344299-98-5Relevant academic research and scientific papers
Thermolysis of free-radical initiators: tert-butylazocumene and its 1,3- and 1,4-bisazo and 1,3,5-trisazo analogues
Engel,Pan,Ying,Alemany
, p. 3706 - 3715 (2007/10/03)
Four tert-butylazocumenes (4-7) were prepared from the corresponding cyanobenzenes, and their nitrogen evolution kinetics and products were analyzed. In combination with TEMPO, the simplest compound, tert-butylazocumene (4), shows promise as a "one-radical" initiator of styrene polymerization. The ABNO-trapped cumyl radical 29 is a particularly stable trialkylhydroxylamine, whose thermolysis half-life is 2.1 h at 150 °C. Taking advantage of this stability, we trapped the cumyl radical centers from 7 to afford tris adduct 32a. While the behavior of the meta bisazoalkane 6 can be mostly predicted from that of 4, the para isomer 5 exhibits both unusual products and kinetics, attributed to the formation of quinodimethane 33 via azo-containing radical 34. In fact, flash vacuum pyrolysis of 5 allowed observation of the 1H and 13C NMR spectra of 33, whose persistence even at ambient temperature showed that this quinodimethane is far more stable than the parent 36. Finally, evidence is presented that 7 is an initiator of star polymerization.
Kinetics of nitroxide radical trapping. 2. Structural effects
Bowry,Ingold
, p. 4992 - 4996 (2007/10/02)
Laser flash photolysis and kinetic competition product demonstrated that in isooctane at ambient temperature the rate constant for coupling of carbon-centered with presistent nitroxides, kT, upon the of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of kT are observed for changes in both the structure of the nitroxide the structure of the carbon radical. Thus, for any particular carbon kT is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the "usual" di-tert-alkyl nitroxides kT decreases along the series, 1,1,3,3-tetra-methylisoindoline-2-oxyl ≥ 2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo) > di-tert-butyl nitroxide, i.e., kT decreased on going from a five-membered ring to a six-membered ring to a noncyclic structure. Cyclopropyl triphenylmethyl are trapped at the fastest and slowest rates, respectively, the corresponding kT value 3.0 × 109 and 1.2 × 108 M-1 s-1 for ABNO and 2.1 × 109 6 M-1 s-1 for Tempo. Steric effects in the carbon radicals are for Tempo than for ABNO. For example, the ratio of kT's for the trapping of nonyl tert-butyl is 1.7 for Tempo but 1.3 for ABNO, while for the trapping of benzyl cumyl the ratio of kT's is 4.1 for Tempo 0.9 for ABNO. The effect of resonance stabilization can be illustrated by the kT values for three sterically unhindered primary radicals, n-nonyl, benzyl, 2-naphthyhmethyl, viz., 1.2 × 109, 4.8 × 108, and 5.7 × 107 M-1 s-1, respectively, for Tempo and 2.2 × 109, 1.2 × 109, and 8.1 × 108 M-1 s-1, respectively, for ABNO.
