A polymer is a molecule made of a very large number of small units joined in a chain by covalent bonds. The small molecules that join to make it are called monomers, and the reaction that joins them is called polymerization.
The unit that appears again and again along the chain is called the repeating unit. Most polymers have a relative molecular mass of about ten thousand or more, compared with 18 for water and 180 for glucose, so they are also called macromolecules.
Some polymers are made by living things, such as starch and cellulose in plants, proteins, nucleic acids and natural rubber.
Others, such as polyethylene, nylon and the polyesters, are synthetic polymers made in factories, mostly from monomers obtained from petroleum.
In addition polymerization, monomers that have a double bond between two carbon atoms open the double bond and join one after another. Nothing else is formed, so the repeating unit contains the same atoms as the monomer.
In condensation polymerization, each monomer has two groups that can react, one at each end. Each time two monomers join, a small molecule such as water is given off, so the polymer contains fewer atoms than the monomers that made it.
Polyethylene is made by addition polymerization, while nylon 6,6 and the polyesters are made by condensation polymerization.
During polymerization, chains start and stop growing at random, so the molecules in a sample of a polymer do not all have the same number of repeating units. This number is called the degree of polymerization and is written n.
The repeating unit \mathrm{{-}CH_2{-}CH_2{-}} of polyethylene has a relative formula mass of 28, so a chain with n repeating units has a relative molecular mass of about 28n.
Because n differs from chain to chain, a polymer has no single relative molecular mass. Instead, it is described by its average relative molecular mass, the average taken over all the chains in the sample.
In a solid polymer, the long chains are not all arranged in the same way. In some regions, parts of the chains lie side by side in an orderly way, and in others the chains are tangled at random.
These are called crystalline regions and amorphous regions, and one chain can pass through both. The higher the proportion of crystalline regions, the harder, stronger and less transparent the polymer is.
Because a polymer contains both kinds of region and chains of many different lengths, it has no sharp melting point.
When it is heated, the chains move more and more, and the polymer softens gradually over a range of temperatures. The temperature at which it starts to soften is called the softening point.
Polymer chains can have different shapes. Some are linear chains with no branches, some have side branches, and in some the chains are joined to each other by cross-links to make a three-dimensional network.
Linear and branched chains can slide past each other when they are heated, so these polymers soften and can be moulded again. In a network, the cross-links hold the chains in place, so the polymer does not soften on heating.
Branches keep the chains from packing closely together, so a polymer with many branches is usually softer and less dense than one with linear chains.