Condensation Polymers - Polyamides

Polyamides are condensation polymers in which the monomers are joined by amide links, -CONH-. They are made from two types of monomer: a diamine (two -NH₂ groups) and a dicarboxylic acid (two -COOH groups). Each time an amine group reacts with a carboxylic acid group, an amide link forms and a molecule of water is eliminated. A diacyl chloride can be used instead of the acid, in which case HCl is eliminated and the reaction is faster.Nylon-6,6 is made from hexane-1,6-diamine, H₂N(CH₂)₆NH₂, and hexanedioic acid, HOOC(CH₂)₄COOH. The name shows that each monomer has six carbon atoms.The repeat unit is -NH(CH₂)₆NHCO(CH₂)₄CO-. Forming one repeat unit eliminates two water molecules.Kevlar is made from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid. Its flat, rigid chains line up and hydrogen bond strongly to each other, making it extremely strong; it is used in body armour.Hydrogen bonds between the N-H and C=O groups of neighbouring chains make polyamides strong fibres. Unlike poly(alkenes), the amide links can be hydrolysed by acids or alkalis, breaking the polymer back into its monomers.Worked example: the repeat unit of nylon-6,6 is C₁₂H₂₂N₂O₂, M_r = 144 + 22 + 28 + 32 = 226. A chain with M_r = 45 200 contains 45 200 ÷ 226 = 200 repeat units.