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 . 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.