Q2(d). Syntheses of nylon 66, Teflon and Terylene

Introduction. Nylon 66 and Terylene are step-growth (condensation) polymers: each link forms with loss of a small molecule (H₂O or CH₃OH), and high molar mass needs exact 1: 1 stoichiometry and removal of the by-product. Teflon is a chain-growth (addition) polymer made by radical polymerisation of tetrafluoroethylene.

(i) Nylon 66 (polyhexamethylene adipamide)

1. Monomers: hexamethylenediamine (hexane-1,6-diamine) and adipic acid (hexanedioic acid), six carbons each (hence “66”).
2. The two are mixed in water to form the 1: 1 nylon salt (hexamethylenediammonium adipate), which is crystallised; this fixes the stoichiometry exactly.
3. The concentrated salt solution is heated in an autoclave under nitrogen, first under pressure (about 220°C) and then at about 270–280°C as the pressure is released; water is removed continuously and the molten polyamide is extruded and melt-spun.

n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH nylon salt -[NH(CH₂)₆NH-CO(CH₂)₄CO]_(n)⁻ + 2n H₂O

Scheme: Repeat unit of nylon 66 (C₁₂H₂₂N₂O₂, 226 g mol⁻¹); amide links give the H-bonded, strong fibre.

(ii) Teflon (polytetrafluoroethylene, PTFE)

1. Monomer: tetrafluoroethylene CF₂=CF₂ (made from CHClF₂ by pyrolysis at about 700°C).
2. Radical polymerisation in water (suspension or emulsion), with a persulfate ((NH₄)₂S₂O₈) or peroxide initiator, moderate temperature (about 50–80°C) and pressure of the gaseous monomer (several atm). The reaction is highly exothermic and is cooled.
3. Initiation R• + CF₂=CF₂ → RCF₂CF₂•; propagation adds monomer; termination by combination.

n CF₂=CF₂ (NH₄)₂S₂O₈ -[CF₂-CF₂]_(n)⁻

(iii) Terylene (Dacron, polyethylene terephthalate, PET)

1. Monomers: dimethyl terephthalate (or terephthalic acid) and ethylene glycol (excess).
2. Transesterification: about 150–200°C with a zinc or manganese acetate catalyst; methanol distils off, giving bis(2-hydroxyethyl) terephthalate.
3. Polycondensation: about 270–290°C under high vacuum with Sb₂O₃ catalyst; the excess ethylene glycol is removed, driving the chain to high molar mass. The melt is spun into fibre or moulded.

DMT + 2 EG Zn(OAc)₂ BHET + 2 CH₃OH n BHET Sb₂O₃ PET + n EG

DMT = CH₃O₂C-C₆H₄-CO₂CH₃ (p); EG = HOCH₂CH₂OH; BHET = HOCH₂CH₂O₂C-C₆H₄-CO₂CH₂CH₂OH; PET = -[OC-C₆H₄-CO-OCH₂CH₂O]_(n)⁻.

Scheme: Repeat units: Terylene (left) and Teflon (right).

Conclusion. Nylon 66: hexamethylenediamine + adipic acid via the nylon salt, melt condensation at about 280°C with loss of water. Teflon: radical polymerisation of CF₂=CF₂ in water with a persulfate initiator under pressure. Terylene: dimethyl terephthalate + ethylene glycol, transesterification and then polycondensation under vacuum with Sb₂O₃.

Application. Nylon 66 is used for ropes, tyre cord and engineering parts; Teflon for non-stick coatings, gaskets and chemically resistant laboratory ware; Terylene for textile fibres and, as PET, for drink bottles.