Chain branching, free radical

The chemical kinetics occur at a finite rate, with a certain time required for reactions to proceed. As the frequency decreases, providing more time at relatively higher temperature and pressure within each cycle, there is time for the chain-branching free-radical species to build up to levels that trigger an ignition. As the frequency increases, the time available... 

Atomic hydrogen and hydroxyl radicals are very important for sustaining com-bnstion. The hydrogen radical is responsible for the chain-branching free-radical reactions in the flame [reaction (1.7)], whereas the hydroxyl radical is responsible for the oxidation of CO to CO2 [reaction (1.8)], which is a highly exothermic reaction and is responsible for the larger part of the heat generation in the flame. 

T. Urbanski and Fal cki [60] reported a spontaneous explosion when a solution of N2O4 in /t-alkanes was kept for a few days at room temperature. This was most likely a consequence of the formation of branched reaction chains through free radicals. 

Polymeric chains, bearing free radicals, combine with each other to give branched structures. Addtions of chains with a free radical to double bonds result in formations of crosslinks. ... 

Tocotrienols differ from tocopherols by the presence of three isolated double bonds in the branched alkyl side chain. Oxidation of tocopherol leads to ring opening and the formation of tocoquinones that show an intense red color. This species is a significant contributor to color quaUty problems in oils that have been abused. Tocopherols function as natural antioxidants (qv). An important factor in their activity is their slow reaction rate with oxygen relative to combination with other free radicals (11). 

During the polymeriza tion process the normal head-to-tad free-radical reaction of vinyl chloride deviates from the normal path and results in sites of lower chemical stabiUty or defect sites along some of the polymer chains. These defect sites are small in number and are formed by autoxidation, chain termination, or chain-branching reactions. Heat stabilizer technology has grown from efforts to either chemically prevent or repair these defect sites. Partial stmctures (3—6) are typical of the defect sites found in PVC homopolymers (2—5). 

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, Free Radical Polymerizations ). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent ... 

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

Isomerization reactions occur frequently in catalytic cracking, and infrequently in thermal cracking. In both, breaking of a bond is via beta-scission. However, in catalytic cracking, carbocations tend to rearrange to form tertiary ions. Tertiary ions are more stable than secondary and primary ions they shift around and crack to produce branched molecules (Equation 4-10). (In thermal cracking, free radicals yield normal or straight chain compounds.)... 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

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