Poly oxidative chain scission

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 reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

Kinetics of Chain Scissions during Accelerated Aging of Poly(ethylene oxide)... 

The rates and products of poly(vinyl alcohol) (PVA) photooxidation were measured over a temperature range of 30°-90°C. Oxidation was initiated with 253.7-nm light, and several model compounds were included. The PVA photooxidation rate was autocatalytic and relatively insensitive to temperature with the major products being carbon dioxide, acids, peroxides, polymeric p-hydroxyketone, and hydrogen peroxide. Acids were mostly formic acid and carboxylic acids at the polymer chain ends. Several molecules of carbon dioxide and acid were formed per statistical chain scission. The mechanism of the photooxidation of PVA is discussed. 

PMMA is compatible with poly(ethylene glycol), PEG, through strong polar interactions, and is found to protect the latter against oxidation by a mechanism which appears to promote increased PMMA chain scission in proportion to the amount of polyether in the blend [Makhija et al., 1992]. Even stronger specific H-bonded interactions exist between poly(acrylic acid) or poly(methacrylic acid) and poly (vinyl alcohol), and both well-... 

The susceptibility of a resin to oxidation depends on the presence of a hydrogen atom that is readily removable in the initiation step. Thus polypropylene is prone to oxidation because the hydrogen atom adjacent to the methyl group is easily removed. On the other hand, poly-tetrafluoroethylene (PTFE), with no hydrogen present at all, is exceedingly oxidation resistant Of course, if oxidation simply involved the addition of an oxygen molecule to the chain backbone its effects would not necessarily be seen as deleterious. Often however, a consequence of oxidation is scission of the chain backbone and a drop in mechanical properties occurs. 

Oxidations can lead to chain scission, to cross-linking, and/or to polymer discoloration. This undesirable chemical aging can be largely prevented by the addition of antioxidants. Hydrogen chloride is also often released from poly(vinyl chloride) for this reason, this polymer must also be stabilized by what are known as heat stabilizers. 

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