Chain reactions characteristics

To understand these reactions, the so-called Bolland and Gee reaction scheme17-18 and its subsequent developments has been applied to explain the chain reaction characteristics of both thermal and photooxidation of polyolefins. The scheme (Scheme 2.1) has been found to be a useful model for many other polymers comprising significant aliphatic character, such as aliphatic polyamides and polyesters and certain polyvinyls including poly(vinyl chloride) (PVC). 

Section 27 5 Ammo acids undergo reactions characteristic of the ammo group (e g amide formation) and the carboxyl group (e g esterification) Ammo acid side chains undergo reactions characteristic of the functional groups they contain... 

One characteristic of chain reactions is that frequentiy some initiating process is required. In hydrocarbon oxidations radicals must be introduced and to be self-sustained, some source of radicals must be produced in a chain-branching step. Moreover, new radicals must be suppHed at a rate sufficient to replace those lost by chain termination. In hydrocarbon oxidation, this usually involves the hydroperoxide cycle (eqs. 1—5). 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

The individual steps in chain reactions involving radicals are characteristically of small activation energy, between about 10 and 50kJmol and so these reactions should occur at an immeasurably high rate at temperatures above 500 K (see Table 2.1), which is a low temperature for a useful combustion process. The overall rate of the process will tlrerefore depend mainly on the concentrations of tire radicals. 

Certain kinetic aspects of free-radical reactions are unique in comparison with the kinetic characteristics of other reaction types that have been considered to this point. The underlying difference is that many free-radical reactions are chain reactions that is, the reaction mechanism consists of a cycle of repetitive steps which form many product molecules for each initiation event. The hypothetical mechanism below illustrates a chain reaction. 

In the schemes considered to this point, even the complex ones, the products form by a limited succession of steps. In these ordinary reaction sequences the overall process is completed when the products appear from the given quantity of reactants in accord with the stoichiometry of the net reaction. The only exception encountered to this point has been the ozone decomposition reaction presented in Chapter 5, which is a chain reaction. In this chapter we shall consider the special characteristics of elementary reactions that occur in a chain sequence. 

According to the authors mentioned above, induced chain reactions (Livingston ) or induced catalysis (Bray and Ramsey ) take place when the very slow reaction between the acceptor and actor is catalyzed by the inductor. However, since the chemical characters of the acceptor and the inductor are the same, the actor reacts with the inductor, too, thus a part of it will be excluded from the catalysis. The principal characteristics of reactions of this type according to Luther and Schilow are that the value of Fj largely exceeds 2 and that the plot of Fj rm 5([Ac]/[I])o rises exponentially. 

The principal characteristic of induced reactions of this type which have not been stressed so far, is that the extent of the induced change greatly decreases and in most cases reaction even ceases in the presence of chain-breaking substances. The induced reaction can be suppressed by any substances reacting with chain carriers at a higher rate than does the acceptor, and the product of the reaction of the suppressor can easily react with the inductor. Since the concentration of the chain carriers is generally low, the supressors of induced chain reactions exert considerable effect even in small quantity. The effect is particularly pronounced when the suppressor reacts reversibly. 

The processes by which unsaturated monomers are converted to polymers of high molecular weight exhibit the characteristics of typical chain reactions. They are readily susceptible to catalysis, photoactivation, and inhibition. The quantum yield in a photoactivated polymerization in the liquid phase may be of the order of 10 or more, expressed as the number of monomer molecules polymerized per quantum absorbed. The efficiency of certain inhibitors is of a similar magnitude, thousands of monomer molecules being prevented from polymerizing by a single molecule of the inhibitor. ... 

Such methods also make it possible to learn about the state of health of the dead before death, the diseases from which they suffered, their age at the time of death, the method used for their mummification, and even the cultural environment in which they lived and were mummified (Cockbum et al. 1998 Harris and Wente 1980). The conception and development of the polymerase chain reaction (PCR) at the end of the twentieth century made it also possible to study the genetic characteristics of the mummies and of the populations to which they belonged (see Textbox 65). 

Another characteristic of many radical reactions is that, once initiated, they often proceed with great rapidity owing to the establishment of fast chain reactions of low energy requirement, e.g. in the halogenation of alkanes (3, cf. p. 323) ... 

The essential characteristic of a chain reaction mechanism is the existence of a closed cycle of reactions in which unstable or highly reactive intermediates react in propagation steps with stable reactant molecules or other intermediates and are regenerated by the sequence of reactions... 

In any attempt to elucidate the mechanism of a chemical reaction, it is very important to determine at an early stage whether or not a chain reaction is occurring. Consequently, we now note some of the characteristics by which the... 

The ability of a nitro group in the substrate to bring about electron-transfer free radical chain nucleophilic substitution (SrnI) at a saturated carbon atom is well documented.39 Such electron transfer reactions are one of the characteristic features of nitro compounds. Komblum and Russell have established the SrnI reaction independently the details of the early history have been well reviewed by them.39 The reaction of p-nitrobenzyl chloride with a salt of nitroalkane is in sharp contrast to the general behavior of the alkylation of the carbanions derived from nitroalkanes here, carbon alkylation is predominant. The carbon alkylation process proceeds via a chain reaction involving anion radicals and free radicals, as shown in Eq. 5.24 and Scheme... 

As already noted (see Chapter 4), autoxidation is a degenerate branching chain reaction with a positive feedback via hydroperoxide the oxidation of RH produces ROOH that acts as an initiator of oxidation. The characteristic features of inhibited autoxidation, which are primarily due to this feedback, are the following [18,21,23,26,31-33] ... 

Reaction with other radicals is characteristic of the triarylmethyls and the chain reaction with oxygen has already been discussed. The absorption of the radical nitric oxide is also used as an analytical tnethod for triphenylmethyl. 

Elemental sulfur is found in the flames of all the sulfur-bearing compounds discussed in the previous subsections. Generally, this sulfur appears as atoms or the dimer S2. When pure sulfur is vaporized at low temperatures, the vapor molecules are polymeric and have the formula Sg. Vapor-phase studies of pure sulfur oxidation around 100°C have shown that the oxidation reaction has the characteristics of a chain reaction. It is interesting to note that in the explosive studies the reaction must be stimulated by the introduction of O atoms (spark, ozone) in order for the explosion to proceed. 

---