Inorganic Polymers

polymers whose main macro-molecular chain is inorganic (that is, it contains no carbon atoms). The side groups are usually also inorganic, although polymers with organic side groups are often considered to be inorganic (there is no rigorous distinction according to this feature).

Inorganic polymers, like organic polymers, are classified according to spatial structure as linear, branched, ladder, and crosslinked (two- and three-dimensional). In terms of the composition of the main chain, they are classified as homochain polymers ([—M—] _n) and heterochain polymers ([—M—M’—] _n or [—M—M’—M”—] _n), where M, M’, and M” are various atoms. For example, polymeric sulfur, [—S—] _n, is a homochain linear inorganic polymer without side groups.

Many inorganic compounds in the solid state form single macromolecules; however, only substances with a certain anisotropy in their spatial structure (and therefore in their properties) are considered inorganic polymers. In this regard, crystals of inorganic polymers differ from the completely isotropic crystals of ordinary inorganic compounds, such as NaCl and ZnS. Most chemical elements are incapable of forming stable homochain inorganic polymers, and only about 15 elements—including sulfur, phosphorus, selenium, tellurium, and silicon—form relatively short (oligomeric) chains, which are considerably less stable than homochain polymers with C—C bonds. Therefore, the most common inorganic polymers are heterochain polymers in which electropositive and electronegative atoms (for example, boron and nitrogen, phosphorus and nitrogen, and silicon and oxygen) alternate and form polar (partially ionic) chemical bonds with one another and with the atoms of the side groups.

The increased reactivity of inorganic polymers, mainly a tendency toward hydrolysis, is caused by polar bonds. Therefore, many inorganic polymers are unstable in air. In addition, some inorganic polymers are easily depolymerized, with the formation of cyclic structures. These and other chemical properties of inorganic polymers may be partially altered by controlled substitution of the side framework; this predominantly affects the nature of the molecular interactions, which determine the elastic and other mechanical properties of the polymers. Thus, upon hydrolysis of the P—Cl bond (and subsequent polycondensation), the linear elastomer polyphosphonitrile chloride, [—Cl₂PN—]_n, is converted into a three-dimensional structure lacking elastic properties. The resistance of this elastomer to hydrolysis may be increased by replacing the chlorine atoms by any of several organic radicals. Many heterochain inorganic polymers are characterized by heat resistance that is substantially higher than that of organic and heteroorganic polymers (for example, polymeric phosphorus oxonitride, [PON]_n, remains unchanged upon heating to 600°C). However, heat resistance of inorganic polymers is rarely combined with valuable mechanical and electrical properties. For this reason, the number of inorganic polymers for which practical use has been found is relatively limited. However, inorganic polymers are a valuable source of new heat-resistant materials.