where 𝑓 ! and 𝐹 ! are the mole fraction of monomer A in the feed and instantaneous incorporation fraction of monomer A into the polymer, respectively. We note a foundational assumption in the derivation of the closed-form copolymer equations considering reversible propagation (e.g., Izu, Wittmer, Krueger, and Lowry). In all cases, the concentrations of all terminal run lengths for reversible comonomer(s) of the same type are treated as geometrically proportional with the ratio 𝛼: