The reactivity ratio of a growing copolymer chain terminating in a given monomer is the ratio of the reaction rate constant for addition of the same monomer and the rate constant for addition of the other monomer. That is,  and , where for example  is the rate constant for propagation of a polymer chain ending in monomer 1 (or A) by addition of monomer 2 (or B).
The composition and structural type of the copolymer depend on these reactivity ratios r and r according to the Mayo–Lewis equation, also called the copolymerization equation or copolymer equation, for the relative instantaneous rates of incorporation of the two monomers.