Thermal oxidation scheme peroxide formation

Perhydroxyl radical, 75 thermal generation from PNA of, 75 Peroxy radical generation, 75 Peroxide crystal photoinitiated reactions, 310 acetyl benzoyl peroxide (ABP), 311 radical pairs in, 311, 313 stress generated in, 313 diundecanyl peroxide (UP), 313 derivatives of, 317 EPR reaction scheme for, 313 IR reaction scheme for, 316 zero field splitting of, 313 Peorxyacetyl nitrate (PAN), 71, 96 CH3C(0)00 radical from, 96 ethane oxidation formation of, 96 IR spectroscopy detection of, 71, 96 perhydroxyl radical formation of, 96 synthesis of, 97 Peroxyalkyl nitrates, 83 IR absorption spectra of, 83 preparation of, 85 Peroxymethyl reactions, 82 Photochemical mechanisms in crystals, 283 atomic trajectories in, 283 Beer s law and, 294 bimolecular processes in, 291 concepts of, 283... 

Production and use of PVC occur in the presence of air, i. e. in the presence of oxygen. Therefore, it is surprising that the mechanistic details of thermooxidative degradation of PVC are still not fiilly revealed. The major reactions of this process are shown in Scheme 1. As indicated in this Scheme, thermal dehydrochlorination yields HCl and simultaneously sequences of conjugated double bonds (polyenes) in the chain. The reactive polyenes lead to peroxides in a reaction with oxygen followed by the formation of radicals. Subsequent chain reactions result in additional initiation of HCl loss and further oxidative processes (/, 8). 

Other pathways of radical generation involve thermal or photochemical fragmentation of perfluoroacyl peroxides [14] or photochemical fragmentation of perfluoro-alkylsulfonyl bromides [15]. The in situ formation of Barton esters from perfluoroacyl chlorides and thiopyridone-N-oxide has also been used as a convenient source of radicals [16] (Scheme 2.99). 

The Baeyer-Villiger oxidation of cyclohexanone with aqueous hydrogen peroxide has been reported to result in a thermally activated radical leading to the formation of adipic acid. An oxidative rearrangement of 2-furylcarbamates to A-Boc-5-hydroxypyrrol-2(5//)-ones has been reported (Scheme 63). ... 

The reverse ATRP using AIBN as the initiator has been performed successfully for copper-based heterogeneous (Xia and Matyjaszewski, 1999) and homogeneous (Xia and Matyjaszewski, 1997) systems in solution and in emulsion as well as for iron complexes (Matyjaszewski and Xia, 2001). A general outline of reverse ATRP is shown in Scheme PI 1.9.1. As shown in this Scheme, the starting materials in reverse ATRP are a thermal free radical initiator (I-I), transition metal halide in the oxidized state (XMt ), and monomer (M), while the propagation step resembles a normal ATRP. It may be noted, however, that the reverse ATRP initiated by peroxides sometimes behaves quite differently than that initiated by azo compounds like AIBN. The differences between the benzoyl peroxide (BPO) and AIBN systems possibly arise due to an electron transfer and the formation of a copper benzoate species in the BPO system (Xia and Matyjaszewski, 1999). 

Photolysis of 3-buten-l-ol nitrite affords no cyclized products (Cy5/Cy4) neither does 5-hexen-l-ol nitrite (Cy6/Cy7). The same result is obtained on peroxydisulphate oxidation of 5-hexen-l-ol. In the Cy6/Cy7 case an important competitive pathway is probably 1,5-intramolecular ally lie hydrogen abstraction and, indeed, esr spin trapping by nitrosodurene " provides evidence of this. Cyclization in the Cy6/Cy7 case was considered to explain the reaction products of tetrahalogeno-o-benzoquinones with 2,3-dimethylbut-2-ene but was discarded in favor of a direct cycloaddition process on the basis of spin trapping and deuteration experiments. As discussed before, cyclization in the Cy3/Cy4 case must be difficult to observe because of the high j5-scission rate of oxyranylalkyl radicals. Nevertheless, this pathway has been used recently to explain the formation of diepoxides in the thermal-, photochemical-, or ferrous-salt-induced decomposition of unsaturated cyclic peroxides. In view of the multistep scheme involved this conclusion must await further confirmation. 

Diels-Alder reaction of cyclopentadiene and butadiene affords a mixture of exo-5-vinyl-2-norbornene (la) and e do-5-vinyl-2-norbornene (lb) [1], Preparative GC separation [1] of these isomers encounters difficulties in obtaining the individual isomer in pure form and in large quantities. An alternative approach of separation via thermal isomerization [2], in which lb gets transferred to 4,7,3a,7a-tetrahydro-lff-indene, whereas la remains unchanged, is also not successful. This is because it is difficult to prevent la being contaminated by unreacted lb. As no other method is available for their separation, Inoue has reported [3] that hydroboration of 1 with 9-BBN, followed by oxidation with alkaline hydrogen peroxide results in the formation of alcohols 2a and 2b. The iodoether cyclization only of the endo isomer takes place. The sequence of approach is delineated in Scheme 29.1. 

A comparative study has been conducted into the thermal and thermooxidative degradation of PET and PBT polymer films and their model compounds, ethylene dibenzoate and butylene dibenzoate, in an oxygen atmosphere at 160 °C 832485. On the basis of the compounds identified by GC-MS, a mechanism was proposed for the degradation of the model compounds that involves oxidation at the a-methylene carbon with the formation of unstable peroxides and carboxylic acids a.l53. From the studies performed under N2 at 160 °C, it was concluded that benzoic acid and esters are products of the thermal degradation as illustrated in Schemes 15 and 16, while benzoic and aliphatic acids, anhydride and alcohols were due to thermooxidative degradation. 

The alkoxy radical is usually described as a typical product of the thermal decomposition of hydroperoxides. Nevertheless, in the post-irradiation oxidation process at room temperature, it cannot originate from this reaction because all the formed products follow a kinetic similar to that of ketone formation [21]. The reaction between the alkyl macroradical and the peroxy macroradical forms peroxides (Scheme 9, Reaction 20), but we can also hypothesize Reaction 21, Scheme 9. Literature studies demonstrate that the alkoxy radical can give beta-scission (Reaction 28) forming a primary alkyl radical and CO, a product that is found during the irradiation of PE (Scheme 10, Reaction 29) [24]. The activation energy of this reaction is around 50kJ/mole. 

The formation of triarylimidazolyl free radicals 2 from the thermal or photolytic dissociation of hexaarylbiimidazoles 1 has been reported [2,3,7,9,10]. These radicals are known to dimerize to regenerate a hexaarylbiimidazole, usually one of the two favored isomers 1 or 3 (Scheme 7.1) [3,7,9]. They are also known to react with nitric oxide to give N-nitrosotriarylimidazoles [2] and to react with hydrogen peroxide to give 4-hydroperoxytriarylimidazoles [10]. 

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