Hydroperoxide group

Thus, chelates are able to dissipate the radiation harmlessly as infrared radiations or heat through resonating structures. Carlsson and Wiles [22] have confirmed in their studies that the quencher slowly migrate through the solid polymer destroying the hydroperoxide group. 

If the grafting is carried out in air, the active sites on the polymeric backbone is attacked by atmospheric oxygen leading to the formation of macroperoxy radical, which might abstract the hydrogen atom from the backbone polymer by an inter- or intramolecular process to give hydroperoxide groups as shown. 

The hydroperoxide groups thus formed undergo decomposition during irradiation producing macroxy radicals that offer sites for grafting. 

Irradiation of the polymer in air prior to grafting introduces hydroperoxide groups in the backbone polymer by the intermolecular, intramolecular, or hydrogen abstraction process. 

These hydroperoxide groups undergo radiolytic cleavage during irradiation of the aqueous polymer monomer mixture. 

Such a reciprocal motion of the kinetic chain (or back reaction) results in the decomposition of hydroperoxide groups without any interruption of kinetic chains and leads to the decrease in hydroperoxy groups. It has... 

On the basis of the above findings, grafting of vinyl monomers onto irradiated polypropylene has been attempted successfully by the mutual method. Upon irradiation hydroperoxide groups are introduced, which provide sites for grafting. During mutual irradiation in the presence of the monomer in aqueous medium, these hydroperoxide groups and water undergo decomposi-... 

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]. 

PGHS accomplishes two transformations, an initial reaction of arachidonic acid with 02 to yield PGG2 and a subsequent reduction of the hydroperoxide group (-OOH) to the alcohol PGH2- The sequence of steps involved in these transformations was shown in Figure 7.9, page 244. 

The final possible mode of action for an antioxidant is as a peroxide decomposer. In the sequences that lead to photodegradation of a polymer the ready fragmentation of the hydroperoxide groups to free radicals is the important step. If this step is interfered with because the peroxide has undergone an alternative decomposition this major source of initiation is removed. The additives which act by decomposing hydroperoxide groups include compounds containing either divalent sulfur or trivalent phosphorus. The mechanism involves... 

Depending on the nature of the sulfur or phosphorus compound used, the product R2S = O or R3P = O may undergo a number of further reactions with ROOH groups, all of which convert the hydroperoxide group into an alcohol. These compounds tend to be only weakly effective so are generally used in conjunction with synergistic promoters. Suitable mixtures are used to stabilise a variety of polymers including poly(alkenes), ABS, and poly(stryrene). 

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. 

These ESR spectra are in good agreement with ESR spectra of ozonized PP published previously (30) The rapid formation of peroxy radicals indicates that ozone reacts with PP without induction period. In the initial stage of reaction the hydroperoxide groups (POOH) concentration increases and the rate of POOH formation is linearly dependent on the ozone concentration (Fig.2). After prolonged ozonization the concentration of POOH remains almost constant. 

The formation of POOH during simultaneous exposure of PP films to ozone and light (LI or L2) can not be obtained kinetically. The experimental results show for rapid formation of hydroperoxide groups which are partially decomposed under UV-irradiation. There is no linear dependence on the ozone concentration. 

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... 

As the first isolable intermediate in the bioconversion of arachidonic acid into prostaglandins and thromboxanes (Eq. 3), PGG2 is a bicyclic peroxide of immense biological importance. It is difficult to obtain pure from natural sources and the presence of the 15-hydroperoxide group adds a further dimension of chemical lability to that associated with the 9,11-peroxide bridge. The chemical synthesis of PGG2 is thus a landmark in prostaglandin chemistry. It also represents a pinnacle of success for the silver-salt route to bicyclic peroxides. 

Nitroxides have been shown to associate quite strongly with model hydroperoxides in the liquid phase (17.). We have now found evidence for NO /hydroperoxide group association in solid PPH films based both on the e.s.r. spectra of N0> species and... 

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. 

When all the dioxygen consumed appears in hydroperoxide groups and the number of decomposed hydroperoxide molecules is low, the kinetics of oxygen uptake can be described by the following equation [3],... 

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). 

The active alkoxyl radicals formed by this reaction start new chains. Apparently, the hydroperoxide group penetrates in the polar layer of the micelle and reacts with the bromide anion. The formed hydroxyl ion remains in the aqueous phase, and the MePhCHO radical diffuses into the hydrocarbon phase and reacts with ethylbenzene. The inverse emulsion of CTAB accelerates the decay of hydroperoxide MePhCHOOH. The decomposition of hydroperoxide occurs with the rate constant k = 7.2 x 1011 exp(-91.0/R7) L mol-1 s-1 (T = 323-353 K, CTAB, ethylbenzene [28]). The decay of hydroperoxide occurs more rapidly in an 02 atmosphere, than in an N2 atmosphere. 

Rate Constants of Polymer Hydroperoxide Groups Decay... 

Rate Constants of Hydroperoxide Group Decomposition into Free Radicals Measured by Kinetics of Autoxidation and by Free Radical Acceptor Method... 

The addition of an acceptor decreases the rate of POOH decomposition. The increase of added [InH] creates a tendency for k-% to decrease to the kA value, i.e., Ax —> A d at [InH] —> DC. Acceptors, which do not react with hydroperoxide groups, were used sterically hindered phenols and stable nitroxyl radicals (TEMPO) were found to be efficient acceptors. The ratio kinA(2kt)m can be calculated from the values Ax and A d according to the formula ... 

AM Tolks. Kinetics and Mechanism of Hydroperoxide Groups Decay in Polymers. PhD Thesis, Polytechnic Institute, Riga, 1973 [in Russian]. 

The BDE values of O—H bond of hydroperoxide depend on the substituent near the hydroperoxide group (see Chapters 2, 7, 8, and 9). The higher the value of D(ROO—H) the faster the exothermic reaction of the peroxyl radical with phenol. The values of AH of reactions of different R02 with several of the monosubstituted phenols (ArjOH) and sterically hindered phenols (Ar2OH) are collected in Table 15.1. 

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