A heterocyclic compound is a cyclic molecule in which one or more of the ring atoms are heteroatoms, that is, atoms other than carbon. The most common heteroatoms are: nitrogen, oxygen, sulfur, boron, phosphorus. Each of them gives rise to large families of heterocyclic compounds.
Pyridine and pyrimidine are heterocyclic analogs of benzene, glucose in cyclic form is a heterocyclic compound in that there is an oxygen atom inside the ring.
With this name are indicated the polymers whose basic structural unit is formed by alkyl radicals and aromatic radicals. Bearing in mind that aliphatic compounds have the possibility to polymerize with the formation of very long, flexible, plastic and malleable chains, the introduction of aromatic rings gives the polymer a rigidity and a mechanical resistance that, if it increases the technological properties making polymers similar to metals, makes it difficult the subsequent processing (poor solubility, poor ductility, high melting point, etc.).
Since it is possible to vary the percentage of the two types of organic compounds in the polymer chains with the use of appropriate catalysts and with particular reaction conditions, it is possible to obtain polymers with aliphatic and aromatic compounds in all desired ratios; and it is also possible to vary the type of aliphatic or aromatic compound depending on the properties that are desired in the finished product.
The most important heterocyclic polymers with their basic units and fields of application are described below.
- Polyimides: this class includes polymers whose main chain has imide bonds between two aromatic nuclei. These polymers, among which we will remember the polyimide obtained by copulation of pyromellitic dianhydride and di-p-aminophenyl ether, must be processed with the same technology used for metals; they present exceptionally valuable mechanical properties comparable to those of metals.
- Polyamidoimides: this class includes polymers with a base unit containing in the main chain an imide bond and an amide bond. They are used in the preparation of paints, high and low temperature electrical insulators; reinforced with glass they have been used for airplane structures.
- Polyesteromides: polymers whose specific property is resistance to relatively high temperatures.
- Poliobenzoimidazoles: they are marketed as prepolymers and remain unaltered for up to three months. The transformation from prepolymers to polymers is carried out by heating. Due to their excellent thermal stability they are used for parts of equipment on airplanes, rockets, spacecraft.
- Polyoxodiazoles: are very stable to light and degrade only at temperatures above 400 ºC.Politriazoles: are stable in air up to 450 ºC.