- Polycondensation and disproportionation of an oligosiloxanol in the presence of a superbase
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Kinetics of reactions of model oligosiloxanols, 1,1,3,3,3-pentamethyldisiloxane-1-ol (MDH) and 1,1,3,3,5,5,5-heptamethyltrisiloxane-1-ol (MD2H), which occur in the presence of phosphazenium superbase, hexapyrrolidine-diphosphazenium hydroxide, in an acid-base inert solvent, toluene, was studied using sampling and gas chromatographic analysis method. In addition, kinetics of reactions of MDH and MD2H with trimethylsilanol (MH) was studied. In the MDH and MD2H systems the oligosiloxanol condensation competes with the oligosiloxanol disproportionation, the latter being the dominating process. The disproportionation products, i.e. MDn+1H and MDn-1H, n=1, 2, ? undergo analogous consecutive disproportionation and condensation reactions. The kinetic law was derived and rate parameters determined from initial rates and by computer simulation to the best agreement with experimental data. Both competing reactions, the disproportionation and the condensation, conform to the same general kinetic law being first internal order in substrate and first order in catalyst. Activation parameters of these reactions were determined. The results were interpreted in terms of a bimolecular mechanism in which nucleophilic attack of the silanolate anion directed to silicon of the silanol group causes the cleavage of one of its geminal bonds to oxygen, either the one to hydroxyl leading to condensation or the one to siloxane which leads to disproportionation. The latter is faster as the silanolate is a better leaving group compared with OH-. Moreover, in the pentacoordinate silicon transition state (or intermediate) the siloxane substituent preferentially enters the apical position, thus driving the OH substituent into the unreactive equatorial position.
- Grzelka, Agnieszka,Chojnowski, Julian,Cypryk, Marek,Fortuniak, Witold,Hupfield, Peter C.,Taylor, Richard G.
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- The hydroxyl radical reaction rate constants and atmospheric reaction products of three siloxanes
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The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of hexamethyldisiloxane (MM, (CH3)3Si-O-Si(CH3)3), octamethyltrisiloxane (MDM, (CH3)3Si-O-Si(CH3)2-O-Si(CH 3)3), and decamethyltetrasiloxane (MD2M, (CH3)3Si-O-Si(CH3)2-O-Si(CH 3)2-O-Si(CH3)3). Hexamethyldisiloxane, octamethyltrisiloxane, and decamethyltetrasiloxane react with OH with bimolecular rate constants of 1.32 ± 0.05 × 10-12 cm3molecule-1s-1, 1.83 ± 0.09 × 10-12 cm3 molecule-1, and 2.66 ±0.13 × 10-12 cm3molecule-1s-1, respectively. Investigation of the OH + siloxane reaction products yielded trimethylsilanol, pentamethyldisiloxanol, heptamethyltetrasiloxanol, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, and other compounds. Several of these products have not been reported before because these siloxanes and the proposed reaction mechanisms yielding these products are complicated. Some unusual cyclic siloxane products were observed and their formation pathways are discussed in light of current understanding of siloxane atmospheric chemistry.
- Markgraf, Stewart J.,Wells
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p. 445 - 451
(2007/10/03)
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- Kinetics and Products of the Gas-phase Reactions of (CH3)4Si, (CH3)3SiCH2OH, (CH3)3SiOSi(CH3)3 and (CD3)3SiOSi(CD3)3 with Cl Atoms and OH Radicals
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Direct air-sampling atmospheric-pressure ionisation tandem mass spectrometry and FTIR spectroscopy were used to analyse the products of the OH radical- and Cl atom-imitiated reactions of hexamethyldisiloxane, hexamethyldisiloxane, tetramethylsilane and trimethylsilylmethanol at room temperature and atmospheric pressure.The data obtained indicate the initial formation of (CH3)3SiOSi(CH3)2OCHO and (CD3)3SiOSi(CD3)2OCDO from hexamethyldisiloxane and hexamethyldisiloxane, respectively, and (CH3)3SiOCHO from both tetramethylsilane and trimethylsilylmethanol.
- Atkinson, Roger,Tuazon, Ernesto C.,Kwok, Eric S. C.,Arey, Janet,Aschmann, Sara M.,Bridier, Isabelle
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p. 3033 - 3040
(2007/10/03)
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- Reaction of O(3P) with Hexamethyldisilane
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The title reaction has been investigated at room temperature in a discharge flow system and also by stationary Hg(3P1)-sensitized N2O photolysis experiments.By far the most important reaction is the insertion of O(3P) atoms into the Si-Si bond yielding a vibrationally excited hexamethyldisiloxane.The rate constant for this reaction has been measured to be (1.3 +/- 0.3) x 10-13 cm3 s-1.The vibrationally excited hexamethyldisiloxane could not be stabilized at pressures up to 1 bar, decomposing mainly by Si-C bond cleavage.A reaction mechanism is proposed to account for the observed products.
- Hoffmeyer, H.,Potzinger, P.,Reimann, B.
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p. 4829 - 4831
(2007/10/02)
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