Catenation of Silicon

Polysilanes are polymers in which there is catenation of silicon, that is, where silicon atoms are bonded to each other in a continuous manner. Synthesis of polysilanes involves the Wurtz coupling of diorganodichlorosilanes with sodium metal (Eq. 2-239) [Baldus and Jansen, 1997 Corriu, 2000 Manners, 1996 Miller and Michl, 1989 West, 1986 West and Maxka,... 

In the case of silicon, the tendency for catenation allows halides having the formula Si2X6 to be prepared. The chloride can be prepared by the reaction... 

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom (Structure 1). The groups R and R attached to the silicon chain can be of a large variety. Polysilanes with alkyl and/or aryl substituents have been the most thoroughly investigated [1-3], whereas polysilanes having at least a heteroatom substitution such as H, Cl, OR, NR2 have received much less attention [4]. The number of silicon atoms is usually from several hundreds to several thousands. 

Catenated Organic Compounds of Silicon, Germanium, Tin, and Lead Henry Gilman, William H. Atwell, and Frank K. Cartledge Cyclobutadiene-Metal Complexes P. M. Maitlis Organoarsenic Chemistry W. R. Cullen... 

Many of the physical properties of polysilanes depend on the actual substituents present on silicon. However, polysilanes have some distinct features in comparison to other polymers which is a direct result of the unique characteristics that a catenated chain of silicon atoms provide. These can be summarized as follows ... 

The phenomenon of catenation (/, 21) is observed in compounds where an element forms stable bonds to itself. It finds its fullest expression in the area of organic chemistry where many carbon atoms are united in linear, branched, and cyclic arrangements. Our studies have been largely with Group IVB types, and particularly those of silicon. 

For many years, the limited similarity between silicon and carbon excited the scientific community. Carbon and silicon share the same outer shell electronic structure, s, which permits sp hybridization and dominant tetrahedral coordination, as well as dominance of the tetravalent oxidation state. Nevertheless, silicon chemistry is markedly poorer compared to that of carbon. Double silicon bonds and silicon catenation are scarce, and crystalline silicon, which is so widely used in the electronics industry, is never encountered in nature. Instead, sUicon-oxygen bonds dominate natural silicon chanistry, and solid silica and silicates have no common physicochemical features with carbon dioxide and carbonates. The silicon atom is larger than carbon, it is less electronegative, has lower nuclear electric charge shielding and, perhaps most importantly, it has vacant d-orbitals in its outer shell all these dictate the reactivity of silicon. Several consequences of these differences are especially significant, and they are also relevant to sol-gel electrochemistry. 

A consideration of the hydrides emphasizes the uniqueness of carbon. It is the champion catenator of all the elements. The silanes are more reactive than the alkanes for both thermodynamic and kinetic reasons. The relative ineffectiveness of pTt-pTt overlap as well as the introduction of dit-pir interactions in the compounds of silicon play a large role in distinguishing them from those of its lighter congener. 

Many more complicated organopolygermanes are listed in an article by H. Gilman raititled Catenated organic compounds of silicon, gennanium, tin and lead, in Atb. Organomet. Chem. 4 (1966) 1-94. 

Polysilanes (polysilylenes) consist of a linear chain of silicon atoms carrying two substituents, generally, either allqrl or aryl.(l,2,3) Interest in these materials stems from their unique properties, such as sigma-catenation and thermochromic behavior. They have potentid applications as photoresists, electro-optical devices, non-linear optical mateikls, and also as precursors to silicon carbide. 

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