Origin of penultimate unit effects

Some theoretical justifications for the prevalence of systems which show an implicit penultimate effect have appeared. These are summarized in the recent reviews by Cootc and Davis.2,3 

If the Evans-Polyani rule (Section 2.4.1)63 applies, the activation energy 6ijk will be proportional to the reaction enthalpy (-AT/yO and eq. 17 will hold. 

If it is assumed that penultimate unit effects on the reaction entropy are insignificant, the terms in eqs. 18 and 19 corresponding to the stabilization energy of the reactant propagating radical will cancel and rVli=ryly There should be no explicit penultimate unit effect on copolymer composition. On the other hand, the radical reactivity ratio j (eq. 20) compares two different propagating radicals so 

II also follows from this treatment that 

However, plots which would demonstrate this relationship show considerable 

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