Initiator thermally dissociating

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

The initiation step. Reaction (I), represents the thermal dissociation of bromine, which is brought about by collision with any other molecule, denoted by M. 

The gas-phase reaction between H2 and Br2 to form HBr is considered to be a chain reaction in which the chain is initiated by the thermal dissociation of Br2 molecules. The chain... 

The low pre-exponential factor associated with fc°° is an indication of a non-adiabatic reaction. Since there is good evidence122 against a significant extent of thermal dissociation of N20 to N+NO, the alternative dissociation, reaction (1), has been universally accepted as the initial step in the decomposition. The subsequent fate of the ground-state O atoms is unquestionably reaction with N20, viz. 

Bodenstein explained this result by suggesting that the H2-Br2 reaction was chain in character and initiated by a radical (Ih ) formed by the thermal dissociation of Br2. He proposed the following steps ... 

The description a(CCH) is probably an oversimplification. The same type of spectrum is also obtained from the thermal dissociation at similar temperatures of initially adsorbed C FLt species. Recent scanning tunneling... 

The other potential initiation step, thermal dissociation of H2, is too slow to be competitive, except at very high temperatures. 

The reaction mechanism for this process is fairly well established and may serve as a more complex example of a chain reaction. The initiation step in ethane pyrolysis is thermal dissociation of the ethane molecule,... 

The chemistry in the high-temperature regime is known in more detail than that occurring at lower temperatures. This allows us to be a little more specific. As an example, consider oxidation of propane. Typically the initiation reaction is thermal dissociation of the fuel,... 

This expression has been analyzed for the dissociation of ICN. The initial thermal distribution corresponds to large j. A distribution peaked around j 25 was obtained, in good agreement with experimental data. The analysis has been generalized to describe the case of a bent triatomic molecule (53,5A). Moreover, these authors consider the scalar coupling which corresponds to indirect photodissociation. 

When two radicals are in close association as a pair surrounded by a cage of solvent molecules, the two odd electrons will interact with one another just as two electrons do within a molecule. The interaction will yield either a singlet state, if the two electrons have spins paired, or a triplet, if the spins are unpaired. If, for example, the caged pair arose by thermal dissociation of an ordinary ground state molecule, in which all electrons would have been paired, the state would initially be a singlet, S, whereas if the pair arose in a photochemical reaction from dissociation of an excited molecule in a triplet state, it would be initially a triplet, T. 

It seems that the cavities enclose a vapor of the solute because of the high vapor pressure of these compounds. The primary reaction pathway for these compounds appears to be the thermal dissociation in the cavities. The activation energy required to cleave the bond is provided by the high temperature and pressure in the cavitation bubbles. This leads to the generation of radicals such as hydroxyl radical, peroxide radical, and hydrogen radical. These radicals then diffuse to the bulk liquid phase, where they initiate secondary oxidation reactions. The solute molecule then breaks down as a result of free-radical attack. The oxidation of target molecules by free radicals in the bulk liquid phase under normal operating pressures and temperatures can be presented by a second-order rate equation ... 

Reactions in which the initial substance is simultaneously its product are rather rare. These "propagation reactions were called auto-catalytic steps by Kondratiev and Nikitin. For example, one of the steps in the thermal dissociation of hydrogen is... 

The fraction of radical reactions in the cage is not necessarily the same for photochemical and thermal dissociation of the initiator. Photochemically generated radicals are usually excited and so they have less tendency to produce stable molecules. They may be flung away from each other by excess energy at the moment of generation, and the probability of mutual collision is reduced [124]. 

At 200 C, neither COBr2 nor HCOBr is a stable compound. Propose a radical chain to account for the thermal bromination of CH2O to produce CO -f HBr with the initiation step being the thermal dissociation of Br2. Neglect wall reactions. Compute expressions for the stationary-state concentrations of all radical species and the rate e pression for —dCBT )/dt, Try to justify all steps included in the scheme. 

The most common reaction of carbonyl complexes is CO dissociation. This reaction, which may be initiated thermally or by absorption of ultraviolet light, characteristically involves loss of CO from an 18-electron complex to give a 16-electron intermediate, which may react in a variety of ways, depending on the nature of the complex and its environment. A common reaction is replacement of the lost CO by another ligand to form a new 18-electron species as product. For example,... 

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