Distribution of oxidation products

The reaction of lead tetraacetate (LTA) with monohydric alcohols produces functionalization at a remote site yielding derivatives of tetrahydrofuran (THF) 12). An example is the reaction of 1-pentanol with LTA in nonpolar solvents which produces 30% THF. The reaction, which is believed to proceed through free-radical intermediates, gives a variable distribution of oxidation products depending on solvent polarity, temperature, reaction time, reagent ratios, and potential angle strain in the product. 

Fig. 3 Effect of UV laser intensity on the distribution of oxidation products. DNA was solubilized in 1 mM pH 7 TRIS buffer in order to stabilize the duplex... 

The diffusion-controlled, hydroperoxide-initiated, oxidation regime (oxidation is restricted to a superficial layer). The shape of the distribution of oxidation products depends on the 02 solubility in the polymer (Fig. 14.15). 

Kinetic modelling calculation of the thickness distribution of oxidation products. 

Sample Thickness. It is convenient to study at least two sample thicknesses one ( 100 fj.ni) to determine the kinetic parameters of the non-diffusion-controlled kinetics, the other one (> 1 mm) to determine the distribution of oxidation products along the thickness. 

Figure 7. Spatial distribution of oxidation products for PE after 207 and 255 hours of exposure at lOO C in atmospheric air. Points Experimental carbonyl profiles determined by microspectrophotometry. Continuous line kinetic model...

This reaction indicates a distinct stereospecificity due to the structure of singlet oxygen and its reactivity. It was found, for example, that the quantitative distribution of oxidation products from 1-methyl-2-iso-propylethylene and l,l-dimethyl-2-isopropylethylene differs when molecular or singlet oxygen is used [249]. 

Chemistry of environment. The presence of oxidant atmosphere (usually air) determines the start of oxidative degradation from the start of irradiation. An inert atmosphere like vacuum or nitrogen does not provide any source of secondary reactions with radicals that appear during radiolysis. The diffusion of oxygen into irradiation material influences the distribution of oxidation products that takes a parabolic shape with the maximum amoimt at the both external sides and minimum is placed on the symmetry axe [14]. The penetration of molecular oxygen allows the reactions of free radicals with it and the peroxyl radical are formed. They are the initiators of further oxidation, which advances as a chain process. 

Fig. 14 Distribution of oxidation products in low-crystalline polypropylene exposed to weathering degradation, (open circle) outdoor oxidation, (open square) accelerated oxidation...

There are many similar attributes of thermal-, photo- and radiafion-degradafion. They progress on the similar mechanisms based on the homolytic or heterolytic scissions and the formation of degradation initiators for loop process of oxidative degradation. The discrepancies between these three ageing ways consist of the concentration of primary free radicals and their distribution in the material. The depth of degradation is only some microns, but the diffusion of chain promoters, hydroperoxides, towards the inner layers of materials leads to a parabolic distribution of oxidation products around the symmetry axe. 

X. Cohn, L. Audouin, J. Verdu, M. Le Huy. Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 2 Effect of sulfur vulcanization on mass changes and thickness distribution of oxidation products during thermal oxidation. Polymer Degradation and Stability 92(5), 898-905, May (2007). 

The NMR results indicate the following distribution of oxidation products for the polyethylene sample that initially had 17.7 branches per 1000 CH2 groups ketone. 

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