Thermolysis of polyethylene hydroperoxides in the melt. Part 9: Re-examination of the experimental kinetics of hydroperoxide decomposition

The experimental kinetics of decomposition of polyethylene hydroperoxides in the melt is re-examined. It is found that the rates determined are more accurate if only the “free” hydroperoxides are taken into account instead of the total hydroperoxides that include also the “associated” hydroperoxides. Then, decomposition of polyethylene hydroperoxides in the melt can be attributed unambiguously to a first-order reaction that is valid in the whole time range of the thermolysis experiments. Nevertheless, the first-order rate constant determined this way increases with the initial hydroperoxide concentration. This constitutes a significant difference with the first-order rate constants that are valid in low molecular mass chemistry and are independent of the initial concentration of the reacting species. It has already been concluded previously that this experimental first-order rate cannot be attributed to true monomolecular hydroperoxide decomposition. Hence, another or other reactions must be envisaged for the interpretation of the specific first-order decomposition of the hydroperoxides in polyethylene melts.     

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.     

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 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.     

Radical-Pair Formation in Hydrocarbon (Aut)Oxidation

The reaction profiles for the uni- and bimolecular decomposition of benzyl hydroperoxide have been studied in the context of initiation reactions for the (aut)oxidation of hydrocarbons. The unimolecular dissociation of benzyl hydroperoxide was found to proceed through the formation of a hydrogen-bonded radical-pair minimum located +181 kJ mol⁻¹ above the hydroperoxide substrate and around 15 kJ mol⁻¹ below the separated radical products. The reaction of toluene with benzyl hydroperoxide proceeds such that O−O bond homolysis is coupled with a C−H bond abstraction event in a single kinetic step. The enthalpic barrier of this molecule-induced radical formation (MIRF) process is significantly lower than that of the unimolecular O−O bond cleavage. The same type of reaction is also possible in the self-reaction between two benzyl hydroperoxide molecules forming benzyloxyl and hydroxyl radical pairs along with benzaldehyde and water as co-products. In the product complexes formed in these MIRF reactions, both radicals connect to a centrally placed water molecule through hydrogen-bonding interactions.     

The influence of oxygen pressure on the kinetics of the thermal decomposition of Co3O4

The kinetics of the thermal decomposition of Co₃O₄ has been examined in the 1123–1200 K temperature and 2.66–20.73 kPa oxygen pressure range. The kinetics of this process has been deseribed in terms of a mixed-control model of reaction. The values of activation energies of diffusion and chemical reaction as well as the observed activation energy have been given. The strong dependence of the decomposition rate on temperature and oxygen pressure has been explained.     

Diffusion-Controlled Reactions in Polymer Systems

Abstract

Chemical reactions require that at least two molecules or reacting groups come into close proximity so that energy, electrical charge, or chemical groups may be exchanged between them. The process of bimolecular reaction between A and B is given by Eq. (1), taking into account the diffusion step bringing the two reactions in proximity, (AB)     

Ab initio study of aluminum chemical vapor deposition from dimethylaluminum hydride: a gas phase reaction mechanism

The effect of preheating of dimethylaluminum hydride (DMAH) as a gas on the epitaxial growth in aluminum chemical vapor deposition (Al-CVD) is studied theoretically. The chemical changes of DMAH in the gas phase such as unimolecular decomposition reactions, bimolecular reactions and polymerizations are treated using ab initio molecular orbital method (MP2/6-31G**) and density functional theory (B3P86/LanL2DZ). The gas phase equilibrium composed of the previous reaction products under the usual experimental conditions for Al-CVD is also investigated in detail as the initial stage of the CVD process. From the energetics point of view, unimolecular decomposition reactions and bimolecular reactions hardly occur, however, polymerizations of DMAH take place readily at the low temperatures found in Al-CVD. A large amount of DMAH-dimer and a small amount of DMAH-monomer and trimer coexist in the equilibrium state.     

The Mechanism of the Decomposition of Peroxides and Hydroperoxides by Mineral Fillers

The interaction of fillers and pigments with free radical initiators has been studied. Clay minerals have a marked influence on both the rate and the mechanism of the decomposition of peroxides and hydroperoxides. Kaolinite is a particularly effective catalyst and causes rapid decomposition even at room temperature. The reaction of cumene hydroperoxide with kaolinite is first-order in peroxide and the rate constant is proportional to the ratio of clay to hydroperoxide. From a study of the products of the reaction and the influence of solvent on the decomposition, a mechanism involving an intramolecular rearrangement or closely associated ion pairs has been proposed. The application of these results to polymer filler composites is discussed.     

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.     

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

Thermal 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     

Synthesis of poly(arylene ethylene) oligomeric hydroperoxides and their use as initiating agents of radical polymerization

The oxidation to hydroperoxide of poly(arylene ethylenes) (PAE) by oxygen carried out in solutions at 80–110°C. The effect of initiating additions and the nature of solvent relative to the content of hydroperoxide groups in oxidized PAE were investigated. The oxidation to hydroperoxides in PAE occurs at the methylene groups, and the synthesized hydroperoxides are secondary peroxides. The decomposition of PAE hydroperoxides in toluene and chlorobenzene at concentrations of 0.006–0.03 mole/l. for hydroperoxide in the presence and absence of N-phenyl-α-naphthylamine (PNA) was studied. The decomposition of one hydroperoxide has been studied in the presence of cobaltous and manganese resinates and of PNA in chlorobenzene at 30–50°C. The addition of PNA to a chlorobenzene solution of PAE hydroperoxide containing cobaltous or manganese resinate accelerates the hydroperoxide decomposition, reduces the activation energy, and changes the reaction order from the second-order to first-order. The synthesized hydroperoxides initiate the radical polymerization of styrene and methyl methacrylate. The initiating activity of one of the synthesized hydroperoxides of PAE for polymerization of styrene (60°C) in the presence and absence of activating addition of manganese resinate was also evaluated.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 32: Formal kinetics of γ-lactone formation from secondary products. Heterogeneous kinetics

Oxidation of aldehydes and γ-hydroxy-trans-vinylene groups can yield γ-lactones. These intermediates account for γ-lactone formation in the advanced stages of polyethylene processing in air. The acyl-peroxy radical formed on free radical induced oxidation of aldehydes can abstract intramolecularly a δ-hydrogen atom to yield a peracid. Reaction of the alkyl radical formed in this reaction with the hydroperoxide group of the peracid gives a γ-lactone with simultaneous release of a hydroxyl radical. The calculated rate of γ-lactone formation according to the mechanism envisaged decreases slightly with increasing temperature (activation energy of about −5 kcal/mol). It is in agreement with the experiments that do not show significant activation energy in the high temperature range for the advanced stages of polyethylene processing. The calculated rate of γ-lactone formation is found to increase by a factor of about 2.7 if the processing experiments are performed in pure oxygen instead of in air. This is close to the experimental factor of about 2.Peroxidation of γ-hydroxy-trans-vinylene groups can also yield γ-lactones. The first possibility involves addition of a peroxy radical to the double bond followed by oxygen addition to the alkyl radical. This reaction possibly yields an α-peroxy-hydroperoxide. Intramolecular decomposition involving the two reactive groups of the α-peroxy-hydroperoxide can give an ozonide that on thermal decomposition yields among others an acid group in 4-position to the alcohol. The activation energy calculated is strongly negative so that the rate should decrease strongly with increasing temperature. Hence, the mechanism cannot contribute significantly to γ-lactone formation in the whole temperature range of the experiments. This is so in spite of the fact that the rate is estimated to increase by a factor of about 1.7 on passing from air to pure oxygen, which is close to the experimental value of approximately 2. The second possibility of transformation of γ-hydroxy-trans-vinylene groups is based on stress-induced oxygen addition to the double bond. Acid catalyzed decomposition of the allylic hydroperoxide that is formed in the reaction yields a pair of aldehydes with one of the aldehyde groups in 4-position to the alcohol group. Peroxidation of the aldehyde pair can give an acid group in 4-position to the hydroxyl group so that a γ-lactone can be formed. The activation energy calculated for the process is very small and the effect of the oxygen concentration corresponds to an increase by a factor of approximately 4.5 on passing from air to pure oxygen. It is postulated that simultaneous contribution by different mechanisms might well account for the experimental value of about 2.The heterogeneous kinetics discussed in detail allows for complementary data interpretation. It is especially suited for the understanding of the advanced stages of polyethylene processing, after some induction time.     

Kinetics of thermal decomposition of Co3O4 powder and single crystals

The kinetics of the thermal decomposition of Co₃O₄ is described using a mixed-control model in which the reaction rate is controlled simultaneously by both a diffusional process and a chemical reaction. The observed dependence of the activation energy on the degree of decomposition, and the activation energies of the diffusion and of the chemical reaction are given.

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Zusammenfassung Es wurde die Kinetic der thermischen Zersetzung von Co₃O₄ mittels eines Modelles beschrieben, bei dem die Reaktionsgeschwindigkeit sowohl durch Diffusionsvorgänge als auch durch chemische Reaktionen beeinflut wird. Die gefundene Abhängigkeit der Aktivierungsenergie vom Grad der Zersetzung, von Diffusion und chemischer Reaktion wird gegeben.

     

二苯甲烷二氨基甲酸甲酯热分解制备二苯甲烷二异氰酸酯的表观动力学研究

在癸二酸二(2-乙基己基)酯溶剂中研究了二苯甲烷二氨基甲酸甲酯(MDC)的分解反应机理,建立了二苯甲烷二氨基甲酸甲酯的分解反应动力学模型。通过实验测定,并对不同温度下体系中各物质的浓度数据进行线性拟合得到反应速率方程。结果表明：MDC的热分解分为两个步骤,均为一级反应。两步反应的活化能分别为：138.82kJ/mol,167.78kJ/mol;指前因子分别为：1.51×1012min-1,5.33×1014min-1。     

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.     

焦炭溶损反应动力学及其模型研究

利用未反应核收缩模型对高炉焦炭与CO2的反应动力学进行了研究，建立了以可测参数（R）表达的焦炭与CO2的反应动力学关系式。并对反应速率常数和有效扩散系数、表观反应活化能和有效扩散活化能及反应过程中各步骤阻力进行了分析。结果表明，（1）焦炭与CO2的反应符合未反应核收缩模型。（2）反应的表观活化能Ea=124.5kJ/mol，有效扩散活化能ED=642.4 kJ/mol；界面化学反应的阻力随反应温度升高而增加；残余灰层内的内扩散传质阻力相对比例随温度升高而下降。（3）焦炭溶损反应在低温区主要受内扩散控制，随着温度升高，反应由外扩散、化学反应和内扩散三步控制；当进入高温区，反应进行一段时间后主要受内扩散控制。     

用协同学方法研究化学反应速率和机理问题

用非平衡统计理论中的协同学方法分析了化学动力学中的反应速率和机理问题．对于任何给定的机理，通过引入一种非奇异变换，将相应的反应速率方程组直接地转换成Ｈａｋｅｎ形式，从而得到反应体系序参量以及其它变量的演化解析表达式．讨论了有机分子热分解的Ｒｉｃｅ－Ｈｅｒｚｆｅｌｄ机理．对醚的热分解链反应得到了一级动力学方程，理论结果与实验结果相符合．利用这种方法能解析地求解复杂反应的反应速率方程组，而不必引用在化学动力学中的“稳态近似”假设     

Reaction Kinetics of 2-Propanol Dehydration in Supercritical Water

A study of the kinetics and mechanism of chemical reactions in supercritical fluids is considered. An experimental procedure was proposed for examining reversible chemical reactions in supercritical water. The reaction kinetics of 2-propanol dehydration in supercritical water was studied. It was found that the uncatalyzed reactions of olefin hydrogenation by hydrogen dissolved in supercritical water occur at high rates near the critical point of water. The experimental data on the dehydration of 2-propanol in supercritical water are adequately described by first-order reaction rate equations. The rate constants and activation energies of 2-propanol dehydration near the critical point of supercritical water were found.     

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.