trometer is operated by the Brisbane NMR Centre.

The penultimate unit effects (PUEs) on the propagation, termination, and reversible addition‐fragmentation chain transfer (RAFT) processes in free radical copolymerization are discussed on the basis of recent publications. The propriety of the implicit and explicit PUE models in propagation and chain transfer processes is commented. The penultimate termination model with the geometric‐mean approximation and the related rate equation are highlighted.

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ABSTRACT: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 , 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) In former times, experimental data on copolymerization velocities were analyzed, in almost all cases, by assuming that the terminalmodel propagation scheme 1 was correct.Among the classical velocity equations along this line, the following one commonly termed Walling's equation 2 has been most widely used:where kP and k1 are the rate constants of propagation and termination, respectively, with the subscripts 1 and 2 referring to thehomopolymerizations, r/s are the monomer reactivity ratios, andf/s are the feed monomer compositions. This equation allows the determination of the cross-termination factor