Polymer radical scavenger

Uses Fragrance intermediate fixing agent for perfumes synthetic flavoring agent in foods and pharmaceuticals antioxidant, stabilizer in plastics, food-grade polymers radical scavenger... 

Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

Vinyhdene chloride polymers containing stabilizing features have been prepared. In general, these have been polymers containing comonomer units with functionahty that can consume evolved hydrogen chloride and do so in such a manner that good radical scavenging sites are exposed (145,146). 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

From the earliest days of radiation chemistry it has been known that acetylene polymerizes to a cuprene-like ( alprene ) solid (5, 6,25,28). The characteristics of the polymerization—e.g., lack of effect of temperature, doso rate, and pressure on polymer yield and negligible effect of radical scavengers—led Lind (24) to postulate an ion cluster mechanism. 

Most kinetic treatments of the photo-oxidation of solid polymers and their stabilization are based on the tacit assumption that the system behaves in the same way as a fluid liquid. Inherent in this approach is the assumption of a completely random distribution of all species such as free radicals, additives and oxidation products. In all cases this assumption may be erroneous and has important consequences which can explain inhibition by the relatively slow radical scavenging processes (reactions 7 and 9) discussed in the previous section. 

Stabilizing additives for polymers usually scavenge free radicals or hydroperoxides from the system. The effect of such additives is depicted in reactions... 

UV-filtered light, as in a museum, effectively reduces the amount of oxidation to almost the same level that occurs in darkness. The polymer fraction of mastic was found to reduce the degree of oxidation in the material, possibly by acting as a radical scavenger. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

Jeng US, Lin TL, Chang TS, Lee HY, Hsu CH, Hsieh YW, Canteenwala T, Chiang LY (2001) Comparison of the aggregation behavior of water-soluble hexa(sulfobutyl)fullerenes and polyhydroxylated fullerenes for their free-radical scavenging activity. Prog. Colloid Polym. Sci. 118 232-237. 

As shown in Fig. 9, irradiation of neat polystyrene does not result in the efficient scission but increases the molecular weight due mainly to cross-linking, though a small bump at the low molecular weight side of the SEC pattern indicates the dissociative electron attachment. The cross-linking is suppressed by addition of hydroquinone as a radical scavenger, which suggests that combination reaction of polymer radicals takes place in the neat polymer, as... 

Cross-linking is a predominant process during irradiation of siloxane polymers. Chain scissions are negligible. ° ° The cross-link density increases linearly with a dose up to 160 Mrad (1,600 kGy). ° At 5.0 MGy (500 Mrad) the G(X) value is 0.5. Free radical scavengers, such as n-butyl and frrf-dode-cyl mercaptan and diethyl disulfide, are the most effective antirads. ° - ° At a concentration of 10%, two-thirds of the cross-links were prevented from forming however, the scission yield was also increased. 

Low concentrations of radical scavengers can greatly reduce the density of crosslinking of liquid polymers although most radicals will be formed some distance from a scavenger. For a solid polymer much higher concentrations are needed, but crosslink densities can still be approximately halved. 

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