Polymers are chainlike molecules that are made of the same repetition unit. With a few exceptions such as proteins, a polymer consists of a mix of molecules with different chain lengths. Therefore, average values are given for the molecular weight like the number average, the weight average or the viscosity average molar mass. A measure for the width of the molecular weight distribution is the polydispersity index. The targeted manipulation of the molecular weight distribution of a polymer by removing short and/or long chain material is called polymer fractionation.