It is often useful to alter the copolymer equation by expressing concentrations in terms of mole fractions. Mole fractions of monomers  and  in the feed are defined as  and  where
Similarly,  represents the mole fraction of each monomer in the copolymer:
These equations can be combined with the Mayo–Lewis equation to give
This equation gives the composition of copolymer formed at each instant. However the feed and copolymer compositions can change as polymerization proceeds.