Role of Cu0 in Controlled/“Living” Radical Polymerization

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Several propositions have been made about the mechanism in which Cu0 mediates controlled radical polymerization that include (1) exclusive activation of an alkyl halide initiator by exceptionally active Cu0 to generate a propagating radical and a CuI species, (2) instantaneous disproportionation of CuI into Cu0 and CuII in “catalytic” solvents such as DMSO, and (3) deactivation of the radical by CuII to establish an equilibrium between active and dormant polymer chains. It was further postulated that the activation and deactivation processes in this technique, entitled single-electron-transfer living radical polymerization (SET-LRP), occur via outer-sphere electron transfer (OSET) to produce alkyl halide radical anion intermediates. We report herein on our own investigation of the aforementioned mechanism using Cu complexes of tris[2-(dimethylamino)ethyl]amine (Me6TREN). Model studies were employed to quantify disproportionation of CuI/Me6TREN in DMSO, DMF, and MeCN, where comproportionation of Cu0 with CuII to form CuI was slow but dominant in all three solvents. Relative activation rates of alkyl halides by Cu0 and CuI with Me6TREN were studied; reactions catalyzed by CuI/Me6TREN were significantly faster than those employing Cu0. Polymerization of methyl acrylate proceeded in a similar manner in both DMSO and MeCN at 25 °C initiated by an alkyl halide using either Cu0 and Me6TREN, CuI/Me6TREN, or a slow dosing of CuI/Me6TREN. These studies ultimately indicate that in addition to slowly activating alkyl halides Cu0 also acts as a reducing agent, regenerating CuI activator from accumulated CuII, thereby emulating the mechanism activators regenerated by electron transfer in atom transfer radical polymerization (ARGET ATRP). The possibility of OSET among copper species and alkyl halides was evaluated on the basis of literature data and found to be negligible in comparison to an atom transfer process (i.e., inner-sphere electron transfer).