An Overview of the Polymerization of Cyclosiloxanes

Abstract

A review of the polymerization of cycloorganosiloxanes is provided. Cycloorganosiloxanes such as octamethyl cyclotetrasiloxane are the principal intermediate for the formation of high molecular weight polyorganosiloxane polymers. The ring opening reaction can be initiated through the use of suitable basic or acidic catalysts. The transformation of the cyclosiloxanes into linear chains is an equilibrium process characterized by a very low heat of polymerization. At equilibrium conversions one produces 12-15% of cyclic oligomers, which are predominantly but not exclusively the cyclic tetramer. Equations from the literature which define these equilibrium are reviewed. The principal mechanisms involved in the anionic ring opening polymerization via initiators such as potassium hydroxide are discussed. A host of other related initiator types have also been used, including the so-called transient catalysts which are based upon quaternary silanolates. The transient catalysts are so described since they rapidly decompose above 130°C to yield inactive byproducts. The anionic polymerization of siloxanes is relatively well understood and currently accepted mechanisms are presented. Cationic polymerization of organosiloxanes is also well known but is much less understood. In general, molecular weight vs. conversion curves for the two processes are considerably different and these aspects are reviewed. Molecular weight is often controlled by disiloxane or low molecular weight siloxane molecules terminated with triorganosiloxy groups. These materials (which are known as end blockers) control the molecular weight due to the fact that their silicon-oxygen bonds can exchange with and incorporate the growing chain. By contrast, the silicon-carbon bonds in such materials are incapable of reacting. Thus at equilibrium one incorporates most of the linear chains between the two triorganosiloxy terminals. Such a process is possible with either anionic or cationic intermediates. This approach has recently permitted the introduction of functional groups such as aminopropyl, carboxy propyl, epoxy propyl, etc. Such oligomers are interesting intermediates for novel segmented copolymers.