Chemical structure of the silicone polydimethylsiloxane (PDMS).
More precisely called
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 (
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
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 Kippings' 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 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 usually forms single bonds with each of two oxygen atoms, 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.
|This section needs expansion
adding to it
. (April 2015)
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
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