Addition polymers

A polymer is a substance of high relative formula mass, made up of small repeating units.

Poly(ethene)

Poly(ethene) is a polymer made from very many ethene molecules combined together.

Ethene molecules make long polyethene molecules.Forming poly(ethene) from ethene

The reaction is called a polymerisation reaction:

  • ethene is the monomer
  • poly(ethene) is the polymer

The C=C double bond in ethene is involved in the polymerisation reaction. It allows ethene molecules to join together to form a single product, so it is an example of an addition reaction. Poly(ethene) is an addition polymer.

Modelling addition polymers

It is too difficult to model a complete addition polymer molecule, as it contains many atoms. Instead, we show the structure of its repeating unit, the part that is repeated many times. To deduce the structure of a polymer from the monomer:

  1. draw the structure of the monomer but use C-C instead of C=C
  2. draw brackets around the structure with a long bond passing through each one

The table shows the structure of ethene and its polymer.

Structures of a monomer and polymer including covalent bonds.

Modelling addition polymerisation

Equations use repeating units to model addition polymerisation reactions. The letter n stands for a large number. In general:

n(monomer) → [repeat unit]n

For example:

Structure shift from ethene to poly(ethene) and choloroethene to poly(chloroethene).
Question

The diagram shows the structure of propene. Deduce the structure of poly(propene), and use this to show an equation for the polymerisation of propene.

Covalent structure of propene.
Structure of the polymerisation of propene.