Organic chemistry · GCSE Chemistry
Condensation polymers
GCSE Chemistry (Triple) revision on condensation polymerisation: monomers with two functional groups, small-molecule by-products such as water, nylon and polyester, and how they differ from addition polymers.
Two functional groups on each monomer, join and lose a small molecule (often H₂O). Nylon from diamine + dicarboxylic acid. Polyester from diol + dicarboxylic acid. Not the same as alkene addition — no C=C, and there is a by-product.
The important bits
What you need to know
- 1
Condensation polymerisation joins monomers when each monomer has two functional groups that can react. A small molecule, often water, is also produced — unlike addition polymerisation, which has only the polymer as product.
- 2
Addition polymers come from alkene monomers (C=C opens, one product). Condensation polymers come from monomers with –OH, –COOH, –NH₂ or similar groups that can link in a chain while releasing H₂O or HCl.
- 3
Nylon is a condensation polymer. A diamine (two –NH₂ groups) reacts with a dicarboxylic acid (two –COOH groups): amine + acid → amide link + water. The repeating unit contains an amide bond, –CONH–.
- 4
Polyester (for example PET) forms when a diol (two –OH groups, such as ethylene glycol) reacts with a dicarboxylic acid (such as terephthalic acid). An ester link, –COO–, joins the monomers and water is released.
- 5
Proteins are natural condensation polymers: amino acid monomers join by peptide bonds (also condensation), releasing water. Each amino acid has –NH₂ and –COOH (in the same molecule), so the chain grows with peptide links.
- 6
To draw or recognise a condensation polymer repeat unit, show the link between monomers (amide or ester) and remember that the small molecule lost is usually water. Do not show a C=C in the backbone.
- 7
Condensation polymers can often be broken down by hydrolysis (reaction with water), reversing the condensation. Addition polymers are harder to break down biologically, which links to disposal issues.
- 8
Uses: nylon fibres for clothing and ropes (strong, flexible); polyester (PET) for drink bottles and fabric; proteins for structure and enzymes in living things. Properties depend on chain length and intermolecular forces between chains.
Quotations worth analysing
Short evidence. Real method.
“In condensation polymerisation, monomers join and a small molecule such as water is also produced.”
The by-product is the scoring phrase addition polymer answers miss. Name water unless the question specifies HCl (some nylon lab preparations).
“Nylon is formed from a diamine and a dicarboxylic acid, with water as a by-product.”
Both monomers need two reactive groups. The link is an amide, –CONH–, not an ester.
“Polyester monomers include a diol and a dicarboxylic acid; water is released when the ester link forms.”
PET bottles are a named context. Ethylene glycol plus terephthalic acid is the industrial story simplified for GCSE.
Go deeper
How do I tell addition and condensation apart in one sentence?
Ask what the monomer is. Alkene with C=C → addition, polymer only, repeat unit has two carbons from the double bond. Monomer with two –COOH, two –OH, or –NH₂ and –COOH on different molecules → condensation, small molecule lost, backbone has ester or amide links. Poly(ethene) repeat unit: –CH₂–CH₂–. Nylon repeat unit: –NH–(chain)–CO–NH–(chain)–CO– with trailing bonds through brackets. If a question says “water is produced”, it is condensation. If it says “ethene”, it is addition. Never write H₂O as a product of poly(ethene) formation.
Go deeper
Nylon and polyester — what do I actually draw?
You rarely draw full displayed formulae at GCSE, but you must name the monomer types and the link. Nylon: hexanediamine (H₂N–(CH₂)₆–NH₂) plus hexanedioic acid (HOOC–(CH₂)₄–COOH) is a classic teaching pair. Each –NH₂ reacts with a –COOH, forming –CONH– and H₂O. Polyester: HO–CH₂–CH₂–OH plus HOOC–benzene–COOH simplified as diol + dicarboxylic acid → –COO– link + H₂O. The repeat unit is bracketed with n. Trailing bonds through the bracket show continuation. Side groups stay on the same atoms as in the monomer. If asked for a use, nylon → fibres; PET → bottles and fabric.
Go deeper
Why does hydrolysis matter for exam evaluation?
Condensation polymers contain ester or amide bonds that water can break in acid or alkaline hydrolysis — this is how polyester can eventually be recycled chemically in some processes, and how proteins are digested to amino acids. Addition polymers lack those hydrolysable links in the backbone; they persist in landfill. A balanced compare question: addition from crude-oil alkenes, high atom economy, not biodegradable; condensation from different monomers, water by-product, some links break with water. Do not claim all plastics compost quickly. Name the bond type you are breaking.
See the idea in action
Compare poly(ethene) and nylon. Poly(ethene): monomer ethene, C=C addition, repeat unit –CH₂–CH₂–, only product is the polymer, from crude oil, not biodegradable. Nylon: diamine and dicarboxylic acid monomers, amide links –CONH– form, water is also produced, used for strong fibres, amide bonds can be hydrolysed. Both are long chains, but the monomer type, by-product and disposal story differ. A student who writes “both are polymers so the same” loses the mechanism marks.
Exam technique
Turn knowledge into marks
State “small molecule produced” and name water for nylon and polyester. Contrast with addition polymerisation explicitly. Name monomer types (diamine, dicarboxylic acid, diol) and the link (amide or ester). Do not put C=C in a condensation repeat unit.
Common mistakes
Do not give these marks away
- 01
Saying condensation polymerisation has only one product, like addition.
- 02
Drawing nylon with ester links or polyester with amide links.
- 03
Using ethene as the monomer for nylon, or claiming all polymers are addition polymers.
What is produced when monomers form a condensation polymer such as nylon?
AOnly the polymer
BThe polymer and a small molecule such as water
CCarbon dioxide and water only
DShorter alkanes and an alkene
Show the answer
The polymer and a small molecule such as water. Condensation polymerisation releases a small molecule (often H₂O) as well as the polymer. Addition polymerisation from alkenes has only the polymer as product.
Quick questions
If this is the bit you searched
What is condensation polymerisation GCSE?
Monomers with two functional groups join into a long chain and a small molecule such as water is also produced. Nylon and polyester are examples.
How is nylon formed?
A diamine reacts with a dicarboxylic acid. Amide links join the monomers and water is released.
What is the difference between addition and condensation polymers?
Addition polymers form from alkene monomers with no by-product. Condensation polymers form from monomers with two functional groups and release a small molecule such as water.
What link is found in polyester?
An ester link, –COO–, formed when a diol reacts with a dicarboxylic acid, with water released.