Siloxane

Not to be confused with Silicone.

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)nO3n/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]

Cyclomethicones

Cyclomethicones are a group of methyl siloxanes, a class of liquid silicones (cyclic polydimethylsiloxane polymers) that possess the characteristics of low viscosity and high volatility as well as being skin emollients and in certain circumstances useful cleaning solvents.[6] Unlike dimethicones, which are linear siloxanes that do not evaporate, cyclomethicones are cyclic: both groups consist of a polymer featuring a monomer backbone of one silicon and two oxygen atoms bonded together, but instead of having a very long "linear" backbone surrounded by a series of methyl groups (which produces a clear, non-reactive, non-volatile liquid ranging from water-thin to taffy-thick), cyclomethicones have short backbones that make closed or nearly-closed rings or "cycles" with their methyl groups, giving them many of the same properties of dimethicones but making them much more volatile. They are used in many cosmetic products where eventual complete evaporation of the siloxane carrier fluid is desired. In this way they are useful for products like deodorants and antiperspirants which need to coat the skin but not remain tacky afterward.[7] Most cyclomethicone is manufactured by Dow Corning.[8]

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 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

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.[9]

Cyclomethicones are ubiquitous because they are widely used in biomedical and cosmetic applications and can be found at high levels in American cities and can be toxic to aquatic animals in concentrations often found in the environment.[10][11] The cyclomethicone D4 is bioaccumulative[12] D4 can cause liver damage, has an estrogenic effect and is known to impair fertility in mammals.[12]

In the European Union, D4 and D5 have been deemed hazardous as per the REACH directive. D4 is regulated as a pollutant in Canada.[10]

D5 and D4 have attracted attention because they are pervasive. A scientific review in Canada in 2011 concluded that "Siloxane D5 does not pose a danger to the environment."[13]

Literature

References

  1. ↑ Siloxanes, IUPAC Gold Book
  2. 1 2 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.
  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. ↑ "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
  5. ↑ 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
  6. ↑ Barbara Kanegsberg; Edward Kanegsberg (2011). Handbook for Critical Cleaning: Cleaning agents and systems. CRC. p. 19. ISBN 978-1-4398-2827-4.
  7. ↑ Amarjit Sahota (25 November 2013). Sustainability: how the cosmetics industry is greening up. Wiley. p. 208. ISBN 978-1-118-67650-9.
  8. ↑ Meyer Rosen (23 September 2005). Delivery System Handbook for Personal Care and Cosmetic Products: Technology, Applications and Formulations. William Andrew. p. 693. ISBN 978-0-8155-1682-8.
  9. ↑ Moretto, Hans-Heinrich; Schulze, Manfred; Wagner, Gebhard (2005). "Silicones". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_057.
  10. 1 2 Karpus, Jennifer (20 June 2014). "Exec: Silicone industry must focus on safety, environment". Rubber & Plastic News. Retrieved 8 April 2015.
  11. ↑ Bienkowski, Brian (30 April 2013). "Chemicals from Personal Care Products Pervasive in Chicago Air". Scientific American. Retrieved 8 April 2015.
  12. 1 2 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.
  13. ↑ 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

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