RDRP in the presence of Cu0: The fate of Cu(I) proves the inconsistency of SET-LRP mechanism

Abstract

Metallic Cu in the presence of an amine ligand has become very popular as a catalyst in reversible-deactivation radical polymerization (RDRP). Two contrasting mechanisms were proposed for this process. In SET-LRP, Cu0 is the exclusive activator, while Cu(I) instantaneously undergoes disproportionation to give Cu(II) and Cu0. Conversely, in SARA ATRP, Cu0 plays the role of a supplemental activator as well as a reducing agent for the conversion of Cu(II) to Cu(I), which is the principal activator. One of the cardinal differences between the two mechanisms is whether Cu(I) primarily undergoes disproportionation or reacts with the initiator RX. To provide a clear answer to this question, the kinetics of Cu(I) disproportionation and RX activation were investigated in various experimental conditions that match the polymerization environment (different amine ligands and initiators, effect of solvent, halide ions and Cu0). In all investigated systems, reaction of Cu(I) with alkyl halides is much faster than disproportionation (vact > 102vdisp). This result is in line with SARA ATRP and in clear disagreement with SET-LRP.