Siloxanes cyclotrisiloxanes

Figure 12.14 Pyrogram of the silicone coating from Mello s Tavolo erba. Peak assignments I, hexamethyi cyclotrisiloxane 2, octamethyl cyclotetrasiloxane 3, decamethyl cyiclopenta siloxane 4, dodecamethyl cyclohexasiloxane 5, tetradecamethyl cyclooctasiloxane...

Chojnowski, J. Kazmierski, K. Cypryk, M. Fortuniak, W. Cationic Ring Opening Polymerization of Cyclotrisiloxanes with Mixed Siloxane Units. In Silicones and Silicone-Modified Materials-, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. ACS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 20-37. 

The anionic ring-opening copolymerization of hexamethyl-cyclotrisiloxane (Dj) with 2,4,6-trivinyl-2,4,6-trimethyl-cyclotrisiloxane (Vj) was performed under dry Nj atmosphere in a glass ampoule equipped with a teflon stopcock. All other reactions, except the preparation of Pt(0)-[poly(vinylmethyl-co-dimethyl)siloxane]-carbosilane complexes, were performed using standard Schlenk s or syringe techniques under an atmosphere of argon. 

Keywords polysiloxanes, cyclotrisiloxanes, anionic polymerization siloxane copolymers, block copolymers, dendritic polymers... 

All siloxane block copolymers may be synthesized by the sequential copolymerization of two cyclotrisiloxane monomers. For example, unsubstituted hexamethyicyclotrisiloxane, D3, was first polymerized, and, before full conversion of this monomer, 2,4,6-trivinyltrimetiiylcyclotrisiloxane, V3, was added to the polymerization system [11]. The obtained block copolymer showed a high topological purity and a low polydispersion, MJM = 1.1. 

Anionic polymerization of functionalized cyclotrisiloxanes is a good method for the synthesis of well-defined functionalized polysiloxane with control of molecular mass, polydispersion, and density and arrangement of functional groups. These polymers may serve as reactive blocks for the building of macromolecular architectures, such as all-siloxane and organic-siloxane block and graft copolymers, star-, comb- and dendritic-branched copolymers and various polysiloxane-inorganic solid hybrids. 

Most of the compounds generated from the pyrolysis of poly(dimethyl-siloxane-co-methylphenylsiloxane) are cyclosiloxanes with methyl or phenyl substituents. They are generated by a similar mechanism as shown for the formation of hexamethyl-cyclotrisiloxane during the pyrolysis of poly(dimethyl siloxane). 

By the same token, kryptated potassium hydroxide initiates polymerization of hexamethyl-cyclotrisiloxane in benzene but not in polar solvent. About 10 4 M solution of kryptated KOH converts in about five minutes 60% of 5 10 2 M siloxane into its polymer. Other examples are given in Ref. 116. 

Cyclic polysiloxanes also absorb in this region, with a characteristic cyclotrisiloxane band at 9.8 /z281. Cyclic tetramers and pentamers have a single band at about 9.2// which gradually widens and splits with larger rings284. The positions of these bands are essentially independent of the other silicon substituents. The shift of the siloxane band to... 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

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