In polymer science, nylon-66 is identified as a polyamide formed from which specific pair of monomers? Select the correct diamine–diacid combination used industrially to produce nylon-66 (do not confuse with nylon-6 made from a single lactam).

Introduction / Context:
Nylons are polyamides named using a numbering system tied to carbon counts in their monomers. Nylon-66 is a benchmark engineering polymer used in fibres and molded parts. The question checks recall of the exact monomer pair used for nylon-66 and contrasts it implicitly with nylon-6 (from caprolactam).

Given Data / Assumptions:

• Nylon-66 is a condensation (step-growth) polyamide.
• Monomers are a diamine and a diacid, both with six carbons.
• Byproduct (often water or a salt intermediate) may form during polymerisation.

Concept / Approach:
Polyamides form when amine groups react with carboxyl groups to make amide linkages. Nylon-66 uses hexamethylene diamine (6 carbons) and adipic acid (6 carbons). The “66” denotes 6 carbons in each monomer. This differs from nylon-6, which is produced by ring-opening polymerisation of caprolactam (a single monomer).

Step-by-Step Solution:
Identify nylon-66 as a diamine–diacid system.Match “6,6” to hexamethylene diamine (6C) + adipic acid (6C).Eliminate distractors involving vinyl chloride, formaldehyde, or single-monomer systems.

Verification / Alternative check:
Industrial nylon salt formation (stoichiometric mix of the diamine and diacid) prior to polycondensation confirms the monomer pair.

Why Other Options Are Wrong:
Adipic acid + methyl amine: wrong stoichiometry and carbon count.Vinyl chloride + formaldehyde: unrelated chemistry (PVC/acetals).Caprolactam and water: describes nylon-6 route.None of these: incorrect because a correct pair is listed.

Common Pitfalls:
Confusing nylon-66 with nylon-6 or assuming “66” refers to something else (e.g., chain length).

Final Answer:
Hexamethylene diamine and adipic acid