Aqueous RDRP in the Presence of Cu0: The Exceptional Activity of CuI Confirms the SARA ATRP Mechanism

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Abstract

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 CuII species, rather than previously reported ca. 10 000 ppm of CuII and Cu0 particles formed by predisproportionation of CuI 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 CuI than by Cu0 (contribution of Cu0 to activation is <1%). Because of the high activity of CuI species toward alkyl halide activation, [CuI/Me6TREN] in solution is very low (<5 μM) and classical ATRP equilibrium between CuI and CuII species is maintained. Although in aqueous media disproportionation of CuI/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 [CuI/Me6TREN]2 and [CuI/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 CuI/Me6TREN in aqueous media occurs 8 orders of magnitude faster than disproportionation.

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