Chain propagation effect

Eqs. (1) to (3) indicate that conversion studies under conditions where thermal polymerization prevails can only yield e and the product nyo, whereas the photon-induced reaction provides information on the product nq. To disentangle chain initiation and chain propagation effects an independent determination of the kinetic chain length is required. 

Eor antioxidant activity, the reaction of aminyl radicals with peroxy radicals is very beneficial. The nitroxyl radicals formed in this reaction are extremely effective oxidation inhibitors. Nitroxides function by trapping chain-propagating alkyl radicals to give hydroxylamine ethers. These ethers, in turn, quench chain propagating peroxy radicals and in the process regenerate the original nitroxides. The cycHc nature of this process accounts for the superlative antioxidant activity of nitroxides (see Antioxidants). Thus, antioxidant activity improves with an increase in stabiUty of the aminyl and nitroxyl radicals. Consequendy, commercial DPA antioxidants are alkylated in the ortho and para positions to prevent undesirable coupling reactions. 

Effects of compounds observable at lower concentrations ai e probably connected with the effect on the initiation/termination stages (transition metals in TMB-0, reaction with photoinitiation, UDMH in the same reaction with chemical initiation), while the compounds influencing only at higher concentrations may affect chain propagation stages. 

Much of the CI2O manufactured industrially is used to make hypochlorites, particularly Ca(OCl)2, and it is an effective bleach for wood-pulp and textiles. CI2O is also used to prepare chloroisocyanurates (p. 324) and chlorinated solvents (via mixed chain reactions in which Cl and OCl are the chain-propagating species).Its reactions with inorganic reagents are summarized in the scheme opposite. 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

Both of these reactions involve the production of two active centers where there was only one before. When reactions of this type occur to a significant extent, the total number of active centers present in the system can increase very rapidly, since a multiplication effect sets in as the chains propagate. The growth of chain carriers in a branched chain reaction is pictured below. 

Mechanistic details of the microwave-induced oligomerization of methane on a microporous Mn02 catalyst were studied by Suib et al. [67], with emphasis on fundamental aspects such as reactor configuration, additives (chain propagators, dielectrics), temperature measurements, magnetic field effect, and reaction conditions. 

The peroxyl radical of a hydrocarbon can attack the C—H bond of another hydrocarbon. In addition to this bimolecular abstraction, the reaction of intramolecular hydrogen atom abstraction is known when peroxyl radical attacks its own C—H bond to form as final product dihydroperoxide. This effect of intramolecular chain propagation was first observed by Rust in the 2,4-dimethylpentane oxidation experiments [130] ... 

Since two different peroxyl radicals, namely, H02 and a-aminoalkyl peroxyl, take part in chain propagation and termination, the values of kp and kt rate constants are effective. They depend on the ratio of the concentrations of these two peroxyl radicals. The decay of... 

We see that the increment of the steric influence AEs ranges from 22 to 28 kJ mol 1 for the Ar20 reaction with the C—H bond and amounts to 8 kJ mol 1 for the reaction of Ar20 with hydroperoxide. Due to the steric effect, sterically hindered phenoxyl practically does not participate in the chain propagation. The same effect is observed in the comparison of diphenylaminyl and picrylhydrazyl reactions with R3H [34] ,e0(Ph2N ) = 81.7kJ mol-1, CeofDPPH ) = 99.3 kJ mol-1, and AES = 17.6 kJ mol-1. 

It should be taken into account that the reaction of chain propagation occurs in polymer more slowly than in the liquid phase also. The ratios of rate constants kjlkq, which are so important for inhibition (see Chapter 14), are close for polymers and model hydrocarbon compounds (see Table 19.7). The effectiveness of the inhibiting action of phenols depends not only on their reactivity, but also on the reactivity of the formed phenoxyls (see Chapter 15). Reaction 8 (In + R02 ) leads to chain termination and occurs rapidly in hydrocarbons (see Chapter 15). Since this reaction is limited by the diffusion of reactants it occurs in polymers much more slowly (see earlier). Quinolide peroxides produced in this reaction in the case of sterically hindered phenoxyls are unstable at elevated temperatures. The rate constants of their decay are described in Chapter 15. The reaction of sterically hindered phenoxyls with hydroperoxide groups occurs more slowly in the polymer matrix in comparison with hydrocarbon (see Table 19.8). 

As in the case of H2—02 limits, certain general characteristics of the defining curve in Fig. 3.5 may be stated. The lower limit meets all the requirements of wall destruction of a chain propagating species. The effects of vessel diameter, surface character, and condition have been well established by experiment [2],... 

Generally, radical chain reactions are carried out in nonpolar solvents although strong solvent effects on propagation steps are rare. Apart from the polarity, a much more important criterion for the solvent choice is the solvent s inertness towards the chain propagating radicals involved. 

An increase in the cA-stilbene concentration favors the chain propagation and decreases the probability of termination when the DCNA anion-radicals react with the stilbene cation-radicals. A decrease in the irradiation intensity has a similar effect The chain propagation is the first-order process, whereas termination of the chains is the second-order process. A temperature rise accelerates the accumulation of the stilbene cation-radicals. In this system, the free energy of electron transfer is -53- —44 kJ moD (the cation-radical generation is in fact an endothermal process). If a polar solvent is substituted for a nonpolar one, the conversion of the cii-stilbene cation-radical into the trani-stilbene cation-radical deepens. Polar solvents break ion pairs, releasing free ion-radicals. The cA-stilbene cation-radicals isomerize more easily on being released. The stilbene cation-radical not shielded with a counterion has a more positive charge, and therefore, becomes stabilized in the... 

Electroreductive one-electron initiation of cyclization was described for the series of E,E-, 1-dibenzoyl-l,6-heptadiene and its derivatives (Roh et al. 2002, Felton and Bauld 2004). In this case, the catalytic effect was also observed (the actual consumption of electricity was substantially less than theoretical). The same bis(enones) can also be cyclized on the action of the sodium salt of chrysene anion-radical in THF, but with no catalytic effect. Optimum yields were obtained only when 70-120 mol% of the initiator was used, relative to a substrate (Yang et al. 2004). The authors suggest that tight ion pairing of the sodium cation with the product anion-radical in THF (which is a somewhat nonpolar solvent) slows down the intermolecular electron transfer to the bis(enone) molecules. Such an electron transfer would be required for chain propagation. 

By using the differential form of the copolymer composition equation (26, 28) the products of oxidation of mixtures at low conversions permit comparison of rates of chain propagation in autoxidations of various compounds, essentially free from effects of chain initiation, chain termination, and over-all rates. 

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