Peter Cheshire Hupfield

Ring-Opening Polymerization of Siloxanes Using Phosphazene Base Catalysts

Phosphazene bases such as {(NMe2)3P=N–)3P=NBut} have been reported in the literature to be strongly basic materials with basicities up to 1×1018 times stronger than that of diazabicycloundecene (DBU) a strong hindered amine base used in organic reactions. A study of these phosphazene bases as catalysts revealed that they can be activated by small amounts of water, which all silicone feed stocks contain, to form an active ionic base catalyst [(NMe2)3P=N–)3P–NHBut]+[OH]−. This paper discusses the use of these types of base catalysts, and their analogues, as ring-opening polymerization catalysts for cyclosiloxanes. Phosphazene base catalysts can be used at low concentrations to make high molecular weight polydimethylsiloxanes with short reaction times over a wide temperature range. Molecular weight can easily be controlled in the presence of suitably functionalized endblockers. Water and carbon dioxide have been shown to have a significant impact on the polymerization rates. Polymers prepared show excellent thermal stability by thermogravimetric analysis (TGA), following neutralization of the catalyst, with decomposition onset temperatures >500°C in some cases. As a result of the extremely low levels of catalyst used, the polymers often do not require filtration.

Polycondensation and disproportionation of an oligosiloxanol in the presence of a superbase

Abstract 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.

The influence of perfluoropolyether silane structural modification on performance when used as surface treatment agents

SummaryImparting oil and water repellency on surfaces is easily achieved with the use of fluoroalkyl-modified silanes. Whilst perfluoroalkyl-modified silanes exhibit high static and advancing water and oil contact angles, their low receding contact angles mean that water and oil will not readily slide over treated surfaces. However, modified perfluoropolyether (PFPE) silanes, whilst exhibiting high static and advancing water and oil contact angles, also exhibit high receding contact angles, resulting in surfaces where the sliding contact angle is very low. The influence of structure modification of the silane and the perfluoropolyether component are examined along with the organic linking group. Their impact on performance using both the dip coating method and the chemical vapour adsorption method is discussed.