You should know what addition polymerisation is and how addition polymers are formed before you read this page.

Polymerisation reactions can be carried out at different temperatures and pressures and also with different catalysts. This results in polymers with different properties e.g. Poly(propene) is a very useful polymer and has many uses including ropes, car bumpers and plastic crates. However when chemists first produced poly(propene) there was a problem! The -CH₃ groups as shown in the image below were arranged randomly around the backbone of the molecule. Some of the -CH₃ were above the carbon backbone and some below, unfortunately this resulted in a soft, flexible polymer which melted at a fairly low temperature and it was of little use.

However by using a specially developed catalyst the Italian chemist Giulio Natta was able to produce a polymer where all the -CH₃ groups were on the one side of the carbon backbone. This produce a much stronger, stiffer polymer with a melting point high enough (160⁰C) to make plastic plates and containers which won't melt in the dishwasher. This new stronger and stiffer polymer was also suitable for use in making plastic crates and ropes.

Example 2- Polythene (poly(ethene)

There are two types of polythene in common use, high density polythene, HDPE and low density polythene, LDPE. HDPE is used to make many plastic containers and bottles e.g. the shampoo and soap bottles opposite are made from HDPE. Low density polythene is used to make mainly plastic carrier bags for shopping but some bottles and washing up bowls are also made from LDPE.
The difference between the two types of polymers is down to the way they are made. The table below summaries the differences in the way these two variants of the same polymer are made.

HDPE manufacturers use medium temperature and low pressure and a catalyst whereas LDPE manufacturers use a much higher temperature and pressure but no catalyst. These different conditions produce polymers with different structures. HDPE has straight polymer chains which pack together tightly resulting in a polymer with lots of intermolecular bonding. This produces a polymer with a higher melting point, which is stiffer, stronger and more dense. LDPE on the other hand has chains with lots of branches on it, this is shown in the image below. The presence of these branches means that the polymer chains cannot pack together very well and so it has a low density. It also has a lower melting point, is softer, more transparent, more flexible and cheaper to produce than HDPE.

If you could see the polymer chains in a typical plastic what do you think they might look like? Well a good picture to have in your mind is a plate of noodles or spaghetti ! Here the long strands are all tangled and mixed up together. Well in a typical polymer this is probably how the long polymer molecules or chains would look. If you pull on any of the strands of spaghetti it is possible to tease them out and pull them off the plate. This is because each strand is separate and not actually joined to any other strands.

Most polymers or plastics when you heat them to fairly low temperatures will melt easily. This is easily demonstrated if you take say a plastic bowl or a plastic bottle and place it in an oven at around 150-200⁰C. The plastic will melt and form a sticky liquid. This liquid could be processed again in a factory to make a new plastic object- it could be recycled. Plastics which melt when they are heated are called thermosoftening polymers or plastics. Most plastic are thermosoftening.

However the image below shows some plastic objects which are used in high temperature environments yet they do not melt. Car engines contain many plastic components which are exposed to very high temperatures, even at home for example pots have plastic handles, plastic kitchen utensils, electrical plugs and even the plastic trim on many ovens seem to be able to resist high temperatures without melting. These polymers are called thermosetting polymers. They will not melt on heating, however if they are heated to very high temperatures they will start to char and eventually they will decompose and they may catch fire and burn.

The reason for these differences in the properties of these two types of polymers is in the way the polymers or plastics are made and their structure.

Thermosoftening polymers contains of long polymer chains which are all tangled and mixed together but there are only weak intermolecular forces between the polymer chains. So when they are heated the polymer chains begin to move and vibrate more, this breaks the weak intermolecular bonds and the chains become free to move- the polymer melts.
Thermosetting polymer- have strong covalent bonds or cross-links holding the polymer chains in place. These covalent bonds are much stronger than the weak intermolecular bonds which hold the chains in place in thermosoftening polymers. When a thermosetting polymer is heated the chains are prevented from moving by these cross-links, so the polymer cannot melt.
The image below shows how the polymer chains are arranged in thermosetting and thermosoftening polymer.

Key Points

• The properties of polymers can be changed by processing or manufacturing them using different catalysts, temperatures or pressures.
• Thermosoftening polymers consist of long polymer chains which are attracted to each other by weak intermolecular bonds. These weak bonds are easily broken and this means that when heated these polymers will melt.
• Thermosetting polymers have chains which are cross-linked by strong covalent bonds. This type of polymer will not melt when heated.

Check your understanding - Questions on addition polymers and their properties.

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