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Condensation polymerisation is the second major route to macromolecules studied at A-Level. Unlike addition polymerisation, where alkene monomers stitch together by breaking C=C π-bonds with no by-product, condensation polymerisation joins two difunctional monomers together with the elimination of a small molecule — most often water, but sometimes hydrogen chloride or methanol. The functional groups produced at each link are therefore not bonds left over from a double-bond opening, but newly-formed ester or amide linkages built by nucleophilic acyl substitution. This single mechanistic distinction explains why condensation polymers can be hydrolysed back to their monomers, why their atom economy is below 100%, and why nature uses this strategy almost exclusively to build proteins, nucleic acids, and polysaccharides.
This lesson develops the two industrially dominant families — polyesters (PET / Terylene, PLA) and polyamides (nylon-6,6, nylon-6, Kevlar) — alongside their biological counterparts. It will explicitly contrast condensation with addition polymerisation (developed in lesson 5 of the organic-foundations course), and connect to the ester and amide preparation chemistry from lesson 2 of this course and the amino-acid / DNA chemistry from lesson 6. By the end you should be able to look at any condensation polymer and instantly identify the monomer pair that built it, draw a single repeat unit cleanly, and predict the hydrolysis products that emerge when the chain is broken.
Spec mapping (AQA 7405): This lesson anchors to §3.3.12 (condensation polymers). It builds directly on lesson 2 of this course (§3.3.9 carboxylic acid derivatives — esterification and amide formation) and lesson 6 (§3.3.13 amino acids, proteins and DNA, which are natural condensation polymers). It explicitly contrasts with lesson 5 of the organic-foundations course (§3.3.5 addition polymerisation of alkenes). Recycling, biodegradability and atom-economy connect to §3.1.2 (mass and moles) and §3.10 (green chemistry / sustainability themes that recur throughout the AQA specification). Refer to the official AQA specification document for the exact wording of each section.
Assessment objectives: AO1 — define a condensation polymer and identify the ester or amide repeat unit in a given polymer. AO2 — draw the repeat unit of a polyester or polyamide given the monomers (or vice versa, deduce monomers from a repeat unit), and predict the hydrolysis products. AO3 — compare condensation polymers with addition polymers in terms of mechanism, by-product, atom economy, recyclability and biodegradability; evaluate the sustainability case for replacing PET with PLA or related biodegradable alternatives. Drawing repeat units is a near-guaranteed exam item; quantitative atom-economy calculations and biodegradability evaluations regularly appear in the longer six-mark questions.
Polymer chemistry at A-Level divides cleanly into two mechanisms:
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| Monomers | One type, contains C=C | Usually two types, each difunctional |
| By-product | None | Small molecule (H₂O, HCl, MeOH) |
| Linkage formed | C–C single bond | Ester (–COO–) or amide (–CONH–) |
| Atom economy | 100% | Typically 85–95% |
| Recyclability | Mechanical only (mostly) | Mechanical + chemical (hydrolysis) |
| Biodegradability | Generally poor | Variable; many are biodegradable |
| Examples | Poly(ethene), poly(propene), PVC | PET, nylon-6,6, Kevlar, proteins, DNA |
The two monomers in a condensation polymerisation are described as difunctional: each carries two reactive groups so that, once one end reacts to extend the chain, the other end is still available to react with the next monomer. A diol (HO–R–OH) reacting with a dicarboxylic acid (HOOC–R'–COOH) gives a polyester; a diamine (H₂N–R–NH₂) reacting with a dicarboxylic acid gives a polyamide. If one monomer carried only a single reactive group it would simply cap the chain and polymerisation would not propagate — exactly why monofunctional acids and alcohols are sometimes added deliberately to control chain length.
Key Point: The general signature of a condensation polymer in an exam question is the presence of an ester (–COO–) or amide (–CONH–) linkage in the repeat unit and the loss of n small molecules (n × H₂O, n × HCl) in the balanced equation. If you see C=C in the repeat unit and no by-product, you are looking at an addition polymer instead.
A polyester is built from a diol plus a dicarboxylic acid (or its acyl chloride or diester equivalent). The –OH of the diol attacks the carbonyl carbon of the acid by nucleophilic acyl substitution; tetrahedral intermediate collapse expels water; an ester linkage is left behind. Repeated 2n times across n monomer pairs, the chain grows.
The dominant commercial polyester is PET, made from:
The balanced overall equation is:
n HOOC–C₆H₄–COOH + n HO–CH₂–CH₂–OH → [–OC–C₆H₄–CO–O–CH₂–CH₂–O–]ₙ + 2n H₂O
The repeat unit, drawn properly, is:
–O–CO–C₆H₄–CO–O–CH₂–CH₂–
Notice the structural features:
This particular balance — rigid aromatic spacer plus flexible aliphatic hinge — gives PET its commercial profile: high tensile strength (suitable for fibre as Terylene / Dacron), good barrier properties to gases (drinks bottles), excellent dimensional stability, and the ability to be drawn (oriented) to crystallise and become tougher still.
PET is the most-produced polyester in the world: roughly 30 Mt per year for fibre, 20 Mt per year for bottles. Critically it is also one of the most successfully recycled plastics, partly because the ester linkage can be cleaved both mechanically (melt and remould) and chemically (hydrolysis or transesterification back to the monomers).
Exam Tip: When asked to "draw the repeat unit", show exactly one monomer-pair worth of chain enclosed in square brackets with an "n" subscript outside. The bond extending out of each end of the bracket indicates continuation. Do not draw the by-product molecules (they are eliminated, not in the repeat unit), and do not draw –OH groups at the ends of the repeat unit (those belong to the chain ends, not the repeat).
A polyamide is built from a diamine plus a dicarboxylic acid (or the corresponding diacyl chloride). The lone pair on nitrogen attacks the carbonyl carbon; tetrahedral intermediate collapse expels water (or HCl from an acyl chloride); an amide linkage is left behind.
The classic teaching example is nylon-6,6, made from:
The "6,6" denotes the carbon count of each monomer (six in each). The balanced equation:
n HOOC–(CH₂)₄–COOH + n H₂N–(CH₂)₆–NH₂ → [–OC–(CH₂)₄–CO–NH–(CH₂)₆–NH–]ₙ + 2n H₂O
The repeat unit:
–OC–(CH₂)₄–CO–NH–(CH₂)₆–NH–
Nylon-6,6 was first commercialised by DuPont in 1935 (the Carothers research programme) and remains a workhorse engineering polymer. Drawn into fibres it makes ropes, parachutes, stockings, and reinforcement cord for tyres. Moulded as a solid it makes gears, bearings, and zip teeth. Critically, the regular repeat allows extensive inter-chain hydrogen bonding between the N–H of one chain and the C=O of the next, giving high tensile strength and a sharp melting point (~265 °C).
Nylon-6 is made from a single monomer — caprolactam (azepan-2-one, a seven-membered cyclic amide) — by ring-opening polymerisation. The –NH–CO– bond in the ring breaks and reforms with the next monomer's NH, building a polymer with repeat unit:
–NH–(CH₂)₅–CO–
Note that, like nylon-6,6, this is a polyamide; the "6" indicates the six carbons per monomer. Nylon-6 has marginally lower crystallinity than nylon-6,6 and a slightly lower melting point (~220 °C), making it easier to process. It is widely used in sleeping bags, gears, and carpet fibres.
Kevlar (poly-p-phenylene terephthalamide) is made from:
Repeat unit:
–OC–C₆H₄–CO–NH–C₆H₄–NH–
Two structural features make Kevlar exceptional:
The result is a fibre with tensile strength about five times that of steel by weight, used in bulletproof vests, helmets, brake-pad reinforcement and aerospace composites. Kevlar is not practically biodegradable and not easily recycled — the cost of its extraordinary mechanical performance.
Common Misconception: Students often draw nylon repeat units with –CONH– "backwards" at one end or with one too many –CH₂– groups. Count atoms carefully: nylon-6,6 has four –CH₂– between the two C=O (from adipic acid, since the two acid carbons are counted as part of CO) and six –CH₂– between the two N (from hexane-1,6-diamine).
The single most-tested skill in this topic is drawing a clean repeat unit. Use this four-step method:
Worked example — polyester from butanedioic acid and propane-1,3-diol:
Reverse direction — deduce monomers from this repeat unit: –OC–(CH₂)₈–CO–NH–(CH₂)₆–NH–. The –OC...CO– fragment with eight methylenes between is decanedioic acid (or its acyl chloride), HOOC–(CH₂)₈–COOH; the –NH...NH– fragment with six methylenes between is hexane-1,6-diamine, H₂N–(CH₂)₆–NH₂. The polymer is therefore "nylon-6,10" (six from the diamine, ten from the diacid).
The textbook condensation routes above use carboxylic acid + alcohol or amine, eliminating water. Industrially this is fine because elevated temperature and removal of water drive the equilibrium forward. In the laboratory, however, the reactions are slow and the equilibria unfavourable, so the acyl chloride form of the acid (RCOCl in place of RCOOH) is often used:
n ClOC–C₆H₄–COCl + n H₂N–C₆H₄–NH₂ → [–OC–C₆H₄–CO–NH–C₆H₄–NH–]ₙ + 2n HCl
The reaction is rapid at room temperature; HCl rather than H₂O is the by-product, usually neutralised by a base such as triethylamine or aqueous NaOH so that it does not deactivate any unreacted amine by protonation. Kevlar is manufactured by this acyl-chloride route.
The classic schools demonstration — the nylon rope trick — exploits the same chemistry:
The trick illustrates three things at once: (i) the speed of acyl-chloride condensation, (ii) the 2D nature of the reaction zone (the two reactive monomers are in immiscible phases, so only the interface contributes), and (iii) the fibre-forming character of an aligned polyamide.
Practical-Skills Box — Nylon Rope Trick: Hazards: decanedioyl dichloride is corrosive and lachrymatory; perform in a fume cupboard. Glassware: 100 cm³ beaker, tweezers, glass rod. Procedure: dissolve 1.0 g of hexane-1,6-diamine and 0.5 g of NaOH in 25 cm³ water (lower layer); separately dissolve 1.0 cm³ of decanedioyl dichloride in 25 cm³ hexane (upper layer). Pour the hexane layer carefully on top of the aqueous layer down a glass rod to avoid mixing. A milky film forms at the interface. Touch the film with tweezers and lift slowly — a continuous filament can be drawn out and wound onto a glass rod. Rinse the rope with water and ethanol before disposal in solid waste.
Atom economy is the percentage of reactant atoms that end up in the desired product:
atom economy = (Mᵣ of desired product / Σ Mᵣ of all reactants) × 100%
For an addition polymer such as poly(ethene), every reactant atom ends up in the polymer — atom economy = 100%. For a condensation polymer, the small molecule by-product is "wasted" mass.
Worked example — PET from terephthalic acid and ethane-1,2-diol:
Worked example — nylon-6,6 from adipic acid and hexane-1,6-diamine:
Both polyesters and polyamides typically come in around 85–95% atom economy — better than many small-molecule syntheses but inferior to addition polymerisation's perfect 100%. The acyl-chloride route is worse in atom economy terms because HCl (Mᵣ = 36.5) is eliminated instead of H₂O (Mᵣ = 18.0), but it is faster and milder in lab conditions — a classic process-chemistry trade-off between yield/rate and sustainability.
Three of life's four major biomolecule classes are condensation polymers. The fourth — lipids — are not polymers but the ester linkages in triglycerides arise from the same chemistry.
Amino acids condense via the carboxyl of one and the amine of the next, eliminating water and forming a peptide bond (–CONH–). A protein with n residues contains n–1 peptide bonds and represents the loss of (n–1) water molecules from the constituent amino acids. The peptide bond is planar and rigid (partial C=N double-bond character from amide resonance, as developed in lesson 2), which constrains protein backbone geometry to the Ramachandran-allowed φ/ψ angles and underpins secondary-structure preferences for α-helix and β-sheet. Proteins are therefore polyamides — but with twenty different amine-acid side chains rather than a single monomer pair.
Nucleic acids polymerise via phosphodiester bonds between the 3'-OH of one sugar and the 5'-phosphate of the next. A water molecule is eliminated at each link. The backbone is therefore a polyester (more strictly, a poly-phosphodiester), although students should note the by-product is water and the linkage is –O–P(=O)(O⁻)–O–. The genetic information is carried not in the backbone itself but in the sequence of nucleobases (A, T/U, C, G) hanging off it.
Polysaccharides condense glucose monomers via glycosidic bonds — the anomeric –OH of one glucose reacts with the C-4 –OH of the next, eliminating water. The α(1→4) linkage in starch gives a helical, easily-hydrolysed polymer; the β(1→4) linkage in cellulose gives a straight, hydrogen-bonded fibre that mammals cannot enzymatically digest. Both are polyethers (one C–O–C link per glycosidic bond), formed by condensation.
The deep insight is that nature has converged on the same chemistry humans rediscovered in the 1930s: difunctional monomers, nucleophilic acyl substitution, water as by-product. The reason is thermodynamic: in aqueous biology the equilibrium for amide and ester formation is unfavourable, but ATP-driven enzymes (peptidyl transferase, DNA polymerase, glycogen synthase) couple condensation to nucleotide-triphosphate hydrolysis, paying for the unfavourable bond formation with the cost of pyrophosphate release.
Condensation polymers are far more amenable to chemical recycling than addition polymers, because the ester or amide linkage can be hydrolysed back to the original monomers under suitable conditions.
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