Solvent Effects on the Activation Rate Constant in Atom Transfer Radical Polymerization
2013
Horn, Markus | Matyjaszewski, K. (Krzysztof)
Rate constants of activation (kₐcₜ) for the reactions of tertiary alkyl halides with the ATRP catalyst CuᴵBr/1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) have been determined in 14 different solvents. The measurements have been performed at 25 °C by spectrophotometrically following the time-dependent absorbances of the Cuᴵᴵ species. A large excess of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), which quantitatively trapped the alkyl radicals, ensured the irreversible generation of Cuᴵᴵ. The rate constant for the least active solvent butanone is 30 times smaller than that of the most active solvent DMSO. In addition, the effect of increasing amounts of monomer in a solvent on the activation rate has been analyzed. A linear correlation of activation rate constants with previously determined equilibrium constants (KATRP) provides a Leffler–Hammond coefficient of 0.45. However, the activation rate constants do not correlate with dielectric constants and Dimroth’s and Reichardt’s ET(30) values. Application of the linear solvation energy relationship of Kamlet and Taft revealed that the dipolarity/polarizability π* of the solvent, i.e., nonspecific solvent–solute interactions, mainly accounts for solvent effects on kₐcₜ, while the ability to donate a free electron pair is important for some solvents. Quantum chemical calculations showed that more polar solvents stabilize the Cuᴵᴵ product complex to a higher degree than the Cuᴵ starting complex.
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