Essentially, linear high polymers comprising heterocyclic rings, or groups of rings, linked together by one or more covalent bonds. As the search has continued for polymeric materials having useful properties at high temperatures (500°C or 930°F, or higher), much attention has been given to heterocyclic polymers. As a group such polymers are often both mechanically rigid and inherently resistant to thermal degradation. See also Polymer.
Some of these polymers form molecules in which the rings are fused together, as shown symbolically in the illustration (ladder polymers), and some form molecules in which fused rings are joined by single bonds (stepladder polymers). Similar considerations hold for simple aromatic systems, for example, linear polymers of benzene, but the heterocyclic systems have, in general, been more useful in application.
polymer. (b) Stepladder polymer. (c) Ladder polymer.">
Structural units in linear polymers. (a) Simple linear polymer. (b) Stepladder polymer. (c) Ladder polymer.
Three heterocyclic polymers have been developed to the point of commercial availability; polyimides, polybenzimidazoles, and polybenzothiazoles. At least the first two appear to have established specialized markets.
Major applications for these rather expensive polymers are as metal-to-metal adhesives and as laminating resins for fibrous composites for structural applications in the aerospace industry. Other applications requiring both strength and resistance to oxidation at elevated temperatures have developed, including valve seats, bearings, and turbine blades. See also Polymeric composite.
