Polymer chemistry – feedstock for polyamide: Nylon-6 is industrially manufactured from which single-monomer precursor (ring-opening polymerization to caprolactam-derived 6-carbon repeat units)?

Introduction:
Nylon-6 is a widely used engineering thermoplastic. The question tests understanding of how different nylons are produced and the distinction between single-monomer ring-opening routes and salt/condensation routes used for other polyamides.

Given Data / Assumptions:

• Nylon-6 has a six-carbon repeat unit.
• Industrial routes prefer economical, high-conversion processes.
• We compare candidate monomers and their polymerization mechanisms.

Concept / Approach:
Nylon-6 is manufactured by ring-opening polymerization of ε-caprolactam. In contrast, Nylon-6,6 is produced by step-growth polycondensation of adipic acid with hexamethylene diamine. Recognizing monomer–polymer relationships is key to polymer identification questions.

Step-by-Step Solution:
Identify Nylon-6 route: ring-opening polymerization of ε-caprolactam produces –[NH–(CH2)5–CO]– units.Differentiate from Nylon-6,6: adipic acid + hexamethylene diamine → salt → condensation polymer.Eliminate unrelated pairs (maleic anhydride or sebacic acid with diamines) which form other polyamides or specialty copolymers, not Nylon-6.Conclude ε-caprolactam is the correct single precursor.

Verification / Alternative check:
Industrial literature consistently cites ε-caprolactam as the sole precursor to Nylon-6 via ring-opening polymerization under controlled temperatures and catalysts.

Why Other Options Are Wrong:

• Adipic acid + hexamethylene diamine: produces Nylon-6,6, not Nylon-6.
• Maleic anhydride + hexamethylene diamine: not the commercial route to Nylon-6.
• Sebacic acid + hexamethylene diamine: leads to Nylon-6,10 type materials.
• Acrylic acid: unrelated homopolymer produces poly(acrylic acid), not a polyamide.

Common Pitfalls:
Confusing Nylon-6 with Nylon-6,6 because both have sixes in the name; the first refers to carbon count per repeat from a single monomer, the second to 6 + 6 carbons from two monomers.

Final Answer:
ε-Caprolactam (caprolactam)