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 kc for modeling studies of nitroxide-mediated polymerization (NMP) experiments were determined, and the influence of steric and electronic factors on kc values was addressed by using a Hammett linear free energy relationship. The systematic changes in kc due to the varied steric (Es,n) and electronic (sigmaL,n) characters of the substituents are well-described by the biparameter equation log(kc/M- 1s(-1)) = 3.52sigmaL,n + 0.47Es,n + 10.62. Hence, kc decreases with the increasing steric demand and increases with the increasing electron-withdrawing character of the substituents on the nitroxide.
Zubenko
Dmitry
D
International Tomography Center SB RAS, Novosibirsk, Russia, Institute of Organic Chemistry SB RAS, Novosibirsk, Russia.
Tsentalovich
Yuri
Y
Lebedeva
Nataly
N
Kirilyuk
Igor
I
Roshchupkina
Galina
G
Zhurko
Irina
I
Reznikov
Vladimir
V
Marque
Sylvain R A
SR
Bagryanskaya
Elena
E
D002087:Butyrates / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D048289:Imidazolidines / Q000737:chemistry*; D007834:Lasers; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry*; D010087:Oxides / Q000737:chemistry*; D010782:Photolysis; D011422:Propionates / Q000737:chemistry*; D013696:Temperature; D013997:Time Factors
D002087:Butyrates; D005609:Free Radicals; D048289:Imidazolidines; D010087:Oxides; D011422:Propionates; D002244:Carbon; D009584:Nitrogen