Polymers are a class of high molecular weight materials, with a structure that is characterized by "building blocks" of repeat units or monomers. The monomers react together to form long chains of repeating chemical units. The polymer chains that result may be linear or form a branched or three-dimensional network. Polyurethane copolymers are an important subclass of the family of thermoplastic elastomers. The molecular structure of the polyurethanes must be taken into account when trying to understand and explain their physical behavior. Methods of polymer synthesis generally fall into two categories: addition polymerization and condensation polymerization. The most common method for synthesis, particularly of biomedical polyurethanes, consists of two steps, and is commonly referred to as the prepolymer method. In diisocyanates, the rate of reaction of the second isocyanate group is slower than that of the first group, although this difference is reduced if the isocyanate groups are separated by an aliphatic chain or aromatic rings.