Poly silane

J. M. Zeigler and F. W. G. Fearon, eds., Silicon-BasedPoljmer Science, Advances in Chemistry Series, Vol. 224, American Chemical Society, Washington, D.C., 1990 regarding poly(silanes). 

Cured phenolics are universally brittle in nature. This is true of both resoles and novolacs and does not depend much on the source of methylene used to promote cure. Consequently, the fillers used in molded articles are highly important to the design of the manufactured product. With resoles, the fiber or filler are usually the primary component of the final composite, with the resole acting as a binder or impregnating agent. With novolacs the resin may be the major component in the molded part. Poly-silanes and other organic polymers are also added in some applications to promote impact resistance and toughness [192]. 

Kjrushin Soichiro and Matsumoto Hideyuki, Ladder Poly silanes. 

Poly silanes represent an interesting class of radiation sensitive polymers for which new applications have been discovered. 

The most important process so far has been the reductive elimination of halogens with the formation of Si-Si bonds. Kipping used this reaction and discovered the first perphenylated cyclosilanes, yielding polysilanes as a by-product [8]. Similarly dodecamethylcyclohexasilane was found using dimethyldichlorosilane as a starting material for this reaction by Burkhardt in 1949, but 90% of the yield appeared as poly silane by-products [9]. 

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. 

The systematic catenation of cyclotetrasilane rings gives rise to a double helix structure of two poly silane main chains. 

Before the explanation of ladder poly silanes is started, ladder compounds of other Group 14 elements are briefly mentioned. Hydrocarbons with a ladder-shaped carbon framework are known as ladderanes. The study on ladderanes goes back to 1927, when bicyclo[2.2.0]hexane ([2]ladderane) was synthesized by the reduction of dy-l,4-dibromocyclohexane with sodium.19 Ladderanes so far reported are tricyclo[4.2.0.02,5]octane ([3]ladderane),20 tetracyclo[4.4.0.02,507,1°]decane21 ([4]ladderane), and a number of their derivatives (Fig. 3). 

Fig. 10. Lowest transition energies for ladder and linear polysilanes. Values for linear poly silanes are cited from reference 44.

The radical anions 1, anti-2, anti,anti-3, and anti,anti,anti-4 were generated by the reduction of the corresponding ladder polysilanes with potassium (Scheme 8). When the ladder poly silanes were treated with potassium in tetrahydrofuran (THF) at ca. — 70 °C, the solutions were immediately colored 1, purple anti-2, brown anti,anti-3, blue and anti,anti,anti-4, black. In the UV-visible-NIR spectra, several absorption bands appeared (Fig. 15). The intense absorption of anti,anti-3 and anti,anti,anti-4 in the near-infrared region is noted because it has been... 

Figure 4.2 Comparison of UV absorption spectra of four optically active poly silanes in THF at 30°C poly methyl-(iS )-2-methylbutylsilane (1, a of 0.59), poly -hexyl-(5 )-4-methylpentyl-silane (2, a of 0.75), poly -hexyl-(5 )-3-methylpentylsilane (3, a of 0.92), and poly -hexyl-(5)-2-methyl-butylsilane (4, a of 1.25).

Figure 4.7 UV and CD spectra of poly silane homopolymer 4 (solid lines) and copolymer 10 (dotted lines) containing 10 mol % of (6 )-2-methylbutyl and 90 mol % of 2-methylpropyl groups in isooctane at —5°C.

The emissive counterpart to CD is circularly polarized photoluminescence (CPPL). Where the fluorophore is chiral, then the photo-excited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/r and /L, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality of the photo-excited state of the material. CPPL studies on poly silanes are extremely rare, however, due to the low CPPL intensity and rapid sample degradation in solution, and problems due to artifacts in the solid state. 

An important property of the alkyl poly silanes is their electronic absorption at relatively low energy. This was particularly surprising because such molecules lack tt, d ot lone pair electrons they were the first substances containing only bonding cr-electrons to show such long-wavelength absorptions. The unusual electronic properties of the polysilanes have sparked considerable interest in the chemophysical properties of these molecules. The UV spectra of cyclic polysUanes follow a quite different pattern . 

Figure 6. Trapping experiments for poly silane photodegradation.

Figure 9. Poly silane bilayer resist, top layer 0.25 am experimental polysilane, bottom layer 1.0 am of hardbaked photoresist, projection printed at 313 nm with 02-RIE image transfer.

On the other hand, Shono, Kashimura and coworkers found that alternating the polarity of Mg electrodes in LiC104/THF is quite effective for the formation of Si—Si bonds (Table 19)87,88. This method was successfully applied to the preparation of polysilanes. Sonication resulted in a marked increase in the yields of polysilanes as shown in Table 2087,88. Under higher concentration of dichlorosilane, poly silane of higher molecular weight was obtained. 

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