Siloxane bridges

Figure 1c shows the spectrum of aerosil that has been slurried in water and then dried at 100 C. This treatment initiates gel formation, so that the sample is no longer a chain of silica particles held together by electrostatic forces, but a porous network held together by siloxane linkages. The most obvious features in this spectrum are an increase in the water adsorption features at 3400 cm"" and 1632 cm". In addition, a band at 976 cm" is evident, that was much less obvious in the spectra of the other two silica samples. This feature is due to siloxane bridges formed during gel formation (10,12). 

Scheme 2 Different siloxane bridge structures formed upon dehydroxylation of silica surface. The increasing dimension of silicon rings and, consequently, of the Si - O - Si angle reflects a decreasing of the strain of these structures...

Scheme 2 is still oversimplified, because it does not take into consideration that the two silicon atoms directly involved in the hydroxyl condensation are also linked to other rings in a three-dimensional mode and that part of the surface strain could be localized on these rings. The appearance in the IR spectra of new vibrations in the 880-940 cm region, attributed to the modes of strained siloxane bridges in two membered rings [26,28-32], well evidences this fact. 

In the presence of H2, perhydrocarbyl surface complexes loose their ligands through the hydrogenolysis of their metal carbon bonds to generate putative hydride complexes, which further react with the neighbouring surface ligands, the adjacent siloxane bridges (Eqs. 8-9) [46,47]. 

The most common method of crosslinking via short bridges is a two-step process involving trimethoxysilane, which is shown in Fig. 18.9. In the first step, we graft siloxane branches onto polyethylene with the aid of peroxy radicals. The second step consists of a condensation reaction, which occurs in the presence of hot water or steam. A siloxane bridge is created and methanol is released. 

Shim and coworkers [320] synthesized cross-linked PF copolymers, containing siloxane bridges 221 and 222. Ni-mediated copolymerization of 9,9-dihexyl-2,7-dibromofluorene in... 

H.-J. Cho, B.-J. Jung, N.S. Cho, J. Lee, and H.-K. Shim, Synthesis and characterization of thermally stable blue light-emitting polyfluorenes containing siloxane bridges, Macromolecules, 36 6704—6710, 2003. 

A structural investigation of [Ta(=CH Bu)(CH2 Bu)3] grafted on siUca partially dehydroxylated at 700 °C using EXAFS [18] has revealed a short-range interaction of 2.64 between the O-atom from a siloxane bridge and tantalum. This O-atom acts as a two-electron donor ligand to stabiUze the formally ten-electron surface complex [(=SiO)Ta(=CH Bu)(CH2 Bu)2j, yielding the more stabiUzed twelve-electron species [(=Si0)Ta(=CH Bu)(CH2 Bu)2(=Si-0-Si j (Scheme 11.9). 

Scheme 12.7 Effect of thermal pretreatment of the support on the nature of the immobilized silylamide surface complex and determination of the bonding strength of siloxane bridges by displacement with triphenylphosphine oxide on an AS-3 0 m hybrid material.

Figure 14.5 DR-UV spectra of (a) Ti-silsesquioxanes complexes, with the assigned major species for each maximum, and (b) the Ti(OPr )4 grafted on MCM-41 and deconvolution. The fifth ligand in the pentacoordinated species is represented as an adjacent surface siloxane bridge, but water or surface silanols are also possible (see text). (From Reference [64].)...

It is seen that the trichlorosilane reacts with the silanol groups to form siloxane bridges. Subsequently the residual chlorines are hydrolyzed. Under carefiiUy controlled reaction conditions it is possible to obtain a product in which the hydrocarbonaceous layer at the surface is similar to that in a corresponding monomeric bonded phase. However, the hydrolysis of chlorines that did not react with surface silanbis may result in a silanol concentration at the surface that is higher than that in the silica gel proper used as the starting material for the reaction with alkyltri-chlorosilanes. 

Because of the polarization by the —Si—OH bond, the —Si—Si—back bonds are weakened and attacked by H2O molecules to generate new — Si—H and—Si—OH terminations. Again, the Si—H are oxidized into silanol, and simultaneously these silanol groups are condensed into siloxane bridges ... 

It was found that the radical type catalytic sites, which crack isohexane via the radical mechanism and adsorb NH3 dissociatively to form NH2 species, are generated on mesoporous silica by the calcination at high temperature. Although the sites, tentatively assigned to the strained siloxane bridges, are generated from purely siliceous structure, the incorporation of aluminum of impurity level enhances the generation of such active sites. 

Experiments to determine the mode of bonding of the metal hydrocarbyl with silica dried above 200 °C were not attempted because of Reaction 4. Strained siloxane bridges are not present on silicas dried below 200°C (63). 

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