Hydroperoxide groups oxide

Thermal aging is another simple pretreatment process that can effectively improve adhesion properties of polymers. Polyethylene becomes wettable and bondable by exposing to a blast of hot ( 500°C) air [47]. Melt-extruded polyethylene gets oxidized and as a result, carbonyl, carboxyl, and hydroperoxide groups are introduced onto the surface [48]. 

Radiation cross-linking of PVC in the absence of additives has been reported by several authors [20,21,267,268]. PVC by itself is not readily cross-linkable by EB radiation. In the study of radiation chemistry of PVC, it is known that the oxidation takes place in the presence of atmospheric oxygen during irradiation [269] and hydroperoxide groups are produced on oxidation. The formation of carbonyl group along with the elimination of HCl from PVC on irradiation in air has also been reported [270,271]. 

Mechanisms of lipid peroxidation that have been implicated in atherosclerosis may be pertinent to RA. Cellular lipoxygenase enzymes may promote LDL modification by inserting hydroperoxide groups into unsaturated fetty-acid side chains of the LDL complex (Yla-Herttuala etal., 1990). 15-Lipoxygenase has been implicated as an initiator of LDL oxidation (Cathcart etal., 1991) whilst 5-lipoxygenase does not appear to be involved (Jessup et al., 1991). Products of activated lipoxygenase enzymes within inflammatory synovial fluid surest that this pathway could be activated in RA (Costello etal., 1992). 

The surface oxidation products are mainly carbonyl and/or carboxyl groups, with lower level of hydroperoxide groups. The mechanisms of oxidation by ozone and atomic oxygen have been proposed. 

C(C=0)C1 group to the precise structure (primary, secondary or tertiary) of the alkyl groups to which it is linked. However, our subsequent work with NO showed that its products are also sensitive to the alkyl structure yet in addition NO reacts with oxidized polymers to give distinctly different products from alcohol and hydroperoxide groups (see below). Consequently the COCl2 products were not explored further. 

Hydroperoxide groups react with NO to give only nitrates as the dominant products, with only traces (< 5%) of nitrite in both oxidized polyolefins and in concentrated solutions of model hydroperoxides (-OOH levels from iodometry -ONO and -ON02 levels by IR). As reported by Shelton and Kopczewski we have confirmed that both nitrate and nitrite result from NO reaction with dilute hydroperoxide solutions (24). Rather than the NO-induced 0-0 scission proposed by these authors, our evidence points to hydrogen abstraction by NO (reaction 4). (A similar scheme may explain nitrite formation from alcohols.) Both e.s.r. and FTIR evidence is... 

The formation and role of hydroperoxide groups, particularly in the early stages of polymer oxidation is well discussed in the introduction to the next chapter and also features in many of the references cited in this chapter. Their detection and quantification is therefore important. Although this can be done directly or implicitly through many of the instrumentation techniques discussed in this chapter, there are several tests that have been developed, some of which are still widely used, that are based more on chemical methods, titration or staining. The majority have been applied to polyolefins, especially polyethylene. 

The hydroperoxide group weakens the a-C—H bonds, and the peroxyl radical of the oxidized hydrocarbon attacks this group with aldehyde formation. 

Another probable reaction of homolytic decomposition of ester hydroperoxide is the intramolecular interaction of the hydroperoxide group with the carbonyl group of ester with the formation of labile hydroxyperoxide succeeded the splitting of the weak O—O bond (see decomposition of hydroperoxides in oxidized ketones in Chapter 8). 

FIGURE 19.2 The correlation of rate constants of various free radical reactions with molecular mobility of nitroxyl radical in the polymer matrix of different polymers with addition of plastificator I in IPP, II in preliminary oxidized IPP, III in PE, and IV in PS. Line 1 for the reaction of 2,6-bis(l,l-dimethy-lethyl)phenoxyl radical with hydroperoxide groups at T — 295 K line 2 for the reaction of 2,2,6, 6-tetramethyl-4-bcnzoyloxypiperidinc-/V-oxyl with 1-naphthol at T = 333 K line 3 for the reaction of 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-iV-oxyl with 2,6-bis(l,l-dimethylethyl)phenol at T = 333 K line 4 for the same reaction at 7 — 303 K line 5 for the same reaction at T = 313 K and line 6 for the same reaction at T — 323 K [18]. 

In addition to this reaction, quinones and other alkyl radical acceptors retard polymer oxidation by the reaction with alkyl radicals (see earlier). As a result, effectiveness of these inhibitors increases with the formation of hydroperoxide groups in PP. In addition, the inhibiting capacity of these antioxidants grows with hydroperoxide accumulation. The results illustrating the efficiency of the antioxidants with cyclic chain termination mechanisms in PP containing hydroperoxide groups is presented in Table 19.12. The polyatomic phenols producing quinones also possess the ability to terminate several chains. 

Hydroperoxide groups can be generated previously on the Nylon monomer, e.g. oxidizing the tertiary carbon atom of the isopropyl group of the 5-isopropyl phthaloyl chloride with potassium permanganate. The corresponding polyamide can be grafted with styrene in a redox system (151). 

Silylation of Ti-MCM-41 materials produces highly active and selective for epoxidation ol olefins using organic hydroperoxides as oxidants, it has been found that the controlling parameters of the final catalytic activity of silylated Ti-MCM-41 materials are the hydrophobicity and the concentration of free silanol groups on the external surface of the mesopores that built up the Ti-MCM-41 structure. 

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