Visualization of the crucial step in SET-LRP

Abstract

The crucial step in SET-LRP is the disproportionation of the CuX generated by activation with Cu(0) wire or powder, into nascent, extremely reactive Cu(0) nanoparticles, and CuX₂. Nascent Cu(0) activates the initiator and dormant chains via a heterogeneous single-electron transfer (SET) mechanism. Here we report model reactions visualizing the disproportionation of CuBr and activation by nascent Cu(0) in protic, dipolar aprotic, and nonpolar solvents, and in protic, polar and nonpolar monomers. The nascent Cu(0) nanoparticles and the green/blue color of the solution of CuBr₂/N-ligand were visible, demonstrating that disproportionation occurs under all SET-LRP and many ATRP conditions. Unexpectedly, nascent Cu(0) nanoparticles and insoluble CuBr₂ were also formed via a surface disproportionation of CuBr/Me₆-TREN in a range of nonpolar solvents and monomers. The effect of solvent polarity on the rate of SET activation was visualized by adding methyl 2-bromopropionate (MBP) initiator to the disproportionation mixture and monitoring the disappearance of the nascent Cu(0) nanoparticles. The consumption of nascent Cu(0) was extremely rapid in DMSO, fast in MeCN, and slower in toluene. This trend confirms the expected dependence of SET activation on solvent polarity. The enhanced stabilization of Cu(0) nanoparticles in DMSO compared to MeOH was visualized, and used to explain the synergistic solvent effect in DMSO–MeOH and other solvent–monomer mixtures. Visualization of the disproportionation and activation also explains the rate acceleration in CuX-catalyzed ATRP in polar media in which the active catalyst is most likely the extremely reactive nascent Cu(0) generated by disproportionation, and therefore, rapidly discriminates between SET-LRP and ATRP.