Condensation polymers

CIE A-Level Chemistry· 45 min read

1. How condensation polymers form★★☆☆☆⏱ 15 min

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📘 Definition

Condensation Polymerization

A step-growth polymerization reaction where bifunctional monomers react to form a polymer chain, eliminating a small stable molecule (usually water or HCl) as a by-product

Example:

Nylon forms from 1,6-diaminohexane and hexanedioic acid, eliminating 2 water molecules per repeat unit

Unlike addition polymers (formed from alkene monomers with no by-product), condensation polymers require reactive functional groups that can react together. Most form from two different monomers, each with two functional groups, though some form from a single monomer with two different functional groups.

📐 Worked Example

Draw the repeating unit of the polymer formed when ethane-1,2-diol reacts with benzene-1,4-dicarboxylic acid, name the polymer and identify the eliminated by-product.

1
Identify the functional groups: a diol (two -OH groups) and a dicarboxylic acid (two -COOH groups). Each -OH reacts with -COOH in a condensation reaction.
2
Remove -OH from the carboxylic acid and -H from the alcohol to create free bonding sites:
3
$-\text{O-CH}_2\text{CH}_2\text{-O-OC-C}_6\text{H}_4\text{-CO}-$
4
The polymer is polyethylene terephthalate (PET, Terylene), and the eliminated by-product is water.

2. Polyesters★★☆☆☆⏱ 10 min

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📘 Definition

Polyester

A condensation polymer where adjacent monomer units are joined by ester linkages (-COO-)

Example:

Common examples are Terylene (PET) and polylactic acid (PLA), a biodegradable polymer

Polyesters can form in two ways: (1) a diol monomer reacting with a dicarboxylic acid monomer, or (2) a single hydroxycarboxylic acid monomer that contains both an -OH and -COOH group in one molecule.

📐 Worked Example

Lactic acid ( $CH_{3} CH(OH)COOH$ ) polymerises to form PLA. Deduce the structure of the repeating unit of PLA.

1
Lactic acid is a single monomer with one -OH and one -COOH group, so it can react with other lactic acid molecules via condensation.
2
Remove H from the -OH group and OH from the -COOH group to form bonding sites, giving the repeating unit:
3
$-\text{O-CH(CH}_3\text{)-CO}-$
4
The linkage between units is an ester bond, confirming PLA is a polyester, with water as the eliminated by-product.

3. Polyamides★★★☆☆⏱ 15 min

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📘 Definition

Polyamide

A condensation polymer where adjacent monomer units are joined by amide linkages (-CONH-)

Example:

Synthetic example: Nylon; natural example: proteins made from amino acids

Polyamides form similarly to polyesters, but use amine groups instead of alcohol groups. They form from either a diamine + dicarboxylic acid, or amino acid monomers that have both an amine and carboxylic acid group. The amide linkage is called a peptide linkage in biological proteins.

📐 Worked Example

Nylon 6,6 forms from 1,6-diaminohexane ( $H_{2} N(CH_{2})_{6} NH_{2}$ ) and hexanedioic acid ( $HOOC(CH_{2})_{4} COOH$ ). Draw the repeating unit and state the by-product.

1
Identify the functional groups: diamine (two -NH $_{2}$ ) and dicarboxylic acid (two -COOH). An amide bond forms when they react.
2
Remove H from the amine and OH from the carboxylic acid to create bonding sites, giving the repeating unit:
3
$-\text{NH-(CH}_2\text{)}_6\text{-NH-CO-(CH}_2\text{)}_4\text{-CO}-$
4
One water molecule is eliminated per amide bond, so two water molecules are eliminated per full repeating unit of Nylon 6,6.

4. Identifying monomers from polymer structures★★★☆☆⏱ 15 min

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A common exam question gives you the structure of a condensation polymer and asks you to find the monomers. This is done by reversing the condensation reaction via hydrolysis, following these steps:

Split the polymer chain at the linkage: split ester between C-O, split amide between C-N
Add an -OH group to the carbonyl carbon (from the carboxylic acid part)
Add a -H to oxygen (for polyester) or nitrogen (for polyamide)
Check that each monomer has two correct functional groups

📐 Worked Example

A condensation polymer has repeating unit $- NH-(CH_{2})_{5} -CO -$ . Identify the monomer structure.

1
Split the amide linkage between the carbonyl carbon and the nitrogen atom.
2
Add -OH to the carbonyl carbon, and add -H to the nitrogen atom, giving:
3
$\text{H}_2\text{N-(CH}_2\text{)}_5\text{-COOH}$
4
This is a single amino-carboxylic acid monomer, which forms Nylon 6 via condensation polymerization.

5. Common Pitfalls

Wrong move:

Forgetting to include the eliminated by-product in polymerization equations

Why:

Marks are often awarded for including the by-product, which is required for a balanced equation

Correct move:

Add one water (or HCl) per linkage; a two-monomer repeat unit eliminates 2 small by-product molecules

Wrong move:

Splitting the polymer linkage at the wrong position when finding monomers

Why:

Students often split the C=O bond instead of the C-O/C-N bond in the linkage

Correct move:

Always split ester between C-O and amide between C-N; never split the carbonyl double bond

Wrong move:

Not drawing free dashes at the ends of the repeating unit

Why:

Examiners penalize drawing full end groups (H/OH) instead of showing the chain continues

Correct move:

Always draw a dash at each end of the repeating unit to represent the extending polymer chain

Wrong move:

Confusing ester and amide linkages

Why:

Students often mix up the position of oxygen and nitrogen in the linkage

Correct move:

Remember: Ester = C-O-C=O (has oxygen between units), Amide = C-N-C=O (has nitrogen between units)

Wrong move:

Thinking all condensation polymers need two different monomers

Why:

Most common examples use two monomers, so students forget single-monomer condensation polymers

Correct move:

Check the repeating unit length: single monomers (like lactic acid, amino acids) give shorter repeating units

6. Quick Reference Cheatsheet

Polymer Type	Monomers	Linkage	By-product	Example
Polyester	Diol + Dicarboxylic acid	Ester (-COO-)	Water	Terylene (PET)
Polyester	Hydroxycarboxylic acid	Ester (-COO-)	Water	Polylactic acid (PLA)
Polyamide	Diamine + Dicarboxylic acid	Amide (-CONH-)	Water	Nylon 6,6
Polyamide/Protein	Amino acids	Peptide (-CONH-)	Water	Natural silk/wool

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 1
Identify linkage in condensation polymer
2022 · 2
Draw repeating unit of polyamide
2021 · 4
Deduce monomers from polymer structure

What's Next

Condensation polymers connect foundational organic chemistry of functional groups to materials science and biological chemistry, two key areas tested in CIE A-Level Chemistry. Mastering this sub-topic prepares you for questions on polymer structure, properties, and environmental impact, which appear in both multiple-choice and extended response papers. Condensation polymerization is also the basis for biological polymers like proteins and carbohydrates, which are covered in the inorganic and biological chemistry sections of the syllabus. Follow the links below to continue your exam preparation.