1. Addition Polymerisation occurs when the double bonds of hydrocarbons called alkenes are broken, and then joined into a long-chain molecule (e.g. Many ethene molecules join to form polyethene in plastic bags.
  2. Condensation Polymerisation occurs when two types of monomers join to form a long-chain polymer and water is given off (e.g. Many molecules of urea and formaldehyde are joined to form glue.)

Types of Plastics

  1. Thermoplastics consist of long chains with no cross-links that allow the molecules to slide over each other when heated. This allows them to be moulded into new shapes. Common examples are polyethene in plastic bags, nylon, polyvinyl chloride (PVC), perspex and polystyrene.
  2. Thermosetting Plastics have cross-linking between many molecules and they cannot slide over each other. Once they are first set into a certain shape, they cannot be remoulded. If they are heated, they will not melt but will eventually burn. Common examples are bakelite in light switches and saucepan handles, polyurethanes and epoxy resins.

Types of Shaping Plastics

Plastics can be moulded into objects in 3 main ways:

  1. Injection moulding
  2. Extrusion
  3. Blow moulding

Injection Moulding


Blow Moulding