Macroradicals from oxidation

Apart from reacting with the growing grafted chain macroradicals, the oxidant cations or the initiator radicals may cause decomposition of cellulose macroradicals formed at the first stage of oxidation. Taking into account the latter process, the rate of graft polymerization initiation may be described as follows62 ... 

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. 

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. 

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 ... 

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. ... 

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. 

It means that in reality the macroradicals are concentrated in a thin layer near the surface of polymer particles. Fig. 8 shows the temperature dependence of the rate constants of oxidation reactions for the three polymers investigated (curves b, c, d). One can see that in the temperature range investigated this dependence is in agreement with the Arrhenius equation. Let us examine the initial sections of oxidation curves. Analysis of the curves shows that they can be represented as a superposition of two different exponents corresponding to two different rate constants of radical oxidation. The temperature dependence of rate constants determined from the initial sections of oxidation curves of polymethyl-metacrylate is shown in Fig. 8 (curve a). Following fact is of interest ... 

The initiation efficiency depends on the relationship between the rate of macroradical formation and that of initiation. In Table 1 are compiled results on certain redox systems employed for the initiation of cellulose grafting either by direct oxidation of cellulose (or its derivative) or chain transfer from active low molecular weight radicals. Table 1 indicates that in systems where the matrix acts as a reducing agent (1st group), the initiation efficiency does not exceed 15%, i.e. only a minor portion of macroradicals formed at the first stage of oxidation initiates graft polymerization while the rest is oxidized to stable... 

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... 

Macromolecular dispersion in HDPE with patch—like transfer is defined by polymer—metal and polymer—polymer adhesive interactions. The major contribution to macromolecular dispersion is from the alternating areas of polymer—polymer and metal-polymer contacts. Macroradicals generated within polymer—polymer contact may recombine on the metallic surface to form chemisorption and coordination complexes with an oxide film. Under the dynamic contact this process may increase the effect of mechanical actions on the macromolecular dispersion of polyolefine. 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

As has been shown, PP oxidation occurs predominantly intramolecularly, the kinetic chain moves along the macromolecule. Macroradical RO2, formed by the oxidation of polypropylene, reacts with a hydrogen atom from the tertiary C atom located in the P-position relative to the peroxide radical of their molecules. As a result, intramolecular transfer of a macromolecule oxidized PP formed "blocks" of several adjacent OH-groups. 

The formation of PP macroradicals is an easy process initiated by more or less any radical initiator. It occurs spontaneously in oxidative processes. Alkyl radiccils (except for methyl) are usually not reactive enough to initiate an efficient macroradical formation in PP. Oxyl radicals, formed by a thermal decomposition of peroxides, are the most convenient species for crosslinking initiation. The transfer of the radical centre to PP is selective to a certain extent. At temperatures usual for peroxide decomposition, the ratio of the rate constants of the abstraction of hydrogen from primary, secondary, and tertiary carbon by the oxyl radical is approximately 1 3 10 [2]. 

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