Aliphatic Polyamides of the Nylon Series

In the nylon series, there are only three polymers that are of large-scale significance commercially nylon 6,6, nylon 6,10, and nylon 6,12. 

Together with these, a series of special nylons is also produced. The formation of polymer occurs by polycondensation of diamines with dicarboxylic acids. The necessary equivalence of the functional groups is achieved by first producing the salt from 1 mol each of dicarboxylic acid and diamine, e.g., neutralization to produce the so-called AH salt from adipic acid with hexamethylene diamine. The amidization equilibrium is so favorable that polycondensation can take place in the presence of water, which is therefore used as a heat sink. The polycondensation of nylon 6,6 is typical of commercial syntheses. A 60-80% suspension of the salt is precondensed for 1-2 h at 220-230°C and 13-17 bar (vapor pressure of steam). After an 80-90% yield has been reached, further condensation takes place above the polymer melting point (264 C) at 270-280°C under vacuum. 

Nylon 6,6 is the polyamide from hexamethylene diamine and adipic 

In the synthesis of adipic acid one can start with benzene, phenol, tetrahydrofuran, butadiene, or cyclohexane. Benzene is converted to phenol (e.g., by the cumene process), this is hydrogenated to cyclohexanol, and the cyclohexanone gained by oxidation is then oxidized to adipic acid, HOOC—(CH2)4—COOH, with nitric acid. Cyclohexane can also be oxidized with air to cyclohexanol, from which adipic acid is obtained by direct nitric acid oxidation. Adipic acid can also be produced by saponification of adipodinitrile (adiponitrile), which in turn comes from tetrahydrofuran or butadiene (see below). 

The four commercial syntheses of hexamethylene diamine are based on tetrahydrofuran, butadiene, adipic acid, and acrylonitrile. 

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