Hydroperoxide groups decomposition

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

Hydroperoxide decomposers play an important role in hydroperoxide group decomposition. The most important are metal complexes of sulfur or phosphorous containing compounds. 

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

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... 

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... 

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. 

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 ... 

Methyl groups, as hydrocarbon surface species, vibrational spectra, 42 214—219 Methylheptane, ring closure, 25 154 3-Methylhexane dehydrocyclization, 30 13 isomerization, 30 7, 14, 39-40 Methylhexane, ring closure, 25 155 Methyl hydroperoxide, catalytic decomposition, 35 161... 

The goal here was to find new solid catalysts for cyclohexyl hydroperoxide (chhp) decomposition in cyclohexanol and cyclohexanone. The requirement list had foreseen a study on silica-supported metals of groups 4 and 5 and the need for a heterogeneous catalyst without metal Bxiviation. 

In order to create a radical center on the backbone one may irradiate with ultraviolet radiation (see Sect. 3.1.4) or with high energy radiation. Autoxidation leads to hydroperoxide groups whose decomposition can also lead to suitable radical centers. 

Another polymer which is easily peroxidized is polyacetaldehyde it has a polyacetalic structure and is characterized by the presence of some side hydroperoxide groups (1 to 4°/00) due to traces of peracetic acid when the polymer is prepared. The homolytic decomposition of these peroxide groups yields macroradicals to which methyl methacrylate could be grafted [73, 74). 

For most polymers, the yield of hydroperoxides is relatively low even in the presence of oxygen excess. The relatively high values were, e.g., obtained during oxidation of atactic polypropylene [79], In the initial phases of oxidation, the yield of hydroperoxide related to 1 mol of oxygen absorbed is 0.6 at 130 °C when passing the maximum concentration it decreases considerably. In isotactic polypropylene, the maximum yield of hydroperoxides attains the value 0.2, only [80]. This may be probably related with a local accumulation of hydroperoxides in domains of defects in the crystalline structure which leads to an increased ratio of participation of hydroperoxide groups in the chain reaction of an oxidation process (induced decomposition of hydroperoxides) and finally to a lower yield of hydroperoxides... 

The core of the kinetic model describes the thermal oxidation at low temperature (typically at T < 200 C) at low conversion ([PH] = [PH]o = constant) and in oxygen excess of unfilled and unstabilized saturated hydrocarbons. It is derived from the closed-loop mechanistic scheme (CLM) of which the main characteristic is that radicals are formed by the thermal decomposition of its main propagation product the hydroperoxide group POOH 12). This closed-loop character explains the sharp auto-acceleration of oxidation at the end of an induction period (Figure 1). 

In the presence of the Group A complexes, the hydroperoxides undergo decomposition rapidly, but with the Group B complexes they are more stable. 

The thermal degradation of cellulose may also involve a free radical mechanism. It was difficult to observe these radicals, but it was possible to monitor the formation of hydroperoxide groups on heating cellulose in air. The hydroperoxide functions are formed and decomposed simultaneously, and their concentration rapidly climbs until a steady state is reached. The decomposition of the hydroperoxide function appeared to follow first-order kinetics with a rate con-... 

As illustrated in Fig. 4, the course of changes in MWD for the additive free HDPE are very similar to those for LDPE at temperatures above its melting range. This is also the case for both the degree of LCB and the formation of insoluble material. However, compared to LDPE the formation of low MW material is more rapid for HDPE. This might be due to catalyst residues, influencing the way of decomposition of hydroperoxide groups. 

Bevllacqua s proposed structure differs from Bolland s by the placement of the hydroperoxide group outside of the peroxy ring. Both of these cyclic peroxy structures are considered to be formed by similar mechanisms. Barnard, Cain, Cunneen, and Houseman (13) have proposed a mechanism for the free radical decomposition of the Bevllacqua structure. 

Since the radicals from (24) are probably generated adjacent to one or more hydroperoxide groups [148], a radical-induced decomposition of hydroperoxides similar to that detected in solution [288, 398] is also possible ((26) and (27)). 

A second possible mechanism of sensitization involves the induced decomposition of hydroperoxides caused by the direct photolysis of groups such as peroxides and ketones to give radicals which later attack hydroperoxide groups to produce peroxy radicals. 

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