Ring-Opening Polymerization of Siloxanes Using Phosphazene Base Catalysts

Abstract

Phosphazene bases such as {(NMe₂)₃P=N–)₃P=NBu^t} 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–NHBu^t]⁺[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.