A polymer is a substance of high average relative formula mass, made up of small repeating units called monomers.
Ethene molecules can combine together in a polymerisation reaction to make poly(ethene). This type of reaction is called addition polymerisation, as we are adding lots of the same monomer together to make one product. The C=C double bond in ethene breaks open to allow ethene molecules to join together.
We don't waste our time drawing out a whole polymer chain, as it contains far too many atoms. Instead, we show the structure of the repeating unit.
When we draw an equation to show an addition polymerisation, it is important that we write n next to both the monomer and the polymer to show large numbers of each are used and made. We must also draw brackets around the polymer structure, with bonds coming through both brackets, to show this is repeating.
Polymers and their Problems
There are many problems associated with polymers including the:
availability of starting materials
persistence in landfill sites, due to non-biodegradability (microorganisms are unable to break them down)
greenhouse gases and toxic gases produced during disposal by combustion (to power homes)
requirement to sort polymers so that they can be melted and reformed into a new product
Recycling polymers is great as it reduces the problems of disposal, and reduces our use of crude oil - but it is difficult and expensive to sort each of the polymers to be recycled.
Reactions of Alkenes
Biological polymers are made naturally by living organisms.
DNA (deoxyribonucleic acid) is a polymer made from four different monomers called nucleotides. Most DNA molecules are made from two polymer chains. These polymer chains join together in the shape of a double helix. Each nucleotide is made of a sugar molecule, a base (A, C, T or G), and a phosphate group.
Carbohydrates include substances such as sugar, starch, and cellulose. These molecules are made from carbon, hydrogen and oxygen. Simple sugars, like glucose, are called monosaccharides. Glucose monomers can join together in long chains to make a polysaccharide called starch.
Proteins are another example of polymers. There are 21 different amino acids that can be arranged to make proteins we need to survive. Many proteins are enzymes, and/or are essential to our biological functions.
Condensation Polymers (Higher Only)
An ester forms when an alcohol reacts with a carboxylic acid. For example:
methanol + ethanoic acid → methyl ethanoate + water
Methyl ethanoate is an ester. For every one ester molecule made, there is a water molecule formed.
Condensation polymerisation is another way of making polymers. Instead of forming just the polymer molecule as the only product, two products form:
a polymer molecule
a small molecule, normally water
A polyester requires two different monomers to form a polymer. There must be one of each of the following different monomers:
a 'dicarboxylic acid' - monomer contains two carboxylic acid groups, –COOH
a 'diol' - monomer contains two alcohol groups, –OH
both ends of each monomer molecule have a functional group, so can react with another monomer molecule
one molecule of water is formed every time an ester link is formed