Molar mass distribution
（重定向自Number average molecular weight）
In linear polymers the individual polymer chains rarely have exactly the same degree of polymerization and molar mass, and there is always a distribution around an average value. The molar mass distribution (or molecular weight distribution) in a polymer describes the relationship between the number of moles of each polymer species (Ni) and the molar mass (Mi) of that species. The molar mass distribution of a polymer may be modified by polymer fractionation.