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Accelerating Effect (accelerating + effect)
Selected AbstractsThe Extraordinary Cocatalytic Action of Polymethylaluminoxane (MAO) in the Polymerization of Terminal Olefins by Metallocenes: Chemical Change in the Group 4 Metallocene Dimethyl Derivatives Induced by MAO,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2005John J. Eisch Abstract In the polymerization of olefins with Group 4 metallocene dichlorides or dimethyl derivatives as procatalysts the use of polymethylaluminoxane (MAO) as the cocatalyst, especially in extreme excess (102,103 times the metallocene equivalent), has been shown to have an extraordinary accelerating effect on the rate of olefin polymerization, when compared with the cocatalytic action of alkylaluminum halides. In attempts at explaining the greatly superior catalytic activity of MAO in olefin polymerization (the MAO conundrum), hypotheses have generally paralleled the steps involved in the cocatalytic action of RnAlCl3,n, namely the alkylation of Cp2MtCl2, ionization of Cp2Mt(R)Cl into the metallocenium cation, [Cp2Mt,R]+, and anion, [Rn,1AlCl4,n], and subsequent ion-pair separation. In order to understand any differences in catalytic action between such cocatalysts, we have studied the individual action of MAO (100 equiv.) and of MeAlCl2 (1,2 equiv.) on each of the Group 4 metallocene derivatives, Cp2TiCl2, Cp2ZrCl2, Cp2Ti(CH3)2 and Cp2Zr(CH3)2. With MeAlCl2 each of the metallocene derivatives appeared to form the cation, [Cp2Mt,CH3]+, with greater (Ti) or lesser (Zr) ease, because an alkyne such as diphenylacetylene was then found to insert into the Mt,CH3 bond stereoselectively. In striking contrast, treatment of each metallocene with MAO gave two reactions very different from MeAlCl2, namely a steady evolution of methane gas upon mixing and a finding upon hydrolytic workup that the diphenylacetylene present had undergone no insertion into the Mt,CH3 bond but instead had been reductively dimerized completely to (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene. To account for this astonishing difference in chemical behavior between MAO and MeAlCl2 in their cocatalytic activation of Group 4 metallocenes to olefin polymerization, it is necessary to postulate a novel, unique sequence of reaction steps occurring between MAO and the metallocene. If one starts with the metallocene dichloride, then the free TMA present in the MAO would generate the Cp2Mt(CH3)2. This metallocene dimethyl derivative, complexed with an oligomeric MAO unit, would undergo a transfer-epimetallation with added olefin or acetylene to form a metallacyclopropane or metallacyclopropene, respectively. With added diphenylacetylene the resulting 2,3-diphenylmetallacyclopropene would be expected rapidly to insert a second alkyne to form the 2,3,4,5-tetraphenyl-1-metallacyclopentadiene. Simple hydrolysis of the latter intermediate would generate (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene while alternative workup with D2O would give the 1,4-dideuterio derivative of this butadiene. Both such expectations were confirmed by experiment. In the case of added olefin, similar metallacyclopropane and metallacyclopentane intermediates should be produced until ring opening of the latter five-membered ring leads to an open-chain zwitterion, a process having ample precedent in the research of Gerhard Erker. The solution to the MAO conundrum then, namely the extraordinary cocatalytic activity of MAO in olefin polymerization by metallocenes, lies in the unique catalytic activation of the Group 4 metallocene dimethyl derivative, which occurs by transfer-epimetallation of the olefin monomer by the Cp2Mt(CH3)2,MAO complex. The most advantageous Lewis acidic sites in the MAO,oligomeric mixture for such metallocene,MAO complexation are suggested to be terminal Me2Al,O,AlMe, segments of an open-chain oligomer. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Mechanism of oxidation of alanine by chloroaurate(III) complexes in acid medium: Kinetics of the rate processesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2009Pratik K. Sen The kinetics of the oxidation of alanine by chloroaurate(III) complexes in acetate buffer medium has been investigated. The major oxidation product of alanine has been identified as acetaldehyde by 1H NMR spectroscopy. Under the experimental conditions, AuCl and AuCl3(OH), are the effective oxidizing species of gold(III). The reaction is first order with respect to Au(III) as well as alanine. The effects of H+ and Cl, on the second-order rate constant k2, have been analyzed, and accordingly the rate law has been deduced: k2, = (k1[H+][Cl,] + k3K4K5)/(K4K5 + [H+][Cl,]). Increasing dielectric constant of the medium has an accelerating effect on the reaction rate. Activation parameters associated with the overall reaction have been calculated. A mechanism involving the two effective oxidizing species of gold(III) and zwitterionic species of alanine, consistent with the rate law, has been proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 473,482, 2009 [source] Kinetics and mechanism of oxidation of antimony(III) by keggin-type 12-tungstocobaltate(III) in aqueous hydrochloric acid mediumINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2003B. D. Bhosale The reaction between Sb(III) and [CoIIIW12O40]5, proceeds with two, one-electron steps; formation of unstable Sb(IV) is the slow first step followed by its reaction with another oxidant in a fast step. The reaction rate is unaffected by the [H+] as there are no protonation equlibria involved with both the reactants, whereas the accelerating effect of chloride ion is due to the formation of an active chlorocomplex of the reductant, SbCl63,. Increase in the ionic strength and decrease in the relative permittivity of the medium increases the rate of the reaction, which is attributed to the formation of an outer-sphere complex between the reactants. The activation parameters were also determined and these values support the proposed mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 9,14, 2003 [source] Some novel accelerating agents for nitroxide-mediated living free-radical polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2005Huang Jianying Abstract Malononitrile (MN), trifluoroacetic acid anhydride, acetylacetone, acetoacetic ester, and diethyl malonate have been identified as novel rate-accelerating additives for nitroxide-mediated living free-radical polymerization. Among these additives, MN has the greatest accelerating effect. Adding MN at an MN/2,2,6,6-tetramethylpiperidine-oxyl (TEMPO) molar ratio of 4.0 results in a nearly 20 times higher rate of polymerization of styrene (St), and adding MN at an MN/TEMPO molar ratio of 2.5 results in a nearly 15 times higher rate of copolymerization of St and methyl methacrylate. The polymerization of St proceeds in a living fashion, as indicated by the increase in the molecular weight with time and conversion and the relatively low polydispersity. The polymerization rate of St is so quick that the conversion reaches 70% within 1 h at 125 °C when the molar ratio of MN to TEMPO is 4:1. Moreover, the reaction temperature can be reduced to 110 °C. A possible explanation for this effect is that the formation of hydrogen bonds between the MN and TEMPO moiety weakens the CON bond at the end of the polymer chain. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5246,5256, 2005 [source] Effect of Divalent Cation Additives on the ,-Al2O3 -to-,-Al2O3 Phase TransitionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2000Kiyoshi Okada The effect on the ,-Al2O3 -to-,-Al2O3 phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu2+, Mn2+, Co2+, Ni2+, Mg2+, Ca2+, Sr2+, and Ba2+, were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu2+ and Mn2+), (2) those with little or no effect (Co2+, Ni2+, and Mg2+), and (3) those with a retarding effect (Ca2+, Sr2+, and Ba2+). The crystalline phase formed by reaction of the additive with ,-Al2O3 at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al3+ and the additive (,r): The transition temperature increased as ,r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al3+ and O2, in ,-Al2O3, suppressing the grain growth of ,-Al2O3, and retarding the transformation into ,-Al2O3. [source] Octreotide enhances the accelerating effect of erythromycin on gastric emptying in healthy subjectsALIMENTARY PHARMACOLOGY & THERAPEUTICS, Issue 8 2002E. Athanasakis Summary Background : Erythromycin exhibits gastrokinetic properties through cholinergic pathways. Reports regarding the action of octreotide on gastric emptying are conflicting. Aim : To assess: (i) the hypothesis that serotonin receptors are involved in the accelerating effect of erythromycin on gastric emptying; and (ii) any modification of the gastrokinetic action of erythromycin induced by octreotide. Subjects and methods : Gastric emptying of a standard meal was estimated in 20 healthy subjects by scintigraphy on three different occasions in a double-blind, placebo-controlled manner and in random order: (i) after placebo; (ii) after 200 mg of intravenous erythromycin; and (iii) after 200 mg of intravenous erythromycin following pre-treatment with either 4 mg of intravenous ondansetron (10 subjects) or 50 µg octreotide. Results : Erythromycin significantly accelerated gastric emptying in all subjects by abolishing the lag phase. Pre-treatment with ondansetron abolished the accelerating effect of erythromycin by restoring the emptying times to placebo levels. Octreotide significantly enhanced the accelerating effect of erythromycin by reducing both the lag and post-lag phases of gastric emptying. Conclusions : Serotonin receptors are involved in the accelerating effect of erythromycin on gastric emptying. This effect seems to be enhanced by pre-treatment with octreotide, possibly as a result of the modification of the gastrointestinal hormonal environment. [source] | |||||||||||||