Q6(c). Nylon-6,6 and Terylene: repeating units and manufacture

Introduction. Both are condensation (step-growth) polymers: each link forms with loss of a small molecule (H₂O or CH₃OH). Nylon-6,6 is a polyamide; Terylene (Dacron, PET) is a polyester.

Nylon-6,6

Scheme: Six carbons from the diamine and six from the diacid give “6,6”.

1. Equimolar hexamethylenediamine and adipic acid form the nylon salt (hexamethylenediammonium adipate), which fixes the exact 1: 1 ratio needed for long chains.
2. An aqueous solution of the salt is heated under pressure; water is driven out and amide (–CO–NH–) links form. Repeating unit: [-NH(CH₂)₆NH-CO(CH₂)₄CO⁻].
3. Hydrogen bonds between the N–H and C=O groups of neighbouring chains give high strength and melting point; the melt is spun into fibres.

Terylene (polyethylene terephthalate)

Scheme: Ester interchange, then polycondensation with loss of ethylene glycol.

1. Dimethyl terephthalate is heated with excess ethylene glycol (with a metal acetate catalyst): ester interchange releases methanol and gives bis(2-hydroxyethyl) terephthalate. (Terephthalic acid itself can be esterified directly.)
2. The bis-ester is heated more strongly under vacuum (Sb₂O₃ catalyst); ethylene glycol is removed as each new ester bond forms: polycondensation. Repeating unit: [-O-CH₂CH₂-O-CO-C₆H₄-CO⁻].
3. The rigid para-phenylene units give a high-melting, crystalline polyester used as fibre and film.

Conclusion. Nylon-6,6: [-NH(CH₂)₆NH-CO(CH₂)₄CO⁻]ₙ from hexamethylenediamine + adipic acid. Terylene: [-OCH₂CH₂O-CO-C₆H₄-CO⁻]ₙ from dimethyl terephthalate (or terephthalic acid) + ethylene glycol.

Application. Nylon-6,6 is used in tyre cord, ropes and gear wheels; PET in textiles, soft-drink bottles and recording film.