Hydroperoxide groups determination

We have determined the number of hydroperoxide groups per chain, analyzed by leucobase titration. The comparison between the analytical determination of hydroperoxide end groups f and the theoretical values F, allows us to know whether all the non-coupled chains are functionalized by a hydroperoxide group. 

Peroxide value. The oxidation of oils and fats leads to the formation of hydroperoxides. The hydroperoxides readily decompose to produce aldehydes, ketones, and other volatile products, which are characteristic of oxidation rancidity. The method for determination of peroxide concentration is based on the reduction of the hydroperoxide group with HI (or KI) to liberate free iodine, which may be titrated. The... 

It was proposed on the basis of the results of ESR and iodometric peroxide determinations that photodegradative modification of the wood surface starts with formation of free radicals that interact with oxygen to form hydroperoxide groups. The latter decompose rapidly to form carbonyl, carboxyl, and similar groups (186). 

The absorption band at 3520 cm-1 due to the unbonded hydroperoxide group (—OOH) is very weak, whereas the absorption band at about 3380 cm-1 is common to the bonded hydroperoxide and hydroxyl (—OH) groups. For this reason it is difficult to determine quantitatively the content of hydroperoxide groups by IR absorption spectroscopy. Mitchel and Perkins [427] devised a rfew method, based on the reaction with S02... 

It should be noted that reaction 6 does not lead to polymer chain scission ie, there is no molar mass reduction here, in contrast to reaction 5 where there is a molar mass reduction. It should also be noted that the formation and reaction of hydroperoxide groups attached to polymer molecules (reactions 3 and 4) are much slower, ie, rate determining, than the other processes shown. It will be apparent from the free-radical products formed in reactions 1 through 4 that this whole peroxidation procedure is a branching chain reaction. Additive chemistry is required to provide polyolefins with any sort of prolonged service fife, and such chemistry is well known and entirely effective. 

Peroxide Value, The method for determination of peroxide concentration is based on the reduction of the hydroperoxide group with HI or Fe +. The result of the iodometric titration is expressed as the peroxide value. The Fe " method is more suitable for detemuning a low hydroperoxide concentration since the amount of the resultant Fe +, in the form of the ferrithiocyanate (rhodanide) complex, is determined photometrically with high sensitivity (Fe-test in Table 14.27). The peroxide concentration reveals the extent of oxidative deterioration of the fat, nevertheless, no relationship exists between the peroxide value and aroma defects, e. g. rancidity (already existing or anticipated). This is because hydroperoxide degradation into odorants is influenced by so many factors (cf. 3.7.2.1.9) which mdkc its retention by fat or oil or its further conversion into volatiles unpredictable. 

The advantages and limitations of these methods in solid polymers have been reviewed [683]. Table 10.7 gives a comparison of the different methods of titration with the determination of hydroperoxide groups by a spectroscopic method in which SO2 is used. (cf. section 10.17.2.3) The sulphur dioxide (SO2) method can be carried out easily, but with some polymers secondary effects are observed which perturb the measurements. 

The individual processes shown above (reactions 3.1 to 3.9) do not by any means represent all the relevant chemistiy that occurs upon the exposure of unstabiUzed polyolefins to heat and/or UV light in the environment. What these equations do emphasize, however, is the involvement of the hydroperoxide groups in the molar mass reduction phenomenon. It must also be emphasized that the formation of these groups is rate-determining in the heat or UV-Iight initiated oxidative degradation. The only major difference between these two types of initiation is that the intermediate ketones that are formed are UV-... 

The complex formation between hydroperoxides and HALS derivatives proposed for the preceding reaction was recently postulated by two different groups of investigators. First, Carlsson determined a complex formation constant for +00H and a nitroxide on the basis of ESR measurements—. Secondly, Sedlar and his coworkers were able to isolate solid HALS-hydroperoxide complexes and characterize them by IR measurements. The accelerated thermal decomposition of hydroperoxides observed by us likewise points to complex formation. It is moreover known that amines accelerate the thermal decomposition of hydroperoxides-. Thus Denisov for example made use of this effect to calculate complex formation constants for tert.-butyl hydroperoxide and pyridineitZ.. 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

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