Copolymers are materials derived from macromolecules containing more than one type of monomer. Such materials have found widespread use in applications from injection molding manufacturing 1 to drug delivery, 2 due in part to the fine control over structure-property relationships that is enabled by simply adjusting the monomer composition at the beginning of a copolymerization reaction. While this approach can provide a convenient handle to tune copolymer composition, the sequence of monomers along a copolymer backbone also strongly influences copolymer properties. For example, chemically dissimilar monomers in blocky sequences can self-assemble, whereas monomers in random or alternating sequences create polymers with properties averaged between the two monomer compositions. In most cases, copolymer sequence cannot be strictly controlled but is instead determined stochastically on a macromolecule-by-macromolecule level based on monomer feed composition and the reactivity of each comonomer's active chain end structure with the other comonomers in solution, which is known as a terminal model of copolymerization. For systems containing two comonomers (monomer A and B) that undergo irreversible propagation reactions, this reactivity is fully described by a pair of reactivity ratios, rA and rB, which are defined as the ratio of homopropagation to cross-propagation rate coefficients: