Silsesquioxanes structure

While all these connected entities still have closed cages of the cube-shaped silsesquioxane type, there exist a number of connected entities that consist of opened silsesquioxane structures with aluminum-oxy species as linkers. One structure type can be expressed by the general formula [(RSi)7011(0-SiR,3)]Al[(R,3Si-0)011... 

From these considerations, the synthesis of silsesquioxanes was optimised, by means of HTE, as a function of the activity of the catalysts obtained after titanium coordination to the silsesquioxane structures. Therefore, this approach aimed at producing any incompletely condensed silsesquioxane that would result in active catalysts after titanium coordination rather than a specific structure (like silsesquioxane ulhS). The epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) as the oxidant was chosen as test reaction for the activity of the catalysts [26]. 

A new parameter space for the synthesis of silsesquioxane precursors was defined by six different trichlorosilanes (R=cyclohexyl, cyclopentyl, phenyl, methyl, ethyl and tert-butyl) and three highly polar solvents [dimethyl sulfoxide (DMSO), water and formamide]. This parameter space was screened as a function of the activity in the epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) [26] displayed by the catalysts obtained after coordination of Ti(OBu)4 to the silsesquioxane structures. Fig. 9.4 shows the relative activities of the titanium silsesquioxanes together with those of the titanium silsesquioxanes obtained from silsesquioxanes synthesised in acetonitrile. The values are normalised to the activity of the complex obtained by reacting Ti(OBu)4 with the pure cyclopentyl silsesquioxane o7b3 [(c-C5H9)7Si7012Ti0C4H9]. 

As shown in the previous sections, HTE techniques proved to be a powerful tool to identify leads and gain knowledge about the system under study, but they did not provide information about the actual silsesquioxane structures that were synthesised [46]. To gain such information, selected HTE leads need to be prepared on a conventional laboratory scale and characterised by appropriate analytical techniques (such as spectroscopy and chromatography) [45, 46, 48, 49]. Here, the results of the up-scaling of the synthesis of cyclopentyl silsesquioxanes in acetonitrile (Sections 9.2.2 and 9.2.3) and of the synthesis of tert-butyl silsesquioxanes in water (Section 9.2.4) are reported. 

Both fractions were characterised by NMR spectroscopy and mass spectrometry. Fraction A mainly consists of silsesquioxane a7b3 (Fig. 9.5) [38], while fraction B is a mixture of different silsesquioxanes, mostly incompletely condensed species, with the main species assigned to silsesquioxane structure 6i>2 (Fig. 9.5). Finally, both fractions were reacted with a titanium alkoxide and tested for catalytic activity in the epoxidation of 1-octene as a function of the reaction time and the results compared with those of HTE lead (all three catalysts are homogeneous) (Fig. 9.6). 

The mechanistic study by means of mass spectrometry was performed by analysing samples of the reaction mixture at regular intervals throughout the experiment. The MS spectra recorded between t=0 and 1440 min show peaks corresponding to cyclopentyl silsesquioxane structures with l[Pg.222]

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.

Therefore, it is proposed that the species identified by MCR is an o=5 or an o=6 structure, which presents an MS concentration profile similar to that obtained by ATR FTIR, or a mixture of more silsesquioxane structures. The hypothesis of an o=6 structure is supported by the fact that the ATR FTIR concentration profile for the silsesquioxane species as a function of time is rather similar to the MS concentration profile obtained for the o=6 silsesquioxane (cf. Figs 9.9 and 9.13). The slightly different position of the maximum in the two plots is considered to be due to the longer time required in the ATR FTIR experiment to reach the reflux temperature. (This explanation was confirmed by repeating part of the MS study of the synthesis of silsesquioxane 7h3 at 50°C (instead of reflux tem-... 

Besides the intrinsic value of the identification of a new, selective and high yield method to synthesise silsesquioxane Bu2Si20(0H)4, this experiment proved that tert-butyl silsesquioxane a2bA is a suitable precursor for titanium catalysts. Thus, silsesquioxane structures different from the known precursor silsesquioxane alb i [R7Si709(0H)3] [25, 26, 28] can effectively coordinate titanium centres to yield almost equally active epoxidation catalysts. 

A single shot-mass spectrum (Figure 31.6b) shows all three masses that make up the three silsesquioxane structures. 

Nitration of 4-TMSPh-Tg by fuming nitric acid was carried out. The reaction at —30 C gave almost pure completely nitrated products. Cleavage of Si-oxygen or Si-phenyl in silsesquioxane structure was not noticed. The reaction is shown in Scheme 4.15. ... 

The polyhedral oligomeric silsesquioxane structure affects the rheological and cure characteristics but was found to have no discernible influence on the properties in the cured state. 

Dehydrogenative Coupling Reactions for Synthesis of the Higher Order Silsesquioxane Structures... 

Brus J, Urbanova M, Strachota A (2008) Epoxy networks reinforced with polyhedral oligomeric silsesquioxanes structure and segmental dynamics as studied by solid-state NMR. Macromolecules (Washington) 41(2) 372-386. doi 10.1021/ma702140g... 

---