The Mayo–Lewis equation or copolymer equation in polymer chemistry describes the distribution of monomers in a copolymer. It was proposed by Frank R. Mayo and Frederick M. Lewis.
The equation considers a monomer mix of two components  and  and the four different reactions that can take place at the reactive chain end terminating in either monomer ( and ) with their reaction rate constants :
The reactivity ratio for each propagating chain end is defined as the ratio of the rate constant for addition of a monomer of the species already at the chain end to the rate constant for addition of the other monomer.
The copolymer equation is then:
with the concentrations of the components in square brackets. The equation gives the relative instantaneous rates of incorporation of the two monomers.