Understanding Condensation Polymerisation Revision Notes | GCSE AQA Higher Chemistry

1. What Makes Condensation Polymerisation Different?

You've probably heard of addition polymerisation where alkenes join together like a zip fastening. Condensation polymerisation works completely differently - it's more like building with LEGO bricks that click together, but each time you connect two pieces, a small bit falls off.

The key difference is in the monomers themselves. Instead of having a C=C double bond like in addition polymerisation, condensation monomers have two functional groups - one at each end of the molecule. When these monomers react, they don't add together - they eliminate small molecules like water in the process.

2. The key scientific idea of Two Functional Groups

Here's where it gets interesting. For condensation polymerisation to work, each monomer needs exactly two functional groups. Think of these as 'sticky ends' that can react with other monomers.

The simplest case involves two different types of monomers, each with two of the same functional groups. For example, one monomer might have two -OH groups, while another has two -COOH groups. When these react, the -OH from one monomer combines with the -COOH from another, eliminating water (H₂O) and forming a new bond.

Worked example: How Condensation Polymerisation Works

Show how two monomers with functional groups join together in condensation polymerisation

1. Monomer A has two -OH groups Monomer B has two -COOH groups — Each monomer must have exactly two functional groups
2. -OH + -COOH → -COO- + H₂O — The functional groups react together
3. Water (H₂O) is eliminated — This is why it's called condensation - water is lost
4. An ester bond (-COO-) forms between monomers — This links the monomers together in the polymer chain

Answer: Two monomers join with elimination of water, forming a polymer chain

Notice how each monomer still has one unused functional group after reacting? This means the reaction can continue, with more monomers adding to both ends of the growing chain.

3. What Gets Eliminated and Why

The 'condensation' part of the name comes from what happens during the reaction. Just like water condenses from steam, small molecules are 'condensed out' of the reaction mixture.

Water (H₂O) is by far the most common molecule eliminated, but it's not the only one. Depending on the functional groups involved, you might also see hydrogen chloride (HCl) or ammonia (NH₃) being eliminated. The key point is that something small always gets removed.

This elimination is crucial because it drives the reaction forward. As the small molecules are removed (often by heating to evaporate them), more monomers can react, making the polymer chain longer and longer.

4. Real Examples You Need to Know

Let's look at some real condensation polymers you encounter every day. Polyester is probably the most familiar - it's made from two types of monomers, each with two functional groups.

The reaction involves a diol (molecule with two -OH groups) reacting with a dicarboxylic acid (molecule with two -COOH groups). Each time they react, water is eliminated and an ester bond forms. This continues until you have a long polyester chain.

Worked example: Formation of Polyester

Show how ethane-1,2-diol reacts with ethanedioic acid to form polyester

1. Ethane-1,2-diol: HO-CH₂-CH₂-OH Ethanedioic acid: HOOC-COOH — Each monomer has exactly two functional groups
2. HO-CH₂-CH₂-OH + HOOC-COOH → HO-CH₂-CH₂-OOC-COOH + H₂O — One -OH reacts with one -COOH, eliminating water
3. More monomers add to both ends of the growing chain — Each end still has a reactive functional group
4. ...OOC-COO-CH₂-CH₂-OOC-COO-CH₂-CH₂-OOC... — Alternating ester bonds and carbon chains

Answer: Polyester forms through condensation reactions eliminating water molecules

5. Exam Technique and Common Mistakes

Exam questions on condensation polymerisation often ask you to identify what type of polymerisation is occurring, or to predict what small molecule is eliminated. The key is to look for the functional groups.

Common exam mistake: Students often confuse addition and condensation polymerisation. Remember - if the question mentions elimination of small molecules or shows monomers with two functional groups, it's condensation polymerisation.

Exam precision note: In Understanding Condensation Polymerisation - Condensation polymerisation (HT only) - Synthetic and naturally occurring polymers (chemistry only), always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Condensation polymerisation joins monomers together while also removing a small molecule such as water or hydrogen chloride. This is different from addition polymerisation, where monomers join without any small molecule being eliminated.