Thermal degradation of poly(vinyl chloride): effect of oxygen on the thermal degradation of poly(vinyl chloride)

On the basis of the simultaneously measured free radical generation and HCl evolution a mechanism accounting for the effect of the oxygen on the degradation of poly(vinyl chloride) (PVC) is proposed. The mechanism is based on the reaction of oxygen with radicals generated in PVC to form unstable peroxy radicals. In addition, it is proposed that oxygen reacts directly with conjugated polyenes to form peroxy linkage.     

The role of the Baeyer–Villiger reaction in the liquid-phase oxidation of organic compounds

The data on the composition of ester products formed in the Baeyer–Villiger reaction in the liquid-phase oxidation of organic compounds with molecular oxygen, on production channel of peroxy acids, as well as on the influence of a carbonyl compound structure on its reactivity in reactions with peroxy acids have been classified and considered. The Baeyer–Villiger reaction was shown to be the main source of accessory primary alcohol esters and lactones in industrial processes of aerobic oxidation of cyclohexane and paraffin hydrocarbons.     

Regiospecific radical polymerization of a tetrasubstituted ethylene monomer with molecular oxygen for the synthesis of a new degradable polymer

A new class of degradable polymers is obtained from a diene monomer and molecular oxygen as the starting materials via a highly controlled radical copolymerization process. We now report the regiospecific copolymerization of a tetrasubstituted ethylene monomer with oxygen. Theoretical calculations support the highly selective propagations observed during the polymerization. The key steps are the regiospecific reactions of a peroxy radical to diene monomers and an allyl radical to molecular oxygen. The well-controlled molecular structure of the resulting polymer leads to the aldehyde-free degradation products during degradation by various stimuli, such as heating.     

Identification of the adsorption sites of molecular oxygen on Si(111) using XPS

Most of the oxygen adsorb dissociatively on Si, however there is also a significant amount of metastable molecular oxygen chemisorbed on Si. The adsorption site/configuration of these molecular oxygen species is still a controversial subject. New XPS results for oxygen adsorption on Si(111) 7×7 (150K) are presented. They reveal four distinct oxygen components; one of the metastable components has never been reported before. We tentatively identify them as: i) stable oxide (Si-O-Si bridges)(˜532 eV), ii) diffusing oxygen atom (˜533 eV) in silicon oxide and iii) metastable molecular oxygen species (˜527.6 eV and ˜530.6 eV). The latter have been attributed to peroxy radical which is defined as a diatomic oxygen bonded to a single Si adatom. Our results allow us to distinguish the two main configurations of peroxy radical: paul-para and grif. Both of them possess a lifetime of ˜ 180 min.     

Ground state oxygen in synthesis of cyclic peroxides. Part 1: Benzo fused ketals

A thiol-olefin-cooxygenation (TOCO) radical chain reaction involving ground state molecular oxygen converts 2′-isopropenyl acetophenones directly into cyclic peroxy hemiketal products with three new bonds. Starting with 4-t-butylbenzenethiol, this TOCO process proceeds reproducibly on gram scale in 86% yield. Hemiketal→ketal and sulfide→sulfone transformations finally provide a series of sulfonyl cyclic peroxy ketals. The in vitro antimalarial activities of some of these structurally simple benzo-fused cyclic peroxides are reported.     

Nature of mechanoradical formation and reactivity with oxygen in methacrylic vinyl polymers

The formation of mechanoradicals under anaerobic conditions and their reactivity with oxygen at room temperature is described for several methacrylic vinyl polymers. Observed electron spin resonance (ESR) spectra of the mechanoradicals formed were all essentially identical and are clearly assigned to a respective endchain radical. The ESR kinetics of the mechanoradical formation of polymethylmethacrylate (PMMA) and polymethacrylamide (PMAAm) exhibit an interesting contrast; the progressive changes in the radical concentration in PMMA as a function of duration of milling gradually decrease after reaching a maximum value, while those of PMAAm show a parabolic increase. This discrepancy has been ascribed to mechanoradicals of PMAAm that are strongly stabilized by intermolecular and intramolecular doubly hydrogen-bonded networks among the amide groups. Such interactions also are to lower the reactivity of the mechanoradicals with oxygen. Thus, the mechanoradicals of both PMAAm and PMAA do not give a single peroxy radical, but rather a mixture of the mechanoradical and peroxy radical even after exposure to air, while the mechanoradicals of other polymers are rapidly converted to the corresponding peroxy radicals. Such a difference was observed in experiments on the mechanical fracture of such polymers under aerobic conditions.     

A new chemiluminescence emitter in the hydroquinone-inhibited reaction of cumene oxidation by oxygen

A new chemiluminescence emitter was detected in the hydroquinone-inhibited reaction of homolytic oxidation of cumene by oxygen. The emission band lies in the range (22-18)·10³ cm^–1, which corresponds to an energy of 230 kJ/mole between levels. It is suggested that this emitter is p-benzoquinone, formed in the reaction between the phenoxyl and peroxy radicals.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 2, pp. 103–107, March–April, 1994.     

Alkane hydroxylation by peroxy acids: a comparison with the cytochrome P450 hydroxylation

Alkane hydroxylation by peroxy acids proceeds by a synchronous nonconcerted peroxy oxygen insertion into the C-H bond according to density functional theory. A comparable reaction sequence, initiated by homolytic peroxy bond cleavage, can be formulated for the alkane hydroxylation by the cytochrome P450 hydroperoxo-heme Compound 0. This hydroxylation reaction proceeds by a two-step process because the formed reactive intermediate, Compound II, is significantly stabilized.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 23: Mechanisms and kinetics of trans-vinylene group consumption

There are many potential reactions for trans-vinylene groups in oxidizing polyethylene melts. The main possibilities are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to the removal of trans-vinylene groups to some extent. This is especially so, for the reactions with hydroperoxides that have been found to be the dominant reactions with vinylidene and vinyl groups in the low temperature range. The reaction with peroxy radicals is thought to be as important relatively as with vinylidene groups. Therefore, the importance of the reaction is decreasing with increasing temperature. However, the most characteristic reaction for trans-vinylene groups can be detected without any doubt only in the advanced stages of processing. It is mechanical stress induced oxygen addition to the double bond. The discussion shows that the reaction should be important from the beginning of processing. The reaction cannot operate with vinyl and vinylidene groups, which are not part of the polyethylene main chain. After oxygen addition to the trans-vinylene group, the “ene” reaction yields an allylic hydroperoxide so that the double bond is not immediately removed. It is acid catalyzed hydroperoxide decomposition that leads to chain scission with aldehyde formation at the new chain ends.     

An ESR study of the decay reaction of isotactic polypropylene peroxy radicals in air

The decay of peroxy radicals trapped in irradiated isotactic polypropylene has been studied by ESR in air at various temperatures between 284 and 309 K. All the ESR spectra obtained at the various reaction stages are shown to be composed of two components arising from a mobile fraction and an immobile fraction. Only the mobile peroxy radicals decay; those belonging to the immobile fraction are stable. Various reaction mechanisms are examined in order to explain the experimental results; it is concluded that the decay reaction is controlled by diffusion of peroxy radicals and that the immobile peroxy radicals play no role in the decay reaction. Intermolecular hydrogen abstraction of the peroxy radicals, rather than intramolecular abstraction, is suggested as the rate-determining reaction.     

Updating the Chemiluminescence Oxygen‐Aftereffect Method for Determining the Rate Constant of the Peroxy‐Radical Self‐Reaction: Oxidation of Cyclohexene

Updating the facile chemiluminescence oxygen‐aftereffect method, most suitable for determining the rate constant (k_t) of the peroxy‐radical self‐reaction (main chemiluminescence channel), pertained to considering the sensitivity of such a method toward a disturbing influence of the peroxy radicals of the initiator of the chain oxidation process. Such a disturbance may derive from the side chemiluminescent reaction, which involves peroxy radicals of both hydrocarbon and initiator. To examine the applicability and limitations of the chemiluminescence method under present scrutiny, cyclohexene was used as the model oxidizable hydrocarbon substrate. Computer simulations of the reaction and chemiluminescence kinetics have demonstrated the validity of the considered methodology at the value of the rate constant of the propagation of the overall chain process by peroxy radicals of the initiator higher than 1 m ^?1 s^?1. Despite that the chemiluminescence time profile and the stationary level of the total chemiluminescence intensity depend on the kinetics of the side chemiluminescence channel and on the ratio of the excited‐state generation yields in the mentioned reaction channel and in the main chemiluminescence process, the value of k_t assessed by the oxygen‐aftereffect method has been found independent of variation of these characteristics.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 22: Mechanisms and kinetics of vinyl and vinylidene group consumption

The reactions of vinyl and vinylidene groups in oxidizing polyethylene melts are partly unexpected. The main possibilities of consumption that can be envisaged are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to some extent to the removal of vinyl and vinylidene groups. However, the dominant reactions are quite specific for the two unsaturated groups and the temperature range. Consumption of vinylidene groups results mainly from reaction with peroxy radicals and with hydroperoxides. It decreases significantly in the high temperature range in which the hydroperoxides do not accumulate. Reaction with hydroperoxides seems also to be the dominant reaction removing vinyl groups in polyethylene melts at low temperature. The reaction with peroxy radicals seems negligible in the whole temperature range of the experiments. The increasing consumption rates in the high temperature range are attributed to dimerisation involving two vinyl groups. The same reaction is thought to account for molecular enlargement in polyethylene types with significant amounts of vinyl groups. In this respect it complements macro-alkyl radical addition to vinyl groups. The contributions of the two mechanisms to molecular enlargement are discussed.     

Aging and degradation of polyolefins. I. Peroxide-initiated oxidations of atactic polypropylene

Efficiencies of polymer radical production by thermal decomposition of di-tert-butylperoxy oxalate (DBPO) have been measured in bulk atactic polypropylene (PP) at 25–55°C; they range from 1 to 26%, depending on [DBPO], temperature, and presence of oxygen. Most of the polymer radicals thus produced disproportionate in the absence of oxygen but form peroxy radicals in its presence. Most of the pairs of peroxy radicals interact by a first-order reaction in the polymer cage. The fraction that escapes gives hydroperoxide in a reaction that is half order in rate of initiation. In interactions of polymer peroxy radicals, in or out of the cage, about one-third give dialkyl peroxides and immediate chain termination, two-thirds give alkoxy radicals. About one-third of the later cleave at 45°C; the rest abstract hydrogen to give hydroxy groups and new polymer and polymer peroxy radicals. The primary peroxy radicals from cleavage account for the rest of the chain termination. Cleavage of alkoxy radicals and crosslinking of PP through dialkyl peroxides nearly compensate. Up to 70% of the oxygen absorbed has been found in hydroperoxides. The formation of these can be completely inhibited, but cage reactions are unaffected by inhibitors. Concentrations of free polymer peroxy radicals have been measured by electron spin resonance and found to be very high, about 10^?3M at 58–63°C. Comparison with results on 2,4-dimethylpentane indicate that rate constants for both chain propagation and termination in the polymer are much smaller than those for the model hydrocarbon but that the ratio, k_p/(2k_t)^½, is about the same.     

New paradigms for the peroxy acid epoxidation of CC double bonds: the role of the peroxy acid s-trans conformer and of the 1,2-H transfer in the epoxidation of cyclic allylic alcohols

RB3LYP calculations, on reaction of performic acid with cyclic allylic alcohols, demonstrate that the less stable s-trans conformer of peroxy acids can be involved in epoxidations of C=C bonds. Transition structures (TSs) arising from s-trans performic acid retain some of the well-established characteristics of the TSs of the s-cis isomer such as the perpendicular orientation of the O-H peroxy acid bond relative to the C=C bond and a one-step oxirane ring formation. These TSs are very asynchronous but collapse directly (without formation of any intermediate) to the final epoxide-peroxy acid complex via a 1,2-H shift. Thus, our findings challenge the traditional mechanism of peroxy acid epoxidation of C=C bonds by demonstrating that the involvement of the s-trans isomer opens an alternative one-step reaction channel characterized by a 1,2-H transfer. This novel reaction pathway can even overcome, in the case of the reaction of cyclic allylic alcohols in moderately polar solvents (e.g., in dichloromethane), the classical Bartlett's mechanism that is based on the s-cis peroxy acid form and that features a 1,4-H shift. However, the latter mechanism remains strongly favored for the epoxidation of normal alkenes.     

Some Problems of Ppontaneous Peroxidation of Proteins

The present paper is devoted to the investigation of spontaneous peroxy fragmentation of poly-peptide chains using human serum albumin as a model. The chain radical mechanism of this reaction hasbeen proposed. It has been shown that the chain process is initiated by the interaction of ionogenic peptidegroups with the dissolved oxygen.     

Zwitterionic intermediates in enamine-singlet oxygen reactions. configuration-interaction studies on the indole-singlet oxygen reactions

The theoretical calculations using INDO-RHF CI method have demonstrated that the reaction of singlet oxygen with 1,2,3-trimethylindoles proceeds via a zwitterionic peroxide intermediate. The calculations provide important predictions for the mechanism of enamine-singlet oxygen reactions.     

Electron spin resonance studies on graft copolymerization of gaseous styrene onto preirradiated polypropylene. I. Preirradiation in the presence of oxygen

Graft copolymerization of gaseous styrene was carried out onto polypropylene preirradiated in the presence of oxygen at ?78°C and at room temperature, respectively. The origin of the graft initiation activities of these polymers was investigated by means of electron spin resonance (ESR) of trapped radicals. More grafting was found for the polymers irradiated at ?78°C than for those irradiated at room temperature. The difference of grafting between polymers irradiated at ?78°C and those irradiated at room temperature was not explained by the total amounts of trapped radicals, and it was found that all radicals are not effective in the grafting reaction. ESR measurements showed that there exist two kinds of peroxy radicals, one has more effective ability of abstracting hydrogen atoms from the surrounding polymer chains to form carbon radicals, and another is less effective at the temperature of grafting reaction (40°C). Although the samples irradiated at ?78°C contain the both types of radicals, those irradiated at room temperature do not contain the former type of radicals. It was shown that the carbon radicals produced by such a hydrogen abstraction reaction are actually the effective centers in the grafting reaction of polymers irradiated in the presence of oxygen.     

The reactions of methyl radicals with atomic and molecular oxygen

The reaction of methyl radicals with atomic and molecular oxygen was studied with a photoionization mass spectrometer. The methyl radicals were generated by reacting oxygen atoms with ethylene in a fast-flow tube reactor. The rate constant for the reaction of methyl radicals with oxygen atoms was (1.0 ± 0.2) × 10^?10 cm³/molec · sec with no significant variation with temperature over the range of 259–341°K. The reaction of methyl radicals with molecular oxygen involves both a two-body reaction, having a rate constant \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c} {k_{{\rm 3a}} = (10^{- 12.54 \pm 0.35})\exp [(- 940 \pm 250)T^{- 1}]} & {{\rm cm}^{\rm 3} /{\rm molec} \cdot {\rm sec}} \end{array}$\end{document} and a three-body recombination having a negative temperature dependence. The methyl peroxy radical could be observed at its steady-state concentration. The rate constants determined at low pressures are compatible with the values determined at higher pressures by flash photolysis. Formaldehyde appears to be a major product of the two-body reaction of CH₃ with O₂, and also of the reaction of CH₃O₂ with oxygen atoms.     

Regioselective oxygenations of s-trans dienes, silyl dienol ethers (SDEs), by triphenyl phosphite ozonide (TPPO) and its mechanistic study

The direct oxygenation of s-trans dienes, silyl dienol ethers (SDEs) 2, by triphenyl phosphite ozonide (TPPO) has been examined in detail. The regioselective oxygenation was found to give hydroperoxide 3, alcohol 4, ketone 5, dimer 6, and peroxy phosphate 7 with concomitant formation of triphenyl phosphate 8 and diphenyl trimethylsilyl phosphate 10. The formation of peroxy phosphate 7 was found for the first time in TPPO oxygenation reactions. The low temperature (31)P and (1)H NMR spectroscopic analyses proved the direct reaction of SDEs with TPPO without generation of singlet oxygen. The formation of the oxygenated products 3-7 is reasonably explained by the intervention of the zwitterion ZI, which can be formed by the nucleophilic attack of SDE to the central oxygen of the ozonide. The regioselective attack of SDE to the central oxygen of the ozonide was supported by the quantum chemical calculation (B3LYP/6-31G).     

High temperature reactions of an aryl-alkyl phosphine, an exceptionally efficient melt stabiliser for polyethylene

Bis(diphenylphosphino)-2,2-dimethylpropane (PMP) is a highly efficient melt stabiliser of polyethylene. This aryl-alkyl phosphine hinders the degradation of the polymer during processing even in small concentrations and in combination with a phenolic antioxidant its consumption rate is considerably slower than that of phosphites and phosphonites. In this study the reactions of PMP were studied at temperatures corresponding to those used for the processing of polyethylene in order to explore the processing stabilisation mechanism of this additive. Thermal and thermo-oxidative stability were determined by DSC and TGA, respectively by heating PMP in argon and oxygen at 200 and 240 °C. Reactions with peroxy, carbon-centred and oxy radicals, as well as with hydroperoxide were investigated at 200 °C. Reaction products were identified by FT-IR and solution-state NMR spectroscopy. The results revealed that the phosphine studied has sufficient thermal- and thermo-oxidative stability under the processing conditions of polyethylene. It oxidises easily with any oxidising agent including molecular oxygen of air. Consequently, PMP does not only decompose hydroperoxide groups and react with oxy macroradicals during the processing of polyethylene, as claimed by most references on phosphorous antioxidants, but it can also hinder the formation of peroxy macroradicals, i.e., the initiation reaction of thermo-oxidative degradation.