Nylon-6,6: a polyamide template

Hexane-1,6-diamine attacks adipoyl chloride (the nylon rope trick): addition, loss of chloride and deprotonation make each amide link of the [–NH(CH₂)₆NH–CO(CH₂)₄CO–]n repeat unit.

Mechanism step by step

Step growth by nucleophilic acyl substitution: amine plus acid chloride gives an amide and HCl

1. Addition – The amine N attacks the acyl carbon; C=O opens to O⁻
2. Elimination – C=O re-forms and chloride leaves
3. Deprotonation – Hydroxide removes H⁺ from N: the neutral amide link

Key points

• Step growth: any two chains with the right end groups can join, so long chains appear only at very high conversion.
• Carothers: Xn = 1/(1 − p); 99% conversion gives only about 100 units, and the 1:1 ratio must be exact.
• Industrially the diamine and adipic acid form "nylon salt", which is heated to drive off water.
• N–H···O=C hydrogen bonds between chains give nylon its strength and high melting point (about 265 °C).

Drawn with 73 atoms, 10 arrows, 11 shapes – every one editable in MolDraw.

Sources

• Odian, Principles of Polymerization, 4th ed., Wiley 2004, ch. 2 Textbook
• Young & Lovell, Introduction to Polymers, 3rd ed., CRC Press 2011 Textbook