The monomers can be identical, or they can have one or more substituted chemical groups. These differences between monomers can have an effect on properties such as solubility, flexibility, or strength. In proteins, these differences can give the polymer the ability to preferentially adopt one conformation over another, as opposed to adopting a random coil (see self-assembly). Although most polymers are organic (based on carbon chains), there are also inorganic polymers, mainly based on a silicon backbone.
The term polymer covers a large, diverse group of molecules, including substances from proteins to high-strength kevlar fibres. A key feature that distinguishes polymers from other large molecules is the repetition of units of atoms (monomers) in their chains. This occurs during polymerization, in which many monomer molecules link to each other.