AQA GCSE Chemistry (8462) — Bonding, structure, and the properties of matter
Imagine a single paperclip. Now imagine linking thousands of them together to make a massive, continuous chain. This is exactly how polymers are built on a molecular level.
A polymer is a very large molecule (also known as a macromolecule) made up of many small, repeating molecules called monomers.
First, these monomers link together to form a long chain. The atoms within a single polymer chain are held together by strong covalent bonds.
Then, these long chains interact with one another. The forces of attraction between separate, individual polymer chains are called intermolecular forces.
Polymers are solid at room temperature (approximately 20°C). This physical state is a direct result of their molecular size and the forces between the chains.
Because polymer molecules are so incredibly large, the total, cumulative intermolecular forces between the chains are relatively strong. This is in direct contrast to simple, small molecules (like water or oxygen) which only have weak intermolecular forces.
Due to these relatively strong forces, more energy is needed to overcome them in order to melt the substance. Therefore, polymers have higher melting points than simple molecular substances.
However, they generally have lower melting points than giant covalent structures (like diamond). This is because melting a polymer does NOT involve breaking any of the actual strong covalent bonds inside the chains.
Because polymers are far too large to draw in full, chemists use a shorthand general formula represented as .
The repeating unit is the smallest section of a polymer chain that, when repeated over and over, forms the entire structure. To recognise a polymer in an exam diagram, look for three key features:
The name of a polymer is simply the name of its monomer placed in brackets, prefixed with 'poly'. For example, joining ethene monomers () creates poly(ethene).
The ethene monomer contains a double bond (). During the formation of addition polymers, this double bond "opens up" to become a single covalent bond () in the repeating unit.
The structural change results in the repeating unit , with the single bonds extending through the brackets and the placed at the bottom right.
Students often say that polymers have 'strong intermolecular bonds' — never use the word 'bonds' here; instead, always write 'relatively strong intermolecular forces'.
In explanation questions asking why polymers are solid, examiners expect a specific three-step logic: 1) They are large molecules, 2) The intermolecular forces are relatively strong, 3) Therefore, large amounts of energy are needed to overcome them.
Never state that covalent bonds are broken when a polymer melts; melting only overcomes the intermolecular forces between the chains, leaving the strong covalent bonds intact.
When checking if a diagram shows a polymer (Paper 1), ensure the extension bonds pass exactly through the brackets. If they stop inside, you are looking at a simple molecule.
Polymer
A large molecule made up of many repeating units (monomers) joined together by covalent bonds.
Monomer
A small molecule that combines with many others to form a polymer.
Intermolecular forces
The forces of attraction that exist between individual molecules (distinct from the covalent bonds inside the molecules).
Repeating unit
The smallest section of a polymer chain that, when repeated over and over, forms the entire polymer structure.
Extension bonds
The single covalent bonds in a polymer diagram that extend through the brackets to show the chain continues.
Put your knowledge into practice — try past paper questions for Chemistry
Polymer
A large molecule made up of many repeating units (monomers) joined together by covalent bonds.
Monomer
A small molecule that combines with many others to form a polymer.
Intermolecular forces
The forces of attraction that exist between individual molecules (distinct from the covalent bonds inside the molecules).
Repeating unit
The smallest section of a polymer chain that, when repeated over and over, forms the entire polymer structure.
Extension bonds
The single covalent bonds in a polymer diagram that extend through the brackets to show the chain continues.
