Solvent Effects on the Activation Rate Constant in Atom Transfer Radical Polymerization

Rate constants of activation (k(act)) for the reactions of tertiary alkyl halides with the ATRP catalyst (CuBr)-Br-I/1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTE-TA) have been determined in 14 different solvents. The measurements have been performed at 25 degrees C by spectrophotometrically following the time-dependent absorbances of the Cu-II species. A large excess of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), which quantitatively trapped the alkyl radicals, ensured the irreversible generation of Cu-II. 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 (K-ATRP) 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 E-T(30) values. Application of the linear solvation energy relationship of Kamlet and Taft revealed that the dipolarity/polarizability pi* of the solvent, i.e., nonspecific solvent-solute interactions, mainly accounts for solvent effects on k(act), while the ability to donate a free electron pair is important for some solvents. Quantum chemical calculations showed that more polar solvents stabilize the Cu-II product complex to a higher degree than the Cu-I starting complex.