Homolytic bond breakage

The initiator RZ undergoes homolytic bond breakage, either simply by heating or by a more complex process of activation by some added reagent, to produce one reactive and one stable free radical (Eq. 3-216). The reactive radicals quickly initiate polymerization (Eq. 3-217), but the stable radicals are too stable to initiate polymerization. The technologically important... 

Homolytic bond breakage (Section 5.2) The kind of bond-breaking that occurs in radical reactions when each fragment leaves with one bonding electron A B A + B-. 

Unlike the carbonyl linkage, the carbon-carbon double bond undergoes polymerization by both radical and ionic initiators. The difference arises because the re-bond of a vinyl monomer can respond appropriately to the initiator species by either homolytic or heterolytic bond breakage ... 

The final mechanism of stress relief is thermomechanically activated chain scission. Primary bond breakage can be homolytic, ionic or by a degrading chemical reaction. It is worthwhile to note that the relative slippage of chains, microfibrils and fibrils reduces or prevents the mechanical scission of chains in quasi-isotropic polymeric solids. In other words, chain scission is an important mode of fracture only in highly oriented thermoplastic fibers or in thermosets. 

Bernhard and co-workers have performed a series of experiments to determine the mechanisms of DNA strand breakage by direct ionization of plasmid DNA. A big surprise in this work was the discovery that the total yield of single strand breaks exceeds the yield of trapped sugar radicals. Even at very low hydration levels (2.5 waters per nucleotide residue) nearly 2/3 of the strand breaks are derived from precursors other than deoxyribose radicals [74], The authors conclude that a majority of the strand breaks observed do not result from dissociative electron capture, homolytic bond cleavage from excited states, or from hydroxyl radical attack. Rather, the authors conclude that doubly oxidized deoxyribose is responsible for the high yield of strand breaks. 

Displacement reactions with disulfides are a second major pathway to generate thiyl radicals. Formally, this constitutes a one-electron reduction of the disulfide. Homolytic sulfur-sulfur bond breakage by carbon-centered or phosphite radicals, for example, has been shown to lead to sulfides with mixed substitution and thiophosphates, respectively [17, 18], reactions (5) and (6) ... 

The third mechanism of releasing axial stresses is the breakage of a chain through homolytic bond scission, ionic decomposition or degrading chemical reactions. 

This process uses light irradiation of wavelengths lower than the UV-C, i.e., lower than 190 nm. Generally, Xe excimer lamps (Xexc = 172 nm) are used. The excitation leads, in the majority of the cases, to the homolytic breakage of chemical bonds, degrading OM in condensed and gaseous phases (for example, fluorinated and chlorinated hydrocarbons) [1,3]. However, its application is limited, and the most important use of VUV radiation is in water photolysis (Eq. 8) ... 

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