Siloxane cages

It has been well recognized that the hydrolysis of alkoxysilanes and chlorosilanes is effectively catalyzed when fluoride anions are present due to formation of hypercoordinated silicon intermediates.803 More in-depth studies by Bassindale et al. showed that the reaction of PhSi(OEt)3 with stoichiometric amounts of Bu4NF surprisingly yields an encapsulation complex, namely tetrabutylammonium octaphenyloctasilsesquioxane fluoride 830, in which the fluorine atom is situated inside the cubic siloxane cage (Scheme 114). The Si--F distance of average 2.65 A is shorter than the sum of van der Waals radii (3.57 A), which renders the coordination number of the silicon atoms at [4+1]. 

Covalent polymer networks or (Class II) crosslinked macromolecular architecture polymers rank among the largest molecules known. Their molecular weight is given by the macroscopic size of the object for instance, a car tire made of vulcanized rubber or a crosslinked layer of protective coating can be considered one crosslinked molecule. Such networks are usually called macronetworks. On the other hand, micronetworks have dimensions of several nanometers to several micrometers (e.g. siloxane cages or microgels). 

A tantalum-siloxane cage complex (163) has been synthesized by two routes and characterized structurally (Scheme 33).3 1 A tetranuclear Ta2Si2 unit is the basis of a distorted bicapped cubane core [Ta2Si2(/i3-0)2(xi2-0)4]6+.331 Other cyclic Nb— and Ta— siloxanes based on But2Si(OH)2 have been prepared. 

There are two technically important methods by which extended Si/0 systems can be formed from molecular precursors. The first is by reaction of chlorosilanes with oxygen at high temperatures, while the second is by hydrolysis and condensation reactions of chloro- or alkoxysilanes. Chapters 32 and 33 deal with the structural evolution of siloxane structures in such reactions from an experimental and theoretical viewpoint. M. Binnewies et al. compare the stepwise formation of Si-0 networks from SiCU for both the combustion and hydrolysis reactions. The stability and reactivity of intermediate chlorosiloxanes is an important issue in this work. Both the initial process in the reaction of SiCfi with O2 and the growth of larger siloxane cages are investigated theoretically in the contribution of K. Jug. 

The crystal structure of 1 is shown in Fig. 1 the atom labeling is omitted for clarity. The molecule contains a cubic SigOi2 framework with a 4-(trimethylsilylethynyl)phenyl substituent bound to each of the silicon atoms. The silicon atoms of the siloxane cage are approximately tetrahedrally coordinated. The whole molecule shows C symmetry. In contrast to the solid state, 1 shows perfect Of, symmetry in solution, which was verified by NMR spectroscopy. 

Chaikittisilp W, Kubo M, Moteki T et til (2011) Porous siloxane-organic hybrid with ultra-high surface area through srmultemeous polymerization-destruction of functionalized cubic siloxane cages. J Am Chem Soc 133 13832—13835... 

Chaddttisilp W, Sugawara A, Shimojima A et al (2010) Microporous hybrid polymer with a certain crystaUinity buUt from functionalized cubic siloxane cages as a singular building unit. Chem Mater 22 4841-4843... 

Other types of metallosiloxane guests for alkali metals are presumably possible, as suggested by the unusual structure of an indium-siloxane cage which incorporates sodium cations [48a]. 

A higher level of structure is illustrated by the preparation of micropo-rous hybrid polymers from functionalized cubic siloxane cages. ... 

Chaikittisilp, W. Sugawara, A. Shimojima, A. Okubo, T., Microporous Hybrid Polymer with a Certain Crystallinity Built from Functionalized Cubic Siloxane Cages as a Singular Building Unit. Chem. Mater. 2010, 22, 4841-4843. 

The concluding fifth chapter touches upon the issues and advances in constructing of the most thermodynamically stable polysiloxanes built of cage-like subunits. The chemistry of siloxane cages has a long history, but their polymer biography is in the very beginning. 

IR spectroscopy was appUed to study interaction among these copolymers. The Tg values of these copolymers were found to depend on three factors (1) the inert role of POSS as a diluent that reduces the self-association dipole-dipole interactions of the parent polymer molecules (a negative confribution), (2) the dipole-dipole interactions between the POSS s siloxane cage and the polar carbonyl... 

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