Siloxane

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Polydimethylsiloxane is a prevalent siloxane. PmdsStructure.png
Polydimethylsiloxane is a prevalent siloxane.

In organosilicon chemistry, a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: Si−O−Si. 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 [−R2Si−O−SiR2−]n, the premier example of which is polydimethylsiloxane (PDMS). [2] The functional group R3SiO− (where the three Rs may be different) is called siloxy. Siloxanes are manmade and have many commercial and industrial applications because of the compounds’ hydrophobicity, low thermal conductivity, and high flexibility.

Contents

Structure

Siloxanes generally adopt structures expected for linked tetrahedral ("sp3-like") centers. The Si−O bond length 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 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

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

The main route to siloxane functional group is by hydrolysis of silicon chlorides:

2 R3Si−Cl + H2O → R3Si−O−SiR3 + 2 HCl

The reaction proceeds via the initial formation of silanols (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:

n R2Si(OH)2 → H(R2SiO)nOH + (n − 1) H2O

Cyclic products have no silanol termini:

n R2Si(OH)2 → (R2SiO)n + n H2O

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

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) structures. The cubic cages are cubane-type clusters, with silicon centers at the corners of a cube oxygen centres spanning each of the twelve edges. [6]

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]

Uses

Polysiloxanes (silicones), upon combustion in an inert atmosphere, generally undergo pyrolysis to form silicon oxycarbide or silicon 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 as diffusion pump fluid.

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. [7] Unlike dimethicones, which are linear siloxanes that do not evaporate, cyclomethicones are cyclic: both groups consist of a backbone of [(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] Dow is a major producer of cyclomethicones. [9]

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

Nomenclature

Decamethylcyclopentasiloxane, or D5, a cyclic siloxane Decamethylcyclopentasiloxane.svg
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 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)CASLinear siloxanesCAS
L2, MM: hexamethyldisiloxane107-46-0
D3: hexamethylcyclotrisiloxane 541-05-9L3, MDM: octamethyltrisiloxane107-51-7
D4: octamethylcyclotetrasiloxane 556-67-2L4, MD2M: decamethyltetrasiloxane141-62-8
D5: decamethylcyclopentasiloxane 541-02-6L5, MD3M: dodecamethylpentasiloxane141-63-9
D6: dodecamethylcyclohexasiloxane540-97-6L6, MD4M: tetradecamethylhexasiloxane107-52-8

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. [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 like PCBs, several other studies in the Oslo Fjord in Norway, Lake Pepin in the US, and Lake Erie in 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 cyclomethicones D4 and D5 are bioaccumulative in some aquatic organisms, according to one report. [17]

In the European Union, D4, D5 and D6 have been deemed hazardous as per the REACH regulation. They were characterized as substances of very high concern (SVHC) due to their PBT and vPvB properties. [18] Canada regulates D4 under 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

Related Research Articles

<span class="mw-page-title-main">Silicone</span> Family of polymers of the repeating form [R2Si–O–SiR2]

In organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane. They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking utensils, thermal insulation, and electrical insulation. Some common forms include silicone oil, grease, rubber, resin, and caulk.

<span class="mw-page-title-main">Silanol</span> Si–OH functional group in silicon chemistry

A silanol is a functional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in all alcohols. Silanols are often invoked as intermediates in organosilicon chemistry and silicate mineralogy. If a silanol contains one or more organic residues, it is an organosilanol.

<span class="mw-page-title-main">Polydimethylsiloxane</span> Chemical compound

Polydimethylsiloxane (PDMS), also known as dimethylpolysiloxane or dimethicone, is a silicone polymer with a wide variety of uses, from cosmetics to industrial lubrication.

<span class="mw-page-title-main">Silicone resin</span> Type of silicone material

Silicone resins are a type of silicone material which is formed by branched, cage-like oligosiloxanes with the general formula of RnSiXmOy, where R is a non-reactive substituent, usually methyl or phenyl, and X is a functional group: hydrogen, hydroxyl, chlorine or alkoxy. These groups are further condensed in many applications, to give highly crosslinked, insoluble polysiloxane networks.

<span class="mw-page-title-main">Silazane</span> Class of chemical compounds

A silazane is a family of compounds with Si-N bonds. Usually the Si and N have organic substituents. They are analogous to siloxanes, with -NR- replacing -O-.

In inorganic chemistry, chlorosilanes are a group of reactive, chlorine-containing chemical compounds, related to silane and used in many chemical processes. Each such chemical has at least one silicon-chlorine bond. Trichlorosilane is produced on the largest scale. The parent chlorosilane is silicon tetrachloride.

<span class="mw-page-title-main">Silicone rubber</span> Elastomer

Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from −55 to 300 °C while still maintaining its useful properties. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including voltage line insulators; automotive applications; cooking, baking, and food storage products; apparel such as undergarments, sportswear, and footwear; electronics; medical devices and implants; and in home repair and hardware, in products such as silicone sealants.

<span class="mw-page-title-main">Organosilicon chemistry</span> Organometallic compound containing carbon–silicon bonds

Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, flammable, hydrophobic, and stable to air. Silicon carbide is an inorganic compound.

<span class="mw-page-title-main">Decamethylcyclopentasiloxane</span> Chemical compound

Decamethylcyclopentasiloxane, also known as D5 and D5, is an organosilicon compound with the formula [(CH3)2SiO]5. It is a colorless and odorless liquid that is slightly volatile.

Trimethylsilanol (TMS) is an organosilicon compound with the formula (CH3)3SiOH. The Si centre bears three methyl groups and one hydroxyl group. It is a colourless volatile liquid.

Dimethyldichlorosilane is a tetrahedral, organosilicon compound with the formula Si(CH3)2Cl2. At room temperature it is a colorless liquid that readily reacts with water to form both linear and cyclic Si-O chains. Dimethyldichlorosilane is made on an industrial scale as the principal precursor to dimethylsilicone and polysilane compounds.

<span class="mw-page-title-main">Methyltrichlorosilane</span> Chemical compound

Methyltrichlorosilane, also known as trichloromethylsilane, is a monomer and organosilicon compound with the formula CH3SiCl3. It is a colorless liquid with a sharp odor similar to that of hydrochloric acid. As methyltrichlorosilane is a reactive compound, it is mainly used a precursor for forming various cross-linked siloxane polymers.

<span class="mw-page-title-main">Silsesquioxane</span> Molecular compound with applications in ceramics

A silsesquioxane is an organosilicon compound with the chemical formula [RSiO3/2]n. Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices. Silsesquioxanes are members of polyoctahedral silsesquioxanes ("POSS"), which have attracted attention as preceramic polymer precursors to ceramic materials and nanocomposites. Diverse substituents (R) can be attached to the Si centers. The molecules are unusual because they feature an inorganic silicate core and an organic exterior. The silica core confers rigidity and thermal stability.

<span class="mw-page-title-main">Disiloxane</span> Chemical compound

Disiloxane has the chemical formula Si
2H
6O. It is the simplest known siloxane with hydrogen only R groups. The molecule contains six equivalent Si-H bonds and two equivalent Si-O bonds. Disiloxane exists as a colorless, pungent gas under standard conditions. However, it is generally safe for human use as evidence in its widespread use in cosmetics. It is also commonly known as disilyl ether, disilyl oxide, and perhydrodisiloxane

In organosilicon chemistry, polysilazanes are polymers in which silicon and nitrogen atoms alternate to form the basic backbone. Since each silicon atom is bound to two separate nitrogen atoms and each nitrogen atom to two silicon atoms, both chains and rings of the formula [R2Si−NR]n occur. R can be hydrogen atoms or organic substituents. If all substituents R are hydrogen atoms, the polymer is designated as perhydropolysilazane, polyperhydridosilazane, or inorganic polysilazane ([H2Si−NH]n). If hydrocarbon substituents are bound to the silicon atoms, the polymers are designated as Organopolysilazanes. Molecularly, polysilazanes [R2Si−NH]n are isoelectronic with and close relatives to polysiloxanes [R2Si−O]n (silicones).

<span class="mw-page-title-main">Polysilane</span>

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. The simplest polysilane would be (SiH2)n, which is mainly of theoretical, not practical interest.

<span class="mw-page-title-main">Octamethylcyclotetrasiloxane</span> Chemical compound

Octamethylcyclotetrasiloxane, also called D4, is an organosilicon compound with the formula [(CH3)2SiO]4. It is a colorless viscous liquid. It is a common cyclomethicone. It is widely used in cosmetics.

<span class="mw-page-title-main">Organosilanols</span> Organic compounds of the form R3–Si–OH

In organosilicon chemistry, organosilanols are a group of chemical compounds derived from silicon. More specifically, they are carbosilanes derived with a hydroxy group on the silicon atom. Organosilanols are the silicon analogs to alcohols. Silanols are more acidic and more basic than their alcohol counterparts and therefore show a rich structural chemistry characterized by hydrogen bonding networks which are particularly well studied for silanetriols.

<span class="mw-page-title-main">Hexamethylcyclotrisiloxane</span> Chemical compound

Hexamethylcyclotrisiloxane, also known as D3 and D3, is the organosilicon compound with the formula [(CH3)2SiO]3. It is a colorless or white volatile solid. It finds limited use in organic chemistry. The larger tetrameric and pentameric siloxanes, respectively octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane, are of significant industrial interest, whereas 1,000–10,000 tonnes per year of the trimer is manufactured and/or imported in the European Economic Area.

In organosilicon chemistry, silanes are a diverse class of charge-neutral organic compounds with the general formula SiR4. The R substituents can any combination of organic or inorganic groups. Most silanes contain Si-C bonds, and are discussed under organosilicon compounds. Some contain Si-H bonds and are discussed under hydrosilanes.

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. ↑ 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. ISBN   978-1-4398-2827-4.
  8. ↑ Amarjit Sahota (25 November 2013). Sustainability: How the Cosmetics Industry is Greening Up. Wiley. p. 208. ISBN   978-1-118-67650-9.
  9. ↑ Meyer Rosen (23 September 2005). Delivery System Handbook for Personal Care and Cosmetic Products: Technology, Applications and Formulations. William Andrew. p. 693. ISBN   978-0-8155-1682-8.
  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. S2CID   239645449.
  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.
  14. ↑ "Siloxanes: Soft, shiny -- and dangerous?" by Christine Solbakken, Science Nordic, August 28, 2015. Retrieved May 31, 2016
  15. 1 2 Karpus, Jennifer (20 June 2014). "Exec: Silicone industry must focus on safety, environment". Rubber & Plastic News. Retrieved 8 April 2015.
  16. ↑ Bienkowski, Brian (30 April 2013). "Chemicals from Personal Care Products Pervasive in Chicago Air". Scientific American. Retrieved 8 April 2015.
  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. PMID   23211328.
  18. ↑ "Candidate List of substances of very high concern for Authorisation". ECHA . Retrieved 2019-12-18.
  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
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