Oxidation macroradicals

The important characteristics of polymers oxidation were obtained as a result of the study of their initiated oxidation. In the presence of initiator (I) which generates the chains with the rate v, = /c,[I], the oxidation of polymer PH occurs with the constant rate v. When the macroradical P of the oxidized polymer reacts with dioxygen very rapidly (at [02]... 

At 373 K, the ratio of the rate constants is kis/kp = 8x 10 5 L-1 mol, where kis and kp refer to the reactions of isomerization and addition of oxygen. Isomerization competes with the reaction of the macroradical P with 02 therefore, intense hydroxylation during the oxidation of PP may imply that isomerization in the solid phase is slower than in the liquid phase. The experimentally measured ratio A[02]/[ROOH] at different partial pressure of oxygen helps to estimate the ratio k jkp in the oxidized polymer. Since the kinetics of chain PP oxidation are characterized by the rate v... 

Due to the high reactivity of cumene, the reaction of the peroxyl macroradical with cumene occurs more rapidly than the intramolecular reaction and the formed POOH is only from the single hydroperoxyl groups. Such POOH decomposes with free radical formation much more slowly than POOH produced in PP oxidation in the solution and solid state. 

As noted above, the kinetics of autoxidation of PP depends on the partial pressure of oxygen. This is the result of the mechanistic peculiarity of PP oxidation. This peculiarity lies in competition between two reactions the addition of oxygen to the 2-hydroperoxyalkyl macroradical and the decomposition of this radical. 

Compelling evidence suggesting that the breakdown of hydroperoxyl groups is not related to polymer destruction, at least in the initial period of oxidation at temperatures below 400 K, comes from experiments on the initiated oxidation of polymers. It was found that the destruction of polymers develops in parallel with their oxidation from the very onset of the process, but not after a delay related to the accumulation of a sufficient amount of hydroperoxyl groups [129]. These experiments also demonstrated that it is free macroradicals that undergo destruction. Oxidation of polymers gives rise to alkyl, alkoxyl, and peroxyl macroradicals. Which radicals undergo destruction can be decided based on the kinetics of initiated destructive oxidation. 

The radiochemical oxidation of PS in a chloroform solution is accompanied by its destruction and formation of products of styrene oxidation, namely, benzaldehyde and styrene oxide [136]. The radiochemical yield of these products was equal to the radiochemical yield of PS macromolecule cleavages. Butyagin [137] analyzed the products of decomposition of the peroxyl radicals of PS and polyvinyIcyclohexane. Alkyl macroradicals were produced mechano- or photochemically, volatile products were evaporated in vacuum, and alkyl radicals were converted into peroxyl radicals using labeled lsO. Peroxyl radicals were then... 

These calculations show that the degradation via decomposition of peroxyl macroradicals prevails inside the tight range of oxidation conditions. 

Acceptors of alkyl radicals are known to be very weak inhibitors of liquid-phase hydrocarbon oxidation because they compete with dioxygen, which reacts very rapidly with alkyl radicals. The situation dramatically changes in polymers where an alkyl radical acceptor effectively terminates the chains [3,49], The study of the inhibiting action of p-benzoquinone [50], nitroxyl radicals [51-53], and nitro compounds [54] in oxidizing PP showed that these alkyl radical acceptors effectively retard the oxidation of the solid polymer at concentrations ( 10-3 mol L 1) at which they have no retarding effect on liquid hydrocarbon oxidation. It was proved from experiments on initiated PP oxidation at different p02 that these inhibitors terminate chains by the reaction with alkyl macroradicals. The general scheme of such inhibitors action on chain oxidation includes the following steps ... 

The phenomena of nitroxyl radicals regeneration has been discovered in the study of the retarding effect of 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-A-oxyl on PP initiated oxidation [51]. It has been shown that the limiting step of chain termination by the nitroxyl radical is the reaction with the alkyl macroradical of PP. The resulting compound AmOP is fairly reactive with respect to the peroxyl radical and nitroxyl radical is regenerated in this reaction. Thus, the cycle includes the following two reactions (mechanism I) [60-64] ... 

One of the possible reasons of the drop of the initiation rate could lie in the fast consumption of the photoinitiator. With a phosphine oxide photoinitiator (LucirinTPO), as much as 75% of this compound was destroyed within 0.2 s of UV exposure at a light intensity of 400 mW cm 2 (Figure 7). When the light was cut off at that time (rj=0), the polymerization was found to continue to proceed nearly as fast as upon continuous irradiation. The fact that the polymerization is only slightly faster upon continuous irradiation than in the dark suggests that rj has already dropped to a low value when (Rp)max is being measured, at 20% conversion. The important post-polymerization, which lasts only a few seconds, is due to the high concentration of macroradicals that continue to polymerize in the dark. 

Metal ions of transition and other elements of variable valency, e.g. Ce, Co, Fe, V, Mn, etc., are known to oxidize polysaccharides rather selectively, producing macroradicals as intermediates which are capable of adding vinyl monomers and form graft copolymers. These initiators are redox systems which differ from those previously described by not producing free radicals of low molecular weight. Only macroradicals on the substrate are formed in the redox reaction. Some homopolymer may still be formed in the process, e.g. due to oxidation of monomer or other side reactions. ... 

Oxidation of Anionic Polymers In the Solid State The ability of the macroradical and of the macroions to diffuse In the mixture, and to interreact Is responsible for the secondary products formation coupling reaction and alcoholate synthesis. To prevent the diffusion phenomenon, we have carried out the deactivation In the solid state. The living polymers have been prepared In benzene, with or without a solvating agent (THF or TMEDA) and the solution has been freeze dried before the oxygen introduction. The experimental results are collected in Table VII. 

Terminal air oxidation of polystyrene has recently been carried out by degradation of polystyrene in the presence of azo-bis-isobutyronitiile and air oxygen the polystyrene dihydroperoxide can initiate the polymerization of methyl methacrylate and acrylonitrile [193, 194). The yield of homopolymer is very low, indicating an exceptional difference of efficiency between the macroradical and the OH radical. 

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. 

It was thought (6) that the freshly oxidized ferric ion might competitively react with the lignosulfonate to generate the lignin macroradical, thus making lignin itself act as an initiator ... 

The overall activation energy for both processes is about 60 kJ/mol which is one half of the value for the activation energy of oxygen diffusion into the crystalline region of HDPE [65]. The lower values of activation energy for oxidation of allyl radicals show on the participation of reactions of such macroradicals which are in the close proximity of amorphous regions. 

The rate constant of a transfer reaction will therefore be the higher, the weaker C-H bond is attacked by a peroxyl radical. From this it is obvious that the maximum rate of oxidation of polyethylene will increase with increasing number of tertiary hydrogens in the polymer [13]. Since the process includes the interaction of a macroradical with a macromolecule which both are of restricted translational mobility, the maximum rate of oxidation does not depend on the low content of reactive allyl hydrogens in polyethylene. 

The transfer of hydrogen to peroxyl radicals may proceed intra- or inter-molecularly. Intramolecular transfer reaction (isomerization) of peroxyl macroradicals of polypropylene occurs during the oxidation of the polymer in a solution of inactive solvent [75] while the intermolecular transfer is preferred during the oxidation in reactive solvent or in the crystalline state [76]. 

P2VP macroradicals were added to multi-walled carbon nanotubes (CNTs), which was proved to be an effective grafting to method [26], In contrast to other methods of CNTs modification, the macroradical addition does not require any hard oxidative pre-treatment of the CNTs, which preserves their original size. Deposition of an amorphous shell at the surface of CNTs was confirmed by TEM observations. Figure 4a shows modified nanotubes with closed... 

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... 

The oxidation of radicals in our experiments was carried out at 760 mm Hg oxygen pressure and at low temperatures. The macroradicals were generated by mechanical degradation of polymers in vacuo. The polymeric particles, made during the degration procedure, fell into a... 

---