Formation

Addition polymers are created from alkenes. With a high pressure and a suitable catalyst it is possible to join alkenes together by breaking the ?-bonds. As no other product is formed this is known as addition polymerisation. The resulting polymers are called polyalkenes.

For example, if ethene goes through this process the resulting polyalkene is polyethene, a very long hydrocarbon chain. Additional polymers can be made from any alkene for instance poly(propene) from propene or poly(but-2-ene) from but-2-ene.

Useful polymers include:

• polyethene for crates and plastic bags
• polypropene for plastic tubing
• polychloroethene (also known as polyinylchloride) for records and waterproof clothing
• polyphenylethene (also known as polystyrene) for packaging

Using Le Chatelier’s principle it is possible to calculate the most favourable conditions for alkene polymerisation. Two factors need to be taken into consideration:

• the reaction only involves breaking ?-bonds and making ?-bonds thus making it an exothermic reaction
• the reaction is a reduction in mole number

As the reaction is exothermic and involves a decrease in moles the best yield will be produced at a:

• low temperature
• high pressure

Therefore, addition polymerisation takes place with a suitable catalyst and at a high pressure.

Properties of addition polymers

Due to the fact polyalkenes are composed of long hydrocarbon chains which are non-polar and saturated, they possess a number of properties.

Their long length means that the Van der Waal’s forces are usually very strong between the chains. This means that the polymers have high melting and boiling points.

However, the majority of polymers contain chains of different lengths so the Van der Waal’s forces also vary which means that these polymers usually melt over a range of temperatures at a gradual rate as opposed to completely at a fixed temperature. In addition, the chains are not rigid in one position meaning that polymers are usual quite soft.

As the chains are non-polar, polyalkenes are insoluble in water. They tend to be insoluble in solvents too because there are strong intermolecular forces between molecules and the chains are generally tangled up together.

In fact, polyalkenes are generally very unreactive. Because of their saturated hydrocarbon chains they also cannot react with electrophiles or nucleophiles and are not able to undergo addition reactions. This inert quality makes them ideal as insulators in packing and container materials. On the negative side, however, it also means that they do not naturally decompose easily and so are said to be non-biodegradable. This means that they are an environmental hazard.

Polyalkenes differ widely in terms of strength and density. These properties depend on hydrocarbon chain length and, more importantly, on the branching of the chains.

• A polymer with only a few branches will be very compact. Therefore, they usually possess a very high density because the closely packed chains are held together strongly by the Van der Waal’s forces. These polymers are generally also stronger and harder.
• A polymer which is highly branched cannot pack together so efficiently. Therefore, they usually possess a lower density because the chains cannot sit closely together meaning that the Van der Waal’s forces are weaker. There polymers are generally softer and weaker.