inorganic polymers

The word polymer is derived from the Greek term poly, which means many, and meros, meaning part. Thus, a polymer is a large molecule (sometimes called a macromolecule) that is composed of many small repeating units (called monomers). Wool, silk, proteins, starch, and cellulose are abundant examples of natural polymers. Many natural polymers are carbon-based—that is, the monomeric unit contains carbon (C) atoms as the central atoms—and are therefore considered organic polymers. Many inorganic polymers also are found in nature, including diamond and graphite, discussed in other sections. (Both are composed of carbon. In diamond, carbon atoms are linked in a three-dimensional network that gives the material its hardness, whereas in graphite, used as a lubricant and in pencil "leads," the carbon atoms link in planes that can slide across one another.) Many oxyacids and oxy anions also polymerize, however, especially such weak acids as boric acid, H3BO3, and silicic acid, H4SiO4. In the anions of these weak acids, BO33- and SiO44- , respectively, a high density of negative charge resides on the oxygen atoms. The process of polymerization can reduce this charge density. The major classes of mixed organic-inorganic compounds called inorganic polymers include borates and three classes of silicon polymers—silicones, silicates, and silanes. The most industrially important representatives of this polymer family are the silicones, polymeric organosilicon compounds containing Si-O-Si linkages and Si-C bonds. Their backbone consists of alternating silicon and oxygen atoms with organic groups attached to each of the silicon atoms. Silicones with low molecular weight are oils and greases. Higher-molecular-weight species are versatile elastic materials that remain soft and rubbery at very low temperatures, and exhibit relative stability at high temperatures because of the presence of strong silicon-oxygen and silicon-carbon bonds. A general formula for silicones is (R2SiO)x, where R can be any one of a variety of organic groups. Silicones may be linear, cyclic, or cross-linked polymers. Stable to many chemical reagents, silicone polymers incorporate some of the properties of both carbon-hydrogen compounds and silicon-oxygen compounds. Depending on their degree of polymerization and the complexity of the attached organic groups, silicones can occur in the form of oils, greases, rubberlike substances, or resins. Used as lubricants, hydraulic fluids, and electrical insulators, they are especially useful as lubricants in applications where there are extreme variations in temperature, because their viscosity changes very little as the temperature changes. Because silicones are also water-repellent, paper, wool, silk, and other fabrics can be coated with a water-repellent film by exposing them for a short time (1–2 seconds) to the vapor of trimethylchlorosilane, (CH3)3SiCl. The -OH groups on the surface of the materials react with the silane, and the surface becomes coated with a thin water-repellent film of (CH3)Si-O- groups. Silicates are salts containing anions of silicon (Si) and oxygen. Although there are many types of silicates, because the silicon-to-oxygen ratio can vary widely, silicon atoms are found in all silicates at the centers of tetrahedrons with oxygen atoms at the corners. The silicon is always tetravalent (i.e., has an oxidation state of +4). The variation in the silicon-to-oxygen ratio occurs because the silicon-oxygen tetrahedrons may exist as discrete, independent units or may in several ways share oxygen atoms at corners, edges, or—in more rare instances—faces. Thus, the silicon-to-oxygen ratio varies according to the extent to which silicon atoms share the oxygen atoms as the tetrahedrons are linked together. Seven different classifications, based on the linkage of these tetrahedrons, are commonly recognized. 1. In some silicates—e.g., silicates of magnesium (Mg2SiO4) and zirconium (ZrSiO4)—individual SiO44- tetrahedrons exist as independent units. 2. Two SiO4 tetrahedrons share one corner oxygen atom to form discrete Si2O76- ions (e.g., Ca2ZnSi2O7 and Zn4(OH)2Si2O7 9 H2O.) 3. SiO4 tetrahedrons may share corners and form closed rings. For example, in BaTiSi3O9 three SiO4 tetrahedrons share corners, while in Be3Al2Si6O18 (beryl, the deep green variety of which is known as emerald) six tetrahedrons share corners to form a closed ring. 4. SiO4 tetrahedrons in which each tetrahedron shares two oxygen atoms from two other tetrahedrons exist as chains in some silicates (e.g., CaMg(SiO3)2. Although the formula indicates the existence of SiO32- ions, these ions do not occur as independent entities. Parallel chains extend the full length of the crystal and are held together by the positively charged metal ions lying between them. 5. When SiO4 tetrahedrons in single chains share oxygen atoms, double silicon-oxygen chains form; metal cations link the parallel chains together. Several of these silicates are fibrous in nature because the ionic bonds between the metal cations and the silicate anions are not as strong as the silicon-oxygen bonds within the chains. A class of fibrous silicate minerals that belongs to this group is collectively called asbestos, of which chrysotile, which has the formula Mg3(Si2O5)(OH)4, is the best known and most abundant kind. This compound, which exists as fibers more than 20 millimeters (0.8 inch) in length, was formerly employed in many fireproofing and insulation applications, but its use for these purposes has been discontinued because it appears that prolonged exposure to airborne asbestos fibers may cause cancer of the lungs. 6. Silicon-oxygen sheets are formed when oxygen atoms are shared between double chains; metal ions form ionic bonds between the sheets. Because these ionic bonds are weaker than the silicon-oxygen bonds within the sheets, silicates with this structure cleave into thin layers. An example of this class of silicates includes talc, Mg3Si4O10(OH)2. 7. The zeolites, an interesting class of silicates, are three-dimensional silicon-oxygen networks with some of the tetravalent silicon ions replaced by trivalent aluminum (Al3+) ions. The negative charge that results—because each Al3+ ion has one less positive charge than the Si4+ ion it replaces—is neutralized by a distribution of positive ions throughout the network. Zeolites, characterized by the presence of tunnels and systems of interconnected cavities in their structures, are used as molecular sieves to remove water and other small molecules from mixtures. They can be employed to separate molecules for which the molecular masses are the same or similar but the molecular structures are different. In addition, they are used as solid supports for highly dispersed catalysts and to promote specific size-dependent chemical reactions. An example of a zeolite is Na2(Al2Si3O10) 9 2H2O. Silanes, which are compounds of silicon and hydrogen, forms a series of hydrides that have the general formula SinH2n + 2, including SiH4, Si2H6, Si3H8, and Si4H10, and contain Si-H and Si-Si single bonds. Because silicon possesses empty valence-shell d orbitals, the chemistry of silanes is quite different from that of the corresponding carbon-hydrogen compounds, or hydrocarbons. For example, silanes inflame spontaneously in air, whereas the corresponding hydrocarbons do not. The simplest silane, SiH4, a colorless gas called silane, has a formula and a tetrahedral structure analogous to the hydrocarbon methane, CH4. Thermally stable at normal temperature, it reacts violently with air to produce silicon dioxide and water. Borates, which are salts of the oxyacids of boron (B), result either from the reaction of a base with a boron oxyacid or from the melting of boric acid or boron oxide, B2O3, with a molten metal oxide or hydroxide. Examples include such compounds as boric acid, H3BO3, metaboric acid, HBO2, and tetraboric acid, H2B4O7. Borate anion structures range from the simple trigonal planar BO33- ion to relatively complex structures containing chains and rings of three- and four-coordinated boron atoms. Calcium metaborate, CaB2O4, consists of infinite chains of B2O42- units, while potassium borate, K[B5O6(OH)4] 9 2H2O (commonly written as KB5O8 9 4H2O), consists of two B3O3 rings linked through a common four-coordinated boron atom. The tetraborates, B4O5(OH)42- , contain both three- and four-coordinated boron surrounded trigonally and tetrahedrally, respectively, by oxygen (O) atoms. The borate of greatest commercial significance is borax, or sodium tetraborate decahydrate, Na2B4O7 9 10H2O. Found naturally in dry lake beds, such as Searles Lake in California, borax can be used to soften water and to make washing compounds. Its usefulness arises from the insolubility of calcium and magnesium borates and the alkaline or basic nature of aqueous solutions of borax. Borax is also employed in the manufacture of borosilicate glass and enamels and as a fire retardant.