Role of Cu0 in controlled/"living" radical polymerization

Several propositions have been made about the mechanism in which Cu-0 mediates controlled radical polymerization that include (1) exclusive activation of an alkyl halide initiator by exceptionally active Cu-0 to generate a propagating radical and a Cu-I species, (2) instantaneous disproportionation of Cu-I into Cu-0 and Cu-II in "catalytic" solvents such as DMSO, and (3) deactivation of the radical by Cu-II 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 outersphere 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 Cul/Me6TREN in DMSO, DMF, and MeCN, where comproportionation of Cu-0 with Cull to form Cut was slow but dominant in all three solvents. Relative activation rates of alkyl halides by Cu-0 and Cut with Me6TREN were studied; reactions catalyzed by Cu-I/Me6TREN were significantly faster than those employing Cu-0. Polymerization of methyl acrylate proceeded in a similar manner in both DMSO and MeCN at 25 degrees C initiated by an alkyl halide using either Cuo and Me6TREN, Cu-I/Me6TREN, or a slow closing of Cu-I/Me6TREN. These studies ultimately indicate that in addition to slowly activating alkyl halides Cu-0) also acts as a reducing agent, regenerating Cu-I activator from accumulated Cu-I, 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., innersphere electron transfer).