Velocity-of-copolymerization data available in the literature were reinterpreted on the basis of the current notions that the penultimate-unit effect with respect to absolute values of propagation rate constant (but not with respect to the monomer reactivity ratios) is a general rule, and that the termination step is diffusion-controlled, i.e., normal. By making a few simplifying approximations, a new velocity equation was derived, which was found to describe experimental data generally better than the classical equation based on the terminal propagation model with a single adjustable parameter φ, the cross-termination factor. The single adjustable parameter s included in the new equation, which measures the penultimate-unit effect, was found to have a strong correlation with the monomer reactivity ratios such that the smaller r1r2, the smaller is s, i.e., the more significant is the penulimate-unit effect. This result is in support of the relation r1r2=s1s2 suggested by the phenomenological theory [T. Fukuda et al., Makromol. Chem., Rapid Commun., 8, 495 (1987)].
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