Polysilane

8 Changes in UV-visible absorption spectrum of a poly(di-n-hexylsilane) spincast film in the course of crystallization to adopt an all-trans-zigzag conformation of the Si backbone at I5°C (a). Photocontrol of the crystallization rate as observed by the absorbance increases at 366 nm, corresponding to the increase of the all-trans-zigzag conformation of the Si chain (b). (From reference 39 copyright permission from the American Chemical Society.) 

9 Photo-orl itatk n of the Si backbone of poty((M-n-hexylsilane) via transfer from an Az monola r i otoalfgned ria exposure to linearly polarized Kght (From reference 39 copyright permis. Sion from the American Cheitiical Society.) 

The energy conversion from light to mechanical response is one of the most fascinating targets in or nic photochromic systems. Section 15.2 already indicated the light-driven droplet displacement. This section introduces the motion of Az polymers themselves. Rapid motions observed here mi t allow for applications of actuators of polymer-based micromachine systems. The photoinduced surface-relief generation described in the latter part of this section (Section 15.4.2) deals with much thicker films than monolayers. Howevei the mass miration in thicker films is closely related to the structure and properties of the monolayer systems. 

1 PhcrttHirieduttiiccd Ejects in Azobenzene Potyrmer Mcmolajfers 15.4. t. I Macrosize Effect 

10 Photoinduced expansion and contraction of 6AzlO-PVA monolayer at the air-water interfeee.The area changes of an isolated domain can be visualized in situ by Brewster angle microscopy. (From reference S2 copyright permission from the American Chemical Society.) 

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The polysilanes oi polysHylenes (25), an unusual class of inoiganic polymers, saw a very intense period of investigation during the 1980s. 

Ring-Opening Polymerization. As with most other inorganic polymers, ring-opening polymerization of cyclotetrasilanes has been used to make polysilanes (109,110). This method, however, has so far only been used for polymethylphenylsilane (eq. 12). Molecular weights (up to 100,000) are higher than from transition-metal catalyzed polymerization of primary silanes. 

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Electrochemical Synthesis. Electrochemical methods have also been investigated for the synthesis of polysilanes, but these have so far yielded low molecular weight materials (113,114). 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Magnesium does not form stable Grignard reagents with siUcon haUdes, although some siUcon halohydrides do react, forming polysilanes (15). 

Dehydrogenative Coupling of Hydride Functional Silanes. The autocouphng of dihydridosilanes was first observed usiag Wilkinson s catalyst (128). A considerable effort has been undertaken to enhance catalyst turnover and iacrease the molecular weight of polysilane products (129) because the materials have commercial potential ia ceramic, photoresist, and conductive polymer technology. 

Another fertile area of current interest is the synthesis of stable homocyclic polysilane derivatives.Typical examples are cyclo-(SiMe2)7, > (cyclo-Si5Me9)-(SiMe2) -(cyclo-SisMeg), n = 2-5, and several new permethy-lated polycylic silanes such as the colourless crystalline compounds bicyclo[3.2.1]-Si8Mei4 (mp 245°), bicyclo[3.3.1]-Si9Mei6 (mp >330°) and... 

Silicon centered radicals can be generated by transfer to silanes and by photolysis of polysilanes. Rate constants for addition to monomer are several orders of magnitude higher than similar carbon centered radicals.453,43 The radicals have nucleophilic character. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

These and similar complexes of Ti and Zrare effective catalysts for the formation of polysilanes from primary silanes [117-120]. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

Recently, the compounds CpCp Hf(Cl)Si(SiMe3)3 and CpCp Zr(Cl)Si(SiMe3)3 have been used as homogenous catalysts for the formation of polysilanes. 

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom. The two groups attached to the silicon chain... 

Analogous to terminal alkenes, the reaction of 123 with valeraldehyde and cyclohexanone under radical-based conditions allowed for the preparation of the corresponding functional polysilanes 126 (Reaction 90). The efficiency of Si-H bond replacement was 80-85%... 

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