Kinetic studies atom transfer radical addition

As a result of CMU s IP focus, 21 of the 28 issued US patents, pins several active applications, protect the fundamental ATRP process or improvements in the fundamental process, (26,27,30,33-48) while only five of the twenty eight address novel polymer compositions. (28,29,36,49,50) Nevertheless, these early material-focused patents disclose a nnmber of materials that were not prepared by other CRP procednres until a later date. The difference in numbers is due to the fact that two issued patents are directed towards improvements in nitroxide mediated polymerization. (20,51) The first discloses an atom transfer radical addition reaction to form an alkoxyamine that has fonnd nse in ATRP kinetic studies, and the other focnses on rate enhancement of a NMP. 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

Addition of excess CH3I to a solution of [Ni (tmc)]+ results in the rapid loss of the absorption (A = 360 nm, e = 4 x 103 M-1 cm-1) and appearance of a less intense band at A = 346 nm. A subsequent slower reaction gives rise to the weaker absorbance profile of [Ni"(tmc)]2+. The data are interpreted in terms of the formation of an organo-nickel(II) species followed by a slower hydrolysis with breaking of the Ni-C bond. Kinetic studies under conditions of excess alkyl halide show a dependence according to the equation — d[Ni1(tmc)+]/cft = 2 [Ni(I)][RX]. The data have been interpreted in terms of a ratedetermining one-electron transfer from the nickel(I) species to RX, either by outer-sphere electron transfer or by halogen atom transfer, to yield the alkyl radical R. This reactive intermediate reacts rapidly with a second nickel(I) species ... 

To give a specific example, the advantages of styrene as a substrate for peroxyl radical trapping antioxidants are well known" (i) Its rate constant, kp, for chain propagation is comparatively large (41 M s at 30 °C) so that oxidation occurs at a measurable, suppressed rate during the inhibition period and the inhibition relationship (equation 14) is applicable (ii) styrene contains no easily abstractable H-atom so it forms a polyper-oxyl radical instead of a hydroperoxide, so that the reverse reaction (equation 21), which complicates kinetic studies with many substrates, is avoided and (iii) the chain transfer reaction (pro-oxidant effect, equation 20) is not important with styrene since the mechanism is one involving radical addition of peroxyls to styrene. 

Quantitative kinetic studies of absolute rate constants for hydrogen atom transfer from substituted phenols to polystyrene peroxyl radicals by Howard and Ingold in the 1960s provided the first reliable data on substiment effects on antioxidant activities of phenols. Later, a very detailed report appeared providing data on substituent and structural effects on various classes of monohydroxy phenols. In addition, detailed reviews were given of substituent etfects ". These reports provide the basis for understanding how substituent and strucmral effects control the antioxidant activities of phenols and will be summarized in part below. 

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