Recombination Of Peroxy Radicals Research Articles

Kinetics and mechanism of the liquid-phase oxidation of n-carboxylic acids

Short reaction chains participate in the oxidation of C3-C6, C8, C10, and C12n-carboxylic acids. The quadratic-law recombination of peroxy radicals occurs both without and with chain termination. The ratio of the rate constants of these reactions increases to k′/kt = 4.5 on passing from propanoic acid to pentanoic acid, and then it decreases almost to zero for dodecanoic acid. The anomalous variation of the k′/kt ratio is explained by the fact that the radicals resulting from carboxylic acid oxidation at the CH bonds nearest to the functional group make different contributions to the recombination process, depending on the carbon chain length. The cross recombination of secondary hydrocarbon peroxy radicals with the HO2· radicals resulting from the oxidation of carboxylic acids at the β-C-H bonds proceeds without chain termination.

Impact of stabilisers on the thermal catalytic activity of micro- and nano-particulate titanium dioxide in oxidizing condensed mediums

This investigation was undertaken to elucidate an interplay between stabilisers and titanium dioxide nano-particles in oxidizing condensed mediums comprising of the initiatied (initiator is 2,2′-azobisisobutyronitrile, AIBN) oxidation of cumene and ethylbezene and an intercomparison with the thermal oven ageing of high-density polyethylene (HDPE). As a preliminary study, the catalytic influence of different nano and micro-particle sized anatase and rutile titanium dioxide pigments on the uninitiated oxidation of cumene at moderate temperature (60 °C) has been established. The oxidation rates depend linearly on the square root of the concentration of the titanium dioxides over a sufficiently wide range and thereby follow the mechanism of oxidation with recombination of peroxy radicals. Kinetic measurements of oxidation rates of the model reactions of initiated oxidation of cumene and ethylbenzene in the absence and presence of a hindered piperidine stabiliser, Chimassorb 119 FL, Cyasorb 3529 and sterically hindered Irganox 1010 stabilisers clearly showed that the stabilisers promote over-additional initiating activity of titanium dioxide pigments. This effect was found to be very large, i.e. the values of afforded additional initiation rates are 2–9 times more than those compared with the value of the initial initiation rate induced by titanium dioxides in the system without stabilisers. In the case of hindered amine stabilisers this phenomenon is more pronounced than that for the Irganox phenolic antioxidants. On the basis of susceptibility towards all the examined stabilisers, resulting in a sharp increase of the initial initiating activity, the titanium dioxides can be ordered as nano-rutile untreated > nano-anatase treated hydroxyapatite ≥ nano-anatase untreated ≥ (nano-anatase (75%) + nano-rutile (25%)) untreated > micro-anatase untreated > micro-rutile treated. These additional initiation rates of titanium dioxides which are affected by the stabilisers can be explained in terms of physical absorption of the stabilisers onto the surface of the titanium dioxide particles. The stabilisers being adsorbed onto the surface of titanium dioxides block titania surface-OH groups and thereby prevent the formation of water in the oxidizing system to promote the additional catalytic capacity of the titanium dioxide pigments. Basic amine functionalities of Chimassorb 119 FL and Cyasorb 3529 are energetically more protonated by the titanium dioxides' surface hydroxyls during oxidation than when compared with phenolic hydroxyl and ester groups of Irganox 1010 and thereby the amine functionalities stimulate a relatively higher level of additional initiating activity of the pigments. This explanation is in good accordance with the data of work elsewhere [Allen NS, Edge M, Sandoval G, Ortega A, Liauw CM, Stratton J, et al. Factors affecting the interfacial adsorption of stabilizers on to titanium dioxide particles (flow microcalorimetry, modeling, oxidation and FTIR studies): nano versus pigmentary grades. Dyes Pigments 2006; 70(3):192–203]. Nano-anatase pigment C is shown to exhibit a sensitizing effect at a concentration of 1 and 2% w/w in the model reaction of cumene initiated oxidation whereas it exhibits a different effect in the thermal sensitizing oxidation at 1% w/w to the stabilising at 2% w/w during thermooxidative ageing in HDPE film.

Kinetics and mechanism of the liquid-phase oxidation of tert-butyl phenylacetate

The oxidation of tert-butyl phenylacetate in ortho-dichlorobenzene at 140°C occurs with short chains. The primary nonperoxide reaction products (tert-butyl α-hydroxyphenylacetate, tert-butyl α-oxophenylacetate, and benzaldehyde) are formed by the decomposition of a hydroperoxide (tert-butyl α-hydroperoxyphenylacetate) and (or) by the recombination of peroxy radicals with and without chain termination. Benzaldehyde and tert-butyl α-hydroxyphenylacetate undergo radical chain oxidation in a reaction medium to result in benzoic acid and tert-butyl α-oxophenylacetate. Homolytic hydroperoxide decomposition is responsible for process autoacceleration and results in benzaldehyde, which is also formed from hydroperoxide by a nonradical mechanism, probably, via a dioxetane intermediate. Both of the reactions are catalyzed by benzoic acid. Benzoic acid has no effect on hydroperoxide conversion into tert-butyl α-oxophenylacetate, which most likely occurs as a result of hydroperoxide decomposition induced by peroxy radicals. The rate constants of the main steps of the process and kinetic parameters have been calculated by solving an inverse kinetic problem.

Oxygen consumption in EPDM irradiated under different oxygen pressures and at different LET

We conceived a novel set-up for measuring the radiochemical yields of oxygen consumption in polymers. The measurement is based on a sampling of the gas mixture with a mass spectrometer, before and after irradiation. We irradiated an ethylene, propylene and 1,4-hexadiene terpolymer (EPDM) with 1MeV electron and 10.75MeV/A carbon beams. Samples were irradiated under oxygen within a wide range of pressure (5–200mbar). The yields under C irradiation are four times smaller than the yields under electron irradiation. This shows that radiooxidation is very sensitive to the linear energy transfer of the projectiles and hence to the heterogeneity of the energy deposition. The oxygen consumption yields do not vary significantly in the range of pressure investigated; even at 5mbar, the kinetics is still governed by the bimolecular recombination of peroxy radicals.

Mechanism of Cyclohexane Ozonolysis Considering Alternative Pathways

Ozonolysis of cyclohexane was considered taking into account the alternative pathway of recombination of peroxy radicals to form cyclohexanol and cyclohexanone. The rate constants of the reactions involved were determined experimentally or by calculations (optimization of the process model).

Thermo-oxidation of isotactic polypropylene in 32O 2– 36O 2: comparison of the mechanisms of thermo- and photo-oxidation

The formation of gaseous products resulting from thermo-oxidation of isotactic polypropylene (iPP) at 95 and 140°C in an atmosphere composed of a blend of 32O 2 and 36O 2 has been investigated using mass spectroscopy (MS). In contrast to the results obtained previously in the case of photochemical oxidation, MS analysis shows that thermo-oxidation does not generate 34O 2. A possible interpretation is that, in thermo-oxidation, the mechanism involving the recombination of peroxy radicals POO • occurs only with a very low probability. As a consequence, it is suggested that, in thermo-oxidation conditions, the main pathway of oxidation involves the formation of hydroperoxides and not recombination of peroxy radicals as proposed in photo-oxidative conditions.

Relationships between ozone photolysis rates and peroxy radical concentrations in clean marine air over the Southern Ocean

Measurements of the sum of inorganic and organic peroxy radicals (RO2) and photolysis rate coefficients J(NO2) and J(O1D) have been made at Cape Grim, Tasmania in the course of a comprehensive experiment which studied photochemistry in the unpolluted marine boundary layer. The SOAPEX (Southern Ocean Atmospheric Photochemistry Experiment) campaign included measurements of ozone, peroxides, nitrogen oxides, water vapor, and many other parameters. This first full length paper concerned with the experiment focuses on the types of relationships observed between peroxy radicals and J(NO2), J(O1D) and √[J(O1D)] in different air masses in which ozone is either produced or destroyed by photochemistry. It was found that in baseline air with ozone loss, RO2 was proportional to √[J(O1D)], whereas in more polluted air RO2 was proportional to J(O1D). Simple algorithms were derived to explain these relationships and also to calculate the concentrations of OH radicals in baseline air from the instantaneous RO2 concentrations. The signal to noise ratio of the peroxy radical measurements was up to 10 for 1‐min values and much higher than in other previous deployments of the instrument in the northern hemisphere, leading to the confident determination of the relationships between RO2 and J(O1D) in different conditions. The absolute concentration Of RO2 determined in these experiments is in some doubt, but this does not affect our conclusions concerned either with the behavior of peroxy radicals with changing light levels or with the concentrations of OH calculated from RO2. The results provide confidence that the level of understanding of the photochemistry of ozone leading to the production of peroxide via recombination of peroxy radicals in clean air environments is well advanced.

Radiation effects on polymer materials in radiation sterilization of medical supplies

Radiation degradations of polypropylene were studied by measuring chemiluminescence of the irradiated samples. The chemiluminescence emitted by recombination of peroxy radicals were found to increase with increasing dose, reflecting degree of oxidation of polymers. The degradation of PP may be attributed mainly to the oxidation, since the degradation of PP irradiated in air were markedly larger than that in vacuo. It was found that degree of oxidation of EB-irradiated PP was only one-half that for γ-ray-irradiated one. And also the degree of oxidation was found to be very high at the surface area of the film where oxygen can diffuse during irradiation and decrease sharply with increasing depth from the surface. The degradation during storage after irradiation were estimated by the decay curves of chemiluminescence. The degradation during storage occurred at earlier stage up to three months, while in later stage, it hardly proceeded. The degradation of EB-irradiated samples during storage was much smaller than that found in gamma rays irradiation. Irradiation of 2.5 Mrad with EB caused degradation only during storage. It was suggested that the degradation during storage depends on the degree of radiation oxidation. Both degradations, during irradiation and during storage after irradiation, caused by EB irradiation were also found to be smaller than those by γ-rays. The relationship between radiation degradation and degree of crystallinity of polypropylene was also discussed in this paper.

Visible chemiluminescence from rat brain homogenates undergoing autoxidation. II. Kinetics of the luminescence decay

The visible luminescence emitted in the autoxidation of brain homogenates is only partially quenched when antioxidants are added at concentrations such that further oxidation is prevented. From the time course of the emission after antioxidant addition, it can be estimated that nearly 50% of the light arises from an intermediate that decays with a first order kinetics and with a lifetime of ca. 40 s at 32°C. The remaining light arises from the decomposition of one or several intermediates, and show a kinetics that is independent of the incubation time. From the data obtained it is concluded that bimolecular free radical processes, such as the recombination of peroxy radicals, do not significantly contribute to the observed luminescence.

Recombination of polyatomic ester peroxy radicals in solution

Termination rate constants of peroxy radicals of seven polyatomic esters in benzene and perflouorooctane media have been measured by pulse photolysis. Recombination of peroxy radicals for all esters examined proves to be diffusion controlled. Reactivity of peroxy radicals in the termination reaction grows with increasing number of ester groups.

Singlet oxygen production by soybean lipoxygenase isozymes.

The oxidation of linoleic acid catalyzed by soybean lipoxygenase isozymes was accompanied by 1268 nm chemiluminescence characteristic of singlet oxygen. The recombination of peroxy radicals as first proposed by Russell (Russell, G.A. (1957) J. Am. Chem. Soc. 79, 3871-3877) is a plausible mechanism for the observed singlet oxygen production. Lipoxygenase-3 was the most active isozyme. Under the optimal aerobic conditions of p2H 7, 100 micrograms/ml lipoxygenase-3, 100 microM linoleic acid, 100 microM 13-hydroperoxylinoleic acid, and air-saturated buffer, the yield of singlet oxygen was 12 +/- 0.4 microM or 12% of the amount predicted by the Russell mechanism. High yields of singlet oxygen required the presence of 13-hydroperoxylinoleic acid. Systems containing lipoxygenase-2 and lipoxygenase-3 produced comparable yields of singlet oxygen without added 13-hydroperoxylinoleic acid, since the lipoxygenase-2 served as an in situ source of hydroperoxide. Lipoxygenase-1 was active only at low oxygen concentrations. Its singlet oxygen-producing capacity was greatly increased by the addition of acetone to the system. Lipoxygenase-2 did not produce detectable quantities of singlet oxygen.

Decay constants of peroxy radicals from polypropylene and polypropylene model compounds: A kinetic ESR study

The rate constants and the activation energy for the mutual recombination of peroxy radicals generated from atactic polypropylene (PP) and for model compounds of PP and PP autoxidation products have been determined by kinetic ESR. The type of peroxy radicals and the preferred sites of attack for the hydrogen abstraction by t-butoxyl radicals were identified. The results are of interest in respect to the mechanism and the kinetic simulation of PP autoxidation.

Polymer crosslinking by a bimolecular peroxide decomposition

AbstractThe present paper examines the possibility of the utilization of the bimolecular cumyl hydroperoxide redox decomposition, catalyzed by Co11 naphthenate for the polymer crosslinking. The study of the mechanism and the kinetics of this reaction was performed both in a model medium heptane and polymer or polymer solution. In the case of redox initiation, the value of the crosslinking efficiency was found to be about 0.4, and its decrease can be attributed mainly to the formation of ions and, partly, to recombination of peroxy radicals producing dialkyl peroxide. On the basis of data obtained concerning the efficiency and rate of the process, conclusions are made on its applicability, especially for the crosslinking below the melting point of a crystalline polymer, as is the case of the crosslinking of polyethylene films.

Fast Reactions in Solution

ing power because of the larger delocalization of excitation energy, Schwab & Dorr ( 166) studied the disproportionation of the semiquinone radical derived from excited anthraquinone and derivatives by H-abstrac­ tion from a substrate. The radical is not formed with 1 -oxy-anthraquinone, probably because of an inner H-bond blocking the keto-oxygen. Triplet decay via the excited singlet level was reported by Parker & Hatchard (167) in a study of normal and delayed fluorescence of eosin in ethanol and glycerol. If the collision factor for thermal activation is assumed to be about 2 X 1012 sec-1 in both cases, a rate constant for intersystem cross­ ing (thermally activated triplet-..upper singlet) of about 4 X 107 sec:-1 (in ethanol) or 5 X 108 sec.-1 (in glycerol) is obtained. The activation energy (10 kcaI/mole) for triplet decay via the excited singlet (delayed fluorescence) , as determined from temperature dependence, agrees with the spectroscopically determined energy difference between triplet and singlet levels. A toms and radicals.-Bimolecular radical or atom recombination reac­ tions are much faster in the liquid than in the gaseous state since bond energy dissipation is much more effective. Only a few recombination reactions in liquids have been studied, however. On investigating photodissociation of halogens in aromatic solvents, Porter & Smith ( 168) , Bridge (169) , Strong, Rand & Britt ( 1 70, 1 71) observed strong transient absorption spectra in the FAST REACTIONS IN SOLUTION 2 7 visible region which are attributed to charge transfer complexes involving halogen atoms. Gover & Porter (172) found that the rate of disappearance of the iodine complexes is second order in various solvents. This was confirmed by Strong ( 1 73), who obtained rather large second order rate constants for recombination of about 1010 M-I sec-to In the case of bromine, addition reactions occur, and the rate of disappearance of the atom complex is not strictly second order. The efficiency of benzene as a ternary collision partner for the recombination of iodine atoms was al ready recognized in the gas phase [ef. Schumacher ( 1 74)]. Recombination kinetics of the a-ethanol radical CHaCHOH · in water and ethanol were studied by Taub & Dorfman ( 1 75) ; k = 5. 5 X 108 M-l seCl is obtained, leading (for about 80 per cent) to formation of 2.3 butanediol and (for 20 per cent) to disproportionation into acetaldehyde and alcohol. Re­ combination of OH radicals in water to give H202 was studied by Schwarz (1 76) . From an observation of the H202 steady state concentration as a function of the pulse rate'-of a pulsed 1.S MeV electron beam a rate constant of recombination 4 X l09 M-I sec-I is obtained. Using this value. rate con ­ stants for a number o f abstraction reactions o f the OH radical are derived. Dorfman, Buhler & Taub ( 177, 1 78) investigated electron pulse irradia­ tion of aqueous solutions of benzene. The reaction of OH radicals with benzene leads to the formation of the hydroxocyclohexadienyl radical. This addition reaction is diffusion controlled (k = (4.3 ± 0.9) X 109 M-l seCl at 23° C) , no isotope effect is observable with deuteratecl benzene. Hydrogen abstraction reactions involving the OH radical seem to be slower ; in the cor­ responding irradiation of toluene (which readily undergoes H-atom abstrac­ tion at the methyl group in photo halogenation reactions) the benzyl radical is not formed. Similarly, Schwarz obtained a value two orders of magnitude below the diffusion controlled limit for the reaction OH+ H2-4H20 + H (176). The recombination rate of hydrogen peroxy radicals 2H02->H202+02 is much less than diffusion controlled. Schmidt (1 79) obtained a rate constant of 1.S X 107 M-l sec-1 for this reaction. McCarthy & MacLachlan ( 180. 181) , who studied transient spectra and kinetics induced by pulsed 6 MeV elec­ trons in cyclohexane and cyclohexanol, also observed that the rate constants of recombination of peroxy radicals are in some cases 10,000 times smaller than the diffusion controlled limit. Hydroxocyclohexyl radicals, on the other hand, recombine with a diffusion controlled rate constant of 3.4 X 108 M-l sec-I, The same authors also reported a less than diffusion controlled rate constant for the formation of dibenzyl from two benzyl radicals (182).9 Hydrogen atoms are among the most important intermediates in radioly­ sis and in many photochemical processes. The kinetics of atomic hydrogen in liquid saturated hydrocarbon media. based upon steady state radiolysis experiments, has been discussed by Hardwick (183-186) . The relative rate 9 Cr., however, Matheson. M. S. , Annual Review of Physical Chemistry. 13, 89 (1962) indicates the possibility of an error in rate constant assignment.