When R is methyl, the four possible functional siloxane monomeric units are described as follows:
Note that a network of only Q groups becomes fused quartz.
The most abundant silicone resins are built of D and T units (DT resins) or from M and Q units (MQ resins), however many other combinations (MDT, MTQ, QDT) are also used in industry.
Silicone resins represent a broad range of products. Materials of molecular weight in the range of 1000-10 000 are very useful in pressure sensitive adhesives, silicone rubbers, coatings and additives.
Silicone resins are prepared by hydrolytic condensation of various silicone Precursors. In early processes of preparation of silicone resins sodium silicate and various chlorosilanes were used as starting materials. Although the starting materials were the least expensive ones (something typical for industry), structural control of the product was very difficult. More recently, a less reactive tetraethoxysilane - (TEOS) or ethyl polysilicate and various disiloxanes are used as starting materials.