png) A repeat unit is the section of a polymer chain that repeats many times to make the full polymer molecule. ## Polymers are not small simple molecules Simple molecular substances, such as oxygen, water, and carbon dioxide, have small molecules. They often have low melting and boiling points because the forces between their molecules are weak. Polymers are different because their molecules are very long. Even though each individual intermolecular force may not be as strong as a covalent bond, there are many contact points between long polymer chains. Imagine two short pieces of thread lying next to each other: they separate easily. Two long tangled pieces of thread are much harder to pull apart. Polymer molecules behave a bit like long chains with many points of attraction between them. ## Intermolecular forces between polymer molecules An **intermolecular force** is a force of attraction between separate molecules. An intermolecular force is an attraction between molecules. It is not a covalent bond and does not join atoms together inside a molecule. In polymers, the intermolecular forces are described as **relatively strong**. This means they are stronger than the intermolecular forces in many small simple molecular substances. They are not stronger than covalent bonds. Covalent bonds are still much stronger. Do not say polymers are solid at room temperature because “the covalent bonds between polymer molecules are strong”. The covalent bonds are within each polymer molecule. The forces between separate polymer molecules are intermolecular forces. ## Why polymers are solids at room temperature At room temperature, most polymers are solids because the relatively strong intermolecular forces between their large molecules need a lot of energy to overcome. The long polymer chains sit close to each other. Because they are so long, there are many attractions between neighbouring chains. Altogether, these attractions make it difficult for the molecules to move freely past each other. A student is asked: “Explain why poly(ethene) is a solid at room temperature.” 1. Identify the structure: poly(ethene) is a polymer, so it contains very large molecules made from many repeating units. 2. Link the structure to forces: the long polymer molecules have relatively strong intermolecular forces between them because there are many points of attraction between the chains. 3. Link the forces to the property: at room temperature, there is not enough energy to overcome these forces, so the polymer molecules cannot move freely past each other. Therefore, poly(ethene) is a solid. ## Recognising polymers from diagrams In the exam, you may be shown diagrams of different substances and asked which one is a polymer. Look for these features: - A **long chain** of atoms, often mostly carbon atoms. - A **repeating pattern** along the chain. - Atoms joined by **covalent bonds** within the chain. - Sometimes brackets around a repeat unit, with an $n$ outside the brackets. A polymer diagram may show only part of the chain. Dotted or broken ends can mean the chain continues further. If the diagram shows a covalent molecule that keeps repeating the same small section again and again, it is probably a polymer. A diagram shows a long chain of carbon atoms. Each carbon atom is covalently bonded to hydrogen atoms, and the pattern repeats many times. 1. Check whether the structure is made from molecules: the atoms are joined by covalent bonds, so it is a covalent molecular structure rather than an ionic lattice. 2. Check the size and pattern: the chain is very long and the same arrangement of atoms repeats along the