Monomers and polymers
Polymers have many applications and new uses are being developed. However, they can
be difficult to dispose of.
Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller
molecules join together, end-to-end. The smaller molecules are called monomers.
In general: lots of monomer molecules → a polymer molecule.
This animation shows how several chloroethene monomers can join end-to-end to make
poly(chloroethene), also called PVC.
Alkenes can act as monomers because they are unsaturated (they have a double bond):
ethene can polymerise to form poly(ethene), also called polythene
propene can polymerise to form poly(propene), also called polypropylene.
Displayed formulas of polymers
Polymer molecules are very large compared with most other molecules, so the idea of a repeating unit
is used when drawing a displayed formula. When drawing one, starting with the monomer:
change the double bond in the monomer to a single bond in the repeating unit
add a bond to each end of the repeating unit.
Addition polymerisation
Different polymers have different properties, so they have different uses. The table below gives some
examples.
Polymer
Typical use
polythene
plastic bags and bottles
polypropene
crates and ropes
polychloroethenewater pipes and insulation on electricity cables
Polymers have properties that depend on the chemicals they are made from, and the conditions in
which they are made. For example, there are two main types of poly(ethene): LDPE, low-density
poly(ethene), is weaker than HDPE, high-density poly(ethene), and becomes softer at lower
temperatures.
Modern polymers have many uses, including:
new packaging materials
waterproof coatings for fabrics (such as for outdoor clothing)
fillings for teeth
dressings for cuts
hydrogels (for example for soft contact lenses and disposable nappy liners)
smart materials (for example shape memory polymers for shrink-wrap packaging).
One of the useful properties of polymers is that they are unreactive, so they are suitable for storing food
and chemicals safely. Unfortunately, this property makes it difficult to dispose of polymers. They can
cause litter and are usually sent to landfill sites.
Biodegradable plastics
Watch
You may wish to view this BBC
News item (2002) about
degradable carrier bags.
Watch
Most polymers, including poly(ethene) and poly(propene) are not biodegradable, so they may last for
many years in rubbish dumps. However, it's possible to include substances such as cornstarch that
cause the polymer to break down more quickly. Carrier bags and refuse bags made from such
degradable polymers are available now.
Recycling
Polymers have recycling symbols
like this one for PVC to show
what they are
Many polymers can be recycled. This reduces the disposal problems and the amount of crude oil used.
But the different polymers must be separated from each other first, and this can be difficult and
expensive to do.
Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It's also useful as a fuel.
For use in cars and other vehicles, it is usually mixed with petrol.
Ethanol molecules contain carbon, hydrogen and oxygen atoms.
Formula Displayed formula
Molecular model
C2H5OH
Making ethanol from ethene and steam
Ethanol can be made by reacting ethene (from cracking crude oil fractions) with steam. A catalyst of
phosphoric acid is used to ensure a fast reaction.
ethene + steam → ethanol
C2H4 + H2O → C2H5OH
Notice that ethanol is the only product. The process is continuous – as long as ethene and steam are
fed into one end of the reaction vessel, ethanol will be produced. These features make it an efficient
process, but there is a problem. Ethene is made from crude oil, which is a non-renewable resource. It
cannot be replaced once it is used up and it will run out one day.
Fermentation
Sugar from plant material is converted into ethanol and carbon dioxide by fermentation. The enzymes
found in single-celled fungi (yeast) are the natural catalysts that can make this process happen.
C66H12O6 → 2C2H5OH + 2CO2
Unlike ethene, sugar from plant material is a renewable resource.
Now try a Test Bite.