Silanol oligomers, synthesis

Host 24 also stabilises highly reactive cyclic silanol oligomers by a similar in situ synthesis [66]. Phenyltrimethoxysilane, 26, rapidly forms polymeric material in aqueous solutions. The presence of 24 prevents this from occurring and isolates the cyclic trimer intermediate, 27. Several molecules of the silane starting... 

M. Yoshizawa, T. Kusukawa, M. Fujita, S. Sakamoto, K. Yamaguchi, Cavity-directed synthesis of labile silanol oligomers within self-assembled coordination cages, J. Am. Chem. Soc., 2001, 123, 10454-10459. 

The synthesis of cyclic polydimethylsiloxane was first achieved through ring-chain equilibration of siloxane oligomers in the presence of potassium silanolate, as shown in Fig. 51 [163-165]. Cyclics recovered from ring-chain equilibration reactions have been fractionated by preparative GPC, yielding... 

Condensation reactions of silanols provides another method for polysiloxane synthesis. Hydroxy-terminated siloxane oligomers or polymers can be further condensed to polymers of higher molecular weight by condensation with loss of water (equation 23). This reaction is catalyzed by various metal salts tin salts have been widely used. Mixtures of two different a, co-dihydroxysiloxane oligomers may be condensed to give block copolymers. 

In contrast to the hydrolysis technology, the methanolysis process allows for the one-step synthesis of organosiloxane oHgomers and methyl chloride without formation of hydrochloric acid (64,65). The continuous methanolysis can also yield quantitatively linear silanol-stopped oligomers by recycle of the cycHc fraction into the hydrolysis loop. 

The synthesis of oligomeric peralkylated polyaminophosphazenes based upon the P3 and P4 templates is an elegant (but complicated), hazardous, and expensive multi-step process. To exploit catalysts of this type in silicone synthesis on an industrial scale would require a much simplified and lower-cost synthetic protocol. One of the simplest and lowest-cost routes to a conjugated -P=N- template that forms the framework for the synthesis of phosphazene base materials is via phosphonitrilic chloride oligomers (Fig. 3). These are well-known acidic catalysts used in the silicone industry for the condensation polymerization of silanol-terminated polydimethylsiloxanes. Catalysts of this type are most commonly prepared by the reaction of PCI5 with NH4CI or HMDZ,... 

Attempts to isolate 204 and 205 provide only the silanols 200 or 32. The sign and magnitude of the Si-NMR chemical shift is indicative of tetracoordinate silicon species [77], yet it does not match any of the previously synthesized tetracoordinate silanes. The possibility that 204 and 205 are oligomers was eliminated by a Si-NMR crossover experiment [30]. Structural possibilities that remained for 204 and 205 required that they be monomeric and tetracoordinate. The tetrabutylam-monium silyloxide salt ( )-206 and a TBAF hydrogen bond complex ( )-205 were proposed (Figure 7.2). The silyloxide salt is ruled out by independent synthesis of the corresponding tetramethylammonium salt, the Si-NMR chemical shift (—26.23 ppm) of which does not correspond to that for 206. Because compound... 

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