Polysilane polymers physical properties

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. 

This section will demonstrate the first sergeants and soldiers-type helix command surface experiment, in which thermo-driven chiroptical transfer and amplification in optically inactive polysilane film from grafted (or spin-coated) optically active helical polysilane onto quartz substrate [92]. Although helix and optical activity amplification phenomena based on the sergeants and soldiers principle was mainly investigated in polymer stereochemistry, the orientation and physical properties of a thick layer deposited onto a solid surface and controlled by a monolayer command film based on command surface principles was established in photochemical material and surface science [93,94]. Both sergeants and soldiers and command surface experiments appear to have been developed independently. 

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 chemical and physical properties of polysilanes are strongly influenced by substituents attached to the polymer backbone. In this respect, heteroatom-substituted polysilanes should be very much intriguing on their properties. However, heteroatom-functional substituents such as amino and alkoxy groups on silicon cannot survive under the vigorous synthetic conditions of polysilanes by the conventional Wurtz-type condensation of dichlorosilanes. Therefore, it is difficult to prepare heteroatom-functional polysilanes. We have recently found that amino-substituted masked disilenes can be prepared and polymerized successfully to unprecedented amino-substituted polysilane of the completely alternative structure, poly[l,l,2-trimethyl-2-(dialkylamino)disilene]. 

The chemistry of compounds containing Si-Si bond(s) is an intriguing subject in the field of organosilicon chemistry because Si-Si bonds have unique physical and chemical properties. The reactivities of Si-Si bonds is often compared with those of carbon-carbon double bonds. The current interest in polysilanes in material science stems from the fact that they exhibit unusual properties implying considerable electron delocalization in the polymer chain [62]. This section concerns with the unique elecrochemical properties of compounds containing Si-Si bonds (Sect. 2.4). 

Interest in polysilanes was reawakened in 1975, when Yajima and Hayashi found that permethylpolysilane could be transformed into silicon carbide by heating at high temperatures. Soon afterward, papers on soluble, meltable polysilanes began to appear. The literature on polysilanes has grown rapidly since that time. The early focus on the synthesis and simple characterization of polysilanes has given way to detailed physical studies of the structure of these polymers, and of their electronic and photophysical properties. 

Polysilane derivatives constitute a new class of radiation-sensitive materials with interesting physical and electronic properties. The photochemical decomposition of polymers containing disilanyl units seems adequately explained by silicon-silicon bond homolysis and subsequent radical reactions. The solution photochemistry of longer silicon catenates results in the extrusion of substituted monomeric silylenes, as well as the formation of silyl radicals produced by chain homolysis. Recent studies indicating that the... 

Another important class of photonic materials is polysilanes (Fig. 49.15) [217-222]. In polysilanes the delocalization of (T orbitals of the Si atom along the polymer backbone plays a significant role in their NLO properties. Their physical and chemical properties can be tailored by the choice of an appropriate substituent, which also determines the conformation of the polymer and its solubility. What is of more interest is their transparency in their NLO properties, values of a series of polysilanes are given in Table 49.9. Tables 49.10-49.13 contain y and values of some other third-order materials, namely metallophthalocyanine, bis-metallophthalocyanine, metallonaphthalocyanines, fuller-enes, and j8-carotenes. Although these molecules are not considered as polymers, they are macromolecules that have drawn a considerable amount of attention as third-order NLO materials. For example, the highly stable icosa-... 

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