Contains the full lesson along with a supporting toolkit, including teachers’ notes.
The molecules of synthetic polymers are typically composed of hundreds or even thousands of relatively simple molecules (known as monomers) bonded together to form very large long-chain molecules. For example, polyethylene is made from repeating units of ethene (old name: ethylene) as shown below.
Hydrocarbons in which all the chemical bonds are single bonds are called alkanes: e.g. methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, etc.
Hydrocarbons that have a double carbon-carbon bond in each molecule are called alkenes: e.g. ethene, propene, butene etc.
At a suitable temperature and pressure and in the presence of a suitable catalyst, the double bond in ethene molecules can break.
• the plastics are insoluble in water and are often hydrophobic
• they are generally solid and so only the surface molecules are accessible
• often their molecules do not have enough variety to provide likely ‘attack sites’ for bacteria.
Which one of the following would be most likely to be attacked by bacteria: polythene, PVC, PVA and polystyrene, assuming that they were formed as thin sheets?
If you draw out the structure of each of these polymers you will see that PVA (poly-vinyl-acetate) has an ‘exposed’ oxygen-containing group on every second carbon atom. This could form hydrogen bonds with water molecules and so would be a likely site for bacteria to attack it. In fact a variety of algae, yeasts and bacteria can slowly break down polyvinyl acetate. (PVA is commonly used as glue for wood and paper but is not used to make bottles etc.)
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