Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.
The polymerization of styrene mediated by a polystyryl dithiobenzoate was studied by electron spin resonance spectroscopy to determine the concentration of the intermediate radical produced by the addition of polystyryl radical to the dithiobenzoate. The polymerization was also followed by dilatometry to estimate the concentration of the growing radical. The results showed that the fragmentation of the intermediate radical is a fast process with a relevant rate constant on the order of 10 4 s -1 (at 60 °C) and that the intermediate radical undergoes the cross-termination with polystyryl radical to form a 3-arm star chain, thus causing a retardation in the rate of polymerization. The rate constant of cross-termination was estimated to be similar to (somewhat smaller than) that of the termination between polystyryl radicals. The formation of the star was evidenced by an independent model experiment.
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).
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