Aqueous RDRP in the Presence of Cu⁰: The Exceptional Activity of Cu^I Confirms the SARA ATRP Mechanism

Polymerizations and mechanistic studies have been performed to understand the kinetic pathways for the polymerization of the monomer oligo­(ethylene oxide) monomethyl ether acrylate (OEOA) in aqueous media. Typically, the medium consisted of 18 wt % OEOA in water, in the presence of Cu catalysts coordinated by tris­[2-(dimethylamino)­ethyl]­amine (Me₆TREN). Well-controlled polymerization of OEOA can be achieved in the presence of halide anions and Cu wire with ≲600 ppm of soluble Cu^II species, rather than previously reported ca. 10 000 ppm of Cu^II and Cu⁰ particles formed by predisproportionation of Cu^I prior to monomer and initiator addition. The mechanistic studies conclude that even though disproportionation is thermodynamically favored in aqueous media, the SARA ATRP, not SET-LRP, mechanism holds in these reactions. This is because alkyl halides are much more rapidly activated by Cu^I than by Cu⁰ (contribution of Cu⁰ to activation is <1%). Because of the high activity of Cu^I species toward alkyl halide activation, [Cu^I/Me₆TREN] in solution is very low (<5 μM) and classical ATRP equilibrium between Cu^I and Cu^II species is maintained. Although in aqueous media disproportionation of Cu^I/Me₆TREN is thermodynamically favored over comproportionation, unexpectedly, in the presence of alkyl halides, i.e., during polymerization, disproportionation is kinetically minimized. Disproportionation is slow because its rate is proportional to [Cu^I/Me₆TREN]² and [Cu^I/Me₆TREN] is very small. Thus, during polymerization, comproportionation is 10⁴ times faster than disproportionation, and the final thermodynamic equilibrium between disproportionation and comproportionation could be reached only after polymerization is completed. Activation of alkyl halides by Cu^I/Me₆TREN in aqueous media occurs 8 orders of magnitude faster than disproportionation