Chemistry

More precisely called polymerized siloxanes or polysiloxanes. Silicones consist of an inorganic silicon–oxygen backbone chain (⋯−Si−O−Si−O−Si−O−⋯) with two organic groups attached to each silicon center. Commonly, the organic groups are methyl. The materials can be cyclic or polymeric. 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.^{[citation needed]} The second-largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.^{[citation needed]}

Terminology and history[edit]

F. S. Kipping coined the word silicone in 1901 to describe the formula of polydiphenylsiloxane, Ph₂SiO (Ph denoting phenyl, C₆H₅), by analogy with the formula of the ketone benzophenone, Ph₂CO (his term was originally silicoketone). Kipping was well aware that polydiphenylsiloxane is polymeric^{[citation needed]} whereas benzophenone is monomeric and noted the contrasting properties of Ph₂SiO and Ph₂CO.^[3][4] 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 siloxane is preferred according to the nomenclature of modern chemistry.^[5]

Silicone is often confused with silicon, but they are distinct substances. Silicon is a chemical element, a hard dark-grey semiconducting metalloid, which in its crystalline form is used to make integrated circuits (“electronic chips”) and solar cells. Silicones are compounds that contain. Silicon, carbon, hydrogen, oxygen, and perhaps other kinds of atoms as well, and have many very different physical and chemical properties.

Compounds containing silicon–oxygen double bonds, now called silanones, but which could deserve the name “silicone”. Have long been identified as intermediates in gas-phase processes such as chemical vapor deposition in microelectronics production, and in the formation of ceramics by combustion.^[6] However, they have a strong tendency to polymerize into siloxanes. The first stable silanone was obtained in 2014 by A. Filippou and others.^[7]

Synthesis[edit]

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.^[1]

For consumer applications such as caulks silyl acetates are used instead of silyl chlorides. The hydrolysis of the acetates produces 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.

n Si(CH₃)₂(CH₃COO)₂ + n H₂O → [Si(CH₃)₂O]_n + 2n CH₃COOH

Branches or crosslinks in the polymer chain can be introduced by using organosilicone precursors. With fewer alkyl groups, such as methyl trichlorosilane 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.

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