Octamethylcyclotetrasiloxane

Octamethylcyclotetrasiloxane is a member of the cyclic siloxane, or cyclomethicone, family. It is often called simply D4 because of its four dimethylsiloxane units. It exists as an oily liquid or low-melting solid.

In 1946, Winton Patnode and Donald F. Wilcock at General Electric (Schenectady, NY) prepared D4 as part of an extensive study of methylpolysiloxanes made by hydrolyzing methylchlorosilanes. In one the many reactions they reported, they mixed dimethyldichlorosilane¹ (Me₂SiCl₂) with large volumes of water to produce a colorless oil with a composition that had an empirical formula close to that of Me₂SiO.

Analysis of the oil showed that it was primarily a mixture of cyclic oligomers of the formula (Me₂SiO)_x, with isolated compounds ranging from x = 3 to x = 10. The relative amounts of each of the cyclosiloxanes varied with hydrolysis conditions such as the Me₂SiCl₂/H₂O ratio and the presence of cosolvents, acids, or bases. In general, D4 (x = 4) was the most abundant product.

Today, D4 is commercially produced by the same general method. The product mixture is separated into its components via distillation. D4 and the other oligomers are used to prepare poly(dimethylsiloxane)² [(Me₂SiO)_n], familiarly known as silicone oil. D4 itself is used as an additive to plastic and rubber products, paints, adhesives, cosmetics, food packaging, and many other product areas. Its worldwide production is in the hundreds of thousands of tonnes.

Because of its widespread production and use, D4 has come under scrutiny for its adverse environmental and health effects. Last September, the US Environmental Protection Agency announced an evaluation of the environmental risks of its industrial uses. It will be up to the Food and Drug Administration to determine whether its food, health care, and personal care uses are evaluated.

1. CAS Reg. No. 75-78-5.
2. CAS Reg. No. 9016-00-6.