Inorganosilicon chemistry, asiloxane is anorganic compound containing afunctional group of twosilicon atoms bound to anoxygen atom:Si−O−Si. Theparent siloxanes include theoligomeric andpolymerichydrides with the formulaeH[OSiH2]nOH and[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 thebackbone ofsilicones[−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.
Siloxanes generally adopt structures expected for linked tetrahedral ("sp3-like") centers. TheSi−Obond length is 1.64 Å (vs Si–C distance of 1.92 Å) and theSi-O-Si angle is rather open at 142.5°.[3] By contrast, the C−O distance in a typicaldialkyl 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 lowglass transition temperatures.
The main route to siloxane functional group is byhydrolysis ofsilicon chlorides:
The reaction proceeds via the initial formation ofsilanols (R3Si−OH):
The siloxane bond can then form via a silanol + silanol pathway or a silanol + chlorosilane pathway:
Hydrolysis of a silyldichloride can afford linear or cyclic products. Linear products are terminated with silanol groups:
Cyclic products have no silanol termini:
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/2 with 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]
Oxidation of organosilicon compounds, including siloxanes, givessilicon dioxide. This conversion is illustrated by the combustion ofhexamethylcyclotrisiloxane:
Strong base degrades siloxane group, often affordingsiloxidesalts:
This reaction proceeds by production of silanols. Similar reactions are used industrially to convert cyclic siloxanes to linear polymers.[2]
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 pump fluid.
Cyclomethicones are a group of methyl siloxanes, a class of liquid silicones (cyclic polydimethylsiloxane polymers) that possess the characteristics of lowviscosity and highvolatility as well as being skinemollients and in certain circumstances useful cleaning solvents.[7] Unlikedimethicones, which arelinear siloxanes that do notevaporate, cyclomethicones arecyclic: both groups consist of abackbone 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, 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]
The wordsiloxane is 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:
| 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 |
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 likePCBs, several other studies in theOslo Fjord in Norway,Lake Pepin in the US, andLake 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 cyclomethiconesD4 andD5 arebioaccumulative in some aquatic organisms, according to one report.[17]
In the European Union, D4, D5, D6 and octamethyltrisiloxane have been deemed hazardous as per theREACH regulation. They were characterized assubstances of very high concern (SVHC) in 2024 due to theirPBT (persistent,bioaccumulative and toxic) andvPvB (very persistent and very bioaccumulative) properties and were subsequently added to the REACH Candidate List in 2025.[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]
