聚苯乙烯热降解机理的理论研究

采用密度泛函理论B3LYP/6-311G（d）方法,对聚苯乙烯（PS）热降解反应机理进行了研究。PS热降解的主要产物是苯乙烯,其次是甲苯、α-甲基苯乙烯、乙苯和二聚体等芳烃化合物。PS热降解反应主要包括主链C-C键均裂、β-断裂、氢转移和自由基终止等反应。针对以上各类反应进行了路径设计和理论计算分析,对参与反应的分子的几何结构进行了优化和频率计算,获得了各热降解路径的标准动力学和热力学参数。计算结果表明,苯乙烯主要由自由基的链端β-断裂反应形成;二聚体主要由分子内1,3氢转移的反应形成;α-甲基苯乙烯由分子内的1,2氢转移后进行β-断裂形成;甲苯由苯甲基自由基夺取主链上的氢原子形成;乙苯由苯乙基自由基夺取氢原子形成。动力学分析表明,苯乙烯形成所需要的能垒低于其他产物形成所需要的能垒,故苯乙烯为主要的热降解产物;这与相关实验结果基本一致。     

Reactivity of white phosphorus with compounds of the p-block

This review covers the activation, aggregation and degradation of white phosphorus by molecules containing reactive p-block centers. The chemistry has been divided into a number of reaction types and possible mechanisms are used to aid in the understanding of these reactions.     

TG-DTA-FTIR analysis and isoconversional reaction profiles for thermal and thermo-oxidative degradation processes in black chokeberry (<Emphasis Type="Italic">Aroniamelanocarpa</Emphasis>)

The thermal and thermo-oxidative processes in Aroniamelanocarpa (black chokeberry) were investigated using combined thermo-analytical (TG-DTA) and spectroscopic (FTIR) experimental techniques. Isoconversional analysis revealed that the process in an inert (argon) atmosphere was probably governed by chlorogenic acid degradation, where autocatalysis (described by the empirical ?esták-Berggren model) might occur due to water already present in the early stages of the process through hydrolysis. Thermal degradation is described by the intrinsic kinetic parameters, where the degradation rate increases proportionally with an increase in the heating rate. Under oxidative conditions, the process was found to be primarily driven by neochlorogenic acid degradation. The thermo-oxidative degradation of Aroniamelanocarpa fresh samples can be described by two competitive reactions, where it was established that a cyanidin-3-glucosylrutinoside degradation made a significant contribution to a comprehensive kinetics. This study showed the targeting of the neochlorogenic acid in Aroniamelanocarpa fresh samples to have a strong hydrogen-donating activity, thereby rendering it capable of very efficiently entrapping the peroxy radicals. Current research has demonstrated that the relative contribution of the two competitive reactions to the overall process is highly dependent on the heating rate of the system under consideration.     

Aromatic bromination of aldehydes and ketones using 1,3-di-n-butylimidazolium tribromide [BBIm]Br3 ionic liquids under solvent-free conditions

An environmentally benign and efficient process for the preparation of monobromo derivatives of aryl aldehydes and ketones was developed by simple and practical reactions of aryl aldehydes or ketones with 1,3-di-n-butylimidazolium tribromide ([BBIm]Br₃), as a brominating reagent under solvent-free conditions in very high yields. The process has several advantages: high conversions, short reaction time, mild reaction conditions, simple workup with good to quantitative yields and re-usable ionic liquid.     

A calculation of thermal degradation initiated by random scission,unsteady radical concentration

Changes in molecular weight distribution and in sample volume were calculated for thermal degradation of a polymer. The thermal degradation scheme consists of random scission initiation, depropagation and disproportionation termination reactions. An unsteady radical concentration was considered. There are two parameters, normalized zip length z/x₀ and radical number per initial chain length z′x₀, describing the thermal degradation scheme with an unsteady radical concentration. The effects of the initial number-average molecular weight and order of the disproportionation termination reaction on changes in molecular weight, the sample volume and polydispersity are not significant as long as these two parameters have the same value for each polymer sample. Molecular weights of a degrading sample calculated from the steady state radical concentration tend to be over-estimated and sample volumes tend to be underestimated compared to those calculated with an unsteady radical concentration. The validity of approximations used in the calculation assuming a steady state radical concentration is examined by comparing with results calculated with an unsteady radical concentration for various values of the two parameters. An unrealistically large build-up of monomer radicals is found for both calculations based on the steady state and the unsteady radical concentrations. Two special treatments of monomer radicals can dissipate the build-up of monomer radicals: (1) their immediate vaporization, or (2) an enhanced rate of the termination reaction for the monomer radicals. As a guide, the model based on an unsteady radical concentration is preferred, if the value of z′x₀ exceeds 0.1.     

4-Dimethylaminopyridine-catalyzed multi-component one-pot reactions for the convenient synthesis of spiro[indoline-3,4′-pyrano[2,3-c]pyrazole] derivatives

An efficient and clean reaction process was developed for the convenient and cheap synthesis of spirooxindole derivatives. One-pot reactions of isatins, malononitrile (or ethyl cyanoacetate), hydrazine hydrate (or phenylhydrazine), and 1,3-dicarbonyl compounds were compared with one-pot reactions of isatins, malononitrile (or ethyl cyanoacetate), and 5-pyrazolone derivatives. Both sets of reaction were conducted in EtOH in the presence of 4-DMAP catalyst. A series of spiro[indoline-3,4′-pyrano[2,3-c]pyrazole] derivatives were quickly obtained in excellent yields by using the four-component one-pot reaction method.     

One-pot approach to the synthesis of novel 12H-chromeno[2′,3′:4,5]imidazo[1,2-a]pyridines in aqueous media

The chromeno-imidazo[1,2-a]pyridine scaffold was generated in an one pot condensation/cyclization reaction involving a salicylaldehyde and 1-(cyanomethyl)pyridinium chloride, in aqueous sodium carbonate solution. These novel compounds were isolated in 47-71% yield. The reaction pathway was followed by ¹H NMR spectroscopy allowing a clear understanding of the side reactions involved in the process.Different mono-substituted pyridinium chlorides were synthesized and reacted with mono-substituted salicylaldehydes and a detailed discussion of the scope of the synthetic method is also presented.     

A thermal degradation mechanism of polyvinyl alcohol/silica nanocomposites

The thermal degradation mechanism of a novel polyvinyl alcohol/silica (PVA/SiO₂) nanocomposite prepared with self-assembly and solution-compounding techniques is presented. Due to the presence of SiO₂ nanoparticles, the thermal degradation of the nanocomposite, compared to that of pure PVA, occurs at higher temperatures, requires more reaction activation energy (E), and possesses higher reaction order (n). The PVA/SiO₂ nanocomposite, similar to the pure PVA, thermally degrades as a two-step-degradation in the temperature ranges of 300-450 °C and 450-550 °C, respectively. However, the introduction of SiO₂ nanoparticles leads to a remarkable change in the degradation mechanism. The degradation products identified by Fourier transform infrared/thermogravimetric analysis (FTIR/TGA) and pyrolysis-gas chromatography/mass spectrometric analysis (Py-GC/MS) suggests that the first degradation step of the nanocomposite mainly involves the elimination reactions of H₂O and residual acetate groups as well as quite a few chain-scission reactions. The second degradation step is dominated by chain-scission reactions and cyclization reactions, and continual elimination of residual acetate groups is also found in this step.     

Kinetic studies on lactic acid extraction with amine using a microporous membrane-based stirred cell

Rates and mechanism of the liquid-liquid extraction of lactic acid from aqueous solution with tri-n-octylamine in xylene were studied using a microporous membrane-based stirred cell. It was shown that this operation could give intrinsic rate equations for the formation and dissociation of the acid-amine complexes. A possible reaction mechanism was also proposed. In addition, the effect of temperature on the reaction rates was studied and the activation energies were obtained. Finally, the results of transport parameters and reaction kinetics indicated this extraction process (either forward or backward) to be predominantly controlled by interfacial chemical reactions, under the conditions investigated.     

Developing high-quality g-C3N4 film electrode for the photoelectrocatalytic degradation of methylene blue in water

Developing a high-quality photoelectrode for photoelectrochemical applications is still an ongoing challenge. In this study, we prepared the g-C₃N₄ film on the indium tin oxide (ITO) glass through conventional coating, liquid-based growth, in-situ calcination, and vapor deposition methods, respectively. These electrodes were characterized and used as photoanodes to degrade methylene blue (MB) in water. Among these methods, the in-situ calcination method was most appropriate for preparing the continuous and organized g-C₃N₄ film electrodes with uniform g-C₃N₄ coverage and strong adhesion to the ITO substrate. It also had the highest activity in the photocatalytic (PC), electrochemical (EC), and photoelectrocatalytic (PEC) degradation processes of MB. In the PEC reaction, at an applied potential of 1.0 V and a light intensity of 0.96 W/cm², the removal rate of MB was 62.5%, which was much higher than those in the PC and EC reactions. The high degradation rate was due to the synergistic effect of PEC degradation, wherein the PC and EC reactions promote and optimize each other. In the PC reaction, MB was degraded by ?CH₃ elimination, while the EC degradation pathway mainly included the conversion of sulfhydryl into sulfoxide and the opening of the central aromatic ring. Both methyl loss and aromatic ring opening occurred in the PEC reaction. Moreover, some monocyclic compounds were formed, and MB showed more complete degradation in the PEC reaction.     

Thermal degradation of bis(azidomethyl)oxethane oligomer

The kinetics, composition of products, and mechanism of thermal degradation of the bis(azidomethyl)oxethane oligomer in different media (vacuum, argon, air) in the temperature range 150 to 210°C has been studied by thermogravimetry, volumetry, IR spectroscopy, and mass spectrometry. It has been shown that the kinetic parameters of degradation are practically independent of the medium wherein the degradation proceeds. The process obeys the first-order kinetics and may be described by the Arrhenius equation k = 10^{14.1 ± 1.1}exp[?161 400 ± 4600)/RT] s^?1. Mass spectrometric analysis provides evidence that nitrogen prevails in gaseous reaction products. A comparison of the IR spectra of the initial oligomer and the solid residue of its thermolysis testifies that azide groups are destroyed, while the backbones of polymer heterochains remain intact. The closeness of kinetic and activation parameters for the reaction under study to those available for aliphatic azides, the data on the composition of gases being formed, and the structure of the solid residue of thermolysis allow one to suggest that the mechanism of oligomer thermal degradation involves elimination of a nitrogen molecule in the rate-determining stage of the process and subsequent crosslinking of macrochains.     

New domino approach for the synthesis of 2,3-disubstituted benzo[b]furans via copper-catalyzed multi-component coupling reactions followed by cyclization

An efficient domino process for the construction of 2,3-disubstituted benzo[b]furans has been developed via copper-catalyzed three-component coupling reactions of salicylaldehydes, amines, and alkynes followed by base-assisted O-annulation reaction.     

Predictions of Enzymatic Parameters: A Mini-Review with Focus on Enzymes for Biofuel

Enzymatic reactions are very basic processes in biological systems, and parameters related to enzymatic reactions always provide good indicators for understanding of mechanisms underlined in enzymatic reactions, for better controlling of enzymatic reactions, and for comparison of different enzymes. In this mini-review: first, parameters in enzymatic reactions were briefly reviewed from three different standpoints; second, predictions of parameters in enzymatic reactions without information on enzyme structure were shortly reviewed from viewpoints of geometric approach, graphic approach and compartmental approach; third, predictions of parameters in enzymatic reaction with information on enzyme structure were reviewed from the points of view of modeling, with 19 currently available databases, and 17 software packages and web servers; fourth, the current state of prediction on parameters in enzymatic reaction in biofuel industry with respect to cellulolytic enzymes were reviewed; fifth, the pros and cons for future development were discussed; and finally, a worked example was given in the Appendix to describe the whole procedures of prediction of enzymatic parameters in reactions.     

Catalytic arylsulfonyl radical-triggered 1,5-enyne-bicyclizations and hydrosulfonylation of α,β-conjugates

A catalytic bicyclization reaction of 1,5-enynes anchored by α,β-conjugates with arylsulfonyl radicals generated in situ from sulfonyl hydrazides has been established using TBAI (20 mol%) and Cu(OAc)₂ (5 mol%) as co-catalysts under convenient conditions. In addition, the use of benzoyl peroxide (BPO) as the oxidant and pivalic acid (PivOH) as an additive was proven to be necessary for this reaction. The reactions occurred through 5-exo-dig/6-endo-trig bicyclizations and homolytic aromatic substitution (HAS) cascade mechanisms to give benzo[b]fluorens regioselectively. A similar catalytic process was developed for the synthesis of γ-ketosulfones. These reactions feature readily accessible starting materials and simple one-pot operation.     

An old story with new insight into the structural transformation and radical production of micron-scale zero-valent iron on successive reactivities

It is generally recognized that the formation and accumulation of iron oxides on the surface of zero-valent iron (Fe⁰) resulting in significant decrease of contaminant degradation rates during the long-term reactions. However, in this study, we found that the removal efficiencies of p-nitrophenol (PNP) by micro zero-valent iron (mFe⁰) could maintain at the satisfactory level in the process of continuous reactions (20 cycles). The removal rate constant (0.1779 min⁻¹) of the 5^th cycle was 6.74 times higher than that of the 1^st reaction (0.0264 min⁻¹), even the 20^th cycle (0.0371 min⁻¹) was higher than that of the 1^st reaction. Interestingly, almost no dissolved iron was detected in the solution, and the total iron concentrations decreased dramatically with the process of continuous reactions. The results of scanning electron microscope and energy dispersive spectrometry (SEM-EDS) and X-ray diffraction (XRD) revealed that the structure and composition of corrosion products change from amorphous to highly crystal with the increase of the number of cycles. The corrosion products were mainly magnetite (Fe₃O₄) and a small part of maghemite (γ-Fe₂O₃), which were in the form of microspheres on the surface of mFe⁰. The formation of surface oxidation shell hindered the release of Fe²⁺. X-ray photoelectron spectroscopy (XPS) results illustrated that partial Fe₃O₄ could be converted into γ-Fe₂O₃. Electrochemical analysis proved that the electron transfer rate of mFe⁰ increased with the formation of the oxides shell. However, the consumption of iron core and thicker oxide film weakened the electron transfer rate. Besides, the quenching experiments indicated that the reaction activity of mFe⁰ could be enhanced with the addition of scavengers. This study deepened the understanding of the structural transformation and radical production of mFe⁰ in continuous reactions.     

Synthesis of Polyheterocyclic Dimers Containing Restricted and Constrained Peptidomimetics via IMCR-Based Domino/Double CuAAC Click Strategy

A novel strategy via the triple process (multicomponent reactions (MCR)-domino)/tandem was developed for the synthesis of restricted and constrained bis-1,2,3-triazole-linked pyrrolo[3,4-b]pyridine peptidomimetics dimers in overall yields of 20–55%. This strategy allows the construction of six heterocycles in two stages of the reaction.     

Lumped kinetic model for degradation of chitosan by hydrodynamic cavitation

The lumped kinetic model for the degradation reactions of chitosan by hydrodynamic cavitation was investigated in this study. The molecular weight distributions and mass concentrations were determined by gel permeation chromatograph (GPC). The degradation products were divided into several lumps according to their molecular weights. The appropriate number of lumps for the kinetic study was determined by comparing the residual sum of squares (RSS) values among different models. The RSS of six-lumped model (5.36 × 10^?3) was relatively small and the estimation of parameters was simpler than other models. In this paper, six-lumped kinetic model was adopted and the corresponding reaction network was established. The kinetic parameters were estimated with the Levenberg-Marquardt, and the results showed that the degradation reaction of chitosan was mainly dominated by the formation of degradation products with similar molecular weights. The maximum value of kinetic parameters on the diagonal of the reaction network (1.63 × 10^?1) was much larger than that of non-diagonal kinetic parameters (3.78 × 10^?2). According to the results calculated and measured under different conditions, this model could accurately predict the concentration distributions of lumps in the reaction network, and most of the average absolute deviation were smaller than 9.3%.     

Applications of differential scanning calorimetry to enzyme kinetics

Differential scanning calorimetry has been investigated for its application to the experimental study of the temperature dependence of the kinetics of enzyme—substrate reactions. A reaction cell was developed which allows mixing of enzyme and substrate solutions directly in the calorimeter. The device was used to study the zero-order reaction of acetylcholinesterase with acetylcholine and the first-order reaction of α-chymotrypsin with N-acetyl-l-alanine methyl ester. The reaction cell is found to be satisfactory in the isothermal mode for both first- and zero-order reactions and in the scanning mode for the zero-order reactions but not for the first-order reaction. Limitations of the design are described for general enzyme kinetic studies.