関: polysiloxane

WordNet

any of a large class of compounds that have alternate silicon and oxygen atoms

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/10/21 10:35:31」(JST)

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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)_nOH and (OSiH₂)_n.^[1] 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.^[2] The functional group (RO)₃Si is called siloxy.

Structure

Siloxanes generally adopt structures expected for linked tetrahedral ("sp³-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°.^[3] 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°.^[4] 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.

Synthesis of siloxanes

The main route to siloxane functional group is by condensation of two silanols:

2 R₃Si–OH → R₃Si–O–SiR₃ + H₂O

Usually the silanols are generated in situ by hydrolysis of silyl chlorides. With a disilanol, R₂Si(OH)₂ (derived from double hydrolysis of a silyldichloride), the condensation can afford linear products terminated with silanol groups:

n R₂Si(OH)₂ → H(R₂SiO)_nOH + n−1 H₂O

Alternatively the disilanol can afford cyclic products

n R₂Si(OH)₂ → (R₂SiO)_n + n H₂O

Starting from trisilanols, cages are possible, such as the species with the formula (RSi)_nO_3n/2 with cubic (n = 8) and hexagonal prismatic (n = 12). (RSi)₈O₁₂ structures. The cubic cages is an expanded analogues of the hydrocarbon cubane, with silicon centers at the corners of a cube oxygen centres spanning each of the twelve edges.^[5]

Reactions

Oxidation of organosilicon compounds, including siloxanes, gives silicon dioxide. This conversion is illustrated by the combustion of hexamethylcyclotrisiloxane:

((CH₃)₂SiO)₃ + 12 O₂ → 3 SiO₂ + 6 CO₂ + 9 H₂O

Strong base degrades siloxane group, often affording siloxide salts:

((CH₃)₃Si)₂O + 2 NaOH → 2 (CH₃)₃SiONa + H₂O

This reaction proceeds by production of silanols. Similar reactions are used industrially to convert cyclic siloxanes to linear polymers.^[2]

Nomenclature

Decamethylcyclopentasiloxane, or D₅, a cyclic siloxane.

The word siloxane is derived from the words silicon, oxygen, and alkane. In some cases, siloxane materials are composed of several different types of siloxide groups; these are labeled according the number of Si-O bonds. M-units: (CH₃)₃SiO_0.5, D-units: (CH₃)₂SiO, T-units: (CH₃)SiO_1.5

Cyclic siloxanes (cyclomethicones)	CAS	Linear siloxanes	CAS
D₃: hexamethylcyclotrisiloxane	541-05-9	L₃: octamethyltrisiloxane	107-51-7
D₄: octamethylcyclotetrasiloxane	556-67-2	L₄: decamethyltetrasiloxane	141-62-8
D₅: decamethylcyclopentasiloxane	541-02-6	L₅: dodecamethylpentasiloxane	141-63-9
D₆: dodecamethylcyclohexasiloxane	540-97-6	L₆: tetradecamethylhexasiloxane	107-52-8

Safety and environmental considerations

Because silicones are heavily used in biomedical and cosmetic applications, their toxicology has been intensively examined. "The inertness of silicones toward warmblooded animals has been demonstrated in a number of tests." With an LD₅₀ in rats of>50 g/kg, they are virtually nontoxic.^[6]

Siloxanes such as D4 are pervasive in the environment probably because of their widespread use.^[7]^[8] D4 is toxic to some aquatic organisms, even at low concentrations.^[9] In mammals, it impairs fertility, damages the liver and has an estrogenic effect.^[9] Both D4 and D5 are bioaccumulative.^[9]

In the European Union, D4 and D5 have been deemed hazardous as per the REACH directive. D4 is regulated as a pollutant in Canada.^[8]

Literature

Christoph Rücker, Klaus Kümmerer: Environmental Chemistry of Organosiloxanes. In: Chemical Reviews. 115(1), 2015, p. 466–524, doi:10.1021/cr500319v.

References

^ Siloxanes, IUPAC Gold Book
^ ^a ^b Röshe, L.; John, P.; Reitmeier, R. "Organic Silicon Compounds" Ullmann’s Encyclopedia of Industrial Chemistry. John Wiley and Sons: San Francisco, 2003. doi:10.1002/14356007.a24_021.
^ H. Steinfink, B. Post and I. Fankuchen "The crystal structure of octamethyl cyclotetrasiloxane" Acta Cryst. 1955, vol. 8, 420-424. doi:10.1107/S0365110X55001333
^ "Dichlorosilane–dimethyl ether aggregation: a new motif in halosilane adduct formation" K. Vojinović, U. Losehand, N. W. Mitzel Dalton Trans., 2004, 2578-2581. doi:10.1039/B405684A
^ S. D. Kinrade, J. C. H. Donovan, A. S. Schach and C. T. G. Knight (2002), Two substituted cubic octameric silicate cages in aqueous solution. J. Chem. Soc., Dalton Trans., 1250 - 1252. doi:10.1039/b107758a
^ Moretto, Hans-Heinrich; Schulze, Manfred; Wagner, Gebhard (2005). "Silicones". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_057.
^ Bienkowski, Brian (30 April 2013). "Chemicals from Personal Care Products Pervasive in Chicago Air". Scientific American. Retrieved 8 April 2015.
^ ^a ^b Karpus, Jennifer (20 June 2014). "Exec: Silicone industry must focus on safety, environment". Rubber & Plastic News. Retrieved 8 April 2015.
^ ^a ^b ^c Wang, De-Gao; Norwood, Warren; Alaee, Mehran; Byer, Jonatan D.; Brimble, Samantha (October 2013). "Review of recent advances in research on the toxicity, detection, occurrence and fate of cyclic volatile methyl siloxanes in the environment". Chemosphere 93 (5): 711–725. doi:10.1016/j.chemosphere.2012.10.041. |access-date= requires |url= (help)

External links

EPA report: Siloxane D5 in Dry-cleaning^{[dead link]}
Journal Of Protective Coatings And Linings: Field Performance of Polysiloxanes

English Journal

Physicochemical and biological evaluation of poly(ethylene glycol) methacrylate grafted onto poly(dimethyl siloxane) surfaces for prosthetic devices.

Gonçalves S, Leirós A, van Kooten T, Dourado F, Rodrigues LR.SourceIBB-Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal.
Colloids and surfaces. B, Biointerfaces.Colloids Surf B Biointerfaces.2013 Sep 1;109:228-35. doi: 10.1016/j.colsurfb.2013.03.050. Epub 2013 Apr 9.
Poly(dimethyl siloxane) (PDMS) was surface-polymerized with poly(ethylene glycol)methacrylate (PEGMA) by surface-initiated atom transfer radical polymerization (SI-ATRP) in aqueous media at room temperature. Modification of the PDMS surface followed a three-step procedure: (i) PDMS surface hydroxyla
PMID 23660308

Spectroscopic and chromatographic characterisation of a pentafluorophenylpropyl silica phase end-capped in supercritical carbon dioxide as a reaction solvent.

Ashu-Arrah BA, Glennon JD, Albert K.SourceInnovative Chromatography Group, Irish Separation Science Cluster (ISSC), Department of Chemistry, and the Analytical and Biological Chemistry Research Facility (ABCRF), University College Cork, Cork, Ireland. Electronic address: b.ashuashuarrah@ucc.ie.
Journal of chromatography. A.J Chromatogr A.2013 Jul 12;1298:86-94. doi: 10.1016/j.chroma.2013.05.024. Epub 2013 May 16.
This research uses solid-state nuclear magnetic resonance (NMR) spectroscopy to characterise the nature and amount of different surface species, and chromatography to evaluate phase properties of a pentafluorophenylpropyl (PFPP) bonded silica phase prepared and end-capped using supercritical carbon
PMID 23746646

In Search of the Chemical Basis of the Hemolytic Potential of Silicas.

Pavan C, Tomatis M, Ghiazza M, Rabolli V, Bolis V, Lison DF, Fubini B.AbstractThe membranolytic activity of silica particles toward red blood cells (RBCs) has been known for a long time and sometimes associated to silica pathogenicity. However, the molecular mechanism and the reasons why hemolysis differs according to the silica form are still obscure. A panel of 15 crystalline (pure and commercial) and amorphous (pyrogenic, precipitated from aqueous solutions, vitreous) silica samples differing in size, origin, morphology and surface chemical composition were selected and specifically prepared. Silica particles were grouped into six groups to compare their potential in disrupting RBC membranes, so that one single property differed in each group, while other features were constant. Free radical production and crystallinity were not strict determinants of the hemolytic activity. Particle curvature and morphology modulated the hemolytic effect, but silanols and siloxane bridges at the surface were the main actors. Hemolysis was unrelated to the overall concentration of silanols as fully rehydrated surfaces (such as those obtained from aqueous solution) were inert and one pyrogenic silica also lost its membranolytic potential upon progressive dehydration. Overall results are consistent with a model whereby hemolysis is determined by a defined surface distribution of dissociated/undissociated silanols and siloxane groups strongly interacting with specific epitopes on the RBC membrane.
Chemical research in toxicology.Chem Res Toxicol.2013 Jul 2. [Epub ahead of print]
The membranolytic activity of silica particles toward red blood cells (RBCs) has been known for a long time and sometimes associated to silica pathogenicity. However, the molecular mechanism and the reasons why hemolysis differs according to the silica form are still obscure. A panel of 15 crystalli
PMID 23819533