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PolymerisationOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is addition polymerisation?

Addition polymerisation is a reaction in which many small alkene molecules, called monomers, join together to form one very large molecule called a polymer. No other product is formed - the polymer contains all the atoms of the monomers.

  • Alkenes can undergo addition polymerisation because they contain a reactive carbon-carbon double bond
  • The double bond opens up, allowing each monomer to bond to the next
  • Thousands of monomer units can join to form a single polymer chain
Key termsaddition polymerisationmonomerpolymer

Section 2

How is poly(ethene) formed from ethene?

The simplest example is the formation of poly(ethene) from ethene monomers:

Many C2H4 (ethene) molecules -> poly(ethene), written as (-CH2-CH2-)n

  • The C=C double bond in each ethene molecule opens up to form single bonds linking to neighbouring monomers
  • The name of the polymer is usually formed by putting 'poly' in front of the monomer's name, in brackets, e.g. ethene -> poly(ethene), propene -> poly(propene)
Key termspoly(ethene)
Exam tip

When drawing the repeat unit, put brackets around the structure and a subscript n outside to show it repeats many times, with the double bond replaced by a single bond.

Section 3

How do you represent polymer formation?

To represent addition polymerisation:

  1. Draw the monomer's displayed formula, showing the C=C double bond
  2. Draw the repeat unit in brackets with the double bond changed to a single bond, and bonds extending out of both sides of the brackets to show the chain continues
  3. Write a subscript n outside the bracket to represent 'many'

For example, propene (CH2=CHCH3) monomers join to form poly(propene), with the repeat unit (-CH2-CHCH3-)n.

Key termsrepeat unit
Common mistake

A common error is to leave the double bond inside the polymer's repeat unit - in the polymer, every carbon-carbon bond is a single bond; only the monomer has the double bond.

Must Know

  • Addition polymerisation joins many small alkene monomers into one very large polymer molecule
  • No other product is formed alongside the polymer
  • Ethene monomers form poly(ethene); the polymer name is 'poly' plus the monomer name in brackets
  • The monomer's C=C double bond becomes a single bond in the polymer chain
  • The repeat unit is shown in brackets with a subscript n to represent many repeating units

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