Polymers are high molecular mass compounds formed by polymerization of monomers. The simple reactive molecule from which the repeating structural units of a polymer are derived is called a monomer. A polymer can be described in many ways: its degree of polymerization, molar mass distribution, tacticity, copolymer distribution, the degree of branching, by its end- groups, crosslinks, crystallinity, and thermal properties such as its glass transition temperature and melting temperature. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Natural and synthetic polymers are created via the polymerization of many small molecules, known as monomers.