Fractions from crude oil can be broken down by thermal decomposition in a catalytic cracker. The fraction is vapourised and passed over a hot catalyst, which causes the molecules to split apart and form smaller molecules.
Some of the smaller molecules are alkanes, but some are alkenes that contain carbon carbon double bonds.
- Alkenes are unsaturated, because they contain fewer hydrogen atoms than alkanes with the same number of carbon atoms.
- Their general formula is CnH2n opposed to alkanes formula CnH2n+2.
- They are hydrocarbons, like alkanes, so they burn in air.
- However they are more reactive than alkanes. So alkenes will react with bromine water turning the orange solution colourless.
Making Polymers From Alkenes
Polymers are very large molecules made from many small molecules that have joined together. The small molecules used to make polymers are called monomers. Double bonds in the monomer become single bonds in the polymer when the molecules have bonded together.
Lots of ethene molecules can join together in long chains to from polyethene commonly called polythene. The reaction is called addition polymerisation.
We react other alkenes together in a simiar way to form polymers such as polypropene. Many of the plastics we use as plastic bags, bottles, containers and toys are made from alkenes.
In addition reactions there is only one product because the reactants add together.
Properties Of Plastics
Polymers have very long molecules that form a tangled mass, rather like spaghetti. This gives them strength and flexabilty. Using different monomers produces polymers with different properties.
- Thermosoftening polymers have weak intermolecular forces between their molecules, so when they are heated they become soft. Then they cool down, they harden again. This means that they can be heated to mould them into shape and they can also be remoulded by heating again.
- When thermosetting polymers are heated for the first time, chemical bonds form between the polymer molecules. They link together in a giant network. These strong bonds make the plastic set hard and it cannot be softened again be heating.
REMEMBER: chemical bonds are strong and join atoms together to make molecules, whereas interolecular forces act between molecules and are weaker than chemical bonds. Its best to use the word bond when talking about forces that hold atoms together within molecules.
New and Useful Polymers
We can design the properties of polymers by choosing different monomers and by changing the condition used to make them.
Polymers are widely used for food packaging to keep food in good condition. Some of these polymers are not biodergradable and cause problems with waste disposal.
The polymers used for drinks bottles are strong, flexible, lightweight, clear and non-porous.
Polymers have been developed to coat fabrics that make them waterproff but able to let gases though (breathable). New polmers have been developed for medical use, including hydrogels which are also used in agriculture and food. Smart polymers can be used to control the release of drugs and shape meomory polymers are used for stitching wounds.