11.2 Condensation Polymerisation

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how monomers with two functional groups join to form polyesters, losing small molecules such as water (Higher tier). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain that condensation polymerisation needs monomers with two functional groups (Higher tier)
• Describe how a diol and a dicarboxylic acid form a polyester and water
• Draw the repeating unit of a polyester using simple block diagrams
• Compare addition and condensation polymerisation, and name the small molecules released (H₂O or HCl)
• Calculate the mass of water released and the Mr of a repeating unit

Key words

• Condensation polymerisation: Polymerisation in which monomers join and a small molecule, such as water, is released at each link.
• Diol: A molecule with two –OH (alcohol) groups, e.g. ethanediol, HO–CH₂–CH₂–OH.
• Dicarboxylic acid: A molecule with two –COOH groups, e.g. butanedioic acid, HOOC–CH₂–CH₂–COOH.
• Polyester: A condensation polymer whose monomers are joined by ester links, –COO–.
• Ester link: The –COO– group formed when an –OH group reacts with a –COOH group.

Two functional groups per monomer (Higher tier)

In 10.4 you saw that an alcohol and a carboxylic acid react to make an ester and water. If each molecule has two functional groups, one at each end, the reaction can carry on at both ends. Long chains form: this is condensation polymerisation.

The simplest polyesters use two different monomers: a diol (two –OH groups) and a dicarboxylic acid (two –COOH groups). They join alternately, diol–acid–diol–acid, with an ester link between each pair.

Key fact: Every time an ester link forms, a small molecule is released. For a polyester it is water, H₂O: the H comes from an –OH group and the OH from a –COOH group.

Drawing a polyester

Exam questions often use blocks to stand for the carbon chain in the middle of each monomer, so you only need to show the functional groups.

n HO–□–OH + n HOOC–■–COOH → –[O–□–O–OC–■–CO]n– + 2n H₂O

With real monomers, for example propanediol and butanedioic acid:

n HO–CH₂CH₂CH₂–OH + n HOOC–CH₂CH₂–COOH → –[O–CH₂CH₂CH₂–O–OC–CH₂CH₂–CO]n– + 2n H₂O

Exam tip: The repeating unit is the smallest piece of chain that, drawn over and over end to end, gives the whole chain. For a diol + diacid polyester it holds one diol part and one diacid part, with one bond sticking out through each bracket.

One repeating unit contains one diol and one diacid, joined by an ester link, with a second link to the next unit. So for every repeating unit, two water molecules are released.

Key fact: A real example: polyester (PET) used for drinks bottles and clothing fibres is made from ethanediol and a dicarboxylic acid.

Other small molecules: making nylon

The small molecule is not always water. Nylon is another condensation polymer. In a classic teacher demonstration, two monomer solutions that do not mix are layered in a small beaker. Nylon forms where the layers meet and can be pulled out as a continuous thread and wound onto a glass rod.

In this reaction the small molecule given off is hydrogen chloride, HCl, which forms acidic fumes. So the demonstration is done in a fume cupboard, with gloves and eye protection, and the nylon is not touched with bare hands.

Key fact: Condensation polymer + small molecule: the small molecule is usually water (H₂O) or hydrogen chloride (HCl).

Addition or condensation?

• Monomers; Addition polymerisation contain a C=C double bond; often one type of monomer; Condensation polymerisation have two functional groups each; often two different monomers
• How they join; Addition polymerisation the C=C double bond opens; Condensation polymerisation functional groups react to form links (e.g. ester links)
• Products; Addition polymerisation the polymer only; Condensation polymerisation the polymer and a small molecule such as H₂O or HCl
• Examples; Addition polymerisation poly(ethene), poly(propene), PVC; Condensation polymerisation polyesters, nylon; natural polymers such as proteins

Calculations

Worked example: Mr of a repeating unit

Propanediol (Mr 76) reacts with butanedioic acid (Mr 118) to form a polyester.
Each repeating unit contains one of each monomer, minus 2 H₂O.
Mr of repeating unit = 76 + 118 − (2 × 18) = 194 − 36 = 158

Answer: 158

Worked example: mass of water released

What mass of water is released when 0.25 mol of repeating units forms?
Each mole of repeating units releases 2 mol of H₂O, so 0.50 mol × 18 = 9.0 g.

Answer: 9.0 g of water

Common mistakes

• Forgetting the water. Condensation polymerisation always gives a second product.
• Drawing both monomers in the repeating unit with all their H and OH groups still attached. The H and OH at each link have gone to make water.
• Saying the monomers need a C=C double bond. Condensation monomers need two functional groups.
• Writing n H₂O instead of 2n H₂O for a polyester made from a diol and a diacid: there are two ester links per repeating unit.
• Thinking the small molecule is always water. For nylon made in the lab demonstration it is hydrogen chloride, HCl.

Revision questions

Try these, then check your answers below each one.

Question 1. Higher tier. Which pair of monomers can form a polyester? (Choose from: Ethene and propene, A diol and a dicarboxylic acid, Two alkanes, Ethanol and ethanoic acid.)

Answer: A diol and a dicarboxylic acid

Working: A diol (two –OH) and a dicarboxylic acid (two –COOH) can react at both ends to form long chains. Ethanol and ethanoic acid each have only one functional group, so they make a single ester molecule.

Question 2. Higher tier. Name the small molecule released when a polyester forms.

Answer: water

Working: Water, H₂O, is released at each ester link.

Question 3. Higher tier. True or false? Condensation polymerisation produces two products.

Answer: True

Working: True: the polymer and a small molecule such as water.

Related lessons

• Previous lesson: 11.1 Addition Polymerisation
• Next lesson: 11.3 Natural Polymers
• In this chapter: 11.4 DNA

Continue in the full course

This lesson is section 11.2 (AGCCH1102) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.