Discussion
A polymer has a number average molecular weight of 50000 and a weight average of 125000. What is its polydispersity index?
Answer: C
Correct answer: 2.5
\[ \mathrm{PDI} = \frac{M_w}{M_n} = \frac{125{,}000}{50{,}000} = 2.5. \] The index is dimensionless, being a ratio of two molecular weights, so the options quoted in the tens of thousands cannot be right on units alone. It is always at least 1, which would mean every chain the same length, and a free radical polymerisation typically gives 2 to 5.
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