Polysilane

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General repeating unit of polysilanes, where the R's are the same or different organic groups.

Polysilanes are

organosilicon compounds with the formula (R2Si)n. They are relatives of traditional organic polymers but their backbones are composed of silicon atoms. They exhibit distinctive optical and electrical properties. They are mainly used as precursors to silicon carbide.[1] The simplest polysilane would be (SiH2)n, which is mainly of theoretical, not practical interest.[2]

Synthesis

Dodecamethylcyclohexasilane shares some properties of high molecular weight polysilanes.[3]

The first polysilane, poly(dimethylsilylene), [(CH3)2Si]x, was reported in 1949 by Charles A. Burkhard (1916 - 1991) of General Electric. It was prepared by heating sodium metal with dimethyldichlorosilane:

(CH3)2SiCl2 + 2 Na → [(CH3)2Si]n + 2 NaCl

The modified

potassium-graphite (KC8) can be used at much lower temperatures than those required for traditional Wurtz coupling.[6] This reaction typically produces a trimodal distribution of products: a low molecular weight fraction and two higher molecular weight fractions. The low molecular weight fraction consists of five and six-membered rings, i. e. [SiR2]5 and [SiR2]6. Formation of these rings competes with the growth of the polymer.[6] Another method for the synthesis of polysilanes is dehydrogenative coupling of silanes
.

Properties

The product obtained by Burkhard was difficult to work because it was insoluble in organic solvents. Interest in the polysilanes resumed in the early 1980s when it was reported that [(CH3)2Si]x can be converted to

thermolysis
.

Polysilanes range from highly

NMR spectroscopy
provides insights into the microstructure of a polymer. If resonances are broad, oligomerization is likely; if they are sharp, some sort of pattern in the silicon backbone can be inferred.

Thermolysis to silicon carbide

idealized scheme for conversion of polydimethylsilane to beta-silicon carbide.[1]

Yajima and coworkers discovered that the

additive manufacturing of ceramics.[7]

Spectroscopic characteristics and band structure

Polysilanes exhibit σ-delocalization. This characteristic stems from the low ionization energy for electrons in Si-Si sigma bonds relative to that of C-C sigma bonds, for instance. Accordingly, they absorb strongly in the UV-region (300-400 nm) due to intense σ-σ* electronic transitions.6 Polysilanes degrade in the presence of UV light since σ-σ* electronic transitions can be thought of as bonds breaking, often precluding some applications. Dialkyl polysilanes tend to have a band gap of about 4.5 eV. Introduction of an aryl substituent to each silicon lowers the band gap to about 3.5 eV, making for a borderline semiconductor.[5][6]

Polysilynes

alkyl
). They are more highly cross linked than polysilanes and have been less studied.

See also

References

  1. ^
    ISBN 978-3-527-29390-2
    .
  2. ISBN 0195131193
    .
  3. ISBN 9780470132500
    .
  4. doi:10.1021/cr00096a006
    .
  5. ^ a b c James E. Mark, Harry R. Allcock, Robert West, "Inorganic Polymers," 2nd edition, Oxford University Press, 2005, pp. 201-214.
  6. ^ a b c d West R. (1986) In: Chandrasekhar, V; Inorganic and Organometallic Polymers; Springer: Berlin, 2005; 3-540-22574-9.
  7. ^ Wang X. et al. Additive manufacturing of ceramics from preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry, Additive Manufacturing 2019, volume 27 pages 80-90
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