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Siloxane

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Polydimethylsiloxane is a prevalent siloxane.

Inorganosilicon chemistry,asiloxaneis anorganic compoundcontaining afunctional groupof twosiliconatoms bound to anoxygenatom:Si−O−Si.Theparentsiloxanes include theoligomericandpolymerichydrideswith the formulaeH[OSiH2]nOHand[OSiH2]n.[1]Siloxanes also includebranched compounds,the defining feature of which is that each pair of silicon centres is separated by one oxygen atom. The siloxane functional group forms thebackboneofsilicones[−R2Si−O−SiR2−]n,the premier example of which ispolydimethylsiloxane(PDMS).[2]The functional groupR3SiO−(where the three Rs may be different) is calledsiloxy.Siloxanes are manmade and have many commercial and industrial applications because of the compounds’hydrophobicity,lowthermal conductivity,and high flexibility.

Structure[edit]

Siloxanes generally adopt structures expected for linked tetrahedral ( "sp3-like ") centers. TheSi−Obond lengthis 1.64 Å (vs Si–C distance of 1.92 Å) and theSi-O-Si angleis rather open at 142.5°.[3]By contrast, the C−O distance in a typicaldialkyl etheris 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 lowglass transition temperatures.

Synthesis of siloxanes[edit]

Dimethyldichlorosilane(Si(CH3)2Cl2) is a key precursor to cyclic (D3,D4,etc.) and linear siloxanes.[5]

The main route to siloxane functional group is byhydrolysisofsilicon chlorides:

2 R3Si−Cl + H2O → R3Si−O−SiR3+ 2 HCl

The reaction proceeds via the initial formation ofsilanols(R3Si−OH):

R3Si−Cl + H2O → R3Si−OH + HCl

The siloxane bond can then form via a silanol + silanol pathway or a silanol + chlorosilane pathway:

2 R3Si−OH → R3Si−O−SiR3+ H2O
R3Si−OH + R3Si−Cl → R3Si−O−SiR3+ HCl

Hydrolysis of a silyldichloride can afford linear or cyclic products. Linear products are terminated with silanol groups:

nR2Si(OH)2→ H(R2SiO)nOH + (n− 1) H2O

Cyclic products have no silanol termini:

nR2Si(OH)2→ (R2SiO)n+nH2O

The linear products,polydimethylsiloxane(PDMS), are of great commercial value. Their production requires the production ofdimethylsilicon dichloride.

Starting from trisilanols, cages are possible, such as the species with the formula (RSi)nO3n/2with cubic (n= 8) and hexagonal prismatic (n= 12) structures. The cubic cages arecubane-type clusters,with silicon centers at the corners of a cube oxygen centres spanning each of the twelve edges.[6]

Reactions[edit]

Oxidation of organosilicon compounds, including siloxanes, givessilicon dioxide.This conversion is illustrated by the combustion ofhexamethylcyclotrisiloxane:

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

Strong base degrades siloxane group, often affordingsiloxidesalts:

((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]

Uses[edit]

Polysiloxanes (silicones),upon combustion in an inert atmosphere, generally undergo pyrolysis to form silicon oxycarbide orsilicon carbide(SiC). By exploiting this reaction, polysiloxanes have been used as preceramic polymers in various processes including additive manufacturing.Polyvinyl siloxane (vinyl polysiloxane)is used to make dental impressions and industrial impressions. The use of a poly-siloxane precursor in polymer derived ceramics allows the formation of ceramic bodies with complex shapes, although the significant shrinkage in pyrolysis needs to be taken into account.[citation needed]

Trisiloxanes may be used asdiffusion pumpfluid.

Cyclomethicones[edit]

Cyclomethicones are a group of methyl siloxanes, a class of liquid silicones (cyclic polydimethylsiloxane polymers) that possess the characteristics of lowviscosityand highvolatilityas well as being skinemollientsand in certain circumstances useful cleaning solvents.[7]Unlikedimethicones,which arelinearsiloxanes that do notevaporate,cyclomethicones arecyclic:both groups consist of abackboneof [(CH3)2SiO]n.They are used in many cosmetic products including deodorants and antiperspirants which need to coat the skin but not remain tacky afterward.[8]Dowis a major producer of cyclomethicones.[9]

Cyclomethicones, like all siloxanes, degrade by hydrolysis, producingsilanols.[10]These silanols are produced at such low levels that they do not interfere with hydrolytic enzymes.[11]Even though some cyclomethicones structurally resemblecrown ethers,they bind metal ions only weakly.[12]

Nomenclature[edit]

Decamethylcyclopentasiloxane,or D5,a cyclic siloxane

The wordsiloxaneis derived from the wordssilicon,oxygen, andalkane.In some cases, siloxane materials are composed of several different types of siloxane groups; these are labeled according to the number of Si−O bonds:

M-units: (CH3)3SiO0.5,
D-units: (CH3)2SiO,
T-units: (CH3)SiO1.5.
Cyclic siloxanes (cyclomethicones) CAS Linear siloxanes CAS
L2,MM: hexamethyldisiloxane 107-46-0
D3:hexamethylcyclotrisiloxane 541-05-9 L3,MDM: octamethyltrisiloxane 107-51-7
D4:octamethylcyclotetrasiloxane 556-67-2 L4,MD2M: decamethyltetrasiloxane 141-62-8
D5:decamethylcyclopentasiloxane 541-02-6 L5,MD3M: dodecamethylpentasiloxane 141-63-9
D6:dodecamethylcyclohexasiloxane 540-97-6 L6,MD4M: tetradecamethylhexasiloxane 107-52-8

Safety and environmental considerations[edit]

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 LD50in rats of >50 g/kg, they are virtually nontoxic.[13]Questions remain however about chronic toxicity or the consequences of bioaccumulation since siloxanes can be long-lived.

Findings about bioaccumulation have been largely based on laboratory studies. Field studies of bioaccumulation have not reached consensus. "Even if the concentrations of siloxanes we have found in fish are high compared to concentrations of classical contaminants likePCBs,several other studies in theOslo Fjordin Norway,Lake Pepinin the US, andLake Eriein Canada have shown concentrations of siloxanes decrease at higher range in the food chain. This finding raises questions about which factors influence the bioaccumulation potential of siloxanes. "[14]

Cyclomethicones are ubiquitous because they are widely used in biomedical and cosmetic applications. They can be found at high levels in American cities. They can be toxic to aquatic animals in concentrations often found in the environment.[15][16]The cyclomethiconesD4andD5arebioaccumulativein some aquatic organisms, according to one report.[17]

In the European Union, D4,D5and D6have been deemed hazardous as per theREACHregulation. They were characterized assubstances of very high concern(SVHC) due to theirPBTandvPvBproperties.[18]Canada regulates D4under a pollution prevention plan.[15]A scientific review in Canada in 2011 concluded that "Siloxane D5 does not pose a danger to the environment."[19]

Literature[edit]

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

References[edit]

  1. ^Siloxanes,IUPAC Gold Book.
  2. ^abRöshe, 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.
  3. ^H. Steinfink, B. Post and I. Fankuchen. "The crystal structure of octamethyl cyclotetrasiloxane". Acta Crystallogr. 1955, vol. 8, 420–424.doi:10.1107/S0365110X55001333.
  4. ^K. Vojinović, U. Losehand, N. W. Mitzel. "Dichlorosilane–dimethyl ether aggregation: a new motif in halosilane adduct formation". Dalton Trans., 2004, 2578–2581.doi:10.1039/B405684A.
  5. ^Silicon: Organosilicon Chemistry. Encyclopedia of Inorganic Chemistry Online, 2nd ed.; Wiley: New Jersey, 2005.doi:10.1002/0470862106.ia220.
  6. ^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.
  7. ^Barbara Kanegsberg; Edward Kanegsberg (2011).Handbook for Critical Cleaning: Cleaning Agents and Systems.CRC. p. 19.ISBN978-1-4398-2827-4.
  8. ^Amarjit Sahota (25 November 2013).Sustainability: How the Cosmetics Industry is Greening Up.Wiley. p. 208.ISBN978-1-118-67650-9.
  9. ^Meyer Rosen (23 September 2005).Delivery System Handbook for Personal Care and Cosmetic Products: Technology, Applications and Formulations.William Andrew. p. 693.ISBN978-0-8155-1682-8.
  10. ^S. Varaprath, K. L. Salyers, K. P. Plotzke and S. Nanavati "Identification of Metabolites of Octamethylcyclotetrasiloxane (D4) in Rat Urine" Drug Metab Dispos 1999, 27, 1267-1273.
  11. ^R. Pietschnig, S. Spirk (2016). "The Chemistry of Organo Silanetriols".Coord. Chem. Rev.323:87–106.doi:10.1016/j.ccr.2016.03.010.
  12. ^F. Dankert, C. von Hänisch (2021). "Siloxane Coordination Revisited: Si−O Bond Character, Reactivity and Magnificent Molecular Shapes".Eur. J. Inorg. Chem.2021(29): 2907–2927.doi:10.1002/ejic.202100275.S2CID239645449.
  13. ^Moretto, Hans-Heinrich; Schulze, Manfred; Wagner, Gebhard (2005). "Silicones".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a24_057.ISBN3527306730.
  14. ^"Siloxanes: Soft, shiny -- and dangerous?" by Christine Solbakken, Science Nordic, August 28, 2015. Retrieved May 31, 2016
  15. ^abKarpus, Jennifer (20 June 2014)."Exec: Silicone industry must focus on safety, environment".Rubber & Plastic News.Retrieved8 April2015.
  16. ^Bienkowski, Brian (30 April 2013)."Chemicals from Personal Care Products Pervasive in Chicago Air".Scientific American.Retrieved8 April2015.
  17. ^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.Bibcode:2013Chmsp..93..711W.doi:10.1016/j.chemosphere.2012.10.041.PMID23211328.
  18. ^"Candidate List of substances of very high concern for Authorisation".ECHA.Retrieved2019-12-18.
  19. ^Report of the Board of Review for Decamethylcyclopentasiloxane (Siloxane D5) established under Section 333(1) of the Canadian Environmental Protection Act of 1999, October 20, 2011

External links[edit]

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