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51.
Modeling and simulation of free radical polymerization of styrene under semibatch reactor conditions
The first part of this approach is concerned with the elaboration of a radical polymerization model of styrenne, based on
a kinetic diagram that includes chemical and thermal initiation, propagation, termination by recombination and chain transfer
to the monomer. Furthermore, volume contraction during polymerization is considered, as well as the gel and glass effects.
The mathematical formalism that describes the model in terms of moments is explored in detail. The model was then used to
predict the changes in monomer conversion and molecular weight after intermediate addition of initiator and monomer. The results
of this operation are dependent on the conditions of the reaction mass, quantity, and moment of substance addition. Therefore,
the simulations were performed at different times with respect to the gel effect; before, during and after this phenomenon,
and also with respect to different temperatures and initiators. Increasing the initiator concentration before the gel effect
leads to an earlier appearance of the phenomenon and to a decrease in molecular weight. The ratio
reveals a polydispersity index smaller for the intermediate addition of initiator. No significant changes take place during
or after the gel effect. If along with the initiator, unreacted monomver (used to dissolve the initiator) enters the reactor,
a small dip in conversion is observed. The general conclusion of this paper reveals the intermediate addition of initiator
as a method to control polymer properties and to prevent the “dead-end” polymerization of styrene. 相似文献
52.
Toshio Nishi Hideyuki Nukaga So Fujinami Ken Nakajima 《高分子科学》2007,(1):35-41
Atomic force microscopy (AFM) has the advantage of obtaining mechanical properties as well as topographic information at the same time. By analyzing force-distance curves measured over two-dimensional area using Hertzian contact mechanics, Young's modulus mapping was obtained with nanometer-scale resolution. Furthermore, the sample deformation by the force exerted was also estimated from the force-distance curve analyses. We could thus reconstruct a real topographic image by incorporating apparent topographic image with deformation image. We applied this method to carbon black reinforced natural rubber to obtain Young's modulus distribution image together with reconstructed real topographic image. Then we were able to recognize three regions; rubber matrix, carbon black (or bound rubber) and intermediate regions. Though the existence of these regions had been investigated by pulsed nuclear magnetic resonance, this paper would be the first to report on the quantitative evaluation of the interfacial region in real space. 相似文献
53.
CHEN Kai ZHU Zhong-liang ZHANG De-jun 《高等学校化学研究》2006,22(4):435-438
IntroductionStudies on reversible kinetic systems are consi-dered as a hotspot of chemical and biochemical kineticresearches[1,2]. Of late, some researches have been fo-cused on the simultaneous optimization of the obverseand reverse rate constants[3,4].H… 相似文献
54.
Jian‐Hua Zhao Hsuan‐Liang Liu Hsin‐Yi Lin Chih‐Hung Huang Hsu‐Wei Fang Shau‐Wei Tsai 《中国化学会会志》2007,54(4):835-842
In this study, we present the results from the semiempirical molecular orbital calculations for the acylation step in the lipase‐catalyzed ester hydrolysis. The results reveal that the lowest energy path for the formation of the tetrahedral intermediate is for the serine residue of the catalytic triad to attack the substrate, followed by coupling heavy atom movement and proton transfer. The calculations of four active site models show that the cooperation of the aspartate group and the oxyanion hole is capable of lowering the activation energy by about 16 kcalmol?1. Our results further suggest that the lipase‐catalyzed ester hydrolysis adopts the single proton transfer mechanism. 相似文献
55.
56.
Both the spectra and infrared transition strengths of C6H6 and C6D6 for the C? H stretching overtones up to as high as v = 10 are described in high precision with few parameters (six for the spectra and four for the transition strengths) by the Iachello–Oss algebraic model. The Hamiltonian model is solved in the symmetry adapted bases, which are constructed by the symmetrized boson representation (SBR) technique. The results show that the combination of the algebraic method and SBR technique is a powerful method for describing vibrations of large molecules and high overtones. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
57.
The kinetics of oxidation of ethyldigol by vanadium(V) in aqueous acidic medium has been carried out. The reaction is first order with respect to vanadium(V) and the substrate and is acid catalysed.Hammett acidity function (H
0) andBunnett hypothesis have been applied. The formation of free radicals during the course of the reaction has been indicated. A probable reaction mechanism is proposed.
Die Kinetik der Oxidation von Ethyldigol mit Vanadium(V) in wäßrigem saurem Medium
Zusammenfassung Es wurde die Kinetik der Oxidation von Ethyldigol mittels Vanadium(V) in wäßriger saurer Lösung untersucht. Die Reaktion ist erster Ordnung bezüglich Vanadium(V) und Substrat und ist säurekatalysiert. Es wurden dieHammett-Aciditätsfunktion (H 0) und dieBunnett-Hypothese angewandt. Die Bildung von freien Radikalen während der Reaktion konnte bestätigt werden. Es wird ein Reaktionsmechanismus vorgeschlagen.相似文献
58.
Satoaki OnitsukaHiroshi Nishino 《Tetrahedron》2003,59(6):755-765
The BF3-catalyzed cyclization of 3-acetyl-1-aryl-2-pentene-1,4-diones 1a-e in the presence of water in boiling tetrahydrofuran gave bis(3-acetyl-5-aryl-2-furyl)methanes 2a-e in 26-79% yields along with a small amount of 3-acetyl-5-aryl-2-methylfurans 3a-e. The exact structure of 2a was determined by X-ray crystallography. The use of a half volume of the solvent for the reaction of 1a resulted in the formation of 2,4-bis(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-phenylfuran (4) together with 2a and 3a. A similar reaction of 1a was carried out in the presence of 3-acetyl-5-(4-methylphenyl)-2-methylfuran (3d) to afford 4-(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-(4-methylphenyl)-2-methylfuran (5) in 49% yield. The BF3-catalyzed reaction of 1a with 2,4-pentanedione in dry tetrahydrofuran at 23°C gave 3-(3-acetyl-5-phenyl-2-furfuryl)-4-hydroxy-3-penten-2-one (6a) and 3-(3-acetyl-2-methyl-4-phenyl-5-furyl)-4-hydroxy-3-penten-2-one (7a) in 66 and 24% yields, respectively. The product distribution depended on the reaction temperature. A similar reaction of 1b-e also yielded the corresponding trisubstituted furans 6b-e and tetrasubstituted furans 7b-e in good yields. These results suggested the presence of the furfuryl carbocation intermediate A during the reaction. The one-pot synthesis of 6a and 7a was also achieved by a similar reaction using phenylglyoxal. The deoxygenation of 1a with triphenylphosphine gave 3a in 88% yield, while 1a was treated with concentrated hydrochloric acid to yield 3-acetyl-2-chloromethyl-5-phenylfuran (8) which was quantitatively transformed in ethanol into 3-acetyl-2-ethoxymethyl-5-phenylfuran (9) and in water into 3-acetyl-5-phenylfurfuryl alcohol (10), respectively. In addition, the Diels-Alder reaction of cyclopantadiene with 1a gave the corresponding [4+2] cycloaddition products 11 and 12. 相似文献
59.
Paola Maggiorotti 《Journal of Thermal Analysis and Calorimetry》1992,38(12):2749-2758
The RC1 calorimeter revealed itself a suitable instrument to obtain information about safety and mechanisms involved in the reaction between cyclohexanecarboxylic acid (AEB) and oleum. A previous hypothesis about the existence of an unstable intermediate was confirmed and its heat of formation was calculated. The heat of sulphonation related to undesirable by-products production and the heat of protonation of AEB with H2SO4 were also evaluated. Therefore, it was possible to distinguish the reactions involved in the process and, through their thermal behaviour, to determine the limit conditions to avoid the by-products formation.
Zusammenfassung Das RC1 Kalorimeter erwies sich als ein geeignetes Instrument, um Informationen über Sicherheit und Mechanismen bei der Reaktion von Cyclohexancarbonsäure (AEB) und Oleum zu gewinnen. Eine bereits bestehende Hypothese über die Existenz eines instabilen Zwischenproduktes konnte bestätigt und dessen Bildungswärme berechnet werden. Weiterhin wurde auch die Sulphonierungswärme bezogen auf die Entstehung von unerwünschten Nebenprodukten und die Protonierungswärme von AEB durch H2SO4 ermittelt. Es ist deshalb möglich, die einzelnen Reaktionen innerhalb dieses Prozesses über ihr thermisches Verhalten voneinander zu unterscheiden, um die Grenzbedingungen zur Vermei-dung der Bildung von Nebenprodukten zu ermitteln.相似文献
60.
The dissolution of zinc in 0.48–1.49M HNO3 was studied at 15–25°C, by following simultaneously the concentration changes of the reactants (Zn and HNO3), intermediate (HNO2) and product (Zn2+) with time. Explicit mechanisms were suggested for the dissolution of zinc in nitric acid. The kinetics of the dissolution process show that it is of the first-order with respect to [Zn] and [HNO2]. The data obtained show that the dissolution process is diffusion-controlled. The mechanism of zinc dissolution is compared with the mechanism of copper dissolution.
Die Kinetik der Auflösung von Zink in Salpetersäure
Zusammenfassung Die Auflösung von Zink in 0.48–1.49M HNO3 wurde bei 15–25°C mittels gleichzeitiger Verfolgung der Konzentrationsänderungen der Reaktanden (Zn und HNO3), des intermediären HNO2 und des Produkts Zn2+ untersucht. Es wird ein Mechanismus vorgeschlagen. Die Kinetik der Auflösung ist erster Ordnung bezüglich [Zn] und [HNO2]. Die Daten zeigen, daß der Auflösungsvorgang diffusionskontrolliert ist. Der Mechanismus der Auflösung von Zink wird mit dem der Kupferauflösung verglichen.相似文献