 Polymers almost always appear as mixtures of molecules of multiple molar masses, each of which could be considered a separate chemical substance.However, the polymer may be defined by a known precursor or reaction(s) and the molar mass distribution.For example, polyethylene is a mixture of very long chains of -CH2- repeating units, and is generally sold in several molar mass distributions, LDPE, MDPE, HDPE and UHMWPE.