Chain initiation thermal initiated

Chain initiation is due to reaction (61), i.e. the thermal decomposition of peroxydisulphate into sulphate radicals, viz. 

In 1993, Linford firstly reported a quite useful method to prepare monolayers of alkyl chains by thermal hydros-ilylation of hydrogen-terminated silicon surfaces [25]. Alkyl chains are covalently bound to Si surface by Si-C bonds. This thermal hydrosilylation could be attributed to a free-radical process with 1-alkene. First, a diacyl peroxide initiator was used to produce free radicals. However, at higher temperature, only hydrogen-terminated silicon and a neat solution of 1-alkene or 1-alkyne can form Si-C linkages [26]. Furthermore, lately it is found that such Si-C covalent links can be observed even in dilute solutions of 1-alkenes [27]. In that case, the density of monolayer packing strongly depends on the reaction temperature. 

Chemical combustion is initiated by the oxidation or thermal decomposition of a fuel molecule, thereby producing reactive radical species by a chain-initiating mechanism. Radical initiation for a particular fuel/oxygen mixture can result from high-energy collisions with other molecules (M) in the system or from hydrogen-atom abstraction by 02or other radicals, as expressed in reactions 6.1-6.3 ... 

In thermal oxidation, chain initiation takes place by the reaction of the aldehyde with dioxygen. Two reactions of chain generation in autoxidized aldehydes, namely, bimolecular and trimolecular, were proved [25]. 

The complications which arose in the early photochemical work were due to the presence of impurities in the reactants, notably oxygen, NC13 and water which aided chain initiation or termination. In thermal reactions wall effects were in evidence. 

In contrast to reaction (3.71), at high temperatures the thermal decomposition of the methane provides the chain initiation step, namely... 

Even though there have been appreciably more studies of CS2, COS is known to exist as an intermediate in CS2 flames. Thus it appears logical to analyze the COS oxidation mechanism first. Both substances show explosion limit curves that indicate that branched-chain mechanisms exist. Most of the reaction studies used flash photolysis hence very little information exists on what the chain-initiating mechanism for thermal conditions would be. 

The emission of a helium nucleus in the final stage regenerates the initial carbon-12. The latter thus plays the role of a catalyst. The overall result is the fusion of four protons into a helium nucleus. At high temperatures, this cycle dominates over the proton-proton chain. Indeed thermal agitation facilitates penetration of the relatively high electrical barrier between proton and carbon nucleus. Whatever hydrogen fusion mechanism is prevalent, the star s mass determines the rate at which it consumes its nuclear fuel, and hence also its lifetime. The higher its mass, the more quickly it bums. 

It is the opinion of the present authors that isomerization of a tertiary alkyl radical to a primary radical as in the formation of II from I is improbable. The formation of IV is similarly unlikely. The cycliza-tion of V by intramolecular alkylation seems quite plausible however, equation 9 does not explain either the formation of V or its subsequent cyclization. The following mechanism has the advantages that, like the generally accepted free radical-initiated mechanisms, it postulates a chain reaction and that the intramolecular alkylation step is directly analogous to that proposed for thermal alkylation, namely addition of an alkyl radical to the double bond of the alkene (Frey and Hepp, 12). The method of formation of the chain initiator, R —, again is not critical since R —, merely starts the first cycle of the chain reaction it may be formed by decomposition of the isobutylene. 

Tetralin hydroperoxide (1,2,3,4-tetrahydro-l-naphthyl hydroperoxide) and 9,10-dihydroanthracyl-9-hydroperoxide were prepared by oxidizing the two hydrocarbons and purified by recrystallization. Commercial cumene hydroperoxide was purified by successive conversions to its sodium salt until it no longer increased the rate of oxidation of cumene at 56°C. All three hydroperoxides were 100% pure by iodometric titration. They all initiated oxidations both thermally (possibly by the bi-molecular reaction, R OOH + RH — R O + H20 + R (33)) and photochemically. The experimental conditions were chosen so that the rate of the thermally initiated reaction was less than 10% of the rate of the photoreaction. The rates of chain initiation were measured with the inhibitors 2,6-di-ter -butyl-4-methylphenol and 2,6-di-fer -butyl-4-meth-oxyphenol. None of the hydroperoxides introduced any kinetically first-order chain termination process into the over-all reaction. 

Gray fit Yang (Ref 1), a mathematical model was proposed to unity the chain and thermal mechanisms of explosion. It was shown that the trajectories in the phase plane of the coupled energy and radical concentration equations of an explosive system will oive the time-dependent behavior of the system when the initial temperature and radical concentration are given. In the 2nd paper of the same investigators (Ref 2), a general equation for explosion limits (P—T relation) is derived from a unified thermal and chain theory and from chis equation, the criteria of explosion limits for either the pure chain or pure thermal theory can be deduced. For detailed discussion see Refs... 

Three conditions must be fulfilled obtain complete conversion of the reactants, H2 and CI2. The first condition is that thermal equilibrium of the system be favorable. This condition is fulfilled at low and intermediate temperatures, where formation of the product HC1 is thermodynamically favored. At very high temperatures, equilibrium favors the reactants, and thereby serves to limit the fractional conversion. The second requirement is that the overall reaction rate be nonnegligible. There are numerous examples of chemical systems where a reaction does not occur within reasonable time scales, even though it is thermodynamically favored. To initiate reaction, the temperature of the H2-CI2 mixture must be above some critical value. The third condition for full conversion is that the chain terminating reaction steps not become dominant. In a chain reaction system, as opposed to a chain-branching system discussed below, the reaction progress is very sensitive to the competition between chain initiation and chain termination. This competition determines the amount of chain carriers (batons) in the system and thereby the rate of conversion of reactants. 

Hydrogen abstraction by RH02 could also participate in the process of initiating a chain of thermal oxidation reactions (pathy). In aqueous systems, cations will further react by solvolysis, and superoxide anion will readily disproportionate to yield H202 (path i). This is in contrast to the fate of superoxide anions in ozonation advanced oxidation processes (AOPs), where they react primarily with ozone to produce hydroxyl radical. This description of the chemical pathways of UV/H202 oxidation of organics illustrates that, when oxygen is present, the major paths directly or indirectly create more... 

Chain initiation is readily accomplished by deliberately adding initiators, that is, compounds yielding free radicals on thermal decomposition. In practice, initiators should have substantial rates of decomposition in the temperature range 50°-150°C. The rate of chain initiation, Rt, is given by... 

Two principal classes of antioxidant are effective in thermal oxidation. Chainbreaking or primary antioxidants limit the rate of the chain propagation steps (Eqs. 3-2 and 3-3) by trapping carbon- or oxygen-centered free radicals. Hydroperoxide decomposing or secondary antioxidants prevent chain initiation by interfering with ROOH. Photoantioxidants protect plastics exposed to photo-oxidation. 

The participation of Diels-Alder type intermediates in polymerization was considered by Hill et ah (26) in 1939 as a result of the elucidation of the structures of the butadiene homopolymer and the butadiene-methyl methacrylate copolymer resulting from thermal polymerization in emulsion. The considerable amount of alternating 1,4 and 1,2 structures in the homopolymer and the predominantly 1,4 structure of the butadiene in the copolymer which contained more than 50% alternating units of butadiene and methyl methacrylate led to the proposal that the reaction proceeded through a Diels-Alder dimer complex or activated complex. Chain initiation involved a thermal reaction in which the activated com-... 

This may favor the subsequent complexation with more macromolecules of 23b due to the suppressed thermal fluctuation and the solvation of the cations, and initiates a cascade reaction until most of the 23b is consiuned. In the case of added polyanions, however, the IPC is overall anionic, but with the negative charges on the surface of the complex. Therefore, the solvation is favored and an association of excess 23b is unfavorable. The equilibriiun at step 1 corresponds to chain initiation, and steps 2 and 3 to chain propagation. The observed phenomenon is useful for the selective separation of charged polymer systems. 

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. 

Of particular interest are changes in the chain length occurring in connection with the autocatalytic reaction acceleration in TS-6. Numerous thermal polymerization studies showed that the activation energy is EJl = 1.00 + 0.02 eV, independent of conversion. Consequently the autocatalytic reaction enhancement cannot be the result of an increase of the Boltzmann factor. Instead, an increase in the number of monomers consumed per primary chain initiation event has been postulated. Experimentally n(X = 0.5)/n(X = 0) = 200 is found... 

The essence of the energetic studies on TS and 4-BCMU is contained in Fig. 9. In TS formation of the chain initiating species -- a dimer — requires an energy of 1.0 eV. It can be supplied thermally or optically via monomer excitation. In the former case it is this chain initiation reaction that controls the thermal reactivity and its temperature-dependence. Chain initiation can also be produced optically at a yield of order 10 per absorbed UV-quantum. In this case it is chain propagation that determines the temperature dependence of the polymerization yield. However, the activation energy E" need not be and in general is not identical with the energy... 

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