Chain scission reactions

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

M. Cates. Reptation of living polymers Dynamics of entangled polymers in the presence of reversible chain-scission reactions. Macromolecules 20 2289-2296, 1987. 

The effect of oxidative irradiation on mechanical properties on the foams of E-plastomers has been investigated. In this study, stress relaxation and dynamic rheological experiments are used to probe the effects of oxidative irradiation on the stmcture and final properties of these polymeric foams. Experiments conducted on irradiated E-plastomer (octene comonomer) foams of two different densities reveal significantly different behavior. Gamma irradiation of the lighter foam causes stmctural degradation due to chain scission reactions. This is manifested in faster stress-relaxation rates and lower values of elastic modulus and gel fraction in the irradiated samples. The incorporation of O2 into the polymer backbone, verified by IR analysis, conftrms the hypothesis of... 

SCHEME 31. S (a) Oxidation, dehydrofluorination and crosslinking reactions as a result of EB irradiation of poly (vinylidenefluoride-co-hexafluoropropylene-co-tetrafluoroethylene). (b) Chain scission reactions as a result of EB irradiation of poly (vinylidenefluoride-co-hexafluoropropylene-co-tetrafluoroethylene). 

The term charring refers to the complete degradation of a polymer after which there is no longer any polymeric character to observe. Charring results from chain scission reactions that are left unchecked and is the typical process by which thermosets degrade. The resulting material is typically black and brittle. 

An alternative pathway for entanglement loss is chain scission (Fig. 3.2, process B), in which a covalent bond along the polymer main chain is broken and a stress-bearing, otherwise elastic, chain is lost. Chain scission reactions, for example, homolytic carbon-carbon cleavage, have obviously high activation energies. The stress-free rates of these reactions are therefore typically extremely low. 

In the supercritical wet oxidation, the concentration of oxygen in the reactive system is high and the contact between the reactants is more intimate, thus making chain scission reactions much more active. This appears to be a major reason for more rapid degradation of PVC in an oxygen environment and production of monomers and dimers. 

Alkyl lead compounds are extremely effective gasoline antiknock agents. By decomposing to form lead oxide compounds during the gasoline combustion process, lead alkyls interrupt the rapid chain scission reactions which lead to combustion knock. Also, lead alkyls help to prevent exhaust valve seat wear and may minimize octane requirement increase. However, unless utilized in conjunction with lead scavengers such as 1,2-dichloromethane, lead deposits can accumulate within the gasoline combustion chamber. 

Very recently, attempts have been made to develop PP/EOC TP Vs. In order to make TPVs based on PP/EOC blend systems, phenolic resin is ineffective because the latter needs the presence of a double bond to form a crosslinked network structure. Peroxides can crosslink both saturated and unsaturated polymers without any reversion characteristics. The formation of strong C-C bonds provides substantial heat resistance and good compression set properties without any discoloration. However, the activity of peroxide depends on the type of polymer and the presence of other ingredients in the system. It has been well established that PP exhibits a (3-chain scission reaction (degradation) with the addition of peroxide. Hence, the use of peroxide only is limited to the preparation of PP-based TPVs. Lai et al. [45] and Li et al. [46] studied the fracture and failure mechanism of a PP-metallocene based EOC based TPV prepared by a peroxide crosslinking system. Rajesh et al. 

Volatile Products. The volatile products which result from breakdown of the polymer backbone include carbon monoxide, ammonia and acetamide, which was the major product from the aliphatic amino acids. Side-chain fragments are also found in significant yields. The presence of acetamide as the major product suggests that two related chain scission reactions coupled with elimination of the side chain must be highly favourable. 

Depolymerization, elimination, and statistical chain-scission reactions can be used for polymer analysis. When the monomer is the main degradation product obtained, it can easily be identified by boiling point and refractive index. 

Elimination and chain-scission reactions provide characteristics pyrograms which can often be identified by gas chromatography or IR spectroscopy. 

The linear variation of ln[H(t)/H(0)] suggests that the rate of the global chain scission reaction follows first-order kinetics that can be characterized by the following relation ... 

According to these results the macroradical —CF2—CH—CF2— plays the major role in cross-linking of VDF-HFP copolymer. The dehydrofluorination is witnessed by the findings of polyenyl-type radicals. In the presence of oxygen, the alkyl radicals react with 02 to form peroxy radicals, which intervene both in cross-linking and in chain-scission reactions. 

Lipid alkoxy radicals (LO ) decompose in chain scission reaction to a great variety of reactive aldehydes such as malonaldehyde, hydroxyalkenals, 2-alkenals, 2,4-alkadienals and alkanals. Lipid alkoxy radicals also cause the degradation of the apolipoprotein B (apoB) to smaller peptide fragments. 

Here, the peroxy radical eliminates superoxide to form a carbocation which then undergoes hydrolysis to form a product leading to chain scission. Reactions (23)-(24) show an alternative route for the same peroxy radicals. In this case, there is a bimolecular reaction with another peroxy radical ... 

The prevention of further deterioration requires arresting the chain scission reaction caused by acid or enzymatic hydrolysis of the glycosidic linkages. The reduction in DP through chain scission has a dual character to its negative effect upon properties. In addition to the inherent reduction in the tensile properties of the fibers, the lower DP enhances the opportunity for crystallization with resulting embrittlement. 

Yet another approach to recovery of lost properties could be based on cross-linking. The primary objective here would be to extend the domain spanned by covalently linked molecular entities to counteract the reduction resulting from the chain-scission reaction. Cross-linking, however, is a rather complex process, particularly for native fibers if carried too far it could result in embrittlement, further compounding the effects of age. 

Finally, the effect of ordering processes must be considered in the interpretation of results from accelerated aging tests. The activation energies for the chain-scission reaction and for molecular ordering are likely to be quite different. Furthermore, enhancement of the ordering process through the reduction in DP caused by chain scission is likely to manifest itself as an autocatalytic effect wherein the rate of decay of... 

---