Molar mass distribution
= Molar mass distribution
The distribution of <molar masses> among the chains in a <polymer> sample. Number-weighted and mass-weighted descriptions assign different relative weights to long chains: for populations $N_i$ of chains with <molar mass> $M_i$, the <number-average molar mass> is $M_n=\sum_iN_iM_i/\sum_iN_i$, while the <weight-average molar mass> is $M_w=\sum_iN_iM_i^2/\sum_iN_iM_i$. A single average does not determine the full distribution.