Chain induced decomposition

The complexities of the chain induced decompositions can be minimized by choosing an initiator and solvent without easily abstracted hydrogens under these conditions peroxy and azo compounds decompose unimolecularly at easily measured rates to provide convenient sources of radicals for use in studying other radical processes. 

Addition of compounds (i.e., benzyl ethers) that produce relatively stable radicals and cations, and cause chain induced decomposition of onium salts [108-110,122,134],... 

Several reports have dealt with the reactions of dimethyldioxirane. Spectrophoto-metric studies of acetone solutions of the reaction between dimethyldioxirane and 2-methylbutane have shown an initial radical chain-induced decomposition of the dioxirane, which is inhibited by the presence of dioxygen. The same group has also studied the reaction of dimethyldioxirane with adamantane in an oxygen atmosphere. The first-order rate constants depended only on the concentration of the adamantane and were independent of the dimethyldioxirane concentration. 

The following observations can be made with respect to the data contained in this table. The quantum yields for both onium salts are less than unity, suggesting that chain induced decomposition processes are not occurring in acetonitrile. As expected, the quantum yields are independent of the anions since they are both transparent to ultraviolet irradiation and do not participate directly in the initial bond breaking process. The discrepancy between the quantum yields for acid formation and either iodobenzene or diphenyl sulfide reported by Gatechair is particularly notable. 

In contrast to the photolysis of diaryliodonium salts in acetonitrile which gives quantum yields for the formation of aryliodides of < 1, photolysis in tetrahydrofuran (THF) at 366 nm gives quantum yields of approximetaly Quantum yields greater than unity in this case suggest that a chain induced decomposition of the... 

Eurther reactions of the alkylperoxy radical (ROO-) depend on the environment but generally cause generation of other radicals that can attack undecomposed hydrosend peroxide, thus perpetuating the induced decomposition chain. Radicals also can attack undecomposed peroxide by radical displacement on the oxygen—oxygen bond ... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

Diacyl peroxides are particularly prone to induced decomposition (Scheme 3.16). Transfer to initiator is of greatest importance for polymerizations taken to high conversion or when the ratio of initiator to monomer is high. It has been shown that, during the polymerization of S initiated by BPO, transfer to initiator can be the major pathway for the termination of chains.7,41... 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

It is also known that alkyl cobaloximes are subject to radical-induced decomposition.2 7 This suggests an alternative to the mechanism shown in Scheme 9.28 involving reversible chain transfer (Section 9.5). 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

For the copper-induced decomposition of diazodiphenylmethane in acetonitrile, a fundamental difference in the catalytic action of Cu C104 and Cu ClO was detected. Whilst with CuC104, intermediary copper carbenoids are believed to be responsible for the mainly formed benzophenone azine402, CufClO initiates a chain reaction, promoted by radical cations and yielding mainly tetraphenylethene... 

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... 

It is rather surprising that the induced decomposition of POOH of PP occurs with long chains in a dioxygen atmosphere. The most probable reaction of tertiary hydroperoxyl groups of PP decay is that of hydroperoxyl radicals formed from POOH (see Chapter 19) ... 

This reaction is termed chain transfer to initiator and is considered further in Sec. 3-6b. The induced decomposition of initiator does not change the radical concentration during the polymerization, since the newly formed radical (polymer chain. However, the reaction does result in a wastage of initiator. A molecule of initiator is decomposed without an increase in the number of propagating radicals or the amount of monomer being converted to polymer. 

Chain Termination in the Oxidation of Cumene. Traylor and Russell (35) assume that the acceleration in the rate of oxidation of CH which is produced by added COOH is solely caused by a chain transfer reaction between CO radicals and COOH. This assumption implies that all CH3OO radicals enter into termination via Reaction 13. However, Thomas (32) has found that acetophenone is formed even in the presence of sufficient COOH to raise the oxidation rate of CH to its limiting value. (The receipt of Thomas manuscript prior to publication stimulated the present calculations.) From this fact, and from a study of the acetophenone formed during the AIBN-induced decomposition of COOH, Thomas concludes that the accelerating effect of added COOH is primarily caused... 

The alkylation of quinoline by decanoyl peroxide in acetic acid has been studied kineti-cally, and a radical chain mechanism has been proposed (Scheme 207) (72T2415). Decomposition of decanoyl peroxide yields a nonyl radical (and carbon dioxide) that attacks the quinolinium ion. Quinolinium is activated (compared with quinoline) towards attack by the nonyl radical, which has nucleophilic character. Conversely, the protonated centre has an unfavorable effect upon the propagation step, but this might be reduced by the equilibrium shown in equation (167). A kinetic study revealed that the reaction is subject to crosstermination (equation 168). The increase in the rate of decomposition of benzoyl peroxide in the phenylation of the quinolinium ion compared with quinoline is much less than for alkylation. This observation is consistent with the phenyl having less nucleophilic character than the nonyl radical, and so it is less selective. Rearomatization of the cr-complex formed by radicals generated from sources other than peroxides may take place by oxidation by metals, disproportionation, induced decomposition or hydrogen abstraction by radical intermediates. When oxidation is difficult, dimerization can take place (equation 169). 

The phenomenon of induced decomposition, in which radicals derived from reaction of the solvent with the initiator benzoyl peroxide consume some of the initiator in a chain process, was first elucidated by Bartlett and Nozaki (equation 51), and by Cass. ... 

An initiator must be stable toward induced decomposition from its own radicals or from the growing radical-terminated polymer chain in the reaction medium. If radicals induce initiator decomposition, the resultant products tend to form polymers of low average molecular weight. 

Thermal unimolecular decomposition of perfluorodiacyl peroxides seems to be less prone to cage-recombination, with only 5 % of coupling remaining when such decomposition is carried out in the presence of an excess of a radical scavenger such as CCl3Br [68]. Of course, donor-induced decomposition of diacyl peroxides leads to clean chain processes with virtually no radical recombination being observed [66]. 

This scheme explaining the copolymerization observed has not yet been explained in kinetic aspects. Indeed, if decomposition rate of diacetyl peroxide equals 10-5 s-1 at 60 °C [120], this value does not follow the classic evolution of rate vs temperature (according to Arrhenius law). The authors suggest induced decompositions of these peroxides by CH3 radicals existing in the medium, and also by macroradicals coming from growing chains during... 

As mentioned above, the secondary reaction in the system is caused by a new reaction of free radical interaction (produced in the elementary dissociation of the radical initiator) with the initiator. In this case, one more active intermediate particle (free radical), not observed at usual peroxide decomposition, is generated in the system. Owing to formation of this active site, conjugated reaction proceeds by the radical-chain mechanism. Thus products formed may be analogous to those obtained at usual initiator decomposition, or different products may be formed—this circumstance is of no importance for detection of induced decomposition. 

Formation of protecting surface layer before or after ignition being in competition with the heat-induced decomposition of the polymer chains and oxidation of the formed fragments by radical process... 

For most polymers, the yield of hydroperoxides is relatively low even in the presence of oxygen excess. The relatively high values were, e.g., obtained during oxidation of atactic polypropylene [79], In the initial phases of oxidation, the yield of hydroperoxide related to 1 mol of oxygen absorbed is 0.6 at 130 °C when passing the maximum concentration it decreases considerably. In isotactic polypropylene, the maximum yield of hydroperoxides attains the value 0.2, only [80]. This may be probably related with a local accumulation of hydroperoxides in domains of defects in the crystalline structure which leads to an increased ratio of participation of hydroperoxide groups in the chain reaction of an oxidation process (induced decomposition of hydroperoxides) and finally to a lower yield of hydroperoxides... 

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