Give reason: Why is the molecular mass of a polymer always expressed as an average?

(N/A) The properties of polymers are closely related to their molecular mass,size,and structure.
During the synthesis of polymers,the growth of the polymer chain depends on the availability of monomers in the reaction mixture.
As a result,a polymer sample contains chains of varying lengths,meaning the molecules are not uniform in size.
Therefore,the molecular mass of a polymer is always expressed as an average value,such as the number-average molecular mass $(M_n)$ or weight-average molecular mass $(M_w)$.