Chemical structure of the silicone polydimethylsiloxane (PDMS).
More precisely called polymerized siloxanes or polysiloxanes, silicones consist of an inorganic silicon-oxygen backbone chain (⋯-Si-O-Si-O-Si-O-⋯) with organic side groups attached to the silicon atoms. These silicon atoms are tetravalent. So, silicones are polymers constructed from inorganic-organic monomers. Silicones have in general the chemical formula [R2SiO]n, where R is an organic group such as an alkyl (methyl, ethyl) or phenyl group.
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.
Terminology and history
F. S. Kipping coined the word silicone in 1901 to describe polydiphenylsiloxane by analogy of its formula, Ph2SiO (Ph stands for phenyl, C6H5), with the formula of the ketone benzophenone, Ph2CO (his term was originally silicoketone). Kipping was well aware that polydiphenylsiloxane is polymeric whereas benzophenone is monomeric and noted that Ph2SiO and Ph2CO had very different chemistry. The discovery of the structural differences between Kipping's molecules and the ketones means that silicone is no longer the correct term (though it remains in common usage) and that the term siloxanes is correct according to the nomenclature of modern chemistry.
Silicone is sometimes mistakenly referred to as silicon. The chemical element silicon is a crystalline metalloid widely used in computers and other electronic equipment. Although silicones contain silicon atoms, they also include carbon, hydrogen, oxygen, and perhaps other kinds of atoms as well, and have physical and chemical properties that are very different from elemental silicon.
A true silicone group with a double bond between oxygen and silicon does not commonly exist in nature; chemists find that the silicon atom prefer to form single bonds with each of two oxygen atoms, forming a chain, rather than a double bond to a single atom. Polysiloxanes are among the many substances commonly known as "silicones".
Molecules containing silicon-oxygen double bonds do exist and are called silanones but they are very reactive. Despite this, silanones are important as intermediates in gas-phase processes such as chemical vapor deposition in microelectronics production, and in the formation of ceramics by combustion.
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Most common are materials based on polydimethylsiloxane, which is derived by hydrolysis of dimethyldichlorosilane. This dichloride reacts with water as follows:
- n Si(CH3)2Cl2 + n H2O → [Si(CH3)2O]n + 2n HCl
The polymerization typically produces linear chains capped with Si-Cl or Si-OH (silanol) groups. Under different conditions the polymer is a cyclic, not a chain.
For consumer applications such as caulks silyl acetates are used instead of silyl chlorides. The hydrolysis of the acetates produce the less dangerous acetic acid (the acid found in vinegar) as the reaction product of a much slower curing process. This chemistry is used in many consumer applications, such as silicone caulk and adhesives.
Branches or cross-links in the polymer chain can be introduced by using organosilicone precursors with fewer alkyl groups, such as methyltrichlorosilane and methyltrimethoxysilane. Ideally, each molecule of such a compound becomes a branch point. This process 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.
When silicone is burned in air or oxygen, it forms solid silica (silicon dioxide)(SiO2) as a white powder, char, and various gases. The readily dispersed powder is sometimes called silica fume.