Ring-Opening Polymerization of Cyclosiloxanes

The direct process is also quite versatile and can be fine-tuned to prepare other types of chlorosilanes also. Ethylchlorosilanes could also be similarly prepared analogous to the synthesis of methylchlorosilanes. Preparation of phenylchlorosilanes required a slight modification of the catalyst. Addition of a mixture of HCl and MeCl to silicon affords mixtures of methylchlorosilanes along with MeSiHCU- Simultaneously, along with Rochow, but independently, Richard Mueller in Germany had also come out with a direct process, initially for preparing HSiCls, and later for methylchlorosilanes [7]. 

The discovery of the direct process of the synthesis of organochlorosi-lanes took these compounds from the realm of laboratory curiosity to commercially important materials. The chemistry of the polymerization methods for the assembly of polysiloxanes is discussed in the subsequent sections. Polysiloxanes are synthesized by two prineipal methods (a) ringopening polymerization of cyclosiloxanes and (b) condensation polymerization involving a hydrolysis/condensation reaction of diorganodichlorosi-lanes or condensation reaction between two difunctional diorganosilanes. 

---

Chojnowski, J. Ring-Opening Polymerization of Cyclosiloxanes. In Gelest Catalog Arkles, B., Larson, G., Eds. Gelest Inc. Morrisville, PA, 2004, pp 389-105. 

Boileau and coworkers11 have used a novel trimethylsilylmethyl lithium initiator MesSiCTDLi (1), in combination with a cryptand [211], for the ring-opening polymerization of cyclosiloxanes. Initiation of hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) polymerization has been followed by H, 7Li, 13C and 29Si NMR. 

Sormani PM, McGrath JE (1985) Kinetics and mechanisms of the anionic ring opening polymerization of cyclosiloxanes in the presence of bis(l,3-aminopropyl tetramethyl-disiloxane) In McGrath JE (ed) Ring opening polymerization kinetics mechanisms and synthesis. ACS Symposium Series No 286... 

The reactions of vinylcyclosiloxanes and vinylcyclosilazanes with styrene in the presence of RuHCl(CO)(PCy3)2 opened a new route to functionalized monomers for the ring-opening polymerization of cyclosiloxanes and cyclo-silazanes (Eq. 29) [56]. 

Mechanism of Equilibration. The generally accepted mechanism for the base-catalyzed ring-opening polymerization of cyclosiloxanes involves attack of the basic catalyst at the silicon atom (15). It has been proposed, and generally accepted, that the active species is a partially dissociated siloxanolate anion (13). In the results presented in this chapter, significant differences in reaction rates were observed as the corresponding cation of the siloxanolate species was varied. The more rapid disappearance of D4 and aminopropyldisiloxane in the presence of these catalysts increased in the following order ... 

Summary Phosfdiazene bases represent a new class of highly active non-ionic catalysts that rapidly polymerize cyclosiloxanes with equilibrium attained in very short reaction times at very low catalyst levels. To date, phosphazene base catalysts have been considered an academic curiosity because of the complicated and hazardous synthetic protocol used to prepare them. A facile synthetic process has been developed, which yields ionic phosphazene bases in three steps with an overall yield of qrproximately 75%. This is achieved through nucleophilic substitution of ionic phosphonitrilic chloride oligomers with secondary amines, followed by anion exchange. These ionic phosphazenes were found to exhibit similar reactivity in the ring-opening polymerization of cyclosiloxanes to that of the non-ionic phosphazene base. 

Summary PDMS-6-PEO short-chain diblock copolymers were prepared via anionic ring-opening polymerization of cyclosiloxanes. Applying this method, various well-defined block copolymers with different compositions were synthesized and their phase behavior was investigated. The polymers predominantly showed lamellar phases in aqueous solutions. At small surfactant concentrations, vesicles were formed, as observed via cryogenic TEM. The aggregates of the diblock copolymers were used for the formation of lamellar thin films, applying the evaporation-induced self-assembly approach. 

Molenberg, A. Moller, M., A Fast Catalyst System for the Ring-Opening Polymerization of Cyclosiloxanes. Macromol. Rapid Commun. 1995,16,449-453. 

Ring-opening Polymerization of Cyclosiloxanes by Ionic Initiators... 

The ring-opening polymerization of cyclosiloxanes is a complex equilibrium polymerization and detailed studies have shown that a series of competing reactions exist that allow formation of cyclic as well as pol)mieric products. The overall polymerization is a chain-growth polymerization. 

Ring-opening polymerization of cyclosiloxanes can be carried out by either cationic or anionic initiators. 

---