Radicals polyenylic

On the other hand, carbonyl quenching by polyenyl radicals in PVC has been invoked in order to account for autoinhibition during photooxidative degradation (52). Some workers have mentioned the possibility of singlet oxygen formation in PVC via the quenching of excited polyenes (25,29) or cyclohexadienes (53). 

Despite the enormous importance of dienes as monomers in the polymer field, the use of radical addition reactions to dienes for synthetic purposes has been rather limited. This is in contrast to the significant advances radical based synthetic methodology has witnessed in recent years. The major problems with the synthetic use of radical addition reactions to polyenes are a consequence of the nature of radical processes in general. Most synthetically useful radical reactions are chain reactions. In its most simple form, the radical chain consists of only two chain-carrying steps as shown in Scheme 1 for the addition of reagent R—X to a substituted polyene. In the first of these steps, addition of radical R. (1) to the polyene results in the formation of adduct polyenyl radical 2, in which the unpaired spin density is delocalized over several centers. In the second step, reaction of 2 with reagent R—X leads to the regeneration of radical 1 and the formation of addition products 3a and 3b. Radical 2 can also react with a second molecule of diene which leads to the formation of polyene telomers. 

One side reaction commonly encountered in the reaction of alkyl-substituted polyenes with oxygen-centered radicals is hydrogen abstraction from the alkyl group in positions adjacent to the polyene jr-system. For reactions of the ferf-butyloxy radical, this reaction becomes so dominant that it can be used to form polyenyl radicals by hydrogen abstraction... 

In conclusion, it appears that allyl or polyenyl radicals are much less reactive than alkyl radicals, which restricts the use of polyenes in intermolecular radical chain reactions to simple two-step processes. Allyl radicals are, however, reactive enough to partake in a variety of intramolecular reactions. 

The orbital coefficients obtained from Hiickel calculations predict the terminal position to be the most reactive one, while the AMI model predicts the Cl and C3 positions to be competitive. In polyenes, this is true for the addition of nucleophilic as well as electrophilic radicals, as HOMO and LUMO coefficients are basically identical. Both theoretical methods agree, however, in predicting the Cl position to be considerably more reactive as compared to the C2 position. It must be remembered in this context that FMO-based reactivity predictions are only relevant in kinetically controlled reactions. Under thermodynamic control, the most stable adduct will be formed which, for the case of polyenyl radicals, will most likely be the radical obtained by addition to the C1 position. 

V. REGIOSELECTIVITY IN REACTIONS OF POLYENYL RADICALS A. Trapping with Closed-shell Molecules... 

In addition to ESR spectroscopy, which is a general method for detecting radicals, Dole et al. (9, 10, 11, 12) have developed a method of ultraviolet spectroscopy at low temperatures, which is specific for allylic and polyenylic radicals. Numerous papers have dealt with changes in polymers on irradiation, and all of these conclude that the reactions, in one way or another, arise from the formation of free radicals. Only a few papers describe experiments in which the radicals have been observed directly by ESR or ultraviolet spectroscopy at low temperatures. This article merely summarizes the present knowledge of the nature of radicals formed in polyolefins by irradiation in vacuum (ionizing radiation and ultraviolet light) and discusses some new trends in studying these radicals. 

After irradiation with high doses, the recorded spectrum is of singlet outline (27, 37, 38), and this signal has been interpreted as caused by polyenylic radicals (III) ... 

After the sample had been heated to —95°C. for 10 minutes, the spectrum was quantitatively transferred into a singlet with the line width of 17 gauss (Affmsi). This spectrum was interpreted as caused by polyenylic radicals (III). After heating to room temperature, no detectable concentration of radicals was found. In another study (4, 5) of ultraviolet-irradiated polyethylene, the sextet spectrum was also observed together with a singlet. The singlet spectrum was ascribed to peroxy radicals formed by the air present in the sample tube. 

Exercise 7.2 and later also in Exercise 8.5 (where the spin densities are derived in details). The detailed generalization of the treatment to the entire family of the polyenyl radicals has appeared, and the interested reader may wish to consult the original literature (21). 

Exercise 7.2 The pentadienyl radical is an odd member of the C2 iH2 +i polyenyl radicals (n = 3). It possesses three Kekule-type resonance structures, shown in Fig. 7.Ans.la. One resonance structure has a radical at the central carbon and is labeled Rc, the other two place the radical either on the right- or left-hand carbon atoms and are labeled accordingly as Rr and Ri, respectively. There are other structures with long bonds, but we are going to neglect them in this treatment. This will affect the calculated values of the spin density, and... 

On warming the irradiated sample to -40 °C, the main ESR signal is a singlet. This was found previously for the homopolymer poly-VDF, and was assigned to the polyenyl radicals. 

Singlet 1,6 mT wide being observed at photolysis and radiolysis of cellulose and its derivatives is attributed to radicals with the chain of conjugation these are alkyl and polyenyl radicals [148]. 

Here X - chromophore (CA or photoinitiator) Ri - acetoxyalkyl radical with high reactivity R2 - low-active polyenyl radical with free valency, conjugated with double bond AcOH - acetic acid Ki - Ke - constants of the rate of corresponding reactions. K3 = K3 y -l-Kj , where K3 t and K , - constants of the rate of the process in the darkness and under light action. 

The sharp singlet spectrum observed at about —120° C was also observed after treatment of the irradiated sample at — 78° C for a short time. This spectrum was initially identified as polyenyl radicals (59). Observed instability and g value, however, are not expected for radicals of this kind (65). The g value is very close to the average of g values of several kinds of acyl radicals (43). Therefore it seems that the sharp singlet spectrum observed here is due to some kind of acyl radicals. 

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