Normally, a polymer contains chains of varying lengths and therefore, its molecular mass is always expressed as an average. In contrast, natural polymers such as protein contains chain of identical length and hence, have definite molecular mass.

The molecular mass of a polymer is expressed as

(a) Number average molecular mass `(barM_n)`

`barM_n =(sumN_iM_i)/(sum_i N_i)`

where `N_i` is the number of molecules of molecular mass `M_i`

(b) Weight average molecular mass `(barM_w)`

`barM_w =(sumN_iM_i^2)/(sum_i N_iM_i)`

where `N_i` is the number of molecules of molecular mass `M_i`. Methods such as light scattering and ultracentrifuge depend on the mass of the individual molecules and yield weight average molecular masses. `barM_n` is determined by employing methods which depend upon the number of molecules present in the polymer sample viz. colligative properties like osmotic pressure.

The ratio of the weight and number average molecular masses `(barM_w/barM_n)` is called Poly Dispersity Index (`PDI`). Some natural polymers, which are generally monodisperesed, the `PDI` is unity (i.e. `barM_w = barM_n)`.

In synthetic polymers, which are always polydispersed, `PDI > 1` because `barM_w` is always higher than `barM_n`