Impurity hydroperoxide groups

Polyolefin products undergo photodegradation by the absorption of UV light. This is due to the unstable product formation of polyolefins during fabrication process, due to the presence of impurities like carbonyl or hydroperoxide groups [30, 31]. 

Industrially produced polymers are relatively pure materials immediately after manufacture. The only impurities are normally catalyst residues and unsaturated groups, produced in side reactions during polymerization. Storage, particularly if light approach is limited, leads to the slow introduction of hydroperoxide groups as a result of auto-oxidation. This process is facilitated by the presence of small amounts of unsaturated groups in the polymer. 

Properly end-capped acetal resins, substantially free of ionic impurities, are relatively thermally stable. However, the methylene groups in the polymer backbone are sites for peroxidation or hydroperoxidation reactions which ultimately lead to scission and depolymerisation. Thus antioxidants (qv), especially hindered phenols, are included in most commercially available acetal resins for optimal thermal oxidative stabiUty. 

The avermectins also possess a number of aUyflc positions that are susceptible to oxidative modification. In particular the 8a-methylene group, which is both aUyflc and alpha to an ether oxygen, is susceptible to radical oxidation. The primary product is the 8a-hydroperoxide, which has been isolated occasionally as an impurity of an avermectin B reaction (such as the catalytic hydrogenation of avermectin B with Wilkinson s rhodium chloride-triphenylphosphine catalyst to obtain ivermectin). An 8a-hydroxy derivative can also be detected occasionally as a metaboUte (42) or as an impurity arising presumably by air oxidation. An 8a-oxo-derivative can be obtained by oxidizing 5-0-protected avermectins with pyridinium dichromate (43). This also can arise by treating the 8a-hydroperoxide with base. 

Most synthetic and natural polymers degrade when exposed to solar ultraviolet (UV) radiation (1-5). In synthetic polymers degradation is generally caused by the presence of photosensitive impurities and/or abnormal structural moieties which are introduced during polymerization or in the fashioning of the finished products. The presence of groups such as ketones, aldehydes, peroxides and hydroperoxides are implicated in polymer degradation (1-5). 

A Perkin-Elmer Model 21 infrared spectrophotometer was used to detect and to estimate the hydroxylic and carbonyl functions in the oxidized product mixtures. The organic hydroperoxide and peroxide functional groups in the product mixtures were determined by an iodine liberation and titration procedure (11). In order to get reproducible results, it is necessary to pretreat the olefins with about 10 weight % activated silica or alumina for several hours with agitation to remove adventitious peroxides and impurities. Sodium bisulfite solution rapidly destroys hydroperoxides but does not destroy peroxides completely. The hydroperoxides and peroxides decomposed extensively during attempted distillation at about 1 mm. of Hg partial vacuum. We had some success in concentration by liquid chromatography over silica gel the unconverted olefins are eluted with n-hexanes, and a hydroperoxide-peroxide... 

Several other groups have invoked initial formation of hydroperoxide by abstration of hydrogen from organic substrates (including olefins) by a dioxygen complex. In some cases, the oxygen adducts were known to form under the reaction conditions (67, 68) in other cases, they have been postulated as intermediates (69, 71). One such report on a Pd(O) system (67) was later refuted by a more detailed study, which showed that the autoxidation resulted by decomposition of trace amounts of hydroperoxide impurity (50). (Some Co systems are considered in more detail in the next section). 

Since pure PP does not Incorporate chromophorlc groups, it is clear that photolnltlatlon of radical degradation processes must Involve chromophorlc impurities. There has been a great deal of discussion of this in the past and hydroperoxides or carbonyl structures formed by oxidation of the parent polymer and transition metal residues from the polymerization catalyst seem to be the most likely candidates. It is not appropriate to discuss this aspect in the present paper, suffice it to say that the association of methane with photolnltlatlon, but not thermal Initiation, suggests that photolnltlatlon Involves C-CH3 bond scission to form chain side radicals in contrast to thermal Initiation which involves scission of the C-C bond In the main chains. 

Finally, it can be concluded that the chemical effects of ultraviolet irradiation of polypropylene, as well as polyethylene, are due to the photolysis of impurities such as hydroperoxides and ketonic groups. 

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