⚛ Addition polymerisation
Uses a double bond. No small molecule is lost. The polymer is the only product.
Polymers » Condensation Polymers and Amino Acids
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.7.3.2, 4.7.3.3 (Higher tier only)
- How condensation polymerisation works and why the monomers need two functional groups
- How a polyester is made from ethanediol and hexanedioic acid
- Why amino acids can join to make polypeptides
- How different amino acids combine to make proteins
Higher tier only
Everything in this lesson is for students sitting the Higher tier papers. Foundation tier students do not need it.
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Unlock This CourseCondensation polymerisation
Polyester fibres like these are condensation polymers - every time two monomers link up, a small water molecule is kicked out
In the last polymer lesson, the monomers had a C=C double bond and simply added on to each other. Nothing was lost. Condensation polymerisation is different. Here the monomers each have two functional groups, one at each end. When two monomers react, they join together and usually lose a small molecule, such as water.
Because a small molecule is lost each time, these reactions are called condensation reactions. Making a long chain this way is called condensation polymerisation.
Key terms:
- Condensation reaction: a reaction where two molecules join and a small molecule, usually water, is lost.
- Condensation polymerisation: making a polymer from monomers that have two functional groups each, losing small molecules as the chain grows.
- Polyester: a polymer made by condensation polymerisation, with ester links joining the repeating units.
Why two functional groups?
Think of each monomer as a short rope with a hook at each end. One hook can catch one neighbour and the other hook can catch another neighbour. That is how a long chain forms.
If a monomer had only one functional group, it could join to just one neighbour and the chain would stop. So each monomer needs a functional group at both ends.
💧 Condensation polymerisation
Monomers have two functional groups. A small molecule such as water is lost at every join. There are two products.
Making a polyester
The simplest condensation polymers come from two different monomers. Each monomer has two of the same functional group.
- Ethanediol is an alcohol with two -OH groups, one at each end: HO-CH2-CH2-OH.
- Hexanedioic acid is a carboxylic acid with two -COOH groups, one at each end: HOOC-(CH2)4-COOH.
An -OH group reacts with a -COOH group to make an ester link, and a molecule of water is lost. Because both monomers have two groups, the reaction can happen at both ends of each molecule. Ethanediol joins to hexanedioic acid, which joins to another ethanediol, and so on. The result is a polyester.
The chain alternates between the two monomers. The repeating unit contains one piece from each:
-O-CH2-CH2-O-CO-(CH2)4-CO-
In words: ethanediol + hexanedioic acid → polyester + water
You can draw this with blocks. Use one type of block for each monomer's carbon chain, with its two functional groups as the ends. When two blocks join, show a small molecule (water) leaving.
Worked example
Question: One molecule of ethanediol joins to one molecule of hexanedioic acid. How many molecules of water are lost?
Answer: One. Each time an -OH group and a -COOH group react to make a link, one molecule of water is lost. One join means one water molecule. A chain made by 100 separate reactions loses 100 water molecules.
Amino acids
An amino acid is a molecule with two different functional groups: an amino group (-NH2) at one end and a carboxylic acid group (-COOH) at the other.
The simplest is glycine: H2N-CH2-COOH.
This time only one monomer is needed, because it already has both kinds of group. The -NH2 of one glycine reacts with the -COOH of the next one. A molecule of water is lost and the two molecules join. The same thing happens again and again, so amino acids react by condensation polymerisation to produce a polypeptide.
The repeating unit in poly(glycine) is: -NH-CH2-CO-
Key terms:
- Amino acid: a molecule with two different functional groups, -NH2 and -COOH.
- Polypeptide: a polymer made by joining amino acids by condensation polymerisation.
- Protein: a long chain made from different amino acids combined in the same chain.
From polypeptides to proteins
Eggs, beans and chicken are packed with protein - long chains of amino acids joined in a set order
Glycine is only one of many amino acids. Different amino acids can be combined in the same chain. Chains like this are proteins. The order of the amino acids makes each protein different.
🔗 Same two groups
Every amino acid has -NH2 and -COOH, so any amino acid can join to any other.
🔬 Different middle part
The rest of each amino acid differs, which is why proteins can be so varied.
Common mistakes
1. Saying water is made in addition polymerisation. It is not. Water is lost only in condensation polymerisation.
2. Forgetting the water. A condensation polymer equation always has a small molecule as a second product.
3. Drawing a monomer with only one functional group. It must have two or the chain cannot grow.
4. Mixing up the two types. Amino acids have two different groups. Ethanediol and hexanedioic acid each have two of the same group.
Exam-style question
Glycine is H2NCH2COOH. Explain how glycine forms a polypeptide. [3 marks]
Model answer
Each glycine molecule has two functional groups, an -NH2 group and a -COOH group. The -NH2 group of one molecule reacts with the -COOH group of another, and a small molecule (water) is lost. This happens many times, joining the molecules into a long chain called a polypeptide. This is condensation polymerisation.
Exam tip
When a question asks you to explain condensation polymerisation, always mention both parts: two functional groups on each monomer, and a small molecule being lost.