A polymer is a chemical substance consisting of macromolecules. The macromolecules are composed of one or more structural units (the so-called constitutional repeating units). The adjective polymer means “made from many (same) parts”. Since the number of repeating units and thus the molecular weight of the molecules varies, in many cases a polymer consists of non-identical macromolecules. Synthetic and semi-synthetic polymers are the most important component in the production of plastics. Polymers produced by living organisms (so-called biopolymers) are essential for life.