Reactivity ratio is a traditional parameter quantifying the reaction kinetics in copolymerization

which is important for potentially controlling microstructures of polymers and guiding the copolymerization process. Our recent experiments using tube-NMR technique enable us to

in situ

monitor the concentration profiles of the co-monomers during the anionic copolymerization process. This motivates us to revisit the Mayo-Lewis (ML) equation

which is the basis for derivation of reactivity ratio and has been extensively utilized in addition copolymerization. We found that although an explicit ML expression is desirable for ease of calculation and correlation with experimental data

it fails in our anionic copolymerization experiment as well as some data available in the literature. The origin is ascribed to the validity of the steady state assumption which is essential in the ML equation. This assumption can be released in anionic copolymerization and replaced by the fact that the overall concentration of the living chain ends keeps constant throughout the copolymerization. Alternative numerical method has been utilized to obtain the rate constants and consequently the reactivity ratios. Our work suggests that the ML equation should be applied with caution.