More precisely called polymerized siloxanes or polysiloxanes, silicones are mixed inorganic-organic polymers with the chemical formula [R₂SiO]_n, where R is an organic group such as methyl, ethyl, or phenyl. These materials consist of an inorganic silicon-oxygen backbone (⋯-Si-O-Si-O-Si-O-⋯) with organic side groups attached to the silicon atoms, which are four-coordinate.

In some cases organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common siloxane is linear polydimethylsiloxane (PDMS), a silicone oil. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.

[edit]Synthesis

Silicones are synthesized from chlorosilanes, tetraethoxysilane, and related compounds. In the case of PDMS, the starting material is dimethyldichlorosilane, which reacts with water as follows:

n Si(CH₃)₂Cl₂ + n H₂O → [Si(CH₃)₂O]_n + 2n HCl

During polymerization, this reaction evolves hazardous hydrogen chloride gas. For medical uses, a process was developed where the chlorine atoms in the silane precursor were replaced with acetate groups. The reaction product of the final curing process is the less dangerous acetic acid (the one found in vinegar). As a side effect, the curing process is much slower in this case. This is the chemistry used in many consumer applications, such as silicone caulk and adhesives.

Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce branches or cross-links in the polymer chain. Ideally, each molecule of such a compound becomes a branch point. This can be used to produce hard silicone resins. Similarly, precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain.

Modern silicone resins are made with tetraethoxysilane, which reacts in a more mild and controllable manner than chlorosilanes.

[edit]Chemical terminology

Silicone is not to be confused with "silicon." Although silicones contain silicon atoms, they are not made up exclusively of silicon, and have different physical characteristics from elemental silicon.

F. S. Kipping coined the word "silicone" in 1901 to describe polydiphenylsiloxane by analogy of its formula, Ph₂SiO, with the formula of the ketone benzophenone, Ph₂CO (Ph stands for phenyl, C₆H₅). Kipping was well aware that polydiphenylsiloxane is polymeric whereas benzophenone is monomeric and noted that Ph₂SiO and Ph₂CO had very different chemistry.^[2][3]

A true silicone group with a double bond between oxygen and silicon does not commonly exist in nature; chemists find that the silicon atom much prefers a single bond with each of two oxygen atoms, rather than a double bond to a single atom. Polysiloxanes are still more commonly known as "silicones".

Molecules containing silicon-oxygen double bonds do exist and are called silanones. Several silanones have been studied in argon matrices^[4] and in the gas phase, but they are highly reactive.^[5] Despite their reactivity, silanones are important as intermediates in gas-phase processes such as chemical vapor deposition inmicroelectronics production, in the formation of ceramics by combustion,^[6] and in astrochemistry.^[7]