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siloxane
A siloxane is a functional group in organosilicon chemistry with the Si–O–Si linkage. The parent siloxanes include the oligomeric and polymeric hydrides with the formulae H(OSiH2)''n''OH and (OSiH2)n.〔(Siloxanes ), ''IUPAC Gold Book''〕 Siloxanes also include branched compounds, the defining feature of which is that each pair of silicon centres is separated by one oxygen atom. The siloxane functional group forms the backbone of silicones, the premier example of which is polydimethylsiloxane.〔 The functional group (RO)3Si is called siloxy.
==Structure==
Siloxanes generally adopt structures expected for linked tetrahedral ("''sp''3-like") centers. The Si–O bond is 1.64 Å (vs Si–C distance of 1.92 Å) and the Si–O–Si angle is rather open at 142.5°.〔H. Steinfink, B. Post and I. Fankuchen "The crystal structure of octamethyl cyclotetrasiloxane" Acta Cryst. 1955, vol. 8, 420-424. 〕 By way of contrast, the C–O distance in a typical dialkyl ether is much shorter at 1.414(2) Å with a more acute C–O–C angle of 111°.〔"Dichlorosilane–dimethyl ether aggregation: a new motif in halosilane adduct formation" K. Vojinović, U. Losehand, N. W. Mitzel Dalton Trans., 2004, 2578-2581. 〕 It can be appreciated that the siloxanes would have low barriers for rotation about the Si–O bonds as a consequence of low steric hindrance. This geometric consideration is the basis of the useful properties of some siloxane-containing materials, such as their low glass transition temperatures.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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