Polymerization reactions chain

As the name implies, chain-reaction polymerization is a chain reaction in which the initiator may be a cation, anion, or free radical. An example of cationic polymerization is found in the polymerization of isobutylene (2-methylpropene) in the presence of protic or Lewis acid catalysts to give poly(isobutylene) (16), as depicted in Equation 22.6. The conversion of acrylonitrile to poly(acrylonitrile) (17) using sodium amide, a strong base, represents anionic polymerization (Eq. 22.7). 

Free-radical polymerization is a widely used method to induce chain- 

The reaction is started by the thermal decomposition of an initiator, which in our experiment is tert-butyl peroxybenzoate (18), a compound that produces the free radicals 19 and 20 when heated (Eq. 22.8). If In represents one or both of these free radicals, the course of the polymerization may be illustrated as shown in Equations 22.9-22.12. Equation 22.9 indicates the function of the free radicals in initiating the polymerization. Equations 22.10a and 22.10b represent the propagation of the growing polymer chain. Equations 22.11 and 22.12 show possible termination processes. In Equation 22.11, the free-radical end of one growing polymer chain abstracts a hydrogen atom from the carbon atom next to the end of another polymer radical to produce the unsaturated and saturated polymer molecules 22 and 23, respectively, in a process termed disproportionation. For the termination reaction illustrated by Equation 22.12, Rad may be one of the initiating radicals. In, or another growing polymer chain. 

InCH2CH—(CH2CH) —CH2CH + Rad C6H5 C6H5 C6H5 

In the discovery experiment that follows, you will explore an important aspect of polymer chemistry by preparing polystyrene (24) under different reaction conditions and test whether it, like many polymers, may be produced in a variety of physical forms such as an amorphous solid, a film, and a clear glass. 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

Suppose the chain-reaction mechanism for radical-addition polymerization of a monomer M (e.g., CH2CHC1), which involves an initiator I (e.g., benzoyl peroxide), at low concentration, is as follows (Hill, 1977, p. 124)  

From the summation of (6), (7),.. (8), with the assumption that kpcMCp. is relatively 

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A chain reaction polymerization of vinyl monomer, which is usually carried out by a photoinitiator to produce a primary radical (R ), which can interact with a monomer molecule (M) in a propagating process to form a polymer chain composed of a large number of monomer units (see Eq. [2] and reaction Scheme [3]. 

Synthetic polymers are classified by their method of synthesis as either chain-growth or step-growth. The categories ate somewhat imprecise but nevertheless provide a useful distinction. Chain-growth polymers are produced by chain-reaction polymerization in which an initiator adds to a carbon-carbon double bond of an unsaturated substrate (a vinyl monomer) to yield a reactive inter-... 

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. 

In addition to oxidation, many other reactions occur as free radical chain reactions polymerization, decomposition, fluorination, chlorination, etc. All chain reactions have a few important general peculiarities [1—3]. 

Chain-propagating radical reaction, nonpolymeric, 14 276 Chain propagation, in low density polyethylene, 20 218-220 Chain-reaction polymerizations, 14 244 Chain rule of partial differentiation,... 

A kinetic chain reaction usually consists of at least three steps (1) initiation, (2) propagation, and (3) termination. The initiator may be an anion, a cation, a free radical, or a coordination catalyst. Although coordination catalysts are the most important commercially, the ionic initiators will be discussed first in an attempt to simplify the discussion of chain-reaction polymerization. 

What name is used to describe the negatively charged counterion in cationic chain-reaction polymerizations ... 

What percentage of polymer is usually found when a polymer produced by chain-reaction polymerization is heated above its ceiling temperature ... 

Which t q)e of chain-reaction polymerization is most likely to terminate by coupling ... 

Although the mechanism of copolymerization is similar to that discussed for the polymerization of one reactant (homopolymerization), the reactivities of monomers may differ when more than one is present in the feed, i.e., reaction mixture. Copolymers may be produced by step-reaction or by chain reaction polymerization. It is important to note that if the reactant species are Mi and M2, then the composition of the copolymer is not a physical mixture or blend, though the topic of blends will be dealt with in this chapter. 

Initiators such as BPO are used not only for the initiation of chain reaction polymerization, but also for the curing of polyesters and ethylene-propylene copolymers, and for the grafting of styrene on elastomeric polymer chains. 

Polymers produced by chain-reaction polymerization Copolymerization Kinetics... 

For reasons of simplicity, the polymers discussed so far are related to polyethylene and are usually produced by a chain reaction polymerization of vinyl monomers. Like other chain reactions, the polymerization requires three steps, i.e., initiation, propagation, and termination. Additional information... 

Polymer synthesis by step-reaction involves the coupling of small molecules which are difunctional by virtue of having two reactive functional groups. It is common for the coupling to involve elimination of a small molecule, and typical examples might be the synthesis of a polyester from a hydroxy-acid or of a polyamide by reaction of a diamine with a diacid. Step-reaction polymerizations are often equilibria, whereas chain-reaction polymerizations are more usually irreversible. 

Polymer synthesis is a complicated process that can be carried out applying different methodologies starting from appropriate monomers [193-196], The most important methods of polymer synthesis are step-growth polymerization and chain reaction polymerization [194,196],... 

The most common chain reaction polymerization is free-radical polymerization. A free radical is merely a molecule with an unpaired electron, which has a tendency to add a supplementary electron in order to form an electron pair which makes it extremely reactive. These molecular complexes could be produced by heat or irradiation, or formed by the addition of a compound, named the initiator (I), for example, dialkyl peroxides (R compounds (R — N = N — R), which are not strictly, catalysts, since they are chemically altered during the reaction [196],... 

There are two primary polymerization approaches step-reaction polymerization and chain-reaction polymerization.12... 

In contrast to the slow step-reaction polymerizations, chain-reaction polymerizations are fairly rapid.14 Chain-reaction polymerizations (often referred to as addition polymerizations) require the presence of an initiator for polymerization to occur. Initiation can occur by a free radical, an anionic, or a cationic species, which open the double bond of a vinyl monomer and the reaction proceeds as shown in Fig. 15.11 where may be a radical,... 

Copolymerization can occur through any of the chain reaction polymerization mechanisms described above however, the reactivity of a given monomer toward the second monomer can vary. Thus, not all combinations of monomers may be copolymerized. Each active end will exhibit different reactivity toward each monomer, which can be expressed as reactivity ratios, r and r2.35 These reactivity ratios rl in this example) show the tendency of a given active end, for example Mj, to add its own monomer (Mt) over the other monomer (M2). The copolymer composition at any instant can be determined by the composition of the feedstock and the reactivity ratios by... 

TABLE 14.19 Comparison of Step-Reaction and Chain-Reaction Polymerization... 

Copolymers may be produced by step reaction or by chain reaction polymerization in similar mechanisms to those of homopolymerization. The most widely used synthetic rubber (SBR) is a copolymer of styrene (S) and butadiene (B). Also, ABS, a widely used plastic, is a copolymer or blend of polymers of acrylonitrile, butadiene, and styrene. A special... 

Earlier investigations in our laboratory 56) have shown that several chain reaction polymerizations at temperatures below the melting or dissolution temperature of the final polymer lead to a thermodynamically more stable state than crystallization of the identical polymer from the polymer melt. Bawn and Ledwith mentioned in a review on stereoregular addition polymerization 57) that crystallization of the growing polymer chain might influence the polymerization step such that a more stereoregular polymer results. Ham 58—60) has finally pointed out a possible influence of the crystallization on the tacticity of the preceeding poljonerization which should only be possible when both processes are practically simultaneous. 

When dealing with catalysis it is best, however, to classify polymerization reactions according to the mechanism of chain propagation (2). One may distinguish in this way between chain-reaction polymerization and step-reaction (stepwise) polymerization. The essential features of these classes are shown in Table I (15). The diflFerences between the two types of polymerization are also evident from equations of rate (Rp) and average degree of polymerization (DP). For a free-radical polymerization of vinyl compounds (an example of a chain reaction), Rp and DP are functions of monomer and catalyst concentration (Equations 9 and 10) ... 

Chain-reaction polymerization may be induced by various catalysts. For example, vinyl polymerization (Reaction 5) occurs with free-radical, anionic, or cationic catalysts, as well as with coordinate catalysts (which may be of the anionic or the cationic type) (30, 31). Poly(a-amino acid) formation (Reaction 8) may be carried out with basic catalysts. 

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