Polysilanes special

Reductive dehalogenating coupling can be used for the preparation of other new and interesting compounds, and a brief consideration of the special field of polysilanes, the cyclosilanes, will show this. 

The first step for the synthesis of a melt spinnable polysilane is the alkoxylation and distillation of the residue (Figure 1). 1,2-dimethyltetramethoxydisilane and 1,1,2-trimethyltrimethoxydisilane are mixed in a special ratio and a poly silane will be obtained by a catalytic redistribution reaction. Catalysts for this reaction are alkali alkoxides like sodium methoxylate. Phenylmethoxydisilanes [22] or phenylchloride are used as additives. A mixture of methyltrimethoxysilane and dimethyldimethoxy-silane distils off as a byproduct of the redistribution reaction. Figure 2 shows the mechanism of the catalytic redistribution. 

Numerous synthetic strategies for "preceramics" based on polysilanes are known, but the synthesis of Si-Si chains with different substituents on the silicon skeleton is difficult. In the following the advantages of the triflate method for the synthesis of special substituted oligosilanes are shown by some examples. 

Ladder polysilanes constitute a special case of fused polycyclic silicon macromolecules, in which cyclotetrasilane rings systematically catenate to form a silicon double helix, comprising two multi-linked silicon chains. Work in this area was initiated in this area by Matsumoto in 1987, and now comprises an integral part of the literature on higher-dimensionality polysilanes. 

An alternative view of the polysilane structure is depicted using the worm-like model as proposed for poly(diacetylene)s59, where the linear chain has a large number of small twists without sharp twists playing a special role60-62. In this model, a Gaussian distribution of site energies and/or exchange interactions and the coherence of the excitation is terminated by any of the numerous usual random deviations from perfect symmetry. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

Tacticity is required for the synthesis of crystalline thin polysilane films used for optical and semiconductor devices. Modern synthetic routes allow control over the conformation and tacticity of polysilane molecules used as precursors for thin layers of photoresists, photoconductors and nonlinear optical phases in complex semiconductor and (opto)electronic devices. These properties can be exploited only if the synthesis method ensures a minimal level of contamination, especially with oxygen and metals, and special care is taken to limit electronic-grade polysilanes to a level of contamination on the order of a few ppm in the case of oxygen and in the ppb range for metals. The reactivity of polysilane toward oxygen has forced placing the devices in a helium environment during measurement procedures.36... 

Many kinds of polymers containing silicon atoms in the main chain have been synthesized. A few examples are shown in Figure 1. By a wide margin, the most important are the polysiloxanes, commonly called silicones, which are now the basis for a worldwide industry.Also described in this article are the polysilane polymers, which are of interest because of their special properties reflecting extensive delocalization of the a-electrons along the Si-Si backbone see a-Bond). ... 

As the name implies, polysilane polymers consist of chains made up exclusively of sihcon atoms. (In the ht-erature these polymers are named either as polysUanes or poly(silylenes) thus (Et2Si) can be called polydiethylsilane or poly(diethylsilylene).) Unlike the heteroatomic polysiloxanes, with alternating silicon and oxygen atoms in the polymer backbone, the poly silanes are homoatomic and therefore structurally closer to aUcene polymers. However, because the atoms in the main chain are all silicon, the polysilanes show quite special properties. ... 

The formation of the Si-Si bond occupies a special position in organosilicon chemistry. Silicon is one of the few elements, which can form stable catenated compounds. Besides high-molecular weight polysilanes, oligomeric polysilanes are a very important class of compounds that are related to silylenes,... 

Of these, the siloxanes or silicone polymers have been studied the most and are also of the greatest commercial importance (3-8). Increasingly important, at least for some specialized applications, are the polysilanes (3, 9),... 

In the last years transition metal-silyl complexes have received special attention for several reasons [1, 2], On the one hand, they are assumed to be important intermediates in catalytic processes [2] (transition metal-catalyzed hydrosilylation reaction, dehydrogenative coupling of silanes to polysilanes, etc.), on the other metal-substituted silanes show special properties, which can be tuned systematically by judicious choice of the metal and its ligands [3] Furthermore, silylenes (silanediyls) are stabilized by unsaturated transition metal fragments leading to metal-silicon double-bonds [4]. In the light of a possible application in MOCVD processes some of these complexes are of interest as potential single-source precursors for the manufacture of thin silicide films [5]. 

By the way, although a dominant majority of papers concerning the formation of amorphous SiC layers describes appUcations of CVD or PVD techniques, there have also been some attempts to use the polymer route for preparing SiC films. Starting from solutions of various polysilanes or polycarbosilanes, frequently films are formed by spin-coating and pyrolyzed under inert atmosphere [215-218]. Of course, such a procedure does not form a part of this section SiC layers via gas phase reactions . However, in this connection it should be mentioned that polysilanes are also applied to form films via evaporation, not only with the aim to build amorphous and/or crystalline SiC films, but also to use special properties of the polysilane films themselves, i.e. without a subsequent pyrolysis of these films. Such amorphous films are characterized by non-linear optical effects [219, 220] and their properties may be controlled by the uniformity of the orientation of polysilane chains which is susceptible to epitaxial influences [221-223]. 

PROPERTIES OF SPECIAL INTEREST Relatively low cost, compared with other polysilanes. For general information about polysilme polymers see the entry for... 

Design and synthesis of hybrid polymers and of dendrimers for special applications and as building blocks for carbosilanes, polysilanes catalysis and highly crosslinked coatings. 

However, the structure of the copolymer was blocky with runs of the respective monomer units, and hence the polymer was not the real polysilastyrene [11]. Phenylation of the chloro-substituted polysilane 3 with phenyllittuum gave poly(l,l,2-trimethyl-2-phenyldisilene) in good yield. The structure of the polymer was confirmed with the SEC and NMR spectra. We have now succeeded in the preparation of the real head-to-tail polysilastyrene (4, R = Ph) about 20 years after the first reported synthesis of soluble polysilastyrene. This procedure is a simple method for preparing polysilanes of special structure with functional groups which are otherwise very difficult to prepare. 

PROPERTIES OF SPECIAL INTEREST Transition from crystalline phase to columnar mesophase at 42°C. That is, PDHS is crystalline with an all-frans arrangement of the polysilane chain below the disordering temperature of 42 C above this temperature the polymer exists in a hexagonal columnar Uquid crystalline phase. 

PROPERTIES OF SPECIAL INTEREST Relatively low cost, compared with other polysilanes. 

These catenated cyclic polysilanes, as well as others such as dodecamethyl-cyclohexasilane, are of special value for several reasons. We have shown that A and B, in particular, can be opened by some metals, halogens, and halogen acids to give compounds of high synthetic value. 

The last comprehensive review of polysilanes was given 20 years ago by Miller and Michl [3]. Since then, considerable work has been carried out to find a perfect method for their synthesis. Special interest has also been directed to the characterization of polysilanes to explain their special physical properties. 

The special bonding situation in polysilanes can be used to explain most of their properties and will be described in the following paragraph. 

Because of their special chemical and electro-optical properties, polysilanes have been proposed as materials for many applications. These can be classified into two main categories ... 

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