85461-52-5Relevant articles and documents
Polycondensation and disproportionation of an oligosiloxanol in the presence of a superbase
Grzelka, Agnieszka,Chojnowski, Julian,Cypryk, Marek,Fortuniak, Witold,Hupfield, Peter C.,Taylor, Richard G.
, p. 14 - 26 (2007/10/03)
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.
Disproportionation of oligodimethylsiloxanols in the presence of a protic acid in dioxane
Cypryk, M.,Rubinsztajn, S.,Chojnowski, J.
, p. 91 - 97 (2007/10/02)
The kinetics of the acid-catalyzed disproportionation of α,ω-dihydroxyoligodimethylsiloxanes HOnH, n = 2, 5, and their analogues having one hydroxyl function replaced by methyl MenH, n = 2, 5, were studied in dioxane in the presence of water.The formation of the primary disproportionation products was monitored by gas-liquid chromatography.The reaction in dioxane solution can compete with the condensation process only in the presence of water.It is of first order with respect to the substrate, catalyst, and water.The results are interpreted in terms of a stepwise mechanism involving rate determining cleavage of the terminal siloxane unit by water, followed by condensation of transiently-formed dimethylsilanediol with the substrate.The unzipping mechansim involving the dimethylsilanediol intermediate is suggested to operate in hydrolytic cleavage of polydimethylsiloxane exposed to water or water vapour.
