropylene, and poly(vinyl chloride). 

Addition or chain growth polymers are formed by the direct addition of monomer molecules held together by a covalent bond without loss of any by-product during the polymerisation process. Thus the molecular mass of a monomer molecule and a repeating unit is the same. Examples of this class of polymers are vinyl polymers such as polystyrene, polybutadiene and poly (vinyl chloride), and diene polymers such as polybutadiene, and poly-isoprene, polychloroprene. 

In general, there are two distinctively different classes of polymerization (a) addition or chain growth polymerization and (b) condensation or step growth polymerization. In the former, the polymers are synthesized by the addition of one unsaturated unit to another, resulting in the loss of multiple bonds. Some examples of addition polymers are (a) poly(ethylene), (b) poly(vinyl chloride), (c) poly(methyl methacrylate), and (d) poly(butadiene). The polymerization is initiated by a free radical, which is generated from one of several easily decomposed compounds. Examples of free radical initiators include (a) benzoyl peroxide, (b) di-tert-butyl peroxide, and (c) azobiisobutyronitrile. 

Addition polymer or chain-growth polymer (Section 24.1) A polymer formed by a chain mechanism, where one initiator molecule causes a large number of monomers to react to form one polymer molecule. 

Addition or chain-growth polymerization is the most important industrial process for the production of polymers. Polyethylene, polypropylene and polystyrene are all formed... 

Although sometimes the classifications of condensation and step-growth polymers are considered to be interchangeable, as well as those of addition and chain-growth polymers, one must be aware that the classification of a polymer only by structure or only by mechanism may lead to ambiguities. Odian [8] recommends to classify a polymer attending both, structure and mechanism, in order to avoid this problem. Tables 1.3 and 1.4 contain examples of common addition and condensation polymers, respectively. 

Although the terms addition or chain growth, and condensation or step growth are often used synonymously, they are not exactly the same. The classification of addition and condensation is based on the composition of the repeating unit and monomers or reactants used, whereas the classification of chain growth and step growth is based on the mechanism of the formation of the polymers. ... 

The process by which polymers are formed from their respective monomers by the sequential addition of monomers, without loss of any by-product, is known as addition or chain growth polymerisation. The polymerisation proceeds through three distinct steps ... 

Addition polymers form when monomers undergo an addition reaction with one another. These are also called chain-reaction (or chain-growth) polymers because as each monomer adds to the chain, it forms a new reactive site to continue the... 

Carothers, in 1929, classified synthetic polymers into two classes, according to the method of their preparation, i.e., condensation polymers and addition polymers. In polycondensation, or step-growth polymerization, polymers are obtained by reaction between two polyfunctional molecules and elimination of a small molecule, for example water. Typical condensation polymers are shown in Figure 2. Addition (or chain reaction) polymers are formed from unsaturated monomers in a chain reaction. Examples of addition polymers are shown in Figure 2. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

Chain growth polymers, which are often referred to as addition polymers, form via chain addition reactions. Figure 2.2 presents a generic chain addition mechanism. Chain addition occurs when the active site of a monomer or polymer chain reacts with an adjacent monomer molecule, which is added to the end of the chain and generates a new active site. The active site is the reactive end of a monomer or polymer that participates in the polymerization reaction. 

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. 

A polymer that results from the rapid addition of one monomer at a time to a growing polymer chain, usually with a reactive intermediate (cation, radical, or anion) at the growing end of the chain. Most chain-growth polymers are addition polymers of alkenes and dienes, (p. 370)... 

Synthetic polymers can be divided into two major classes, depending on their method of preparation. Chain-growth polymers, also known as addition polymers, are made by chain reactions—the addition of monomers to the end of a growing chain. The end of the chain is reactive because it is a radical, a cation, or an anion. Polystyrene—used for disposable food containers, insulation, and toothbrush handles, among other things—is an example of a chain-growth polymer. Polystyrene is pumped full of air to produce the material known as Styrofoam . 

Chain-growth polymers are made by chain reactions— by the addition of monomers to the end of a growing chain. These reactions take place by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has an initiation step that starts the polymerization, propagation steps that allow the chain to grow at the propagating site, and termination steps that stop the growth of the chain. The choice of mechanism depends on the stmcture of the monomer and the initiator used to activate the monomer. In radical polymerization, the initiator is a radical in cationic polymerization, it is an electrophile and in cationic polymerization, it is a nucleophile. Nonterminated polymer chains are called living polymers. 

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