Question
Compare Nylon 6,6 and Nylon 6 in terms of their monomers, type of polymerisation, structure, and uses. Why are they called polyamides?
(NCERT Class 12, Chapter 15 — Polymers)
Solution — Step by Step
Monomers: Hexamethylenediamine (6 carbons) + Adipic acid (6 carbons)
That’s why it’s called 6,6 — both monomers have 6 carbon atoms each.
Polymerisation type: Condensation polymerisation (step-growth). Water is released at each step.
Structure:
The amide bond (–CO–NH–) connects the repeating units — this is why it’s a polyamide.
Monomer: Caprolactam (a cyclic amide with 6 carbon atoms)
Polymerisation type: Ring-opening polymerisation. The caprolactam ring opens up and the units link end-to-end.
Structure:
It’s called Nylon 6 because the monomer has 6 carbon atoms (only one number, since there’s only one monomer).
| Property | Nylon 6,6 | Nylon 6 |
|---|---|---|
| Monomers | Two (diamine + diacid) | One (caprolactam) |
| Polymerisation | Condensation | Ring-opening |
| Carbon atoms in name | 6 + 6 | 6 |
| Melting point | ~265°C (higher) | ~215°C (lower) |
| By-product | Water | None |
Both contain the amide linkage (–CONH–) and both are classified as polyamides.
- Textile fibres — clothing, hosiery, carpets
- Ropes and tyre cords — high tensile strength
- Bristles — toothbrushes, paintbrushes
- Engineering plastics — gears, bearings (due to low friction)
Nylon was the first purely synthetic fibre (developed by Carothers at DuPont in 1935).
Why This Works
Both nylons are polyamides — polymers linked by amide bonds. The amide bond is the same bond found in proteins (peptide bond), which is why nylons mimic some properties of natural fibres like silk and wool (both proteins).
The difference in preparation (condensation vs ring-opening) affects the molecular regularity and hence the melting point. Nylon 6,6 has more regular intermolecular hydrogen bonding due to its symmetric structure, giving it a higher melting point than Nylon 6.
Alternative Method — Classification Context
Both Nylon 6 and 6,6 are:
- Synthetic polymers (not natural)
- Fibre-forming polymers (strong intermolecular H-bonding due to amide groups)
- Thermoplastic (can be melted and remoulded)
- Linear polymers (no cross-linking)
For NEET, the most asked question: “What are the monomers of Nylon 6,6?” Answer: hexamethylenediamine and adipic acid. For Nylon 6: caprolactam. Also remember that Nylon 6,6 is a condensation polymer while Nylon 6 is made by ring-opening — this classification is tested directly.
Common Mistake
Students mix up Nylon 6,6 with Nylon 6 in terms of monomers. The rule: two numbers = two monomers (Nylon 6,6 has two monomers with 6 carbons each). One number = one monomer (Nylon 6 has one monomer with 6 carbons). Also, don’t confuse caprolactam (monomer of Nylon 6) with caprolactone (monomer of a polyester) — the names sound similar but they’re completely different compounds.