Nylon (Condensation Polymer)

The link that loses a molecule

Polymers come in two big families, and they are split by accounting. In an addition polymer like polyethylene, every atom of every monomer ends up in the chain — nothing is thrown away. In a condensation polymer like nylon, every new bond between two monomers kicks out a small molecule. That molecule "condenses out" of the reaction. For nylon the discarded molecule is water (or, on the lab bench, hydrogen chloride).

The reactive functional groups are the giveaway. Nylon needs a diamine — a molecule with an amine group (–NH₂) at each end — and a diacid — a molecule with a carboxylic acid (–COOH) at each end. When an amine meets a carboxylic acid, they form an amide bond (–CO–NH–) and release water:

R–COOH  +  H₂N–R'   →   R–CO–NH–R'   +   H₂O
(acid)      (amine)        (amide)        (water)

Because each monomer is difunctional — two reactive ends apiece — the joining doesn't stop at one bond. The amide just made still has a free –COOH on one side and a free –NH₂ on the other, so it can react again at both ends. Repeat that thousands of times and you have a polyamide: nylon. The "di" is the whole trick. Use a monofunctional acid or amine instead and you get a single small amide molecule that can't grow.

Nylon-6,6: the canonical reaction

The original and still most-produced nylon is nylon-6,6. The two sixes count the carbons in each monomer: hexamethylenediamine, H₂N–(CH₂)₆–NH₂ (6 carbons), and adipic acid, HOOC–(CH₂)₄–COOH (6 carbons including the two carboxyl carbons). The balanced step is:

n HOOC–(CH₂)₄–COOH   +   n H₂N–(CH₂)₆–NH₂
        adipic acid              hexamethylenediamine
                    │
                    ▼
  –[ CO–(CH₂)₄–CO–NH–(CH₂)₆–NH ]–ₙ   +   (2n − 1) H₂O
                  nylon-6,6                    water

Industrially this is done as a melt polycondensation. The two monomers are first combined as a 1:1 salt ("nylon salt," from neutralizing the acid with the diamine), which guarantees the exact stoichiometry. The salt solution is then heated to roughly 270–280 °C under pressure and the water is continuously driven off. Removing the water matters: amide formation is an equilibrium, so by Le Chatelier's principle, pulling out the by-product shoves the reaction toward longer chains.

The other common workhorse, nylon-6, is made differently and despite the name is not a two-monomer condensation. It comes from a single monomer, caprolactam, a six-membered ring containing one amide. The ring opens and the molecules link head-to-tail. No small molecule is expelled, so nylon-6 is technically a ring-opening chain-growth polymer that happens to produce a polyamide identical in repeat-chemistry to half of nylon-6,6.

The mechanism, arrow by arrow

Amide formation is a nucleophilic acyl substitution. Walk through the acid-chloride version used in the rope trick, because it is fast and clean. Adipoyl chloride, ClCO–(CH₂)₄–COCl, has two highly electrophilic carbonyl carbons — the chlorine pulls electron density away and makes the carbon hungry for a nucleophile.

1. Addition. The lone pair on the diamine nitrogen attacks the carbonyl carbon. The C=O π bond breaks, electrons go up onto oxygen, and you get a tetrahedral intermediate with a negative oxygen (an alkoxide) and a positive nitrogen.
2. Proton shuffle. The nitrogen loses its extra proton; oxygen reclaims the negative charge briefly.
3. Elimination. The C=O reforms and chloride is ejected as the leaving group. The result is the amide bond, –CO–NH–, plus HCl.

     O                         O⁻                       O
     ‖                         |                         ‖
  R–C–Cl   + :NH₂R'  →   R–C–Cl        →    R–C–NHR'  +  Cl⁻ (+ H⁺)
                              |
                          NH₂⁺R'
   (acyl chloride)   (tetrahedral intermediate)   (amide)

Why use the acid chloride instead of the plain acid? Leaving-group quality. Chloride is a far better leaving group than hydroxide, so the substitution is fast at room temperature and irreversible — the expelled HCl doesn't push back. With the plain carboxylic acid (the industrial route) the leaving group would be –OH, a terrible leaving group, so you need high temperature, you must remove the water, and the reaction stays an equilibrium. Same amide product, completely different conditions.

Step-growth and the Carothers equation

Nylon polymerizes by step-growth (also called step-reaction or condensation polymerization), which behaves nothing like the chain-growth of addition polymers. In chain-growth, a few active centers each grow to full length quickly while most monomer sits unreacted. In step-growth, any two species with compatible ends can react — monomer with monomer, dimer with trimer, hexamer with octamer. Early on you have a soup of short oligomers, and only at very high conversion do the long chains appear.

That makes the math unforgiving. The number-average degree of polymerization (average monomer units per chain) is given by the Carothers equation:

X̄ₙ = 1 / (1 − p)

where p is the fraction of functional groups that have reacted (the "extent of reaction"). To get useful fiber-grade nylon you want X̄ₙ around 100–200, which demands p ≈ 0.99–0.995. In other words, you must drive the reaction to over 99% completion before you get a strong polymer at all. At 90% conversion you have an X̄ₙ of only 10 — a weak, gummy oligomer. This is the central engineering challenge of every condensation polymer.

Conditions, numbers, and trade-offs

	Melt route (industrial)	Interfacial route (rope trick)
Acid monomer	Adipic acid (–COOH)	Adipoyl chloride (–COCl)
By-product	Water	HCl
Temperature	~270–280 °C	Room temperature (~25 °C)
Reversible?	Yes — equilibrium, water must be removed	No — chloride leaves irreversibly
Stoichiometry control	Critical (nylon salt enforces 1:1)	Self-limiting at the interface
Speed	Hours under pressure	Seconds
Use	Bulk fiber and engineering plastic	Classroom demo, thin films

Some real figures to anchor the chemistry. The amide C–N bond has a partial double-bond character (about 40% double-bond), which is why it is planar and rotation-locked, with a rotation barrier near 75–85 kJ/mol. Each interchain amide-to-amide hydrogen bond is worth roughly 20 kJ/mol; nylon-6,6's tight register of these bonds lifts its melting point to about 264 °C (nylon-6 melts lower, near 220 °C, because its chains are less symmetric). World production of polyamides runs around 9 million tonnes a year. And the equilibrium constant for plain amidation is unfavorable enough (K ≈ 1–10 in the melt) that without continuous water removal the chains never get long — a quantitative restatement of why the by-product accounting matters.

Why nylon is strong: hydrogen bonds in register

A nylon chain on its own is just a flexible string. Its strength comes from how neighboring chains grip each other. Every amide group has an N–H that donates a hydrogen bond and a C=O that accepts one. Because the amides repeat at regular spacing along the backbone, adjacent chains can line up so that every N–H finds a C=O on the neighbor — a continuous ladder of hydrogen bonds zipping the chains into sheets.

  ···C=O····H–N···C=O····H–N···   chain 1
        ¦         ¦                  (H-bonds, ~20 kJ/mol each)
  ···H–N····O=C···H–N····O=C···   chain 2

This is exactly the motif that holds the β-sheets of silk and spider thread together — Carothers was explicitly trying to build a synthetic silk. The regular structure also lets the chains pack into crystalline domains. Cold-drawing a freshly spun nylon fiber pulls those crystallites into alignment along the fiber axis, and the tensile strength jumps several-fold. A drawn nylon-6,6 fiber reaches a tenacity of roughly 0.5–0.9 GPa — strong enough for parachute lines, climbing rope, and tire cord.

Where nylon shows up

• Stockings and textiles. Nylon's 1939 debut was women's hosiery; "nylons" outsold every prior fiber on launch. The fine, strong, elastic filament was unlike anything natural.
• Parachutes and ropes. World War II diverted nylon from stockings to parachute canopies, tow lines, and tents — high tenacity plus resistance to mildew that rotted silk and cotton.
• Engineering plastics. Glass-filled nylon-6,6 is molded into gears, bearings, zip ties, and under-hood automotive parts. It is self-lubricating and shrugs off oils and solvents.
• Tire cord and airbags. Nylon's strength-to-weight and heat tolerance suit it for reinforcing radial tires and for woven airbag fabric.
• Kevlar's cousins. Replace the flexible (CH₂) spacers with rigid aromatic rings and you get aramids like Kevlar — same amide condensation chemistry, but the stiff backbone and even tighter hydrogen bonding give bulletproof strength.

Common misconceptions and pitfalls

• "Nylon-6 is made from two 6-carbon monomers." No — the single number means a single monomer. Nylon-6 comes from caprolactam alone, via ring-opening, not a diamine-plus-diacid condensation. Only nylons with two numbers (6,6 · 6,10 · 6,12) are two-monomer condensations.
• "Condensation always loses water." Usually, but not always. The acid-chloride route loses HCl; polyester condensations can lose methanol when a diester is used (transesterification). The defining feature is that some small molecule leaves, not that it is specifically water.
• "Any imbalance is fine; the excess just sits there." Fatal in step-growth. Off-ratio monomers cap chains early and slash the molar mass. A 5% excess of one monomer holds X̄ₙ to about 40 no matter how long you wait. Stoichiometry is the master variable.
• "Nylon and the peptide bond are unrelated." They are the same amide linkage. Nylon is, chemically, a synthetic protein backbone with only two repeating residues and no side chains.
• "It's heat alone that makes nylon strong." The strength is mechanical alignment plus hydrogen bonding. Cold-drawing — pulling the fiber cold to orient the crystallites — is what converts a weak as-spun thread into engineering-grade fiber.
• "Catalysts make the chains longer." A catalyst speeds the reaction to a given extent of conversion, but the chain length is set by p and stoichiometry, not by rate. To get longer chains you push p closer to 1 (remove by-product, exact 1:1 ratio), not add catalyst.