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(CH 3 ) 2 -C(O)OCH 3 and CH(CH 3 )-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 (σ L,n ) characters of the substituents are well-described by the biparameter equation log(k c /M- 1 s -1 ) = 3.52σ L,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.