11.1 Addition Polymerisation

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how alkenes join to form addition polymers, drawing repeating units and monomers, and common polymers and their uses. 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

• Describe addition polymerisation in terms of monomers and polymers
• Draw the repeating unit of an addition polymer from the monomer
• Work out the monomer from the repeating unit of an addition polymer
• Find the repeating unit and monomer from a section of polymer chain
• Name poly(ethene), poly(propene) and PVC and give uses
• Calculate relative formula masses and numbers of repeating units

Key words

• Monomer: A small molecule that joins with many others to make a polymer.
• Polymer: A very large molecule (macromolecule) made from many monomers joined in a chain.
• Addition polymerisation: Polymerisation in which unsaturated monomers (alkenes) join by opening their C=C bonds. The polymer is the only product.
• Repeating unit: The part of the polymer chain that repeats over and over, drawn in brackets with “n”.
• Plastic: A material made mainly from polymers that can be moulded into shape.

Monomers to polymers

Small alkene molecules such as ethene can join together to form very long chains. Each small molecule is a monomer; the long chain is a polymer. Thousands of monomers join in each chain.

In addition polymerisation, one bond of each C=C double bond breaks. Each carbon atom then uses that spare bond to join to a neighbouring monomer. The atoms are only rearranged, so the polymer is the only product and its mass equals the total mass of the monomers.

n CH₂=CH₂ → –(CH₂–CH₂)n– (n is a large number, often thousands)

In industry the reaction needs a catalyst and, for some processes, high pressure. The alkene monomers come from cracking crude oil fractions (see 9.4).

Key fact: The monomer for addition polymerisation must have a C=C double bond. Alkanes cannot form addition polymers. The C=C makes alkenes far more reactive than alkanes; once the double bonds have opened, the polymer chain is saturated, so poly(alkenes) are quite unreactive – useful for bottles, pipes and bags.

Drawing the repeating unit

1. Draw the two carbon atoms of the C=C in a line, joined by a single bond. 2. Keep every other atom or group (H, CH₃, Cl …) in the same place above and below. 3. Add a bond sticking out on each side, passing through square brackets. 4. Write n at the bottom right, outside the bracket.

Exam tip: To find the monomer, do it in reverse: take the two carbons inside the bracket, make the C–C a C=C, and remove the bonds through the brackets.

Spotting the repeating unit in a chain

Exam questions often show a section of a polymer chain and ask for the repeating unit or the monomer. In an addition polymer the repeating unit is always two carbon atoms of the chain (the two carbons that came from one C=C) with their side atoms.

1. Find the pattern of side groups along the chain and box two neighbouring carbons that contain one of each pattern. 2. Draw that pair as the repeating unit, with bonds through the brackets and n. 3. For the monomer, make the C–C a C=C and remove the bonds through the brackets.

Exam tip: Count check: a chain section with 8 carbon atoms in its backbone contains 4 repeating units, and was made from 4 monomers.

Common addition polymers

• ethene; Polymer poly(ethene), “polythene”; Typical uses plastic bags, bottles, cling film
• propene; Polymer poly(propene); Typical uses crates, ropes, carpets, food containers
• chloroethene (vinyl chloride); Polymer poly(chloroethene), PVC; Typical uses window frames, water pipes, insulation on electrical cables
• tetrafluoro- ethene; Polymer poly(tetrafluoroethene), PTFE; Typical uses non-stick coatings on pans

The name of an addition polymer is “poly” followed by the name of the monomer in brackets.

Calculations with polymers

Worked example: number of repeating units (C = 12, H = 1)

A poly(propene) molecule has a relative formula mass of 126 000. How many monomers joined to make it?
Repeating unit C₃H₆: Mr = 36 + 6 = 42.
n = 126 000 ÷ 42 = 3000

Answer: 3000 propene monomers

Key fact: Addition polymerisation has an atom economy of 100%. 1 tonne of ethene can make (up to) 1 tonne of poly(ethene).

Common mistakes

• Leaving the double bond in the repeating unit. In the polymer, the C=C becomes C–C.
• Putting the side group on the wrong carbon or drawing it in the main chain. Only the two C atoms from the C=C are in the chain.
• Forgetting the bonds through the brackets or the n.
• Saying water or another small molecule is made. In addition polymerisation the polymer is the only product.
• Thinking a polymer is a mixture of different monomers. In addition polymers made from one monomer, every repeating unit is the same.
• Boxing three or four chain carbons as the repeating unit. In an addition polymer it is always two chain carbons.

Revision questions

Try these, then check your answers below each one.

Question 1. Which molecule can be used as a monomer for addition polymerisation? (Choose from: Ethane, C₂H₆, Propene, C₃H₆, Ethanol, C₂H₅OH, Methane, CH₄.)

Answer: Propene, C₃H₆

Working: Propene is an alkene with a C=C double bond, so it can undergo addition polymerisation. Ethane, ethanol and methane have only single bonds.

Question 2. Name the polymer made from chloroethene.

Answer: poly(chloroethene)

Working: Poly(chloroethene), also called PVC.

Question 3. True or false? In addition polymerisation, water is released each time two monomers join.

Answer: False

Working: False. The double bond opens and the monomers join. The polymer is the only product; releasing water is condensation polymerisation.

Related lessons

• Previous lesson: 10.4 Carboxylic Acids and Esters
• Next lesson: 11.2 Condensation Polymerisation
• In this chapter: 11.3 Natural Polymers
• In this chapter: 11.4 DNA

Continue in the full course

This lesson is section 11.1 (AGCCH1101) 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.