Upper secondary · 15 min

The nylon rope trick

An interface between two immiscible solutions weaves nylon-6,6; watch the chain grow amide by amide as it is pulled away.

Goal

Distinguish condensation polymerisation (small molecule released at each step) from addition polymerisation, and describe nylon-6,6 repeat unit.

Apparatus and reagents

The chain viewer below; for a real demo: hexamethylenediamine in water over adipoyl chloride in cyclohexane, tweezers, gloves.

Procedure

Watch the chain grow: every second bead (blue) marks an amide $\ce{-CO-NH-}$ formed when an amine attacks an acyl chloride.
Imagine the interface: diamine diffuses up, diacyl chloride down, and the film only exists right at the boundary — pulling it away exposes fresh surface.
Count one repeat unit: $\ce{-[NH-(CH2)6-NH-CO-(CH2)4-CO]-}$ — six carbons from the diamine, four plus two carbonyls from the acid chloride.
Contrast with addition polymerisation: polyethylene grows without releasing any small molecule.

What to observe

The fibre can be pulled continuously because the reaction is fast and interfacial — monomers meet only at the film surface.
Each amide bond releases HCl (neutralised by the amine or added base), which is why this is a condensation, not an addition.

Explanation

Hexamethylenediamine $\ce{H2N-(CH2)6-NH2}$ and adipoyl chloride $\ce{ClCO-(CH2)4-COCl}$ are each difunctional, so every encounter extends the chain by a new amide bond $\ce{-CO-NH-}$ and one HCl. Because the two monomers sit in immiscible layers, polymerisation is confined to the interface: a film forms and pulling it out winds the polymer into a continuous "rope". Carothers used exactly this chemistry at DuPont in 1935 to make the first fully synthetic fibre.

Chemists behind it

Wallace Carothers