Polymers are molecules which consist of a long, repeating chain of smaller units called monomers. Polymers have the highest molecular weight among any molecules, and may consist of billions of atoms. Human DNA is a polymer with over 20 billion constituent atoms. Proteins, or the polymers of amino acids, and many other molecules that make up life are polymers. Polymers are the largest and most diverse class of known molecules. They even include plastics.

Monomers are molecules typically about 4-10 atoms in size, reactive in that they bond readily to other monomers in a process called polymerization. Polymers and their polymerization processes are so diverse that a variety of different systems exist to classify them. One major type of polymerization is condensation polymerization, where reacting molecules release water as a byproduct. This is the means by which all proteins are formed.

Polymers are not always straight chains of regular repeating monomers; sometimes they consist of chains of varying length, or even chains that branch in multiple directions. Residual monomers are often found together with the polymers they create, giving the polymers additional properties. To coax monomers to link together in certain configurations requires a variety of catalysts--secondary molecules which speed up reaction times. Catalysts are the basis of most synthetic polymer production.

In copolymerization, polymers are formed that contain two or more different monomers. Larger, more complex polymers tend to have higher melting points and tensile strengths than others, due to the wealth of intermolecular forces acting between their constituents. Certain polymers are so complex that they cannot be readily identified, so techniques such as wide angle x-ray scattering, small angle x-ray scattering, and small angle neutron scattering are employed.

Most polymers are organic, employing carbon bonds as their backbone. Others use silicon. Because of the great diversity of polymers, there are many that have yet to be discovered, offering a fruitful field for further research and development.