Ring-opening polymerisation (nylon-6): mechanism and explanation
Caprolactam, a cyclic amide, is opened and the open chains link up to nylon-6. No water is released: the repeating unit has the same molecular formula as the monomer.
Example
A little water hydrolyses some lactam rings to the amino acid. Its amino group attacks further lactams and opens them; this is how the chain grows.
Mechanism step by step
- 1
Water opens some caprolactam rings to 6-aminohexanoic acid.
- 2
The free NH₂ group attacks the carbonyl carbon of another lactam, whose ring opens (amide exchange).
- 3
The new chain end again carries an NH₂ group, and the next ring opening follows.
To read
This topic is part of the syllabus but not in the training: polymers consist of repeating units and biomolecules react through enzymes in the cell. There is no clear-cut task with exactly one product for it.
Go to the trainer with 157 reactions to practiseFrequently asked questions
Where does caprolactam come from?
From cyclohexanone via the oxime and the Beckmann rearrangement.
Which other rings are polymerised this way?
Epoxides (to polyethers such as polyethylene glycol) and lactones (to polyesters such as polylactide, a biodegradable plastic).