Initiation of free-radical chain reactions

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

The main industrial use of rert-alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. 

The use of gamma radiation for the Initiation of free radical chain reactions In liquids offers several advantages over the more conventional methods of initiation. With this technique radical reactions that are not readily accessible by other methods can be studied. Because of the wide temperature range within which radiolytic initiation can be applied this method allows an accurate determination of Arrhenius parameters and therefore can bring better understanding and deeper insight to the factors that control the rates and mechanism of free radical reactions. 

The use of gamma radiation for the initiation of free radical chain reactions is not limited to alkane solutions. Reactions of other radicals can be studied by this method. Tor example, silyl radicals are generated in silane solutions and their subsequent reactions with added solutes can be investigated. 

The photooxidation of polypropylene involves the initiation of free radical chain reaction by the photolysis of hydroperoxides, producing peroxyl radicals as well as alkoxy radicals. In the last decades many studies dealing with the types of stabilizers used to delay the polypropylene photodegradation were reported as well as several studied on the experimental methods for assessing their photostability effectiveness [73-78]. 

Hydroxyl radicals are the most reactive free-radical species known and have the ability to react with a wide number of cellular constituents including amino-acid residues, and purine and pyrimidine bases of DNA, as well as attacking membrane lipids to initiate a free-radical chain reaction known as lipid peroxidation. 

In the thermal-catalytic method a peroxide catalyst is usually used to initiate the free radical chain reaction. The main disadvantages are the higher temperatures required for carrying out the polymerizations, the potential hazard of explosion on addition of catalyst to the monomer, and disposal of excess catalyzed monomer after impregnating. Combinations of heat, radiation, and catalyst have been experimented with to reduce the radiation and catalyst requirements and to increase the rate of polymerization. In thermal polymerization a muffle furnace, infrared heating, and microwave heating can be used to provide the thermal energy. 

Additional chemical evidence for the free radical character of N2F4 is provided by the disclosure that it acts as an initiator in free radical chain reactions and may be used as a polymerization catalyst (I). 

In the initiation part of free-radical chain reactions, a small amount of one of the stoichiometric starting materials (i.e., a starting material that is required to balance the equation) is converted into a free radical in one or more steps. An initiator is sometimes added to the reaction mixture to promote radical formation. In the example, though, no initiator is necessary light suffices to convert Br2, one of the stoichiometric starting materials, into a free radical by cr-bond homolysis. 

Visible light (hv) has sufficient energy to cause weak a bonds, such as the Br Br bond to cleave, or to promote an electron from the highest occupied MO of a compound (HOMO), usually a tt bond, to the LUMO, generating a 1,2-diradical. Even ambient light may suffice to initiate a free-radical chain reaction if there is a sufficiently weak bond in the substrate (e.g., (M). 

The molar mass of the polymer depends on the effective number of addition steps that occur per initiation step. In the description of free-radical chain reactions, this is the kinetic chain... 

Another famous appHcation of mechanistic chemistry was contributed by Morris Kharasch (1895-1957) at the University of Chicago. It was well known since the mid-19th century that addition reactions of HX (HCl, H-OH, etc) to unsymmetrical alkenes occur in a manner that places hydrogen on the olefinic carbon attached to more hydrogen atoms than the other olefinic carbon (Markovnikov s rule). However, when HX is hydrogen bromide (HBr), addition is typically anti-Markovnikov. Kharasch found that traces of peroxides (commonly present on glassware surfaces) initiate a free radical chain reaction for HBr (not for other HX). Careful removal of peroxides Ifom glassware prior to reaction removes the potential free radical mechanism and HBr adds in a normal Markovnikov mode (intermediacy of carbocation, rather than Ifee radical, intermediates). 

Besides its commercial importance, the sensitivity of free-radical chain reactions to initiators and inhibitors can be used in mechanistic investigations. Strong variation in reaction rate on addition of relatively small amounts of compounds known to act as chain-reaction inhibitors is evidence for a free-radical chain mechanism. Such additives are often referred to as free-radical scavengers. 

Chlorine atoms may be generated from molecular chlorine under mild conditions using a catalytic amount of an initiator, In-ln. Thus, homolysis of a molecule of initiator occurs upon irradiation or gentle heat to give free radicals, In (Eq. 9.3). These free radicals may then react with molecular chlorine to produce In-Cl and a chlorine atom (Eq. 9.4) to initiate the free-radical chain reaction. For safety and convenience, sulfuryl chloride, SO2CI2, rather than molecular chlorine is used in this experiment as the source of chlorine radicals. 

The first step in our procedure for initiating the free-radical chain reaction is the homolysis of l,r-azobis[cyclohexanenitrile] (1), abbreviated as ABCN, to form nitrogen and the free radical 2 (Eq. 9.5). The rate of this reaction is sufficiently fast at 80-100 °C to generate enough chlorine atoms to initiate the chain process. The radical 2 then attacks sulfuryl chloride to generate chlorine atoms and SO2 according to Equations 9.6 and 9.7. The series of reactions depicted in Equations 9.S-9.7 comprise the initiation steps of the reaction. 

Sulfur-containing amino acids like cysteine, methionine and tripeptide glutathione are very powerful antioxidants that participate in different stages of free-radical chain reactions of biomolecule oxidation [18]. For example, cysteine participates in the synthesis of taurine, the substance that effectively blocks the peroxide oxidation of lipids by binding hypochlorite anion to form chloramine complex. In any organism, cysteine and glutathione reduces the oxidized form of vitamin C to its initial active form while methionine (being an... 

The result of the steady-state condition is that the overall rate of initiation must equal the total rate of termination. The application of the steady-state approximation and the resulting equality of the initiation and termination rates permits formulation of a rate law for the reaction mechanism above. The overall stoichiometry of a free-radical chain reaction is independent of the initiating and termination steps because the reactants are consumed and products formed almost entirely in the propagation steps. 

Free radical chain reactions depend on an easily generated free radical to initiate the chain. One way to generate this radical is to irradiate halogens, such as Ch and Brj. Another way is to add a small amount of an initiator molecule to the reaction mixture, such as AIBN. This molecule, when heated, decomposes into free radicals that react with other molecules to initiate a chain reaction. 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

Packer and Richardson (1975) and Packer et al. (1980) made use of the fact that electrons can be generated in water by y-radiation from a 60Co source (Scheme 8-29) to induce a free radical chain reaction between diazonium ions and alcohols, aldehydes, or formate ion. It has to be emphasized that the radiolytically formed solvated electron in Scheme 8-29 is only a part of the initiation steps (Scheme 8-30) by which an aryl radical is formed. The aryl radical initiates the propagation steps shown in Scheme 8-31. Here the alcohol, aldehyde, or formate ion (RH2) is the reducing agent (i.e., the electron donor) for the main reaction. The process is a hydro-de-diazoniation. 

Hydrogen halides will easily add to unsaturated compounds under radiolysis or photolysis. The free-radical chain reaction process is initiated by the dissociation of the halide or by the radiolytic production of radicals from the halide or the organic compound. Thus, for the radiolysis of a mixture of HBr and ethene the postulated initiation is... 

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