# If $\bar {M}_w$ is the weight - average molecular weight and $M_n$ is the number -average molecular weight of a polymer, the poly dispersity index (PDI) of the polymer is given by

$\begin {array} {1 1} (1)\;\frac{\bar M_n}{M_w} & \quad (2)\;\frac{\bar M_w}{\bar M_n} \\ (3)\;\bar{M}_w \times \bar {M}_n & \quad (4)\;\frac{1}{M_w \times \bar M _n} \end {array}$

$(2)\;\frac{\bar M_w}{\bar M_n}$