Free radical hydroperoxide impurities

Co-oxidation of indene and thiophenol in benzene solution is a free-radical chain reaction involving a three-step propagation cycle. Autocatalysis is associated with decomposition of the primary hydroperoxide product, but the system exhibits extreme sensitivity to catalysis by impurities, particularly iron. The powerful catalytic activity of N,N -di-sec-butyl-p-phenylenediamine is attributed on ESR evidence to the production of radicals, probably >NO-, and replacement of the three-step propagation by a faster four-step cycle involving R-, RCV, >NO, and RS- radicals. Added iron complexes produce various effects depending on their composition. Some cause a fast initial reaction followed by a strong retardation, then re-acceleration and final decay as reactants are consumed. Kinetic schemes that demonstrate this behavior but are not entirely satisfactory in detail are discussed. 

Antioxidants are added to polymers to retard oxidation. Oxidation is initiated by highly reactive free radicals which are formed by the action of heat, ultraviolet radiation, mechanical action and metallic impurities during polymerization, processing or use. The free radical can then react with an oxygen molecule to produce a peroxy radical (ROO ), a process known as propagation. The peroxy reacts with a hydrogen atom in the polymer to form an unstable hydroperoxide (ROOH) and another free radical (FeUer, 2002). If these processes are allowed to continue, the polymer oxidizes. Oxidation causes crosslinking or chain scission. 

The styrene is separated from the product mix, which also contains unreacted ethylbenzene and other impurities, by vacuum distillation. The monomer can easily autopolymerize into a hard solid and is therefore inhibited from polymerization during storage by mixing in a few parts per milhon of a free-radical reaction inhibitor (generally f-butyl catechol). A relatively small amount of styrene is also made by the oxidation of ethyl benzene in a process introduced by Union Carbide. The ethylbenzene hydroperoxide formed by oxidation is reacted with propylene to form propylene oxide and 2-phenyl ethanol. The latter compound is dehydrated to obtain styrene. 

The oxidative degradation of polymers involves free-radical chain reactions. For example, degradation of polyolefins such as PE is commonly initiated by hydroperoxide impurities incorporated during synthesis and processing. 

The oxidative degradation of the polymer proceeds by a free-radical chain reaction mechanism. Initiation usually occurs by exposure to heat, lighf or mechanical stress. The process is sometimes catalyzed by certain transition metal-ion impurities. The oxidation of hydrocarbon or related polymers by oxygen is an autocatalytic process with primary products being hydroperoxides. 

The autoxidation of unsaturated fatty acids is a chain process occurring autocatalytically through free radical intermediates. Although autoxidation implies that this reaction occurs spontaneously under mild conditions, it is generally initiated by trace metals and peroxides or hydroperoxides present as ubiquitous impurities in food and biological lipid systems (see Chapter 1). 

Studies on the photochemical stability achieved of polymeric materials upon addition of mineral fillers yielded in some unexpected results, since the decrease in properties is, in some cases, more intense after the UV exposure, in comparison with unfilled materials [80-82]. Thus, for PP and PE filled with clays (montmo-rillonite, MMT) [9] such behavior was explained considering the very small amounts of metal ions (i.e., Fe " ) present as impurities in clays and which can act as catalysts in UV-initiated oxidation reactions that entails accelerated degradation. At the same time, hydroperoxides formed during photo-oxidation are a source of free radical species able to further promote the degradation. Even more, light stabilizers effectiveness was found to be much reduced in polymer-MMT composites, maybe because these molecules were adsorbed on the surface of clay platelets so that their action was diminished or even blocked. Such results were reported for PC/MMT composites [5], as well as for PA6/MMT [83]. 

The formation of unstable hydroperoxides, which can also be decomposed by heat, UV light, catalyst residues, or other metallic impurities, ultimately leads to the formation of alkoxy and hydroxyl radicals, as depicted in cycle II. Oxygen-centered radicals can reaa further with the polymer, leading to the formation of more carbon-centered free radicals, which feed back into cycle I. The reactions leading to free radicals being formed on the polymer backbone result in chain linking and/or chain scission reactions in an effort to quench free radicals. 

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