Chemistry only
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.
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.
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.
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:
we could simply shorten this to:
where n is the number of monomers in the
polymer chain.
Here the letter "n" in the equation represents the number of monomer molecules that link together to form the polymer chain.