The key idea
Addition polymers form when monomers with a C=C double bond join together with no other product.Condensation polymers form when monomers with two functional groups react, releasing a small molecule (water or HCl) each time.
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Addition polymerisation
In addition polymerisation, alkene monomers each containing a C=C double bond link together.The double bond opens, allowing each monomer to bond to the next with no atoms lost, so the empirical formula of the repeat unit is the same as the monomer.Common examples include poly(ethene) from ethene, poly(propene) from propene and poly(chloroethene) (PVC) from chloroethene.
Condensation polymerisation
g. –OH and –COOH) react to form polymer chains and small molecules are lost in each step.Polyesters are formed from a diol and a dicarboxylic acid, releasing water.Polyamides (nylons) are formed from a diamine and a dicarboxylic acid, also releasing water.
Properties of polymers
Thermosoftening polymers (most addition polymers) have long chains held by weak intermolecular forces; they soften when heated and can be recycled.Thermosetting polymers have covalent cross-links between chains; they remain rigid and hard when heated and cannot be reshaped.The choice of polymer for a given application depends on these properties.
Naturally occurring polymers
Many naturally occurring substances are polymers. DNA is a condensation polymer of nucleotide monomers.Proteins are condensation polymers of amino acid monomers linked by peptide bonds.Starch and cellulose are condensation polymers of glucose monomers linked by glycosidic bonds.Understanding polymer structure helps explain the properties and functions of these biological molecules.
Definitions to learn
Monomer
A small molecule that joins with many others to form a polymer.
Polymer
A large molecule made of many repeating monomer units joined by covalent bonds.
Addition polymerisation
Polymerisation of alkene monomers in which the C=C double bond opens and no by-product is formed.
Condensation polymerisation
g. water) at each step.
Thermosoftening polymer
A polymer that softens when heated, with no cross-links between chains.
Thermosetting polymer
A polymer with covalent cross-links between chains that does not soften when heated.
Draw the repeat unit of poly(ethene) from the monomer ethene.
Ethene is CH₂=CH₂.
In addition polymerisation the double bond opens and the monomers join.
The repeat unit is −(CH₂–CH₂)ₙ−.
Draw brackets with bonds extending from each side and the subscript n.
Repeat unit: −(−CH₂−CH₂−)ₙ−
For polymer questions, always state the type (addition or condensation) and what is or is not released during the reaction.
Do not draw double bonds in the repeat unit of an addition polymer — the double bond opens during polymerisation.
How to score full marks
- 1No small molecule is released in addition polymerisation — always state this explicitly.
- 2For condensation polymers, name the two types of functional group on the monomers and the small molecule released.
- 3Thermosetting vs thermosoftening: the difference is cross-links — thermosetting has them (rigid when hot), thermosoftening does not (can be remoulded).
Try these yourself
Open each answer only after you have explained the full chemical process.
1State what is meant by addition polymerisation.[3 marks]
- 1.Describe the process in terms of monomers and bonds.
2Name the monomer that forms poly(propene).[1 mark]
- 1.Remove the 'poly' prefix and change to monomer form.
3State one difference between addition and condensation polymerisation.[2 marks]
- 1.Compare the reactants and products.
4Name two types of condensation polymer and one example of each.[2 marks]
- 1.Recall polyester and nylon.
5Describe the bonding between monomers in a polyester.[3 marks]
- 1.State the functional groups and the bond formed.
6Explain why most addition polymers are not biodegradable and why this is an environmental problem.[3 marks]
- 1.Link polymer structure to decomposition.
7Compare the properties of thermosoftening and thermosetting polymers.[4 marks]
- 1.Link structure to behaviour when heated.
8Describe the structure of DNA as a naturally occurring condensation polymer.[3 marks]
- 1.Identify the monomers and the bonds formed.