Formula to calculate weight average molecular mass of polymer is : 

Weight average molecular mass of polymer is:

  $\frac{{\sum }_{\mathrm{n}=1}^{\mathrm{i}}{\mathrm{M}}_{\mathrm{i}}^2{\mathrm{N}}_{\mathrm{i}}}{{\sum }_{\mathrm{n}=1}^{\mathrm{i}}{\mathrm{M}}_{\mathrm{i}}{\mathrm{N}}_{\mathrm{i}}}$

Here, N_i and M_i is the number of molecules and molar mass of the molecules present in the sample polymer.