Inorganic polymer

(inorganic chemistry) Large molecules, usually linear or branched chains with atoms other than carbon in their backbone; an example is glass, an inorganic polymer made up of rings and chains of repeating silicate units.

A giant molecule linked by covalent bonds but with an absence or near-absence of hydrocarbon units in the main molecular backbone; these may be included as pendant side chains. Carbon fibers, graphite, and so forth are considered inorganic polymers.

Some special characteristics of many inorganic polymers are a higher Young's modulus and a lower failure strain compared with organic polymers. Relatively few inorganic polymers dissolve in the true sense, or alternatively, if they swell, few can revert. Crystallinity and high glass transition temperatures are also much more common than in organic polymers. In highly cross-linked inorganic polymers, stress relaxation frequently involves bond interchange.

Inorganic polymers can be classified in a number of ways. Some are based on the composition of the backbone, such as the silicones (SiO), the phosphazenes (PN), and polymeric sulfur (SS). Others are based on their connectivity, that is, the number of network bonds linking the repeating unit into the network. Thus the silicones based on R₂SiO, the phosphazenes based on NPX₂, and polymeric sulfur each have a connectivity of two, while boric oxide based on B₂O₃ has a connectivity of three, and amorphous silica based on SiO₂ has one of four.

Among the well-known inorganic polymers are silicones, chalcogenide glasses, graphite, boron polymers, and silicate polymers. See also Graphite; Silicone resins.