Kinetics nitroxide decay

Nitroxide Decay Kinetics. Plots of nitroxide decay versus time are shown in Figure 7 for different nitroxide concentrations. Plots ot nitroxide decay versus time are shown in Figure 8 for two different coatings at similiar initial nitroxide concentrations. Equation 7 predicts that the nitroxide decay is the sum of a first order term (in nitroxide) and a zeroth order... 

The direct correlation observed in this study between gloss loss rates and photoinitiation rates suggests that the nitroxide doping technique could provide a rapid test for the evaluation of coating durability. Measurements of nitroxide decay kinetics to determine coating photoinitiation rates can be made at near ambient exposure conditions in a few days (compared to years for... 

As discussed above, it is necessary to extrapolate the nitroxide loss kinetics to zero nitroxide concentration in order to determine the photoinitiation rate of the coating. Shown in Figure 9 is a plot of initial nitroxide decay rate versus initial nitroxide concentration for the six coatings studied. Over the nitroxide concentration range studied, the decay rate was found to be linear in nitroxide concentration The slopes and intercepts are given coatings. The slope is a measure of abstraction by excited nitroxide in intercept is the rate of formation of... 

Both the nitroxide decay measuranents of free-radical photoinitiation rates and nitroxide kinetics during HALS stabilization depend on accurate, quantitative measurements of nitroxide concentrations in cross-linked polymers. Quantification of radical concentrations by ESR requires a suitable primary standard, careful sample preparation, a reference standard with which to monitor spectrometer performance, and most important, reproducible positioning of the samples in the resonance cavity of the spectrometer. Most of the experiments described here were carried out with a Bruker-IBM ER 200 D spectrometer equipped with an Aspect 2000 Data System. Because these coatings are cured at temperatures as high as 130°C, the primary nitroxide standard, which was introduced into the coating prior to cure, had to be... 

Derivation of Nitroxide Radical Decay Kinetics and Free-Radical Photoinitiation Rates... 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. 

The nitroxide concentration rises rapidly to a maximum then slowly decays. The slow decay of the nitroxide concentration In the hindered amine doped coatings together with the high rate of Initiation demonstrates the Importance of nitroxide regeneration as a stabilization mechanism. The effectiveness of the hindered amine stabilizers Is a function of the kinetic chain length and of the lifetime of the stabilizer In the coating. The results found here are contrasted with degradation and stabilization studies In other polymers such as polypropylene and polyethylene. 

The kinetics of the decomposition of the transient nitroxide radicals derived from aceto-HX and SAHA are similar RC(0)N0 decays via a second-order reaction, which is followed by two consecutive first-order reactions. Typical kinetic traces are shown in Figure 2. 

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