Bridged silsesquioxanes

Scheme 4 Bridged silsesquioxanes. Reprinted with permission from Shea, K. J. Loy, D. A. Chem. Mater. 2001, 13, 3306-3319. Copyright 2001 American Chemical Society.

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].)...

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.

Figure 3.9 Co-condensation method (direct synthesis) employing TEOS and a terminal organotrialk-oxysilane (top) or TEOS and an organo-bridged silsesquioxane (bottom) as mixed precursors for the organic modification of mesoporous pure silica phases. R = organic functional group.

The precursor must not be too voluminous empirically it turned out that very bulky silsesquioxanes have to be diluted with another, much smaller, more flexible network building component, whether a pure inorganic silica precursor like TEOS/TMOS (see above) or another silsesquioxane with a very short organic bridge (methylene or ethane, see below), in order to give well-ordered mesostructured products. 

Figure 18.2 A sample of pendant and bridged silsesquioxane precursors suitable for creating hybrid nanostructured porous materials.

Of special significance in the development of PMO materials was the synthesis of porous materials with crystalline order of the bridged silsesquioxane inside the framework walls, initially achieved through the use of tt-tt interactions between 1,4-phenylene-bridged silsesquioxane precursors.33 Molecular scale periodicity in... 

A second method that has been described as a means to deal with the limited accessibility issue involves a copolymerization route. In this approach, the porous matrix is copolymerized with the template. Burleigh and coworkers, for example, described the preparation of imprinted polysilsesquioxanes for the recognition of metal ions.79 Polysilsesquioxanes are hybrid porous materials synthesized from bridged alkoxide precursors such as shown in Figure 20.2.80 In this example, the bridged silsesquioxane precursors were copolymerized with the metal ion complex in the presence of surfactant. Once the surfactant and metal ion were removed, a porous network was formed that showed a high affinity for the metal ion.79... 

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