Step-growth polycondensation
Step-growth polyamide synthesis (nylon)
How the Step-growth polyamide synthesis (nylon) works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Hexamethylenediamine + adipoyl chloride. Conditions: Interfacial (H₂O/CH₂Cl₂) with NaOH, 25 °C; or solution with base. Bonds formed: Amide C–N bonds (repeatedly); HCl; bonds broken: C–Cl, N–H. Stereochemistry: None; linear chain. Rate and selectivity: Mn rises slowly: Xn = 1/(1−p), high polymer only at p > 0.99.
At a glance
Step 1: Amine attacks the acid chloride
A diamine's nucleophilic nitrogen attacks a diacid chloride's electrophilic carbonyl carbon — ordinary nucleophilic acyl substitution, the exact same mechanism as any simple amide-forming reaction, just with difunctional monomers.
Step 2: Any two chain ends can react
Because both ends of each monomer carry a reactive group, this amide bond formation happens repeatedly and randomly between whatever chain ends happen to encounter each other in solution — unlike chain-growth polymerization, there's no single active chain end propagating.
Step 3: Oligomers couple into high polymer
Over time, short oligomers combine into longer ones, which combine further — high molecular weight is only reached once conversion is nearly complete, a fundamentally different growth pattern (step-growth) from the chain-growth mechanisms covered elsewhere in this library, even though the individual bond-forming step is just an ordinary amide coupling.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- CAN
- Ceric ammonium nitrate. One-electron Ce(IV) oxidant.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
How it compares with related reactions
| Step-growth polyamide synthesis (nylon) | Acetic acid transfers a proton to water | Acid-catalyzed amide hydrolysis | |
|---|---|---|---|
| Reagents | Hexamethylenediamine + adipoyl chloride | Acetic acid (pKa 4.76) + water | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O |
| Conditions | Interfacial (H₂O/CH₂Cl₂) with NaOH, 25 °C; or solution with base | Aqueous solution, 25 °C | 6 M acid, reflux (hours) |
| Stereochemistry | None; linear chain | None (achiral species) | No stereocentre created; α-stereocentres retained |
| Rate / selectivity | Mn rises slowly: Xn = 1/(1−p), high polymer only at p > 0.99 | Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Step-growth polyamide synthesis (nylon)?
Hexamethylenediamine + adipoyl chloride. Conditions: Interfacial (H₂O/CH₂Cl₂) with NaOH, 25 °C; or solution with base.
Which bonds are formed and which are broken?
Formed: Amide C–N bonds (repeatedly); HCl. Broken: C–Cl, N–H.
What is the stereochemical outcome?
None; linear chain
What controls the rate or the selectivity?
Mn rises slowly: Xn = 1/(1−p), high polymer only at p > 0.99
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go