Unlike small molecules, polymers are typically a mixture of differently sized molecules. Only an average molecular weight can be defined. • Measuring molecular weight • Size exclusion chromatography • Viscosity • Measurements of average molecular weight (M.W.) • number-average molecular weight ( n) •Total weight of all chains divided by # of chains • weight-average molecular weight ( w) •Always larger than Mn • viscosity-average molecular weight ( u ) •Average determined by viscosity measurements. Closer to Mw than Mn • z-average molecular weight ( z)