The Nylon Rope Trick: Pulling Plastic From Two Liquids

What is actually happening

The nylon rope trick is a live demonstration of interfacial polymerization: a step-growth (condensation) reaction that runs only at the thin boundary where an aqueous layer and an immiscible organic layer touch. The product is a polyamide — nylon — whose long chains are stitched together from two different small molecules, each carrying a reactive group at both ends.

In the textbook version the two monomers are:

• 1,6-hexanediamine (hexamethylenediamine), H₂N–(CH₂)₆–NH₂, dissolved in water. Its two amine (–NH₂) ends are the nucleophiles.
• Sebacoyl chloride, ClOC–(CH₂)₈–COCl, dissolved in a nonpolar solvent such as hexane, dichloromethane, or tetrachloroethylene. Its two acid-chloride (–COCl) ends are the electrophiles.

Because a 6-carbon diamine is reacting with a 10-carbon diacid unit, the polymer is named nylon 6,10 — the first digit counts carbons in the diamine, the second counts carbons in the diacid. Its repeat unit is [–NH–(CH₂)₆–NH–CO–(CH₂)₈–CO–]ₙ.

The mechanism, one amide bond at a time

Each link in the chain is an amide bond, and it forms by nucleophilic acyl substitution — the same reaction an organic chemist uses to make any amide from an acid chloride.

• Attack: The lone pair on a diamine nitrogen attacks the electrophilic carbonyl carbon of an –COCl group, forming a tetrahedral intermediate.
• Collapse: The intermediate collapses, ejecting chloride (Cl⁻) and reforming the C=O. A new C–N bond — the amide — is now in place: –CO–NH–.
• Acid released: The reaction liberates HCl. In the classic prep this is neutralized by adding a base such as sodium carbonate or sodium hydroxide to the water layer, which mops up H⁺ and keeps the amine unprotonated (a protonated –NH₃⁺ is not nucleophilic and would stall the chain).

Because both monomers are difunctional, every new bond leaves reactive ends free on each side, and the process repeats: diamine + diacid chloride → dimer → tetramer → … → high polymer. This is step-growth polymerization — chains grow by any two ends meeting, not by a single active site adding monomers one by one as in chain-growth (addition) polymers like polyethylene.

Why the film forms only at the boundary

Neither monomer crosses appreciably into the other's solvent — the diamine loves water, the acid chloride loves hexane — so reaction is confined to the razor-thin zone where the liquids meet. There the first molecules react instantly to make an insoluble nylon skin, and that skin becomes the new interface.

• When you pull the skin upward with tweezers or a glass rod, you strip it away and expose fresh monomer on both sides, which reacts on contact to regenerate the film. That is why the strand seems endless: the film re-forms as fast as you remove it.
• The rate is limited by how quickly monomers diffuse to the interface, not by the intrinsic chemistry — the acid-chloride reaction itself is nearly instantaneous at room temperature.
• Acid chlorides are far more reactive than the carboxylic acids used industrially, which is exactly why this reaction needs no heat, no catalyst, and no vacuum. That high reactivity is also why the demo must be done in a fume hood: sebacoyl chloride is a lachrymator and moisture-sensitive.

Doing the trick

The recipe is disarmingly simple. Dissolve about 3 mL of sebacoyl chloride in 100 mL of hexane; separately dissolve about 4.5 g of hexamethylenediamine (with a little sodium carbonate) in 100 mL of water. Carefully layer the hexane solution on top of the aqueous one so the two do not mix.

• A white film appears at once at the interface. Touch it with tweezers, catch the film, and draw it up and over a glass rod or stirring stick.
• Wind the emerging rope onto the rod, turning steadily. A continuous strand — often several meters long — spools out until one of the monomers is exhausted.
• Wash before handling: the freshly pulled rope is soaked in solvent and unreacted, corrosive monomer. It must be rinsed thoroughly (in dilute base, then water or aqueous ethanol) before anyone touches it bare-handed.

The as-pulled fiber is weak because its chains are short and unaligned. Real nylon fibers gain strength from cold drawing: stretching aligns the chains so that the many amide groups can hydrogen-bond neighbor-to-neighbor into a crystalline, load-bearing structure.

The chemistry that makes nylon strong

What turns a floppy strand into a material tough enough for climbing rope and machine gears is hydrogen bonding between chains. Every amide group offers an N–H that donates and a C=O that accepts, so parallel chains lock together in a dense array of hydrogen bonds, roughly one every few carbons along the backbone.

• The flexible –(CH₂)– runs between the amides provide toughness and the ability to draw; the amides provide the intermolecular glue. This mix of crystalline (hydrogen-bonded) and amorphous regions is why nylons are both strong and resilient.
• Melting points track the amide density: nylon 6,6 melts near 265 °C, while the longer-chain nylon 6,10 melts lower, around 215 °C, because its amides are spaced farther apart and hydrogen-bond less per unit length.
• The same amide groups make nylon absorb water, which plasticizes it — one reason nylon parts swell and soften in humid conditions.

History, uses, and common misconceptions

Nylon was invented at DuPont by Wallace Carothers, whose team synthesized the first practical polyamide (nylon 6,6) in 1935; commercial nylon stockings followed in 1939–1940. Carothers built the intellectual foundation of step-growth polymerization that the rope trick puts on vivid display. The interfacial demonstration itself was popularized decades later by Paul Morgan and Stephanie Kwolek at DuPont — Kwolek would go on to invent Kevlar, another interfacially made aramid polyamide.

• Misconception — "the two liquids react everywhere." They react only at the interface; the bulk of each layer stays unreacted, which is precisely why pulling the film exposes fresh reactant and keeps the rope coming.
• Misconception — "this is how commercial nylon is made." Bulk nylon 6,6 is made by melt polymerization of a diamine and a diacid (via a nylon salt) at 255–280 °C, not with acid chlorides. Acid chlorides are too expensive and too corrosive for tonnage production; the rope trick's convenience comes at a price only a classroom pays.
• Misconception — "the rope is finished nylon you could use." The pulled strand is wet, weak, and monomer-contaminated; genuine engineering strength requires washing, drying, and drawing to align and crystallize the chains.

Interfacial polymerization (the rope trick) versus melt polymerization, the industrial route to bulk nylon.

Feature	Interfacial (rope trick)	Melt polymerization
Typical polymer	Nylon 6,10	Nylon 6,6 or nylon 6
Acid partner	Diacid chloride (very reactive)	Diacid or salt (needs heat)
Temperature	≈ 25 °C, at the bench	255–280 °C under vacuum
Water sensitivity	Water is a reactant's solvent, tolerated	Water must be driven off to shift equilibrium
By-product removed	HCl (neutralized by base/water)	H₂O (distilled away)
Speed	Seconds — film reforms as fast as pulled	Hours