Role of Cu⁰ in Controlled/“Living” Radical Polymerization

Abstract

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 (Me 6 TREN). Model studies were employed to quantify disproportionation of Cu I /Me 6 TREN in DMSO, DMF, and MeCN, where comproportionation of Cu 0 with Cu II to form Cu I was slow but dominant in all three solvents. Relative activation rates of alkyl halides by Cu 0 and Cu I with Me 6 TREN were studied; reactions catalyzed by Cu I /Me 6 TREN were significantly faster than those employing Cu 0 . Polymerization of methyl acrylate proceeded in a similar manner in both DMSO and MeCN at 25 °C initiated by an alkyl halide using either Cu 0 and Me 6 -TREN, Cu I /Me 6 TREN, or a slow dosing of Cu I /Me 6 TREN. 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 II , 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).

Use of Gd powder as catalyst for single electron transfer‐living radical polymerization: Applications for synthesis of high molecular weight polymethyl methacrylate

“…32,33 SET-LRP, which was concluded to be an outer-sphere single electron transfer process, was currently being elaborated and explored with respect to mechanisms, reaction conditions and reacting species, initiator, solvents, and ligands. [34][35][36][37][38][39][40][41][42][43][44][45][46][47] As expected, SET-LRP of methyl methacrylate (MMA) in dimethyl sulfoxide (DMSO) as well as in ionic liquids had been performed. 48 Cu0/PMDETA-catalyzed SET-LRP of MMA initiated with ethyl 2-bromoisobutyrate (EBiB) in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was also reported.…”

Controlled/living heterogeneous radical polymerization in supercritical carbon dioxide

“…Cu(0) was added to increase the polymerization rate [Cu(0) reduces Cu(II) to Cu(I)]. 104 The ligand used was 4,4 0 -di(tridecafluoro-1,1,2,2,3,3-hexahydrononyl)-2,2 0 -bipyridine (dR f6 bpy; Fig. 7), which was specifically designed to have enhanced solubility in scCO 2 .…”

Recent developments in metal-catalyzed living radical polymerization

“…[63][64][65][66] In contrast, the Cu(I)-based systems in the presence of reducing agents, such as tin(II) 2-ethylhexanoate, ascorbic acid, Cu(0) and radical initiators, also induced an efficient living radical polymerization, in which the accumulated Cu(II) species changed into the active Cu(I) species via the reduction by these additives. [67][68][69][70] Although the working mechanisms of Cu(0) are still controversial and may depend on the conditions, it is clear that the use of lower oxidation metal species is important for constructing highly catalytic systems.…”