Chain reactions, free-radical steps

As with other chain reactions, free radical polymerization is a rapid reaction which consists of the characteristic steps of initiation, propagation, and termination. Free radical initiators are produced by the homolytic cleavage of covalent bonds as well as numerous radiation-associated methods. 

Free-Radical Chain Reactions. Free-radical chain reactions are sensitive to two types of catalysis a catalyst may increase the rate of initiation of chains by introducing an additional initiation pathway or may lead to new chain propagation steps. For example a chain reaction involving S(IV) and 02 can be initiated by a number of alternative pathways ... 

In general, which is more rapid (a) free radical chain reactions or (b) step reaction pol5merizations ... 

Radical reactions are often called chain reactions. All chain reactions have three steps chain initiation, chain propagation and chain termination. For example, the halogenation of alkane is a free radical chain reaction. 

Another common feature of free-radical reactions is that they tend to be chain processes. Since any chemical reaction must exhibit conservation of spin, the reaction of a free radical widi a closed-shell (fully electron paired) molecule must result in the production of a new free-radical species which can participate in subsequent free-radical reactions. The series of free-radical reactions leading to product is often a cyclic process in which the initial free radical is produced once again in die last step of the cycle so that the reaction sequence starts over again. The process is termed a chain reaction because each step of the process is linked directly to die preceding step. 

Most free-radical reactions of synthetic value are chain reactions, the key steps of which are illustrated in Scheme 4.1. In the initiation step, a reactive radical is generated from a nonradical precursor (initiator). In many cases, this can be accomplished thermally. For instance, peroxides possess a weak oxygen-oxygen bond and, consequently, undergo homolytic dissociation upon heating ROOR —> 2RO . Free radicals can also be generated photochemically, radiolytically, or by electron transfer from appropriate precursors. 

Cumene hydroperoxide formation. The formation of the hydroperoxide proceeds by a free radical chain reaction. A radical initiator abstracts a hydrogen-free radical from the molecule, creating a tertiary free radical. The creation of the tertiary free radical is the initial step in the reaction. 

Most important, the existence of an induction or inhibition period suggests a free-radical step in the decomposition of the thiophene ring. Further evidence for the free-radical nature of the reaction was obtained from experiments conducted under less severe conditions in order to isolate the initial ring-opened intermediate before subsequent loss of the one-carbon fragment. Efforts to isolate the initial decomposition product were unsuccessful. Apparently, the loss of the one-carbon fragment occurs rapidly, consistent with a free-radical chain reaction of some type. 

Like many other radical reactions, free-radical halogenation is a chain reaction. Chain reactions usually require one or more initiation steps to form radicals, followed by propagation steps that produce products and regenerate radicals. 

In a free-radical chain reaction, every propagation step must occur quickly, or the free radicals will undergo unproductive collisions and participate in termination steps. We can predict how quickly the various halogen atoms react with methane given relative rates based on the measured activation energies of the slowest steps ... 

Free-radical polymerization is the most widely used process for polymer synthesis. It is much less sensitive to the effects of adventitious impurities than ionic chain-growth reactions. Free-radical polymerizations are usually much faster than those in step-growth syntheses, which use diFFereiit monomers in any case. Chapter 7 covers emulsion polymerization, which is a special technique of free-radical chain-growth polymerizations. Copolymerizalions are considered separately in Chapter 8. This chapter focuses on the polymerization reactions in which only one monomer is involved. 

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) ... 

These are examples of step-reaction polymerization (Sec. 32.2). Here, reaction does not depend on chain-carrying free radicals or ions. Instead, the steps are essentially independent of each other they just happen to involve more than one functional group in a monomer molecule. 

There are some general features of a free radical reaction. Free radical reactions take three distinct, identifiable steps. The first is formation of the free radical that can happen by enzyme catalysis, homolysis, thermolysis, radiation, light induction, combustion and pyrolysis, or other means. The second step, called propagation, is the heart of a free radical reaction. In this step, free radicals are repeatedly regenerated and can react with neutral molecules to produce new free radicals. If there is no intervention, two free radicals can react to form a neutral molecule and the reaction is terminated, which represents the third step in the general reaction scheme. Because of this repetitive nature of the reaction, free radical reactions are called chain reactions and are often represented as a cyclic process [Nagendrappa (27A78)]. 

FIGURE 3.2 Schematic representation of the development of molar mass, for example, M, with monomer conversion for chain growth (free radical), chain growth (living) and step growth mechanisms for the polymer buildup reaction. 

In all the cases considered above, it is assumed that the growing chain end (free radical or ion) reacts directly with the monomer. Instead, however, the monomer can also form an intermediate product in an equilibrium reaction with the ion pair, and this intermediate can then convert into the true addition product in a rate-determining step ... 

Oxidation reactions are chain reactions which follow a free radical mechanism. In chain reactions three distinct steps are present initiation, propagation and termination. During initiation a free macroradical (P") is generated in the polymer by heating, radiation or stress. This reacts readily with oxygen to yield a peroxy radical (POO"), the peroxy free-radical abstracts hydrogen from another polymer molecule (PH) creating a new macroradical and a hydroperoxide (POOH). Then the hydroperoxide decomposes to two new free radicals, which are also initiators of the chain reaction. These chain reactions have severe consequences for the polymer, and cause extensive localized oxidation. 

The nature of functional groups of a polymer changes as a result of protonation, ionization, quartemization, and alkylation. There are numerous examples of this effect however, only a few will be described here. Particularly, PVA reaction with is accompanied by a partial conversion of hydroxyl to carboxyl groups (scheme 5). In an acidic medium, complex A decomposes, 1,2-glycol units are oxidized, and reduces to V. In addition, complex B is formed from and 1,2-ketol as a result of oxidation, which is accompanied by chain decomposition to reduce vanadium. The reaction is, to a certain extent, similar for Ce +. Furthermore, free-radical steps are observed in these reactions. It should be noted that this occurs only for PVA, which forms with 1,2-glycol units as a result of abnormal head-to-head addition. This illustrates the importance of both functional and structural uniformity of macroligands. 

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

The elementary steps (1) through (3) describe a free radical chain mech anism for the reaction of an alkane with a halogen... 

The reaction proceeds by a free radical chain mechanism involving the following prop agation steps... 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

Termination steps (Section 4 17) Reactions that halt a chain reaction In a free radical chain reaction termination steps consume free radicals without generating new radicals to continue the chain... 

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