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. The possibility of forming rigid molecules that can be ordered into anisotropic arrays having exceptional stiffness may become of even greater interest. As a group such polymers are often both mechanically rigid and inherently resistant to thermal degradation.