An Overview of the Polymerization of Cyclosiloxanes

Abstract

A review of the polymerization of cycloorgano siloxanes is provided. Cycloorganosiloxanes such as octamethyl cyclotetrasiloxane are the principal intermediate for the formation of high molecular weight polyorganosiloxane polymers. The ring open ing reaction can be initiated through the use of suitable basic or acidic catalysts. The transforma tion 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 pre dominantly 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 polymeri zation 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 as pects are reviewed. Molecular weight is often con trolled 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 0097-6156/83/0212-0145$08.00/0

Silicone containing copolymers: Synthesis, properties and applications

“…As shown in Figure 8, a very popular and simple approach isthe acid or base catalyzed ring-chain equilibration or redistribution reactions of low molecular weight α,ω-organofunctionally terminated silicones, preferably a disiloxane as shown in the reaction scheme, which is usually termed as the "end-blocker"and cyclic silicone monomers such as octamethylcyclotetrasiloxane (D 4 ) or others (3,10,13,14).Partially ionic character of the (Si−O) bond is the main rationale behind the equilibration reactions. During these ringchain equilibration reactions the average molecular weight of the oligomer to be synthesized is controlled by ratio of the end-blocker to the cyclic monomer in the reaction mixture (3,10,13,14). Strong acids such as sulfuric acid, trichloroacetic acid and sulfonic acids and strong bases such as sodium and potassium hydroxide or quaternary ammonium hydroxides can be used as catalysts in the equilibration reactions.…”

“…Therefore, at the end of the reactions linear oligomers are functionally terminated and cyclic side products, which are around 10-15% by weight depending on the number average molecular weight of the oligomer prepared (3,13,14), are non-functional. At the end of ringchain equilibration reactions, the catalyst is deactivated and the cyclic side products are removed from the reaction mixture by vacuum distillation.…”

Physical aging of linear and network epoxy resins

“…This rubber was prepared by equilibrating the disiloxane (n~l) with the cyclic tetramer. Details of the synthesis will be reported separately (13).…”

Library synthesis and characterization of 3‐aminopropyl‐terminated poly(dimethylsiloxane)s and poly(ϵ‐caprolactone)‐b‐poly(dimethylsiloxane)s

“…These interesting properties result in PDMS having a wide range of applications spanning from electrical insulators to biomaterials 1–3. Organofunctional PDMS oligomers can be prepared by the acid‐ or base‐catalyzed ring‐opening polymerization of a cyclic tetramer, octamethylcyclotetrasiloxane (D 4 ), in the presence of a disiloxane or low‐molecular‐weight oligomer having the desired end groups with several different catalysts depending on the desired molecular weight, purity, cost, and ease of application 4–11…”