A detailed investigation of the kinetic irreversibility−reversibility concept is presented on the basis of the analysis of four side reactions occurring in the polymerization of poly(vinyl chloride), the intramolecular 1,5- and 1,6-backbiting and 1,2- and 2,3-Cl shift side reactions. Density functional theory-based reactivity indices combined with an analysis of the reaction force are invoked to probe this concept. The reaction force analysis is used to partition the activation and reaction energy and characterize the behavior of reactivity indices along the three reaction regions that are defined within this approach. It has been observed that in the reactant and product regions mainly geometric rearrangements take place, whereas in the transition state region changes in the electronic bonding pattern occur; here most changes of the electronic properties are observed. The kinetic irreversibility−reversibility of the reactions is confirmed and linked to the differences in the Fukui function and dual descriptor of the radical centers associated with the initial and final species.