Siloxane

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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. 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. The functional group R₃SiO- (where the three Rs may be different) is called siloxy.

Video Siloxane

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°. 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°. 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.

Maps Siloxane

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 are cubane-type clusters, with silicon centers at the corners of a cube oxygen centres spanning each of the twelve edges.

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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.

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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. 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 low to high viscosity), 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. Most cyclomethicone is manufactured by Dow Corning. Cyclomethicones have been shown to involve the occurrence of silanols during biodegration in mammals. The resulting silanols are capable of inhibiting hydrolytic enzymes such as thermolysin, acetylcholinesterase, however the doses required for inhibition are by orders of magnitude higher than the ones resulting from the accumulated exposure to consumer products containing cyclomethicones.

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Nomenclature

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)₃SiO_0.5, D-units: (CH₃)₂SiO, T-units: (CH₃)SiO_1.5

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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.

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. The cyclomethicones D₄ and D₅ are bioaccumulative in some aquatic organisms, according to one report. However, Norwegian researcher Nicholas Warner cautioned that findings about bioaccumulation have been largely based on laboratory studies, while field studies of bioaccumulation are still contradicting each other. "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 to decrease as you proceed higher up in the food chain. This raises several questions about which factors influence the bioaccumulation potential of siloxanes."

In the European Union, D₄ and D₅ have been deemed hazardous as per the REACH regulation. Canada regulates D₄ under a pollution prevention plan. A scientific review in Canada in 2011 concluded that "Siloxane D5 does not pose a danger to the environment."

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Literature

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

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References

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External links

• EPA report: Siloxane D5 in Dry-cleaning
• Journal Of Protective Coatings And Linings: Field Performance of Polysiloxanes

Source of the article : Wikipedia