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Spec Mapping — OCR A-Level Chemistry A (H432) Module 6.2.5 (a)–(b) — Condensation polymers, covering: the distinction between addition and condensation polymerisation; formation of polyesters (Terylene/PET from benzene-1,4-dicarboxylic acid + ethane-1,2-diol; loses water) and polyamides (nylon-6,6 from hexanedioic acid + hexane-1,6-diamine; Kevlar from benzene-1,4-dicarboxylic acid + benzene-1,4-diamine; loses water or HCl); the structure of repeating units in both polymer types; the ability to draw the repeat unit given the monomers, and conversely to deduce the monomers given the repeat unit; the role of acyl chlorides as faster reagents (lose HCl rather than water); uses of polyesters (PET drinks bottles, Terylene fibres) and polyamides (nylon textiles and engineering, Kevlar body armour); and the role of proteins as natural polyamides built from α-amino acid monomers (refer to the official OCR H432 specification document for exact wording).
Polymers come in two main flavours. Addition polymers — like polyethene, polypropene and PVC — are made from alkene monomers by opening C=C double bonds and joining them together; no by-product is formed (you met these in Lesson 10 of the OCR basic-organic module). Condensation polymers are made by joining monomers via amide or ester linkages, losing a small molecule (usually water, occasionally HCl) at each linkage. The difference in chemistry has huge consequences for how the final polymer behaves: condensation polymers are almost always biodegradable or at least hydrolysable (this is the subject of Lesson 10), while addition polymers are notoriously persistent in the environment for centuries.
This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.5 (a)–(b): structure, formation and uses of polyesters and polyamides. It synthesises Lesson 3 (carboxylic acids), Lesson 4 (esters and esterification), Lesson 5 (acyl chlorides as faster acid-derivatives), Lesson 6 (amines as the nucleophile that pairs with COOH or COCl) and Lesson 8 (peptide bonds — the natural example of the same amide linkage). The polymers met here — PET, nylon-6,6, Kevlar — are not just exam material; they are some of the most economically important materials in the 21st-century economy, with annual global production measured in tens of millions of tonnes.
n⋅CH2=CHR⟶−(CH2−CHR)n−
graph LR
A[Addition polymer] --> B[C=C opens, no by-product]
A --> C[Polyethene, PVC, PTFE]
D[Condensation polymer] --> E[Ester or amide forms, loses H2O]
D --> F[Polyester, polyamide, polypeptides]
A polyester has a repeating ester linkage –CO–O– in its backbone.
Polyesters can be made from:
The most famous polyester is Terylene, also known as PET. It is made from:
Each ester linkage forms by condensation of one –COOH with one –OH, eliminating water:
nHOOC−C6H4−COOH+nHO−CH2CH2−OH⟶−[CO−C6H4−CO−O−CH2CH2−O]n−+2nH2O
The repeat unit is –CO–C₆H₄–CO–O–CH₂CH₂–O–, with one residue from each monomer and two ester linkages per repeat.
Uses of PET:
Exam Tip: When asked to draw the repeat unit of PET, draw it with brackets showing where it repeats, and explicitly mark the "–O" at one end and "–CO" at the other so the examiner can see the polymer continues on both sides.
A polyamide has a repeating amide linkage –CO–NH– in its backbone. The same logic as polyesters applies: two difunctional monomers join, losing water.
Nylon-6,6 is the classic synthetic polyamide, made from:
Hence the name "nylon-6,6" — six carbons in each monomer.
nHOOC−(CH2)4−COOH+nH2N−(CH2)6−NH2⟶−[CO−(CH2)4−CO−NH−(CH2)6−NH]n−+2nH2O
The repeat unit is –CO–(CH₂)₄–CO–NH–(CH₂)₆–NH–.
Uses of nylon:
Kevlar is a polyamide with an aromatic twist, made from:
nHOOC−C6H4−COOH+nH2N−C6H4−NH2⟶−[CO−C6H4−CO−NH−C6H4−NH]n−+2nH2O
The aromatic rings in the chain make Kevlar chains rigid and enable extensive hydrogen bonding between neighbouring chains. The result is a material with a strength-to-weight ratio five times higher than steel. Uses include:
Proteins are polyamides with a special feature — all the monomers are α-amino acids rather than separate diacid + diamine pairs. Each amide linkage is the peptide bond you met in Lesson 8.
graph TD
A[Polyamides] --> B[Nylon-6,6: aliphatic, flexible, strong textile fibre]
A --> C[Kevlar: aromatic, rigid, ultra-strong armour]
A --> D[Proteins: alpha-amino acid monomers, hydrogen bonding gives 3D structure]
Two skills that OCR tests heavily:
Worked example: Diamine H₂N–(CH₂)₄–NH₂ + diacid HOOC–(CH₂)₂–COOH →
−[NH−(CH2)4−NH−CO−(CH2)2−CO]n−
Worked example: Given repeat unit –[CO–(CH₂)₂–CO–O–CH₂–CH₂–O]– (a polyester), break at the two –CO–O– linkages. The fragments are:
So the monomers are butanedioic acid and ethane-1,2-diol.
In the lab, making a polyester or polyamide from a carboxylic acid is slow — esterification and amide formation are sluggish. To speed things up, chemists often replace the dicarboxylic acid with the corresponding diacyl chloride:
nClOC−(CH2)4−COCl+nH2N−(CH2)6−NH2⟶−[CO−(CH2)4−CO−NH−(CH2)6−NH]n−+2nHCl
This gives nylon-6,6 in minutes at room temperature, rather than requiring prolonged heating. The by-product is now HCl, not water.
The classic demonstration is the nylon rope trick: a diamine in water is layered over a diacyl chloride in cyclohexane. Nylon forms instantly at the interface and can be pulled out as a continuous rope.
Condensation polymers transformed 20th-century industry. Nylon was invented in 1935 by Wallace Carothers at DuPont (paraphrasing the company's account: Carothers had set out to create a synthetic fibre with the strength of silk, and his team's systematic work on polyamides yielded nylon-6,6 on a Tuesday in February 1935). Production began in 1939 for women's stockings, was redirected to parachutes and military rope during the Second World War, and returned to consumer markets in 1945. Today global nylon production exceeds 6 million tonnes per year, mainly for engineering plastics, textile fibres and industrial cordage.
Kevlar was discovered in 1965 by Stephanie Kwolek at DuPont — accounts of her work describe noticing that an aramid polymer solution behaved unexpectedly when stirred (cloudy, almost liquid-crystalline) and pushing for it to be spun into fibre rather than discarded; the resulting filaments showed remarkable tensile strength. Kevlar's strength-to-weight ratio of about five times steel makes it the dominant material for body armour, ballistic protection, fibre-optic cable reinforcement, and aerospace composites.
PET (Terylene in the UK, Dacron in the US) was patented in 1941 by John Whinfield and James Dickson at the Calico Printers' Association in Manchester. Today PET dominates the global drinks-bottle market — there are around 480 billion plastic bottles produced annually, the great majority PET — and is the most widely recycled plastic by mass.
The pattern across all three materials is the same: difunctional monomers + condensation chemistry → repeating ester or amide linkage → high-strength polymer with applications across textiles, packaging and structural materials.
Synoptic Links — Connects to:
ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / carboxylic-acids(Lesson 3 — dicarboxylic acid monomers like hexanedioic acid for nylon and benzene-1,4-dicarboxylic acid for PET / Kevlar are the same –COOH chemistry).ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / esters-esterification-hydrolysis(Lesson 4 — each ester linkage in a polyester is the same –CO–O– chemistry, just on a chain).ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / acyl-chlorides(Lesson 5 — using diacyl chlorides accelerates polyester / polyamide formation, the basis of the nylon rope trick).ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / amines(Lesson 6 — diamine monomers like hexane-1,6-diamine and 1,4-diaminobenzene are the same basic / nucleophilic N chemistry).ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / amino-acids-chirality(Lesson 7 — α-amino acids are the natural monomers for the polyamides we call proteins).ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / peptides-amides(Lesson 8 — the peptide bond is exactly the amide linkage that gives nylon and Kevlar their structure).ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / hydrolysis-and-comparison-of-polymers(Lesson 10 — these condensation polymers can be hydrolysed back to their monomers under harsh enough conditions, which underlies chemical recycling).ocr-alevel-chemistry-basic-organic / addition-polymerisation-polymer-disposal(Lesson 10 of basic organic — the addition-polymer contrast with C–C backbones and no by-product).
Practical Activity Group anchor: PAG 6 — Synthesis of an organic solid. The nylon rope trick is a classic A-Level demonstration of PAG 6 chemistry: 1,6-diaminohexane in aqueous NaOH is layered over hexanedioyl dichloride in cyclohexane (or hexane). The two solutions form a clear interface; tweezers grasp a thin nylon film at this interface and pull it out as a continuous rope — the polymer forms in minutes at room temperature because the acyl chloride is so reactive. The rope is dried, weighed, and may be sectioned for melting-point analysis. This demonstrates condensation polymerisation, interfacial polymerisation, and the role of acyl chloride activation, all in a single visually striking experiment.
Question (9 marks): Two condensation polymers — nylon-6,6 and Kevlar — are produced industrially in millions of tonnes per year.
(a) Draw the repeat unit of nylon-6,6, starting from the monomers hexanedioic acid (HOOC-(CH2)4-COOH) and hexane-1,6-diamine (H2N-(CH2)6-NH2). Write a balanced equation for the polymerisation. (3 marks)
(b) Kevlar is made from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine. Draw the repeat unit of Kevlar. Explain, in terms of structure, why Kevlar has a strength-to-weight ratio about five times that of steel. (3 marks)
(c) In the laboratory, nylon-6,6 is made in seconds at room temperature by reacting hexanedioyl dichloride (ClOC-(CH2)4-COCl) with hexane-1,6-diamine instead of hexanedioic acid. Explain why the acyl chloride route is so much faster than the dicarboxylic acid route, and identify the by-product released. (3 marks)
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