Kinetic analysis of nitroxide radical coupling reactions mediated by CuBr

Abstract

In a previous paper, we described the room temperature rapid, selective, reversible, and near quantitative Cu‐activated nitroxide radical coupling (NRC) technique to prepare 3‐arm polystyrene stars. In this work, we evaluated the Cu‐activation mechanism, either conventional atom transfer or single electron transfer (SET), through kinetic simulations. Simulation data showed that one can describe the system by either activation mechanism. We also found through simulations that bimolecular radical termination, regardless of activation mechanism, was extremely low and could be considered negligible in an NRC reaction. Experiments were carried out to form 2‐ and 3‐arm PSTY stars using two ligands, PMDETA and Me6TREN, in a range of solvent conditions by varying the ratio of DMSO to toluene, and over a wide temperature range. The rate of 2‐ or 3‐arm star formation was governed by the choice of solvent and ligand. The combination of Me6TREN and toluene/DMSO showed a relatively temperature independent rate, and remarkably reached near quantitative yields for 2‐arm star formation after only 1 min at 25 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2214–2223, 2010

Ultrafast and Reversible Multiblock Formation by the SET-Nitroxide Radical Coupling Reaction

“…The peak at M p3 (9490) did not change with time and was most likely attributable to a cyclic structure consisting of two PSTY chains coupled together by two dinitroxide linkers. It was very unlikely that the cyclic structures formed by the bimolecular termination of two chain end radicals [5] because of the very low probability of the simultaneous activation of both ends of a PSTY chain to radicals. The decoupling experiments described below support this assertion.…”

“…Kinetic simulations showed that the concentration of Cu II Br 2 from this type of reaction increased linearly with conversion. [5] In a feed process, Cu I Br has time to disproportionate, allowing the copper species to reach their equilibrium concentrations. This resulted in a high Cu II Br 2 concentration at the initial stage of the reaction, in which deactivation of the chain-end radicals by Cu II Br 2 now successfully competed with nitroxide trapping.…”

“…[5] In fact, this reaction appeared essentially complete upon the addition of Cu I Br. Kinetic simulations also showed: negligible radical-radical bimolecular termination (<10 −4 %), Cu 0 dominated activation, and confirmed that the persistent radical effect (through deactivation by Cu II Br 2 ) had no effect until conversions of greater than 99%.…”

“…Kinetic simulations showed that the overall rate of alkoxyamine formation in polymeric systems was only limited by the rate of carbon-centred radical formation. [5] Our group attained ultrafast radical formation through single electron transfer (SET) to achieve a reaction with 'click'-type attributes at room temperature, which included near quantitative yields, selectivity in the presence of functional groups, rapid rates of alkoxyamine formation, and non-toxic byproducts. [6] Moreover, this reaction was reversible at high temperatures, and we demonstrated the further functionalization of end-groups through competitive exchange with other functional nitroxides.…”

Post‐polymerization modification by nitroxide radical coupling

“…Therefore, milder conditions were investigated and good results were achieved after the discovery of single electron transfer living radical polymerization . Accordingly, SETNRC, which relies on the formation of carbon‐centered radicals by single electron transfer reaction and successive trapping by nitroxide radicals, emerged as a new and efficient strategy for polymer coupling . In SETNRC, Cu(0)/PMDETA is generally used as a catalyst system to generate macroradicals at ambient temperature.…”