Explaining unexpected data via competitive equilibria and processes in radical reactions with reversible deactivation.

Abstract

In the game rock-paper-scissors, each of the three options, or pathways, has an equal chance of dominating, that is, rock beats scissors, scissors beats paper, and paper beats rock. In the classical form of the game, there is no one dominant pathway to follow, but they are all balanced in likelihood. However, in chemical reactions with several competing reagents, the question must be asked, are all competing reactions and pathways accessible? A related question is, if there are two, or more reversible processes that compete for the same reagent, will both processes equilibrate simultaneously, or will one process dominate system? Can these competing processes shed light on otherwise puzzling data? Several unexpected and counterintuitive experiments have been reported in radical reactions with reversible deactivation. These unexpected observations can be illustrated by the near absence of the products of conventional bimolecular radical coupling in the radical transformation of methylcobaltamine to acetylcobaltamine. Another counterintuitive observation is a difference in the copolymer composition in some copolymerizations proceeding via reversible-deactivation radical polymerization (RDRP) vs conventional radical polymerization (RP), for example, nitroxide mediated polymerization of styrene and methyl methacrylate. A similarly puzzling phenomenon is the reduction in the branching fraction in poly(acrylates) polymerized by RDRP vs conventional RP. In the three previously mentioned cases, the radicals formed in reversible deactivation radical reactions are identical to those formed in the corresponding conventional radical process; therefore an explanation for the discrepancy in the outcomes of the reaction is needed. Other unexpected observations include the presence of initial periods of slower monomer consumption in RDRP reactions initiated by a conventional radical initiator with certain chain transfer agents, while changing the nature of chain transfer agent can lead to an acceleration of the reaction during the initial period. Similarly, in Cu mediated atom transfer radical polymerization (ATRP) with initiators for continuous radical regeneration (ICAR) initiated by a conventional radical initiator, the rate of polymerization should not depend on either the alkyl halide concentration or the Cu concentration. However, experiments show that the rate could be 10 times faster with alkyl halide than without alkyl halide. Finally, in aqueous media, the presence of active alkyl halides can appear to stop the disproportionation of Cu(I) complexes. These unusual data point to a more complex mechanism than originally envisioned, and in fact all these counterintuitive observations can be explained by the concept of competing equilibria and processes. In these cases, the presence of two or more reactions competing for the same reagent typically causes one pathway to dominate, while the rate of the other pathways are diminished. Alternatively, the competing pathways and processes can cause one or more reversible or pseudoreversible reactions to be imbalanced and lead to products distinct from the case where rates of forward and reverse reactions are balanced. In this Account, the concept of competitive processes and equilibria is developed and used to explain each of the unusual observations highlighted above.