Physical Properties of Polysilanes

The properties of polysilanes depend very much on the nature of the organic groups bound to silicon. Some polymers are so highly crystalline as to be insoluble and 

The light-scattering behavior of those polysilanes studied indicates that they are slightly extended and stiffened compared with typical polyolefins. One measure of chain flexibility is the characteristic ratio C, which is also shown in the table. The values of C for most polysilanes of about 20 are larger than those for typical hydrocarbon polymers (—10), indicating that the polysilanes are somewhat less flexible than polyolefins. However, poly(diarylsilylene)s are much more rod-like and inflexible, with persistence lengths greater than 100 46 

MWLS weight average M from light-scattering measurement. 

MWGPC weight average M from gel permeation chromatography, relative to polystyrene standards. Rg radius of gyration. 

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  

Most polysilanes are soluble polymers and dissolve in common organic solvents. The solubility decreases with increasing crystallinity of the polymer. The classic example of this feature is Burkhard s polymer [MeaSijn. Other examples include [EtaSijn, [Ph2Si] etc., [1-4]. However, disrupting the crystallinity by means of copolymerization such as that found in [(Me2Si)n(PhMeSi) ,] or 

Polysilanes are radiation-sensitive pol5maers and degrade upon exposure to UV light. This process is accentuated in solution [1-4]. 

Many polysilanes possess helical main-chain structures although the details of these helical structures vary from case to case [5]. 

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We now wish to report the syntheses and physical properties of polysilanes substituted with a phenolic group and carboxylic groups, and polysiloxanes with a phenolic group directly bonded to Si(Figure 2). We also describe basic aqueous development of these materials. 

Physical properties of polysilanes vary greatly depending upon the nature of the organic substituents attached to silicon. Poly(dimethylsilane) (1) and poly(diphenylsilane) (2) are a highly crystalline in-... 

Physical properties of polysilanes prepared by anionic polymerization are quite different to some extent as have been already seen in thermoc omism of alternating copolymers (4). Moreover, the anionic polymerization can afford polysilanes of special structures otherwise very difficult to prepare. In this section, one of the most dramatic examples is shown. 

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 organopolysilanes are those compounds containing at least one silicon-silicon bond and one silicon-carbon linkage. This review is mainly concerned with the chemistry of aliphatic derivatives of polysilanes. Consideration of aromatic organopolysilanes is excluded from this review except as far as they are used as intermediates for synthesis and their properties correlate with the aliphatic silicon-silicon compounds, because the aromatic organopolysilanes have recently been well reviewed elsewhere (31,51, 73, 76a, 212). Physical properties of the polysilanes also are excluded from consideration except for spectral properties of ultraviolet absorption and nuclear magnetic resonance, since they are well summarized in earlier excellent reviews and texts (8, 34, 35, 51,131,132). 

In this section, for convenience, we first deal with the synthesis and some physical properties of permethylated polysilanes, and then synthesis of other peralkylated and partially phenylated methyl polysilanes. Finally, chemical reactions of all such types of compounds will be discussed together. 

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. 

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). 

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. 

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. 

In this study, we investigated a set of model polysilane chain systems that illustrate the basic physics and chemistry of some optical properties of these materials. In particular, we looked at the band structure for unsubstituted polysilane in an all-trans conformation, as well as in a 4/1 helical conformation with four silicon atoms contained in one translational repeat unit. In addition, we compared results for the dimethyl-substituted polysilane in an dl -trans conformation with the results for the unsubstituted poly silane. 

The pyrolysis of polysilanes and polycarbosilanes is usually carried out using inert gas (e.g., argon) as pyrolysis atmosphere. A general problem associated with the pyrolytic formation of carbides is the desired stoichiometry of the calcined products in contrast to nitrides, excess carbon carmot be evaporated during calcining it may therefore contaminate the powders obtained as an elemental impurity and thus influences the physical, especially mechanical and electrical, properties of the sintered ceramic bodies. The volatiles evaporated during pyrolytic treatment of carbosilanes to form a network structure are H2 and CH4, and they depend on the structure of the polycarbosilane used (Fig. 2). 

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