Peroxide macroradicals

During polymer oxidation, concentration of peroxide macroradicals and hydroperoxide groups should be the function of distance from polymer surface, even in that case when oxygen diffusion does not limit oxidation rate. For the same reason constant of the rate of... 

Polymer Degradation in Reactions of Peroxide Macroradicals with NO2... 

To predict the stability of fluorinated polymers in the atmosphere polluted with nitrogen oxides, the ratio of rates of peroxynitrates in polytetrafluoroethylene (PTFE) by two mechanisms has been determined [19]. Chain peroxide radicals R 02 were obtained by photolysis of PTFE powder in a vacuum with the subsequent oxidation of fluoroalkyl macroradicals. The end peroxide radicals R 02 were generated by photolysis of the R 02 containing PTFE in the air. The exposure of PTFE samples containing chain or end peroxide macroradicals to NO2 results in decay of these radicals. The kinetics of the decay of peroxide radicals of both types is practically identical, and is likely to be determined by the rate of diffusion of NO into the... 

An analogous investigation of the action of NO on y-irradiated tetrafluoroethylene-hexafluoropropylene copolymer (TFE-HFP) containing 13 mol% of HFP units has been performed [33], After exposure of powders and films of TFE-HFP to a dose of 10 Gy in air, there are three types of stable peroxide macroradicals ... 

Aminoxyl radicals in pol5rperfluoroalkanes are not formed in these conditions. However, if to carry out a preliminary y-irradiation on air, middle and end peroxide macroradicals appear with conversion into fluoroami-noxyl macroradicals CF2CF(NO )CF2 by the subsequent exposure to NO. Their EPR spectra in oriented films are quintet of triplets with the parameters 4 =0.46 mT, 4 =U1 mT, n=2.006 4=1.12 mT, 4=1.61 mT and 1=2,0071. The following mechanism of formation of aminoxyl radicals in these conditions is offered... 

The presence of sulphonic and carboxylic groups enables the iron ions to be in the vicinity of the cellulose backbone chain. In this case, the radicals formed can easily attack the cellulose chain leading to the formation of a cellulose macroradical. Grafting of methyl methacrylate on tertiary aminized cotton using the bi-sulphite-hydrogen peroxide redox system was also investigated [58]. 

The xanthate method [62] is considered as one of the most promising methods for industrial chemical modification. The principal involved in the xanthate method of grafting is that cellulosic xanthate either ferrated or in acidic conditions reacts with hydrogen peroxide to produce macroradicals. The following reaction mechanism has been proposed ... 

The regeneration of nitroxyl radical from the product of the reaction of nitroxyl radical with the alkyl macroradical was proved in the following experiments [51]. The nitroxyl radical and initiator (dicumyl peroxide) were introduced in a PP powder and this sample was heated to T= 387 K in an argon atmosphere. The concentration of nitroxyl radical was monitored by the EPR technique. The nitroxyl radical was consumed in PP with the rate of free radical generation by the initiator (see Figure 19.3). Dioxygen was introduced in the reactor after the nitroxyl radical was consumed. The generation of peroxyl radicals induced the formation of nitroxyl radicals from the adduct of the nitroxyl radical with the PP macroradical. 

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

Destruction of dissolved polymers under acoustic treatment results in the formation of macroradicals. Insonification of dissolved polymers in the presence of monomers or a mixture of polymers results in their polymerization production of copolymers is also possible. In the presence of oxygen, radicals may combine following a peroxide pattern. 

Another polymer which is easily peroxidized is polyacetaldehyde it has a polyacetalic structure and is characterized by the presence of some side hydroperoxide groups (1 to 4°/00) due to traces of peracetic acid when the polymer is prepared. The homolytic decomposition of these peroxide groups yields macroradicals to which methyl methacrylate could be grafted [73, 74). 

This scheme explaining the copolymerization observed has not yet been explained in kinetic aspects. Indeed, if decomposition rate of diacetyl peroxide equals 10-5 s-1 at 60 °C [120], this value does not follow the classic evolution of rate vs temperature (according to Arrhenius law). The authors suggest induced decompositions of these peroxides by CH3 radicals existing in the medium, and also by macroradicals coming from growing chains during... 

The first stage of polymer oxidation is their combination with a dioxygen biradical. The resulting peroxy macroradical reacts with the surrounding radicular species, leading to the formation of peroxides or hydroperoxides. One of the original applications of photoDSC consists in the combination of the irradiation step and the measurement, by thermal decomposition, of the amount of accumulated peroxides. 

The mechanism of peroxide crosslinking of elastomers is much less intricate than that of sulfur vulcanisation. Crosslinking is initiated by the thermal decomposition of a peroxide, which is the overall cure rate determining step. Next, the active radicals thus formed abstract hydrogen from elastomer chains to form macroradicals. Finally, crosslinking results either from the combination of two macroradicals or from the addition of a macroradical to an unsaturated moiety of another primary elastomer chain. 

FT-IR results also showed that one new (small) absorption at 1659 cm"1 appeared, which could not be attributed to peroxide decomposition products. This absorption also appeared when the peroxide-curing experiments were carried out using an amorphous EPM, indicating that the absorption did not relate to rearrangement of the third monomer moiety (ENB in this case). It is tentatively concluded that the absorption at 1659 cm 1 is related to EPDM main-chain modifications, resulting from disproportionation reactions of EPDM macroradicals with BHT radical fragments. 

The effect of co-agents in the peroxide-curing of EPDM is very similar to the effect of third monomers. It was concluded that the pendent unsaturation of the third monomer acts as a co-agent, i.e., the amount of third monomer governs the amount of chemical crosslinks formed by macroradical addition reactions via the unsaturated moiety of the third monomer, whereas the amount of peroxide governs the amount of crosslinks formed by macroradical combination reactions. 

When oxygen is admitted into the sample containing the polymer particles with macroradicals, the oxidation of radicals takes place almost immediately, and they are transformed into peroxide radicals with their characteristic asymmetrical spectrum (Fig. 3). After the oxidation process is over, at temperatures below — 50° C the amount of peroxide... 

Comparing Fig. 2 and 5, we can see that in polystyrene at 20° C the peroxide radicals disappear 1000 times more rapidly than the macroradicals of polystyrene itself. This is in drastic contradiction with the... 

Two major reactions are involved in the wastage of primary radicals. These are induced decomposition of initiator by radicals and side reactions in the solvent cage. The mechanisms of induced decompositions depend on the structure of the initiator molecule. For benzoyl peroxide the reaction involved could be an 5 2 attack of propating macroradicals on the 0—0 bond ... 

---