Interrupted SET-LRP of methyl acrylate demonstrates Cu(0) colloidal particles as activating species

Abstract

To determine how “nascent” Cu(0) colloidal nanoparticles mediate the single electron transfer-living radical polymerization (SET-LRP), two different series of interruption experiments were performed on SET-LRP of methyl acrylate in DMSO with [MA]₀/[MBP]₀/[Me₆-TREN]₀ = 222/1/0.1 and hydrazine-activated Cu(0) wire. When the polymerization was interrupted by lifting the Cu(0) wire wrapped around the stirring bar out of the reaction medium with a magnet, leaving colloidal Cu(0) particles, soluble CuBr and CuBr₂ formed during the reaction in solution, the polymerization still proceeded, but at a much slower rate. However, by carefully decanting the reaction mixture from one Schlenk tube containing the Cu(0) wire catalyst to a second one without the Cu(0) catalyst, the polymerization reached a complete stop. This decantation experiment demonstrates that soluble CuBr/L cannot be a major contributor to the activation process in SET-LRP. Therefore colloidal Cu(0) particles generated from the disproportionation of CuBr/L are the catalytic species. The presence of the Cu(0) colloidal particles in the reaction medium in the direct lifting setup is most likely responsible for the continued polymerization in the absence of the Cu(0) wire catalyst. Furthermore, SET-LRP was demonstrated to be “immortal” towards catalyst removal. Finally, it was demonstrated that colloidal Cu(0) nanoparticles formed via disproportionation agglomerate into highly visible particles that were immediately consumed upon addition of the methyl 2-bromopropionate initiator, leaving no observable Cu(0) aggregates.