A polymer is a very large molecule made of many small molecules joined together in a long chain. Poly(ethene) is the example to know. It is made from ethene, C2H4.
Each ethene molecule contains two carbon atoms and has a double bond between them. When ethene molecules join, the double bond opens up and each carbon atom bonds to the next molecule. This builds a long chain of carbon atoms joined by single covalent bonds, with hydrogen atoms attached to the carbon atoms. A chain made from 1000 ethene molecules therefore has 2000 carbon atoms in its backbone. The repeating unit is written (CH2CH2)n, where n is a very large number.
All the atoms in a chain are joined by strong covalent bonds, so poly(ethene) is made of simple molecular substances, not a giant lattice. The molecules are very large, so the intermolecular forces between the chains add up to be strong. This is why poly(ethene) is a solid at room temperature.
