Some Important Condensation Polymers

(a) Polyamides — the nylons

Polymers built with amide linkages are the synthetic fibres we call nylons. They are generally made either by condensing a diamine with a dicarboxylic acid, or by condensing an amino acid (or its cyclic lactam form).

• Nylon 6,6 — made by condensing hexamethylenediamine with adipic acid under high pressure and temperature (553 K), releasing water at every amide-forming step:

It is a fibre-forming solid with high tensile strength, thanks to strong intermolecular hydrogen bonding between chains, which also packs the chains closely and gives the material a crystalline character. Used for sheets, brush bristles, and in textiles.

• Nylon 6 — made by heating caprolactam (a seven-membered cyclic amide) with water at 533–543 K, which opens the ring and links the units end to end:

Used for tyre cords, fabrics, and ropes.

(b) Polyesters

Polyesters are the condensation products of dicarboxylic acids and diols. Dacron (terylene) — already met in the previous section — is made by heating ethylene glycol and terephthalic acid at 420–460 K with a zinc acetate–antimony trioxide catalyst. It is crease-resistant, blended with cotton and wool, and used as a glass-reinforcing material in items like safety helmets.

(c) Phenol–formaldehyde polymers (Bakelite)

The oldest synthetic polymer family, made by condensing phenol with formaldehyde under acid or base catalysis. The reaction first produces ortho- and *para-*hydroxymethylphenol, which then react further with more phenol, linking benzene rings together through –CH₂– bridges.

• The initial, still-soluble linear product is called Novolac (used in paints) — formed when o-hydroxymethylphenol units condense under base catalysis into a chain of phenol rings joined by –CH₂– bridges. …