Laser Flash Photolysis and CIDNP Studies of Steric Effects on Coupling Rate Constants of Imidazolidine Nitroxide with Carbon-Centered Radicals, Methyl Isobutyrate-2-yl and tert-Butyl Propionate-2-yl

Time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP) and laser flash photolysis (LFP) techniques have been used to measure rate constants for coupling between acrylate-type radicals and a series of newly synthesized stable imidazolidine N -oxyl radicals. The carbon-centered radicals under investigation were generated by photolysis of their corresponding ketone precursors RC(O)R (R = C(CH3)2−C(O)OCH3 and CH(CH3)−C(O)−OtBu) in the presence of stable nitroxides. The coupling rate constants k c for modeling studies of nitroxide-mediated polymerization (NMP) experiments were determined, and the influence of steric and electronic factors on k c values was addressed by using a Hammett linear free energy relationship. The systematic changes in k c due to the varied steric (E s,n) and electronic (σL,n) characters of the substituents are well-described by the biparameter equation log(k c/M- 1s-1) = 3.52σL,n + 0.47E s,n + 10.62. Hence, k c decreases with the increasing steric demand and increases with the increasing electron-withdrawing character of the substituents on the nitroxide.