Origin of Activity in Cu-, Ru-, and Os-Mediated Radical Polymerization
Résumé
A method is discussed for quantifying and categorizing the activity of an atom transfer radical polymerization (ATRP) catalyst as being derived from the product of its intrinsic reducing power and affinity for halide anions. The reducing power of several copper, ruthenium, and osmium ATRP catalysts was quantified with cyclic voltammetry in tetrahydrofuran, including for MtX2(PPh3)3, MtX(Cp*)PiPr3, and CuX(BPMODA) (where Mt = Ru and Os, X = Cl and Br, and BPMODA = N,N-bis(2-pyridylmethyl)octadecylamine). Spectrophotometric measurements were used to determine ATRP equilibrium constants (KATRP), a measure of catalyst polymerization activity. Ru and Os catalysts of activity comparable to that of Cu are ∼500 mV less reducing. Evaluation of kinetic polymerization data, together with E1/2 and KATRP values, allowed the determination that halide affinities of these Ru and Os compounds must be ∼7−9 orders of magnitude stronger than typical Cu ATRP catalysts to compensate for their comparatively poor reducing power. Additionally, the ability of the coordinatively unsaturated Os compounds to control polystyrene molecular weights under organometallic radical polymerization (OMRP) conditions where the Ru analogues and Cu compounds cannot is discussed in terms of the potential for Os to form stronger Mt−C bonds. DFT calculations, 1H NMR chain-end analyses, and polymer chain extensions were conducted in order to evaluate the likelihood that standard halogen atom transfer (for ATRP) and reversible radical trapping (for OMRP) processes are indeed regulating the growing radical concentrations under the respective appropriate conditions with the new Os catalysts.
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