Polyamides

A polyamide is a condensation polymer in which the repeating units are joined by amide links, -CONH-. The same link joins amino acids in proteins, where it is called a peptide link.Polyamides can be made in two ways:
• from a dicarboxylic acid and a diamine: the -COOH of one monomer reacts with the -NH₂ of the other, losing H₂O
• from a dioyl dichloride and a diamine: -COCl reacts with -NH₂, losing HCl. This reaction is faster and happens at room temperature.A single monomer with an amine group at one end and a carboxylic acid group at the other, such as an amino acid, can also polymerise on its own to give a polyamide.The general equation for a diacid and a diamine is:n HOOC-R-COOH + n H₂N-R'-NH₂ → [-CO-R-CO-NH-R'-NH-]_n + (2n − 1) H₂OWorked example. Which small molecule is lost when hexanedioyl dichloride, ClOC(CH₂)₄COCl, reacts with hexane-1,6-diamine? The -COCl reacts with -NH₂: the Cl from the acyl chloride joins an H from the amine, so HCl is lost at each link and the product is a polyamide.The amide link contains a polar C=O group and an N-H group, so polyamide chains can hydrogen bond to one another. This makes polyamides strong, which is why they are used as fibres. Like polyesters, polyamides can be hydrolysed at their amide links by hot aqueous acid or alkali.