addition polymerisation

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Polymers or plastics

Polymers or plastics are materials that are found almost everywhere in modern life. Simply look around the room where you are sitting and there are probably many different polymers or plastic objects. However most people would struggle to name two or three different polymers or plastics despite the fact that there are around 65 000 different polymers or plastics in everyday use around the world today; they are used to make everything from toy ducks, mobile phone cases, paints, packaging and window frames and many many more items. Polymers are also found in the natural world e.g. cotton, silk, rubber, starch, DNA and many others could be added to this list.

Polymers are giant covalent molecules made when lots of small molecules called monomers join together. Poly is a word normally meaning many and mono normally refers to one or a single object. Polymers are made when lots, normally thousands or even hundreds of thousands of small molecules called monomers join together to give one large molecule - a polymer. There are two basic ways to make polymers, addition polymerisation and condensation polymerisation, we will look at addition polymerisation on this page.

Addition polymerisation

As a simple example of how an addition polymer is made consider the example below. A single jigsaw piece could represent the monomer; the small molecules which join together to form the giant molecule or polymer. While the linked chain of jigsaw pieces could represent the polymer. Addition polymerisation example.  The monomer ethene which is used to make poly(ethene).

Addition polymerisation- Making Poly(ethene)

For addition polymerisation to work the molecules which are used as monomers must have one noticeable feature; they must be unsaturated; that is they must contain a carbon carbon double bond (C=C). They are alkenes, The smallest and simplest unsaturated molecule is called ethene (shown opposite), its molecular formula is C2H4. It contains 2 carbon atoms and 4 hydrogen atoms. It is an unsaturated molecule since it contains a carbon carbon double bond (C=C).

To turn the small unsaturated ethene molecule into the polymer poly(ethene) the C=C double bond needs to be split and the reactive intermediate molecules then link or join together; this is shown in the images below: Model equation showing the polymerisation of ethene to form poly(ethene).

Many thousands or even millions of ethene molecules would normally join up to make a long polymer chain, however only three ethene molecules are shown above. The carbon atoms in different ethene molecules must link together to form the polymer chain. However each carbon atom can only make 4 covalent bonds and cannot make a fifth covalent bond and link to another carbon atom in a different monomer molecule. So a compound called an initiator is added; this starts the reaction by breaking the double bond between the carbon atom in an ethene molecule and this reactive molecule then adds to another ethene molecule which then has its C=C bond split. This process repeats very rapidly and in this way the ethene monomers are able to add together as shown in the image below.

Model equation showing the polymerisation of ethene to form poly(ethene).

With the carbon carbon double bond opened up each carbon now only makes 3 bonds; this leaves the carbon atoms very reactive and they join together to form the polymer. In the image above the carbon atoms at the end of the chain are able to continue to react since they are only making three covalent bonds, in this way the polymer chain rapidly increases in size.

The ends of the polymer chain extend to join with other ethene monomers to complete the polymer chain. We can think of the polymer a simply a series of identical repeating units that link over and over again. Since all the monomers simply add together to form the polymer; this reaction is called addition polymerisation. This is shown below:

Part of the polythene polymer chain showing the repeat unit in the polymer chain.

we could simply shorten this to:

repeating unit of the polymer poly(ethene).

where n is the number of monomers in the polymer chain.

Naming polymers

Naming polymers is very simple. The name of the monomer is enclosed in brackets ( ) and the word poly is placed in front of the brackets. So in the example given the monomer is ethene so the polymer formed by the polymerisation of ethene would be called poly(ethene). This is often shortened to polythene.

An equation for the reaction would be: Equation to show the poilymerisation of ethene to form poly(ethene) or polythene

Here the letter "n" in the equation represents the number of monomer molecules that link together to form the polymer chain.

Key Points

Practice questions

Check your understanding - Questions on addition polymerisation

Check your understanding - Quick Quiz addition polymerisation

Check your understanding - Additional questions on addition polymerisation