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The Chemistry of the Polymerisation of Ethene.

The picture below shows the polymerisation of ethene.

Ethene put under pressure and heated with a catalyst
will polymerise (make long chains of atoms) to form poly(ethene).
Note that there are no double bonds in the polymer.
Poly(ethene) is an alkane. It is a saturated hydrocarbon.

Depending on the reaction conditions and the type of catalyst used,
ethene can make either HDPE or LDPE.
HDPE is High Density Poly(Ethene) and has a higher crystallinity
and a higher melting point than LDPE (Low Density Poly(Ethene).
HDPE is stronger and stiffer than LDPE.

What is an Addition Polymer?

A polymer that is formed from monomers added together
where no other substance is produced is called an addition polymer.

Examples of addition polymers are
poly(ethene),   poly(propene),  poly(tetrafluoroethene) - PTFE,
poly(chloroethene) - PVC  and poly(phenylethene) - polystyrene.

Chloroethene used to be called vinyl chloride.
The polymer is still called polyvinyl chloride, or PVC.

Phenylethene used to be called styrene.
The polymer is still called polystyrene.

What is the Repeat Unit?

A polymer is often written in the form shown below.

The brackets contain the repeat unit.
The small n means that there are many of them.
The repeat unit is repeated over and over again
many times to make up the long chain of the polymer.

Below are some examples of the repeat units of polymers.

See the properties and uses of polymers.

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