Addition polymers and their properties

Polymers are long chain molecules made via addition polymerisation or condensation polymerisation – the result are all the plastics, fibres and elastomers that modern life depends upon.

If you are interested in the chemistry of materials you should definitely watch this Royal Institution lecture by Professor Mark Miodownik on Strange Materials shaping civilisation, and I can highly recommend his popular science books on materials to expand your appreciation of the impact chemistry has on every day real life.

Addition polymers such as poly(ethene), poly(vinylchloride) and poly(propene) are made from alkene monomers. The great thing about polymers is that we can design a plastic or fibre to possess certain properties in terms of its strength, flexibility, melting point or ability to biodegrade.

The first step is to decide on the monomer to be used. The presence of electronegative atoms in side groups influences the types of intermolecular bonding between polymer chains. Side-groups and chain branches determine how closely chains can pack together which has consequences for the strength of the intermolecular bonds between chains.

You need to be able to draw a repeat unit for a polymer given the monomer, and vice versa. Drawing repeat units skeletally takes practice – students often lose marks by drawing too many carbon atoms / more than one repeat unit!

Copolymers are made up from two monomers that repeat periodically e.g. ABABAB or AABAAB or ABBABB etc.

The video runs through some more examples as well as some vital exam tips and tricks 😎.

Addition polymers are formed via a radical reaction mechanism.

Generally speaking, the longer the polymer chains, the stronger and less flexible the polymer material. Longer chains are more likely to become tangled (remember that there is free rotation about C-C single bonds) and with more points of contact between longer chains, there will be stronger intermolecular bonding.

The result is that the chains are held in a 3-D network – it takes a lot of energy to overcome these intermolecular bonds so that chains can slide freely over each other (which needs to happen for a polymer to be flexible and not rigid or brittle).

We can increase the flexibility of a polymer material in two ways:

Co-polymers – introduce monomers to the polymer chain which have bulky side groups as this will prevent chains from packing closely together, reducing the strength of the intermolecular bonds.

Plasticisers – these are small molecules that sit between polymer chains, preventing close packing, disrupting intermolecular bonding between chains.

And finally, we can increase the brittleness or rigidity of a polymer material by a process called cold-drawing. As the polymer is cooled down, the chains are pulled, stretching them so that they align more closely with a very regular structure. This increases the strength of the intermolecular bonding and results in stronger, tougher polymer fibres.

Practice questions

Terpenes are molecules that are partly responsible for the characteristic smell of some plants. An example is geraniol:

(a) Give the details of a chemical test you could carry out to confirm that geraniol is unsaturated.

(b) Terpenes are made by adding molecules of isoprene (shown below) together. Predict, with reason, the number of isoprene molecules that would be needed to form geraniol.

2. Chloroethene is the monomer used to make poly(chloroethene).

(a) Draw a displayed formula of 2 repeat units of the polymer.

(b) Explain why incineration is not a suitable method of disposal of this polymer, aside from economic reasons.

(c) Chloroethene is both carcinogenic and has a boiling point of 260K. Suggest one precaution that chemists should take when using this monomer.

(d) Chloroethene has a melting point of 119K and yet poly(chloroethene) melts at over 373K. Explain why.

(e) Use your understanding of the properties of poly(chloroethene) to explain whether you would expect a plasticiser to be added to the polymer when used to make the insulation for cables.

(f) With reference to the structure of a section of synthetic rubber shown below, draw the displayed formula for the repeat unit.

Name the monomer used to form a polymer with the following repeat unit:

4. 2-chloropropene can be used as a monomer.

(a) Write a balanced equation for the formation of this polymer, including a the structure of the repeat unit in your answer.

(b) Suggest one advantage and one disadvantage in disposing of this polymer by combustion.

5. Draw the structure of the two monomers that were used to form the copolymer shown below:

Answers

(a) shake a sample of geraniol with bromine water and the water will be decolorized from orange to colourless.

(b) each molecule of isoprene has 5 carbon atoms so 2 will be needed to form one molecule of geraniol which has 10 carbon atoms.

2. (a)

(b) Incineration of poly(chloroethene) produces chlorinated molecules which are toxic, corrosive and may contribute to acid rain.

(d) Poly(chloroethene) is a much bigger molecule so will have far more electrons than the monomer resulting in stronger instantaneous dipole – induced dipole intermolecular bonds / London forces between the molecules

(e) A plasticiser would be added to increase the flexibility of the polymer by preventing the close packing and ordered arrangement of polymer chains, reducing the strength of intermolecular bonds.

(f)