Hydroperoxide molar extinction coefficients

Although there is little doubt that all can be important in particular circumstances, it is generally agreed that hydroperoxides are normally the key to the initial photoinstability in hydrocarbon polymers. Hydroperoxides are present in commercial PP at levels of around 10 - 10 mol/kg and have molar extinction coefficients at 310 nm of around 0.4 l/(mol cm). Carbonyl groups are typically present at around 10 mol/kg and have molar extinction coefficients at 310 nm of around 10 l/(mol cm) (112). However, the quantum efficiency for radical generation in hydroperoxide photolysis is 1, whereas it is of the order of 0.05 for carbonyl groups, partly because radicals formed by Norrish I reaction recombine rather easily and partly because of the preference for Norrish II chemistry, which does not give radicals. 

Table 3) that form stable metal complexes with reduced catalytic activity of the metal (108-110). Ultraviolet absorbers (UVA) act by absorbing uv light, hence retarding the formation and photolysis of hydroperoxides. Typical examples that are based on 2-hydroxybenzophenone and 2-hydroxybenzotriazole structures are given in Table 3 (AOs 28-32). These compounds are photostable and have high molar extinction coefficient in the spectral region 300-360 nm. Their activity is... 

Most lipids lack chromophores of value in spectrophotometric detection, but the absorbance of isolated double bonds (and some other functional groups) at about 205 nm in the UV range can be used successfully if care is taken in the choice of the solvents for the mobile phase. UV detection at low wavelengths has its limitations, however. For example, only a few solvents are transparent and can be used In the appropriate range (e.g. hexane, methanol, acetonitrile, isopropanol and water), and the molar extinction coefficient is so low that traces of impurities In the solvents or in the samples (e.g. hydroperoxides) can swamp the signals from the... 

Hydroperoxide groups are transparent at wavelengths > 340 nm and they have very low molar absorptivity (molar extinction coefficient) (e = 10-1501... 

The iodine, generated as I3, is measured spectrophotometrically at a wavelength of 362 nm (the molar absorptivity, i.e. molar extinction coefficient of I3 at 362 nm is 25 280 m " cm ). The concentration of hydroperoxide can be read off a calibration curve set up using cumene hydroperoxide as a standard. In cases where there is interference at 362 nm, e.g. from some additives, it is recommended that absorption be measured at a longer wavelength, i.e. 425 nm. 

There is some uncertainty concerning the molar extinction coefficient of the hydroperoxide groups [362,474,683]. At a first approximation the value 8(OOH) = 901mol hydroperoxide groups... 

It is very common to compare the total amounts of hydroperoxide (OOH) groups formed and decomposed with the sum of the ketone and trans-vinylene formed in polyolefins and their copolymers [1908,1909]. An example of such kinetic measurements is shown in Figure 10.68. The molar extinction coefficient of a rrans-vinylene group at 965 cm is 169moll cm" [1218]. 

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