Silsesquioxane mechanism

Nanocomposite based on polyurethane (PU) is prepared using silica, clay, and Polyhedral Oligomeric Silsesquioxane (POSS). Preparation, characterization, mechanical and barrier properties, morphology, and effect of processing conditions have been reported on polyurethane-based nanocomposites [72,73]. 

Scheme 14.15 Possible grafting mechanism of[W(=Ar)(=CH Bu)(CH5Bu)j] on silica dehydroxylated at 700°C vs silica dehydroxylated at 200°C, proposed through analogy with silsesquioxane chemistry. Intermediates that have not been observed...

The mechanism of electron-impact elimination of the substituents of oligo(organyl-silsesquioxanes) and their homo derivatives is discussed in °- >. 

Methyl- or phenyl-substituted silsesquioxanes obtained by hydrolysis and condensation of RSi(OR )3 or RSiCb (R = Me, Ph) were studied extensively by several groups75. MeSi— and PhSi— units are the classical components of silicones. The molecular weight and structure of methyl- or phenyl-substituted silsesquioxane polymers, and thus their physical properties (solubility, mechanical properties, etc.), depend very much on the reaction conditions. Since silicones are not the topic of this article, the interested reader is referred to the relevant literature. 

Silsesquioxanes are usually synthesised by the hydrolytic condensation of orga-nosilanes RSiX3 (Scheme 9.1) this is a multiple-step reaction for which an overall mechanism is not available. 

To shed light on the mechanism of formation of silsesquioxane a7b3, to identify the species formed during the process, and to try to explain the high selectivity towards structure a7b3 of the optimised synthetic method described above (64% yield in 18 h), the synthesis of cyclopentyl silsesquioxane a7b3 was monitored by electrospray ionisation mass spectrometry (ESI MS) [50-52] and in situ attenuated total reflection Fourier-transform infrared (ATR FTIR) spectroscopy [53, 54]. Spectroscopic data from the latter were analysed using chemometric methods to identify the pure component spectra and relative concentration profiles. 

Fig. 9.10 Proposed mechanism for the synthesis of cyclopentyl silsesqui-oxane a7h3. The silsesquioxane structures are represented schematically. Each circle symbolises a siloxane unit [(c-C5H9)SiC>3] silicon atoms are represented by the circles and the oxygen atoms by the lines non-bridging lines represent -OH groups cyclopentyl groups not shown.

On the basis of all the information collected from this MS study, it is possible to propose a mechanism for the formation of silsesquioxane a7bi (Fig. 9.10). Silsesquioxanes with learly stages of the synthesis and react very quickly to form more condensed species. It is assumed that the very reactive a1 bi will not be formed again by hydrolysis reactions of more condensed species this means that the reactions in which albi takes part are effectively irreversible and that the compound will only be available for the reactions in the early stages of the synthesis. Silsesquioxanes with 2react with each other to form the more condensed silsesquioxanes with 4[Pg.226]

To obtain resin, the hot product of condensation is poured through the lower drain of condenser 18 to obtain varnish, the resin in condenser 18 is dissolved with ethyl alcohol, which self-flows into the apparatus from batch box 21. While the resin is dissolved, cooler 19 operates in the inverse mode. The obtained varnish is loaded by vacuum into settling box 22, where it is settled at ambient temperature for a long time (24-48 hours) to separate mechanical impurities. There is also a possibility for additional centrifuging in ultracentrifuge 23 for complete elimination of mechanical impurities, as well as clarification" of the varnish. The finished poly-methyl(phenyl)silsesquioxane varnish is sent from the centrifuge into container 24 and packaged. 

Polyhedral oligomeric silsesquioxanes (POSS) are known molecules having various structures [1] as seen in Fig. 1. However double-decker shaped silsesquioxane has not been obtained. POSS is an interesting class of monomers for the preparation of silicon-based polymers. In recent years, silsesquioxane-based polymers have received much attention because of their useful properties, which include heat resistance, mechanical stability, low dielectric constant, etc. [2, 3],... 

Previous reports on the reaction of vinyl-substituted silanes [9] and silsesquioxane [15] with vinyl allQrl ethers catalyzed by ruthenium complexes containing or generating Ru-H and/or Ru-Si bonds show that the process proceeds according to the nonmetallacarbene mechanism as in SC and yields (usually in 5-fold excess of alkene) l-silyl-2-alkoxy-ethenes with a high preference for the E-isomer (Eq. 1). 

On the other hand, GC/MS studies on the polycondensation of C2HsSiCl3 [11] suggest a two step mechanism, including rapid hydrolysis of the monomer and formation of linear, cyclic, and polycyclic products. The polyhedral silsesquioxanes are generated during a second and considerably slower step via cleavage and rebuilding from (strained) polysiloxane molecules. 

The observation of such an egg-shaped stmcture led us to an extension of the idea that such stmctures could be precursors of nanotubes. We therefore studied even larger silsesquioxanes with 72, 84, 96, 144, 192, and 240 silicon atoms in tube-like shapes.Figure 33.3 shows the three largest stmctures. We suggested a growth mechanism similar to the C2 insertion into fullerenes. 

Ethylene oxide/propylene oxide triblock copolymers have been successfully used to template pore formation in ultra low-k films, and dielectric constants as low as 1.5 have been observed with polymer loading levels of 50 wt% (5). These films have good mechanical properties, a high breakdown voltage and a low moisture uptake. We have characterized the films with high-resolution solid-state proton NMR and found that the triblock copolymers form nm-sized core-shell structures with the propylene oxide block at the interface between the ethylene oxide block and the methyl silsesquioxane matrix (14). 

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