Polysilane high polymers

The polysilanes are compounds containing chains, rings, or three-dimensional structures of silicon atoms joined by covalent bonds. Recently, polysilane high polymers have become the subject of intense research in numerous laboratories. These polymers show many unusual properties, reflecting the easy delocalization of sigma electrons in the silicon-silicon bonds. In fact, the polysilanes exhibit behavior unlike that for any other known class of materials. 

In any event, between 1951 and 1975, no papers appeared on polysilane high polymers. However, linear permethylpolysilanes of the type MelSiMezhiMe were prepared and studied, especially by Kumada and his students,(5) and cyclic polysilanes were being investigated in several laboratories.(6,7) Studies of the permethyl-cyclosilanes, (Me2Si)n where n = 4 to 7, showed that these compounds exhibit remarkable delocalization of the ring sigma electrons, and so have electronic properties somewhat like those of aromatic hydrocarbons.(6)... 

In summary, the production of substituted silylenes and silyl radicals upon exhaustive irradiation at 254 nm of polysilane high polymers suggested that the polymer photochemistry resembled that previously reported for short chain acyclic and cyclic oligomers (39). More recent experiments, however, have suggested that the photochemical mechanism for the degradation of the high polymers is more complex than first envisioned (vide infra) (48). 

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

The photochemical studies on polysilane high polymers indicate that they constitute a new class of radiation sensitive materials which undergo primarily... 

For recent reviews of polysilane high polymers see West, R., J. Organometallic Chem. 1986, 300. 327 West, R. In The Chemistry of Organic Silicon Compounds Patai, S. Rappoport, Z. Eds. Wiley-Interscience New York, 1989 Vol. 2, Chapter 19, 1207. 

The most common route to polysilane-high polymers is by the Wurtz-type alkali metal-mediated polycondensation of dichlorosilanes. However, , u-dihalooligosi lanes can also be coupled in a similar manner. Wurtz-type polymerization of BrSiMe2SiR2SiMe2Br led to bimodal molecular weight distributions of alternating co-polymers of the type (SiMe2SiR2SiMe2) (R = hexyl or Bu),17 as in Equation (1). 

West, R., and J. Maxka, Polysilane High Polymers An Overview, Chap. 2 in Inorganic and Organometallic Polymers, M. Zeldin, K. J. Wynne, and H. R. Allcock, eds., Am. Chem. Soc. Symp. Ser., Vol. 360, American Chemical Society, Washington, DC, 1988. 

A detailed study of the electrochemistry of polysilane high polymer films will be reported in the future. 

Polysilane high polymers possessing fully saturated all-silicon backbone have attracted remarkable attention recently because of their unique optoelectronic properties and their importance in possible applications as photoresists, photoconductors, polymerization initiators, nonlinear optical materials etc. A number of review articles have been published on this topic4-9. The studies in this field have stimulated both experimental and theoretical chemists to elaborate on understanding the excited state nature of polysilanes and oligosilanes and of their mechanistic photochemistry. 

Dehydropolymerization, for polysilane high polymers, 3, 562 (/-Electron band ionization energy series, and photoelectron spectroscopy, 1, 397... 

Disilane monomers, into polysilane high polymers, 3, 573 Disilanes... 

Polyselenido nitrosyl complexes, with chromium, 5, 303 Polysilane high polymers... 

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