Addition Polymerisation

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe addition polymerisation as exemplified by poly(ethene) and poly(chloroethene), PVC
  2. Deduce the repeat unit of an addition polymer obtained from a given monomer
  3. Identify the monomer(s) present in a given section of an addition polymer molecule
  4. Recognise the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products

1. What is Addition Polymerisation?

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Addition polymerisation is a chemical reaction where many small molecules join together to make one very long molecule. This process is extremely important because it forms the basis of the plastic industry.

Key Definition

Addition polymerisation is a reaction in which many monomers (small molecules) containing at least one carbon-carbon double bond (C=C) join together to form long chains called polymers. The polymer is the only product formed.

Understanding the Key Terms

  • Monomer = a small, reactive molecule that can join with other monomers (think of it like a single bead)
  • Polymer = a very long chain molecule made up of many repeating units (think of it like a necklace made of many beads strung together)
  • The name "addition" comes from the fact that monomers simply add to each other without losing any atoms

How Addition Polymerisation Works

The process follows these steps:

  1. Each monomer contains a C=C double bond (carbon-carbon double bond)
  2. The π-bond (pi-bond) in the C=C double bond breaks open
  3. The monomers link together by forming new C-C single bonds
  4. This process repeats thousands of times, creating a very long chain

Important: Just like other addition reactions of alkenes, addition polymerisation produces only one product – the polymer itself.

2. Key Examples: Poly(ethene) and Poly(chloroethene)

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Example 1: Poly(ethene)

Monomer: Ethene (C₂H₄)

The general formula for this polymerisation is:

n C₂H₄ → [C₂H₄]ₙ

Where:

  • n = a very large number (can be up to 10,000 or more)
  • The square brackets [ ] show the repeat unit of the polymer
  • The subscript n outside the brackets means this unit repeats n times

Displayed formula (structural formula):

When we draw out the structures:

  • Monomer (ethene): H₂C=CH₂ (shows the C=C double bond)
  • Polymer (poly(ethene)): A long chain where the double bond has opened up to form single C-C bonds

The polymer chain looks like: —CH₂—CH₂—CH₂—CH₂—CH₂—CH₂— and continues for thousands of units.

Common name: Poly(ethene) is also known as polythene or polyethylene – this is what plastic bags and bottles are often made from.

Example 2: Poly(chloroethene) or PVC

Monomer: Chloroethene (H₂C=CHCl)

The general formula for this polymerisation is:

n H₂C=CHCl → [H₂C—CHCl]ₙ

Displayed formula:

  • Monomer (chloroethene): H₂C=CHCl (C=C double bond with one chlorine atom attached)
  • Polymer (poly(chloroethene)): A long chain where each repeating unit has one chlorine atom attached

Common name: Poly(chloroethene) is better known as PVC (which stands for polyvinyl chloride). PVC is used to make water pipes, window frames, and flooring.

Key Feature Present in Both Ethene and Poly(ethene)

Both the monomer and the polymer contain σ (sigma) covalent bonds. These are strong chemical bonds formed by the overlap of atomic orbitals. The difference is:

  • Ethene has C=C (one σ bond + one π bond)
  • Poly(ethene) has only C-C (single σ bonds)

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