Macroradicals peroxy

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. 

According to these results the macroradical —CF2—CH—CF2— plays the major role in cross-linking of VDF-HFP copolymer. The dehydrofluorination is witnessed by the findings of polyenyl-type radicals. In the presence of oxygen, the alkyl radicals react with 02 to form peroxy radicals, which intervene both in cross-linking and in chain-scission reactions. 

The macroradicals (R ) formed during the initiation can easily react with oxygen molecule producing peroxy radicals [23]... 

The antioxidant acts as a chain terminating agent. The antioxidants react faster with peroxy radicals (ROO) than with macroradicals (R) and the activity depends upon their structure [114-116]. 

As intramolecular hydrogen abstraction from the backbone is not possible in this case, the above process is believed to originate from the cleavage of peroxy radicals formed by reaction of oxygen with phosphorus macroradicals derived from residual unreacted P-Cl groups in the phosphazene main chain (Scheme 6). 

Some chemicals retard or suppress free-radical polymerization by reacting with primary radicals or macroradicals to yield radicals that are very stable to further reaction or yield nonradical products. These materials could be retarders or inhibitors. Retarders slow down the formation of polymer but inhibitors completely eliminate it. Oxygen is one of the most commonly known inhibitors for vinyl polymerization and good practice requires the removal of air from the reactor vessels before the reaction is started. It combines with active radicals to form unreactive peroxy radicals. 

A further point of discussion, however, is whether the often cited reactions (6, 7, 8) of TMP derivatives (and their conversion products) with peroxy radicals may sufficiently compete with the propagation steps (5) in Scheme I. Indeed, TMP-derivatives including NOR are known to be rather weak scavengers of peroxy radicals (7) in the liquid phase. Based on considerations which take into account that rapid randomization of macroradicals is largely restricted in the solid polymer, Carlson and Wiles, however, concluded that fast radical scavenging would in fact not be needed for efficient inhibition of long chain polymer photooxidation processes (2, 8, 9). 

These radicals can abstract hydrogen atoms from the polyethylene chains and give rise to new polymer alkyl radicals. This contributes to the increase of crosslinking efficiency. TTie train of subsequent reactions of radicals leads, moreover, to the appearance of a new precureor of free radicals, hydrogen peroxide. On the other hand, macroradicals of polyethylene may react with oxygen directly to form peroxy radicals which will reduce the yield of carbon-carbon crosslinks. 

It was found that the metal tear to the dynamical contact with the polymers is diminished in the oxygen presence than in argon atmosphere, even if the peroxy macroradicals are active with respect to metallic surfaces. Probably in the competition of the chemical events occurring there, the oxygen succeeds to protect in a way the metal surface, by increasing its stability against to the polymer radicals. 

Subsequently, the generated macroradicals react either with the chains that were not affected by destruction or with other components of the surrounding medium. In the case of polysaccharides, the glucosidic bonds are split with the highest probability and the amide bonds, -HN-CO-, in the case of polypeptides or proteins. If the cryolitic degradation occurs in the presence of oxygen, this one stimulates the formation of peroxy and hydroperoxy compounds. 

SCH EME1 Entrapment of oxygen by the hyaluronan C-(macroRadical (A ) yielding a peroxy 1 (macro )radical (A-0-0"). 

In the absence of oxygen in the rigid PTFE matrix, the reverse reaction of these radical proceeds efficiently. In the presence of oxygen, the terminal alkyl macroradicals can be oxidised to form terminal peroxy radicals which interact with double bonds much slower. Under the action of NO on samples containing neighbouring terminal double bonds and peroxy radicals, the latter are converted into macromolecular nitrates and nitrites [47] ... 

A similar procedure was used to obtain spin-labelled TEE-HEP [49]. The presence of hexafluoropropylene (HFP) groups in this polymer leads to disturbance of the structural ordering typical of PTFE to more complex dynamics of their motion. After y-irradiation of powders and films of TFE-HFP copolymer in air, there are three types of stable peroxy macroradicals in the samples end radicals CF -CF O, secondary middle-chain radicals CF -CF(00 )-CF2, and tertiary middle-chain radicals CF2-C(CF3)(00 )-CF2. In contrast to PTFE, prolonged exposure (>100 hours) of these samples in a NO atmosphere at room temperature does not lead to the formation of aminoxyl macroradicals. However, two types of macroradicals are formed if TFE-HFP is heated with evacuation after the decay of radicals in a NO atmosphere. At 90 °C, the ESR spectrum demonstrates the presence of tertiary alkyl macroradicals CF2-C (CF3)-CF2 formed upon decay of the tertiary nitroso compounds [57]. On further increasing of the temperature up to 180 "C, the tertiary alkyl macroradicals... 

In air, alkyl or allyl macroradicals produced in reactions with NO can oxidise to give peroxide radicals. The reactions of the latter with NO result in relatively unstable peroxy nitrates similar to low-molecular peroxide radicals [15, 16] ... 

The reaction of perfluoroperoxide macroradicals also yields peroxy nitrate. In this case, the decomposition similar to (Equation 6.21) results in destruction of the macromolecule because the resultant alkoxyl radicals in fluorinated polymers cannot enter into the substitution reaction [18] ... 

Light induced PP chemiluminescence arises due to the termination of PP peroxy macroradicals formed under the action of light and oxygen [22], It is known that, at low concentrations of radicals, this reaction is first order in the concentration of radicals (linear termination of oxidation chains), resulting in the chemiluminescence intensity I from PP at room temperature being proportional to the product of the rate constant of peroxy macroradicals decay and their concentration [22] ... 

In the presence of oxygen, the terminal alkyl macroradicals can be oxidised to form terminal peroxy radicals ... 

Oxidation reactions are chain reactions which follow a free radical mechanism. In chain reactions three distinct steps are present initiation, propagation and termination. During initiation a free macroradical (P") is generated in the polymer by heating, radiation or stress. This reacts readily with oxygen to yield a peroxy radical (POO"), the peroxy free-radical abstracts hydrogen from another polymer molecule (PH) creating a new macroradical and a hydroperoxide (POOH). Then the hydroperoxide decomposes to two new free radicals, which are also initiators of the chain reaction. These chain reactions have severe consequences for the polymer, and cause extensive localized oxidation. 

The stabilizing effect of amines on radicals from polymers was extended to plastomers by Dubinskaya et al. [41]. The polymers used, polystyrene, poly-(methyl methacrylate), poly(vinyl acetate), and poly(a-methylstyrene), were put in solution with primary, secondary, and tertiary amines at concentrations from 2 to 5%. These solutions were then degraded in a vibratory mill at 80°K in vacuum and in air. It was found that the reactivity of amines with macroradicals in the solid state at low temperature decreases in the order secondary > primary > tertiary. The authors were also able to rank the reactivity of the several macroradicals studied with the same amine. The order of reactivity was peroxy poly(vinyl acetate) > polystyrene > poly(methyl methacrylate) = poly(a-methylstyrene). The last two polymers do not react with amines at low temperatures. They only react with secondary amines at room temperature. 

Low oxygen concentration In this case, termination occurs almost exclusively by the recombination of macroradicals (P ) (equation 2.133). Under these conditions, the concentration of polymer radicals (P ), particularly in the terminating stages of the reaction, is far in excess of the polymer peroxy radicals (POO ). It follows from equation 2.133 that ... 

In the propagation stage the resulted radicals react with oxygen from air to yield new reactive oxi- or peroxi- stmctures or other macroradicals which finally transform in hydroperoxide and peroxide stmctures as shown in Eq. (12) ... 

The mechanism of oxidation of short-chain hydrocarbons is widely known under the name of Bolland s cycle (Scheme 7) [20], and it was studied at 70-80°C in the liquid phase, where the hydrocarbon chains and radicals have high mobility. The first step involves the formation of macroradicals, which, in the presence of oxygen, are easily transformed into peroxy radicals (ROO ). These species evolve to hydroperoxides (ROOH) via hydrogen extraction from a hydrocarbon chain, leaving behind a new radical, which reenters the cycle. 

In the post-irradiation oxidation process, the rate of oxidation decreases by more than one order of magnitude in the first 100 hours, although alkyl macroradicals are continuously formed along with the formation of hydroperoxides (Scheme 9, Reaction 23). The termination reaction of thermo-oxidative processes is generally described as a Russell reaction between two peroxy species. The relative inunobility and the stability of the peroxy radical makes the Russell bimolecular termination strongly disfavored in the solid state at room temperature [22, 23]. More likely. 

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