What you'll learn
Polymers are very large molecules built from many small repeating units, and they make up plastics, fibres and many natural materials. In this guide you will learn what polymers are, how addition polymerisation joins alkene monomers, how to draw repeating units and identify monomers, the difference between addition and condensation polymerisation, and a little about natural polymers such as DNA, proteins and starch. These ideas build on covalent bonding and organic chemistry.
Key terms and definitions
Polymer — a very large molecule made of many small repeating units joined together.
Monomer — the small molecule that joins with others to form a polymer.
Addition polymerisation — joining unsaturated monomers (alkenes) with no other product formed.
Condensation polymerisation — joining monomers with the loss of a small molecule (usually water).
Repeating unit — the part of a polymer that repeats along the chain.
Unsaturated — containing a carbon–carbon double bond (C=C).
Core concepts
What polymers are
A polymer is a long-chain molecule formed when many small monomers join together by covalent bonds. Common synthetic polymers include poly(ethene), poly(propene) and poly(chloroethene) (PVC), used in bags, bottles and pipes. Their properties depend on the monomers used and the conditions of polymerisation.
Addition polymerisation
In addition polymerisation, the monomers are alkenes — they contain a carbon–carbon double bond (C=C). The double bond opens up and the monomers join together to form a single long chain, with no other product formed. For example, many ethene (C₂H₄) molecules join to form poly(ethene). The repeating unit has the same atoms as the monomer, but the double bond becomes a single bond and the unit has bonds extending out on each side.
Drawing repeating units
To draw the repeating unit, take the monomer, change the double bond to a single bond, and add a bond extending from each carbon out of the brackets, with "n" outside to show it repeats many times. Conversely, to find the monomer from a repeating unit, redraw it with the double bond restored and remove the extending bonds.
Condensation polymerisation
In condensation polymerisation, monomers with two functional groups join, and a small molecule (usually water) is lost each time a bond forms. This requires two different monomers (or one monomer with two different reactive ends). Examples include polyesters, formed from a diol and a dicarboxylic acid. The key difference from addition is that a small molecule is released.
Comparing addition and condensation
- Addition: one type of monomer (an alkene), C=C bond, no by-product, repeating unit has the same atoms as the monomer.
- Condensation: two monomers (or two functional groups), each with two reactive ends, and a small molecule (water) is lost.
Natural polymers
Many important molecules in living things are polymers. Proteins are polymers of amino acids; starch and cellulose are polymers of sugars; and DNA is a polymer made of two strands of nucleotides forming a double helix. These show that polymerisation is fundamental to life as well as industry.
Worked examples
Example 1: Identifying the monomer
A repeating unit is –CH₂–CHCl–. What is the monomer?
Restore the double bond and remove the extending bonds: the monomer is CH₂=CHCl (chloroethene), which polymerises to PVC.
Example 2: Why no by-product in addition
Why is no other product made in addition polymerisation?
The alkene C=C double bond opens up and the monomers simply join end to end; no atoms are lost, so there is no other product.
Example 3: Recognising condensation
A polymer is made from two monomers and water is given off. Which type of polymerisation is this?
The loss of water when two monomers join shows it is condensation polymerisation.
Common mistakes and how to avoid them
Forgetting to open the double bond. In addition polymers the C=C becomes a single bond in the repeating unit.
Leaving out the extending bonds and "n". The repeating unit must show bonds going out of the brackets and the subscript n.
Saying a by-product forms in addition polymerisation. No other product is made; only condensation loses a small molecule.
Confusing monomer and repeating unit. The monomer has a double bond; the repeating unit has a single bond and extending bonds.
Mixing up addition and condensation. Addition = one alkene monomer, no by-product; condensation = two functional groups, water lost.
Exam technique for Polymers
Draw repeating units carefully — single bond, extending bonds, brackets and n.
Deduce monomers by restoring the double bond.
State "no other product" for addition and "small molecule (water) lost" for condensation.
Use the words monomer, polymer, repeating unit and unsaturated accurately.
Recognise natural polymers (proteins, starch, DNA) as examples.
Quick revision summary
A polymer is a very large molecule made of many small monomers joined by covalent bonds. In addition polymerisation, the monomers are alkenes with a C=C double bond; the double bond opens and the monomers join into a long chain with no other product formed (e.g. ethene → poly(ethene)). The repeating unit is drawn by changing the double bond to a single bond, adding a bond extending from each carbon out of the brackets, and writing n outside; reverse this to find the monomer. In condensation polymerisation, monomers each have two reactive ends, and a small molecule such as water is lost each time a bond forms (e.g. polyesters from a diol and a dicarboxylic acid). The key contrast: addition uses one alkene monomer and makes no by-product, while condensation uses two functional groups and releases water. Natural polymers include proteins (from amino acids), starch and cellulose (from sugars), and DNA. Draw repeating units precisely, deduce monomers, and state clearly whether a small molecule is lost.