Bimolecular Hydroperoxide Decomposition Research Articles

Spin Trapping Analysis of Radical Intermediates on the Thermo-Oxidative Degradation of Polypropylene.

The purpose of this study is to investigate the thermo-oxidative degradation behavior of polypropylene (PP) by comparing three types of pristine PP granules (consisting of homopolymer, random copolymer, and block copolymer) with their corresponding oxidized analogues. These analogues were intensely oxidized under oxygen at 90 °C for 1000 h by using the electron spin resonance (ESR) spin trapping method that can detect short-lived radical intermediates during the degradation. The degrees of oxidation could be evaluated by chemiluminescence (CL) intensity, which was related to the concentration of hydroperoxide groups generated in the PP chain. In the pristine PP samples, a small amount of hydroperoxides were found to be formed unintentionally, and their homolysis produces alkoxy radicals, RO•, which then undergo β-scission to yield chain-end aldehydes or chain-end ketones. These oxidation products continue to take part in homolysis to produce their respective carbonyl and carbon radicals. On the other hand, in the oxidized PP granules, because of their much higher hydroperoxide concentration, the two-stage cage reaction and the bimolecular decomposition of hydroperoxides are energetically favorable. Carbonyl compounds are formed in both reactions, which are then homolyzed to form the carbonyl radical species, •C(O)-. PP homopolymer produced the largest amount of carbonyl radical spin adduct; thus, it was found that the homopolymer is most sensitive to oxygen attack, and the presence of ethylene units in copolymers enhances the oxidation resistance of PP copolymers.

Atmospheric Fate of Criegee Intermediate Formed During Ozonolysis of Styrene in the Presence of H2O and NH3: The Crucial Role of Stereochemistry.

A gas-phase mechanistic investigation of the unimolecular, water/ammonia-assisted decomposition reactions of the α-hydroxy hydroperoxides (HPs) and hydroperoxide arylamines (a-HPs) produced during the styrene ozonolysis has been carried out theoretically in the present article. The instrumental role of stereochemistry in controlling the outcome of individual reactions has been discussed. Thermodynamic parameters (Δ G298K, Δ H298K, Δ E0K) associated with individual reactions have also been computed. The rate constants estimated for individual reactions using conventional transition state theory (TST) combined with statistical mechanics provide a comprehensive understanding of the reaction mechanism and also elucidate the atmospheric fate of Criegee intermediates. Considering the feasibility of reactions from thermodynamic and kinetic points of view, while aldehyde (PhCHO) formation pathway originating from bimolecular decomposition of HP is found to be kinetically favored, benzoic acid formation pathway remains favored thermodynamically. A similar consideration for the bimolecular reactions of a-HP reveals the phenylmethanimine formation pathway to be kinetically favored, while the benzamide formation pathway is favored thermodynamically. Our findings appear to be in excellent agreement with the experimental observations.

Oxidation of unvulcanized, unstabilized polychloroprene: A kinetic study

Thermal oxidation in air at atmospheric pressure, in the 80–140 °C temperature range and in oxygen at 100 °C in the 0.02–3 MPa pressure range, of unvulcanized, unstabilized, unfilled polychloroprene (CR) has been characterized using FTIR and chlorine concentration measurement. The kinetic analysis was focused on double bond consumption. A mechanistic scheme involving unimolecular and bimolecular hydroperoxide decomposition, oxygen addition to alkyl radicals, hydrogen abstraction on allylic methylenes, alkyl and peroxyl additions to double bonds and terminations involving alkyl and peroxy radicals was elaborated. The corresponding rate constants were partly extracted from the literature and partly determined from experimental data using the kinetic model derived from the mechanistic scheme in an inverse approach. Among the specificities of polychloroprene, the following were revealed: The rate of double bond consumption is a hyperbolic function of oxygen pressure that allows a law previously established for the oxidation of saturated substrates to be generalized. CR oxidation is characterized by the absence of an induction period that reveals the instability of hydroperoxides. The kinetic analysis also reveals that peroxyl addition is faster than hydrogen abstraction but slower in CR than in common hydrocarbon polydienes.

Thermooxidative reactions of polypropylene wax in the molten state

AbstractThermooxidation of polypropylene wax was investigated in the molten state under an air‐flowing atmosphere. Calorimetric measurements were performed to investigate the effect of the inclusion of 1‐dodecanol as an efficient primary alcohol and to reveal the typical outlines of thermooxidation, that is, the induction time and oxidation rate. The same parameters were obtained and compared on the basis of the Fourier transform infrared (FTIR) peaks of major oxidation products, such as ketones and hydroxyl groups. The results showed that 1‐dodecanol caused a change in the shape of the calorimetric thermogram and also yielded oxidized products with more clarity in color. With the Arrhenius equation, the apparent activation energy was calculated on the basis of the induction time and oxidation rate for carbonyl and hydroxyl groups individually. The obtained apparent activation energy was in agreement with the proposed mechanism based on bimolecular hydroperoxide decomposition involving dodecanol. With FTIR spectroscopy, no changes in the chain helical conformation or double‐bond concentration were observed during oxidation up to 500 min. An acid value of up to 6 mg of KOH per gram was obtained at this time. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Induction period in the low‐temperature thermal oxidation of saturated hydrocarbons: Example of polyethylene

AbstractThermal oxidation of hydrocarbon substrates at low‐to‐moderate temperature, typically T ≤ 150°C, results from a radical chain process initiated by hydroperoxide decomposition and displays an induction period. A reliable model exists to simulate oxidation kinetics, but an incertitude remains on initial steps because they are out of reach of all available analytical methods. This work is aimed to have a kinetic approach of the problem, by comparing various mechanisms, i.e., (A) bimolecular decomposition of initially present hydroperoxides; (B) combined uni‐ and bimolecular decomposition of hydroperoxides; (C) the presence of radicals at the beginning of the exposure; and (D) radicals generation at (low) constant rate from irradiation, for instance by ionizing radiation linked to natural radioactivity or from a direct oxygen–substrate reaction. Scheme A is not realistic at low initial hydroperoxide concentrations. All the other mechanisms generate similar behaviors: the induction time tends toward a constant value almost independent of the nature of initial steps, when the concentration of precursors (initially present hydroperoxides or radicals) or the rate of their initial production (from species other than hydroperoxides) tends toward zero. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 769–777, 2008

Thermolysis of polyethylene hydroperoxides in the melt, Part 11: Relative yields of thermolysis products

The experimental ratios of the main products from polyethylene hydroperoxide thermolysis are examined. Comparison with the corresponding theoretical ratios calculated for different hydroperoxide decomposition reactions allows discriminating between the main hydroperoxide decomposition reactions. The experimental values can usually be explained best by the true bimolecular reaction involving two hydroperoxide groups. Mostly these values are significantly different from the theoretical ratios calculated for the bimolecular reaction with an alcohol group and for the pseudo-monomolecular reaction with a segment of the polymer. The bulk of the results points unequivocally to true bimolecular hydroperoxide decomposition for explaining thermolysis of polyethylene hydroperoxides.

Thermolysis of polyethylene hydroperoxides in the melt, Part 10: Product yields from bimolecular and pseudo-monomolecular hydroperoxide decomposition

The quantitative aspects of some specific decomposition reactions of polyethylene hydroperoxides are re-examined. New data have shown that β-scission of primary alkoxy radicals is negligible in the temperature range of the thermolysis experiments. This is important for the true bimolecular hydroperoxide decomposition for which, in a first approximation, β-scission of primary and secondary alkoxy radicals had been taken into account. The calculation shows that the yields of the main oxidation products such as secondary alcohols, ketones, trans-vinylene groups and aldehydes are not considerably affected by the change. However, the theoretical yields of some minor products such as primary alcohols and of some combination reactions are strongly affected. For the pseudo-monomolecular hydroperoxide decomposition involving a segment of the polymer, the main novelty in comparison with previous work consists in taking into account β-scission of the secondary alkoxy radicals. It allows improving the accuracy of the calculated product yields. Moreover, all the theoretical calculations are on the same level of accuracy and can be used for comparison with the experimental product yields.

Thermal Oxidation Kinetics of Unvulcanized Unstabilized Polyisoprene

Abstract The thermal oxidation of an unvulcanized, unstabilized polyisoprene rubber (IR) has been studied in the 40–140 °C temperature range. Ageing was monitored by FTIR determination of double bonds and carbonyl groups, mass uptake measurement, and weight average molar mass determination. A mechanistic scheme based on the standard scheme for radical chain oxidation, but taking into account the diversity of initiation processes and the existence of inter and intramolecular radical additions to double bonds, was built. The kinetic model derived from this scheme is composed of seven differential equations to be solved in discretized thickness layers to take into account the kinetic control by oxygen diffusion. This system was numerically solved using a Matlab program dedicated to stiff systems of differential equations. The elementary rate constants and other kinetic parameters were then determined from experimental data, using an inverse approach. A set of physically reasonable parameter values was obtained, allowing thus to envisage lifetime predictions at low temperature (long term). The results lead to observations difficult to make from classical analytical studies, for instance the predominance of bimolecular hydroperoxide decomposition among other initiation modes or the competition between intermolecular hydrogen abstraction and intramolecular addition of peroxy radicals to double bonds.

Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 1: Unvulcanized unstabilized polyisoprene

The thermal oxidation of an unvulcanized, unstabilized polyisoprene rubber (IR) has been studied in the 40–140 °C temperature range. Ageing was monitored by FTIR determination of double bonds and carbonyl groups, mass uptake measurement, and weight average molar mass determination. A mechanistic scheme based on the standard scheme for radical chain oxidation, but taking into account the diversity of initiation processes and the existence of inter- and intramolecular radical additions to double bonds, was built. The kinetic model derived from this scheme is composed of seven differential equations to be solved in discrete thickness layers to take into account the kinetic control by oxygen diffusion. This system was numerically solved using a Matlab program dedicated to stiff systems of differential equations. The elementary rate constants and other kinetic parameters were then determined from experimental data, using an inverse approach. A set of physically reasonable parameter values was obtained, thus allowing us to envisage lifetime predictions at low temperature (long term). The results led to observations difficult to make from classical analytical studies, for instance the predominance of bimolecular hydroperoxide decomposition among other initiation modes or the competition between intermolecular hydrogen abstraction and intramolecular addition of peroxy radicals to double bonds.

Physico-chemical aspects of polyethylene processing in an open mixer: Part 27: Formal kinetics of aldehyde and carboxylic acid formation in the initial stages

There are many reactions susceptible to yield aldehydes and acids in polyethylene melts. It is β-scission of the alkoxy radicals formed on bimolecular hydroperoxide decomposition that is expected to be one of the main sources of the aldehydes that are formed at increasing rates in the early stages of polyethylene processing. Acid-catalyzed decomposition of allylic hydroperoxides is another source of substantial amounts of aldehydes. Formation and decomposition of α,γ- and α,β-di-hydroperoxides should yield acids. The activation energy estimated for these different processes is very large (about 57 kcal/mol) so that their contribution could be significant in the high temperature range only. This is different for the reaction of aldehydes with hydroperoxides to yield peroxy-hemiacetals. These intermediates can be expected mainly in the low temperature range where hydroperoxides are accumulating. Decomposition of the peroxy-hemiacetals gives acids as one of the main products. Free-radical induced oxidation of aldehydes is likely to yield peracids as far as oxygen addition is competitive with decarbonylation. The main problem is the transformation of the peracids into acids. The reaction with double bonds is expected to yield significantly more acids than thermal decomposition of peracids. If the last occurs, it will be followed mainly by decarboxylation. The overall activation energy for both processes of acid formation is negative (−18 to −20 kcal/mol). It is some combination of the various mechanisms examined that might account for the experimental activation energy for acid formation in the initial stages that is close to 18 kcal/mol.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 25: Mechanisms of aldehyde and carboxylic acid formation

Aldehydes and acids can be formed in numerous reactions in oxidizing polyethylene melts. Significant amounts of aldehydes result from β-scission of alkoxy radicals that are formed on bimolecular hydroperoxide decomposition. There are also large amounts of aldehydes expected from acid-catalyzed decomposition of allylic hydroperoxides as soon as enough acids have accumulated for efficient catalysis. There are difficulties in explaining the formation of aldehydes at a constant rate in sufficient amount for explaining the experimental data. There are much less difficulties with the constant rate of carboxylic acid formation. The α,γ-keto-hydroperoxides that are formed on chain propagation might account for the bulk of the acids formed at a constant rate. The foremost problems with the acids pertain to their formation at increasing rates in the initial as well as in the advanced stages. Formation and decomposition of α,β-di-hydroperoxides and α,γ-di-hydroperoxides is a possibility in this respect. Similarly, α,β-keto-hydroperoxides might be formed on peroxidation in the α-position to ketone groups in the advanced stages. There are considerable difficulties in elucidating the exact role of the aldehydes that are usually seen as the main precursors of the acids. Although there are many possibilities for transformation of aldehydes into acids, the free radical mechanisms envisaged usually have considerable disadvantages. These disadvantages result essentially from fast decarbonylation of acyl radicals and even faster decarboxylation of acyl-oxy radicals. Direct transformation of peracids into acids on reaction with double bonds is always a possibility. Moreover, in the low temperature range (150–160 °C) where hydroperoxides are accumulating, direct reaction of aldehydes with primary and/or secondary hydroperoxides will also yield acids.

The classical kinetic model for radical chain oxidation of hydrocarbon substrates initiated by bimolecular hydroperoxide decomposition

AbstractKinetic modeling of the low‐temperature (typically T ⩽ 200°C) thermal aging of polymers is a problem of great technological importance, owing to the continuously increasing needs of industry in terms of reliable methods for lifetime prediction. In the temperature domain under consideration, for most hydrocarbon substrates, oxidation proceeds by a radical chain reaction initiated by bimolecular hydroperoxide decomposition. In other words, the reaction generates its own initiator, which explains its strong autoaccelerated character. The most pertinent model is, to our opinion, the model elaborated by Tobolsky et al. (J Am Chem Soc 1950, 72, 1942) in the early 1950s. This model is, however, based on three questionable assumptions: the existence of a stationary state for radical concentrations (hypothesis S), the presence of oxygen in excess (hypothesis E), and the fact that the onset of steady state can be observed in the domain of low conversions, where the substrate consumption can be neglected (hypothesis L). One hypothesis (S) lacks consistency. A sounder alternative, which does not modify significantly the mathematical expressions of the model, will be proposed. The other hypotheses (E and L) can be justified in certain cases, but the limits of their domain of validity were never established to our knowledge. It is tried, here, to express these limits in function of fundamental parameters: rate constants and concentrations of reactants. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38:666–676, 2006

一次構造制御によるポリプロピレンの高安定化

Thermal oxidative degradation behavior of isotactic polypropylene (iPP) with various degree of isotacticity was studied based on the activation energy (ΔE) data obtained by thermogravimetric analysis (TGA). The ΔE value displayed a negative proportion to the content of meso pentad fraction (mmmm). Since the value of mmmm was directly related to polymer chain conformation, the ΔE value was thought to have close connection with the concentration of 31 helix conformation in the iPP. The ΔE changes would be caused by the competition between uni- and bimolecular hydroperoxide decomposition, which was controlled by the concentration of 31 helix conformation. In conclusion, this result suggested that the stability of iPP could be controlled by the primary structure such as isotacticity.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 21: Improved product yields on true bimolecular hydroperoxide decomposition

The product yields from the reaction between two hydroperoxide groups have been re-calculated. This is a consequence of the fact that β-scission of secondary alkoxy radicals cannot be neglected in the high temperature range of the polyethylene processing experiments (170–200 °C). It must be taken into account in addition to disproportionation/combination and hydrogen abstraction by alkoxy radicals. The increased complexity caused by the additional reaction results mainly from the larger number of caged radical pairs involved in the reactions and also in the calculations. Among other products it becomes possible to calculate the yields of aldehyde and vinyl groups that would not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols, ketones and trans-vinylene groups are reduced to some extent in comparison with the values calculated if β-scission is neglected. The vinyl group yield corresponds to slightly more than 10% of the yield of trans-vinylene groups in the temperature range of the experiments. The aldehyde yield is significantly larger than the vinyl group yield and is important in the whole temperature range examined. Main-chain scissions are important at the temperatures of the experiments. They become more important than the sum of the different combination reactions from a temperature of 200 °C on.

New kinetic aspects on the mechanism of thermal oxidative degradation of polypropylenes with various tacticities

Thermal oxidative degradation behavior of polypropylene (PP) with different tacticities was studied based on the activation energy (Δ E) data obtained by thermogravimetric analysis (TGA). The Δ E value showed a negative proportion to the content of meso pentad fraction ( mmmm) in all of isotactic PP (iPP) samples, and that of syndiotactic PP sample considerably deviated from this negative proportion relationship. Since the value of mmmm was directly related to polymer chain conformation, the Δ E value was thought to have close connection with the concentration of 3 1 helix conformation in the iPPs. The Δ E changes would be caused by the competition between uni- and bimolecular hydroperoxide decomposition, which was controlled by concentration and character of conformations of PPs.

Chemiluminescence from the thermal oxidation of polyisoprene and polybutadiene I. Influence of oxygen pressure on the chemiluminescence of polyisoprene during its oxidation

The influence of oxygen pressure p (0.016–0.1 MPa) on polyisoprene thermal oxidation at 100 °C has been studied by chemiluminescence (CL). The maximum CL intensity was found to be almost proportional to p, whereas the induction time is influenced in the same way but to a lesser extent. Assuming that the classical mechanistic scheme with initiation by bimolecular hydroperoxide decomposition and termination by bimolecular radical combination, partially studied by Tobolsky and co-workers in the 1950s, is valid, it was established that chemiluminescence originates essentially from hydroperoxide decomposition. A theory which tries to reconcile this result with the most popular hypothesis according to which light is emitted by terminating peroxyl bimolecular combination is proposed.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 20: Additional product yields on bimolecular hydroperoxide decomposition with an alcohol group

Most products formed on polyethylene oxidation result from hydroperoxide decomposition. The product yields can be calculated for various mechanisms of hydroperoxide decomposition. This work concerns the reaction of a hydroperoxide with an alcohol group thought to be dominant in the advanced stages of polyethylene processing in the high temperature range (170–200 °C). Besides hydrogen abstraction by caged alkoxy radicals already envisaged previously, the possibility of β-scission is taken into account. This additional reaction introduces significant complexity into the reaction schemes. This is especially so because additional caged radical pairs must be included into the schemes and the calculations. It becomes possible to calculate the yields of aldehyde and vinyl groups that do not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols and ketones are not much affected by taking into account β-scission. The yield of aldehydes is important in the whole temperature range and increases considerably if the temperature is raised from 170 to 200 °C. It becomes more important than the ketone yield. The vinyl groups are formed in amounts corresponding roughly to 10–15% of the trans-vinylene groups in the temperature range of 170–200 °C.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 19: Mechanisms and kinetics of vinyl and vinylidene group formation

Vinyl and vinylidene group formation is detected in the initial stages of polyethylene processing. In the high temperature range (170–200 °C) the amount formed is small but significant. Formation of these double bonds is usually obscured by their rapid consumption. Bimolecular hydroperoxide decomposition does not seem to be an important source for these products in the early stages of processing. Vinyl and vinylidene group formation can be attributed mainly to intramolecular decomposition of special hydroperoxide groups. The data suggest vinyl groups to arise from secondary hydroperoxide groups formed in α-position to methyl branching. Intramolecular hydroperoxide decomposition involving a primary hydrogen atom from the methyl group yields a vinyl group and an aldehyde. Vinylidene groups seem to arise from secondary hydroperoxide groups formed in α-position to quaternary structures that necessarily include one methyl group. Intramolecular hydrogen abstraction of a primary hydrogen atom from the methyl group yields a vinylidene group and an aldehyde. The calculated rate parameters are in agreement with the thermochemical estimations relative to intramolecular abstraction of primary hydrogen atoms for both reactions. Vinyl groups are also formed on bimolecular hydroperoxide decomposition. The yield of vinylidene groups from the last reaction is negligible.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 16: Mechanisms and kinetics of ketone formation at low temperature

Ketones are formed essentially on various reactions of polyethylene hydroperoxide. In the low temperature range (150–160 °C), some reactions are the same as in the high temperature range (170–200 °C) of the experiments. However, there is much more complexity at low temperature than at high temperature. The experimental kinetics reveals three significantly different processes compared to only two at high temperature. Ketone formation according to a constant rate results from the cage reaction between a peroxy radical and a hydroperoxide group during the chain propagation reaction. One of the reactions envisaged for the ketones formed according to the rate increasing with processing time in the initial stages is similar from the chemical point of view to the reaction yielding ketones according to the constant rate. The reaction proceeds between the hydroperoxides accumulated in an elementary volume and the peroxy radicals responsible for an additional oxidation of this volume. It cannot be accounted for by formal homogeneous kinetics that accounts well for the constant rate but by the heterogeneous kinetics taking into account repeated oxidation of elementary volumes. The second possibility of ketone formation according to the rate increasing with time is based on bimolecular hydroperoxide decomposition involving mainly associated hydroperoxides. The kinetic treatment for this process combines the heterogeneous kinetics of oxidation volume overlapping with monomolecular decomposition of associated hydroperoxides. The rate constant deduced from the data is in agreement with literature values as well as with the values deduced previously from the experiments with PE melts.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 15: Product yields on bimolecular hydroperoxide decomposition

The product yields are important characteristics of the various hydroperoxide decomposition reactions. The true bimolecular hydroperoxide decomposition is thought to be the dominant reaction in the early stages of polyethylene processing. The reaction probabilities and theoretical yields calculated for the reaction account for the ratio of alcohols to ketones in these early stages. The calculations yield a rate constant for the disproportionation between the alkoxy and peroxy radical that is of the same order as published data for corresponding very low molecular mass radicals. The other reactions of the caged radical pairs are the same as those discussed for the reaction between a hydroperoxide and an alcohol group. They easily account for the experimental data. Pseudo-monomolecular hydroperoxide decomposition involving a hydroperoxide group and a segment of the polymer requires only parameters already envisaged previously. It does not seem to be important for polyethylene processing. However, it has been envisaged for thermolysis of polyethylene hydroperoxides in a press. The product yields calculated in this work for this reaction do not agree with the data from thermolysis of polyethylene hydroperoxides. The calculations allow concluding that pseudo-monomolecular hydroperoxide decomposition cannot be the only reaction on thermolysis of polyethylene hydroperoxides.