Propagation step in chain

Figure 34 Example of propagation step in chain depolymerisation mechanism (unzipping) of polymethylmethacrylate (PMMA).

Like the bimolecular reactions, unimolecular reactions are often found as individual steps in complex reactions. These include the unimolecular breakdown of molecules into radicals often found as first initiation steps and propagation steps in chain reactions, e.g. 

In most instances, phenoxy radicals do not engage in propagation steps in chain reactions, consistent with their use as free radical inhibitors. They are, however, reactive enough to oxidize ascorbic acid or to reduce some quinones (Neta and Steenken, 1981). 

Step 1 is called the chain-initiating step. In the chain-initiating step radicals are created. Steps 2 and 3 are called chain-propagating steps. In chain-propagating steps one radical generates another. 

By about 1947 many of the major themes of free radical chemistry had emerged from this war-initiated polymer research data on structure and reactivity and establishment of the utility of competitive kinetics for determining relative rates of chain propagation steps in chain reactions by product analyses as I ve just described th first good rate constants for elementary steps in chain processes - here the first really reliable results were on vinyl acetate polymerization from Paul Bartlett s laboratory ( ) degradative chain transfer, first demonstrated by Bartlett and Altschul and showing how the length of kinetic... 

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

In the next three sections we consider initiation, termination, and propagation steps in the free-radical mechanism for addition polymerization. One should bear in mind that two additional steps, inhibition and chain transfer, are being ignored at this point. We shall take up these latter topics in Sec. 6.8. 

Rea.CtlVltyRa.tlO Scheme. The composition of a copolymer at any point in time depends on the relative rates that each monomer can add to a chain end. If it is assumed that the chemical reactivity of a propagating chain depends only on the terminal unit and is not affected by any penultimate units, then four possible propagation steps in the copolymerisation of two monomers, and M2, with two growing chain ends, M and M2, can be written as follows ... 

The ratio describes the relative reactivity of polymer chain M toward monomer M and monomer M2. Likewise, describes the relative reactivity of polymer chain M2 toward M2 and M. With a steady-state assumption, the copolymerisation equation can be derived from the propagation steps in equations 3—6. 

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

The elementary reactions comprising the chain reaction mechanism are generally classified as initiation, propagation, or termination reactions. In the initiation reaction an active center or chain carrier is formed. Often these are atoms or free radicals, but ionic species or other intermediates can also serve as chain carriers. In the propagation steps the chain carriers interact with the reactant molecules to form product molecules and regenerate themselves so that the chain may continue. The termination steps consist of the various methods by which the chain can be broken. 

Ionic chain polymerisations follow the same basic steps as radical chain polymerisation and are said to be either cationic or anionic depending on the nature of the ion formed in the initiation step. Schemes of the insertion during the propagation step in cationic and anionic chain polymerisations are shown in Figure 24. 

Transfer reaction to the monomer, leading to the insertion of an unsaturated end group, is an important reaction in cationic chain polymerisation. As the activation energies of both termination and transfer reactions are higher than that of the propagation step, cationic chain polymerisation can only lead to high molecular masses when undertaken at low temperatures, typically — 100°C. 

Reactions of phenoxyl and aminyl radicals with RH and ROOH are chain propagation steps in oxidation inhibited by phenols and amines (see Chapter 14). Both reactions become important when their rates are close to the initiation rate (see Chapter 14). Mahoney and DaRooge [57] studied the oxidation of 9,10-dihydroanthracene inhibited by different phenols. He went on to estimate the values of rate constants ratio of the reaction of ArO with RH and the reaction In + In (reactions (9) and (10), see Chapter 14) by the kinetic study. The values of kw for the reaction... 

Reaction (3.1) is the initiation step, where M is a reactant molecule forming a radical R. Reaction (3.2) is a particular representation of a collection of propagation steps and chain branching to the extent that the overall chain branching ratio can be represented as a. M is another reactant molecule and a has any value greater than 1. Reaction (3.3) is a particular chain propagating step forming a product R It will be shown in later discussions of the hydrocarbon-air... 

Radical chain hydrosilylation of alkenes using RsSiH, where R = alkyl and/ or phenyl, is not very useful in synthesis because the hydrogen abstraction step is slow under standard experimental conditions (cf. Section 3.1). These reactions proceed under drastic conditions (Reaction 5.9) [3], but can be promoted under milder conditions by the presence of catalytic amounts of a thiol [27]. Thus, the propagation steps in Scheme 5.1 are replaced by those reported in Scheme 5.4, where the thiol acts as the catalyst and the H transfer agent (see Iso Section 4.5). 

The essential feature of the chain reaction is the propagation step. In this step the intermediate is not consumed but acts simply as a catalyst for the conversion of material. Thus, each molecule of intermediate can catalyze a long chain of reactions, even thousands, before being finally destroyed. 

This is a major chain propagation step in the overall reaction mechanism for ozone formation in photochemical air pollution. Because H02 is intimately tied to OH through reaction (17) and cycles such as that in Fig. 1.4, when NO is present the sources and sinks of H02 are, in effect, sources or sinks of the OH radical. 

The methyl radical CH3 is comparatively unreactive and is only of secondary importance as chain carrier. The major chain-propagating steps in ethane pyrolysis are... 

The foregoing discussion adds further to our understanding of the selectivity observed in the halogenation reactions discussed in Chapter 4. When propene is chlorinated in sunlight, the product is 3-chloropropene, and we may explain this on the basis that the radical-chain reaction involves propagation steps in which a chlorine atom attacks the hydrogen corresponding to the weakest C-H bond ... 

Step 2 A chain-propagation step that results in the formation of a hydrogen atom Step 3 A chain-propagation step in which the hydrogen atom reacts with Cl2 Steps 4, 5, 6 Three chain-termination steps that occur between the two radicals... 

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