Condensation Polymerisation and Nylon-6,6

Condensation polymerisation (also called step-growth polymerisation) builds a polymer by repeatedly reacting together two bifunctional (or polyfunctional) monomers -- each carrying two reactive functional groups, one at each end -- with the elimination of a small molecule, most often water, at every linking step. Unlike addition polymerisation, where one chain grows continuously by adding monomers one at a time to a single reactive end, condensation polymerisation proceeds through many independent, simultaneous small steps: any two monomers, or any monomer and a partly-formed short chain, or two partly-formed chains, can react with each other and eliminate a small molecule at any stage, so the average chain length grows slowly and steadily across the whole reaction mixture rather than shooting up quickly in a few chains.

Nylon-6,6, the classic example prescribed by the syllabus, is formed by heating together two different bifunctional monomers under pressure: hexamethylenediamine, $\text{H}_2\text{N--}(\text{CH}_2)_6\text{--NH}_2$ (a diamine, six carbons between its two $-\text{NH}_2$ groups), and adipic acid, $\text{HOOC--}(\text{CH}_2)_4\text{--COOH}$ (a diacid, six carbons total counting both carboxyl carbons, four $-\text{CH}_2-$ between them). At each linking step, one $-\text{NH}_2$ group of the diamine reacts with one $-\text{COOH}$ group of the diacid to form an amide (peptide-type) linkage, $-\text{CO--NH}-$, releasing one molecule of water:

so the repeat unit is $\big[-\text{NH--}(\text{CH}_2)_6\text{--NH--CO--}(\text{CH}_2)_4\text{--CO--}\big]_n$, containing two amide linkages per repeat unit. The polymer's name records exactly how many carbon atoms each monomer contributes: 'nylon-6,6' because the diamine has 6 carbons and the diacid also has 6 carbons (this naming convention distinguishes it from other nylons, such as nylon-6, made from a single monomer, caprolactam, with 6 carbons). …