A polymer sample has a number average molecular weight (Mn) of 50,000 g/mol and a weight average molecular weight (Mw) of 150,000 g/mol. If the monomer unit has a molecular weight of 100 g/mol, what can be inferred about this polymer sample?
Correct: C
The polydispersity index (PDI) is defined as the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), i.e., PDI = Mw / Mn. In this case, PDI = 150,000 g/mol / 50,000 g/mol = 3.0. A PDI value greater than 1.0 indicates that the polymer sample consists of chains of varying lengths, meaning it has a broad molecular weight distribution. A PDI of 1.0 would indicate a perfectly monodisperse polymer (all chains having the same length), which is rarely achieved in synthetic polymerization. Therefore, a PDI of 3.0 indicates a broad molecular weight distribution. The degree of polymerization is also variable, so option D is incorrect.