In an assembly of macromolecules, a continuous distribution with the differential mass-distribution function of the form: \[f_{\rm{w}}(x)\ \rm{d}x = \frac{a^{b\,+\,1}}{\varGamma\left(b\,+\,1\right)}\ x^{b}\ \rm{e}^{-a\ x}\ \rm{d}x\] where \(x\) is a parameter characterizing the chain length, such as relative molecular mass or degree of polymerization, \(a\) and \(b\) are positive adjustable parameters, and (\(\mathit{\Gamma}\left(b+1\right)\)) is the gamma function of (\(b+1\)).