Thermodynamic Components of the Atom Transfer Radical Polymerization Equilibrium: Quantifying Solvent Effects

A thermodynamic scheme representing the atom transfer radical polymerization (ATRP) equilibrium as the formal sum of equilibria involving carbon-halogen bond homolysis and three additional distinct thermodynamic contributions related to the catalyst is rigorously evaluated. The reduction/oxidation of both the metal complex and the halogen atom, and the affinity of the higher oxidation state of the catalyst for halide anions (or “halidophilicity”), are measured. The validity and self-consistency of the model are verified by independently measuring, computing, or calculating the overall ATRP equilibrium constant and all four contributing equilibrium constants for one catalyst/alkyl halide combination in acetonitrile. As a thorough demonstration of the value and effectiveness of the scheme, the different equilibrium constants were measured or calculated in 11 different organic solvents, and a comparison of their values was used to both understand and predict catalyst activity in ATRP with high accuracy. The scheme explains quite well, for example, why the ATRP equilibrium constant is greater in dimethyl sulfoxide than in acetone by a factor of about 80 and why in acetonitrile and three different alcohols it is nearly identical. The solvent effects are also quantitatively analyzed in terms of Kamlet-Taft parameters, and linear solvation energy relationships are employed to extrapolate catalyst activity over 7 orders of magnitude in 17 more organic solvents and water.