Macroradicals alkyl

The induced decomposition of POOH in an inert atmosphere is the result of the alkyl macroradical reaction with POOH. 

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... 

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 ... 

Nitroxyl radicals, p-benzoquinone, and dinitrotoluene terminate chains only by the reaction with alkyl macroradicals. They form the following series according to their activity nitroxyl radical > quinone > nitro compound. 

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] ... 

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. 

Two products can be formed in the reaction of the nitroxyl radical with the alkyl macroradical. 

Special experiments with preliminary withdrawal of all low-molecular products formed by the reaction of nitroxyl with alkyl macroradicals from the polymer sample were performed. These... 

This calculation shows that the discussed reaction is very exothermic. The activation energy of this reaction calculated by the IPM method (see Chapter 6) is equal to 8.7 kJ mol 1 and rate constant is k = 7.3 x 106 L mol-1 s-1 at T= 400 K. This rate constant is close to that of the acceptance of the alkyl macroradical by the nitroxyl radical. Hence, this reaction is rapid enough to be the efficient step in cyclic chain termination in polymer. 

Similar reactions may also occur on addition of polyfiinctional low molecular monomers to a polymer system. For instance, during addition of a three futKtional monomer to alkyl macroradicals... 

We may expect that polymers originating from monomers with several double bonds will give an increased yield of crosslinks on irradiation [61], Besides consideration of the contact between reactive radicals one supplementary fact should be of importance, namely the residual number of double bonds in the polymer chain whfch may react further with primary alkyl macroradicals and thus increase the efficiency of crosslinking [62],... 

If irradiation of polypropylene is carried out in the presence of acetylene, the efficiency of crosslinking increases substantially [72]. This may be explained by a reaction of alkyl macroradicals with acetylene to give radicals of the allyl type which are known to combine preferentially. 

In the works ( Kiryushkin S.G., Shlyapnikov Y.A., 1975 1986 ), reaction ( ) is considered as induced" decay of hydroperoxide. The kinetic chain is moved along macromolecules formed alkyl macroradical who takes a step "backwards", reacts with the preceding neighboring hydroperoxide group to form alcohol and alkoksiradikal. 

Low molecular weight hydrocarbons obtained from the terminal alkyl macroradicals ... 

The next stage posited is scission of the hydroperoxide which, in theory, can take place in one of two ways 0-0 bond scission to form an alkoxy macroradical and HO, or C-O bond scission to form an alkyl macroradical and HOOa... 

The alkyl macroradical may also undergo chain scission, allegedly via a homolytic route ... 

Alkyl macroradicals (R ) originate either by direct interachon of the polymer with gamma rays (or e-beam) or by hydrogen abstraction from reactive radicals as ROO , HOO , RO , OH , and H. The amoimt of R is proportional to the absorbed dosage, and their actual concentration depends on the dosage rate. 

Experimental studies have shown that vinylene concentration is constant with sample thickness (Costa et al. 1998). Alkyl macroradicals produced by gamma radiation are homogeneously distributed within the UHMWPE component because vinylene are the primary product of interaction between polyethylene and high-energy radiation. In e-beam irradiation, the radical distribution is a function of depth because of the cascade effect, which leads to a subsurface maximum in the absorbed dosage. 

How does high-energy radiation produce allyl macroradicals in UHMWPE Explain the most relevant chemical reactions starting with the formation of alkyl macroradicals. 

In the medical praxis, UHMWPE have been used successfully as one half of the bearing couple (against metallic alloys or ceramics) in total hip and total knee joint replacements crosslinked under irradiation [82]. Even y-irradiated UHMWPE at doses higher than 100 kGy presents the incipient ffacmre development [83], the alkyl macroradicals are involved in crosslinking and in a smaller proportion they promote oxidation [84, 85]. The prosthesis manufactures by the irradiation of UHMWPE have long durability, because the application of radiation treatment induces an increased crystallinity and promotes sterilization in the whole volume of material. 

Bmnella, V., Bracco, P., Carpentieri, I., Paganini, M.C., Zanetti, M., Costa, L. Lifetime of alkyl macroradicals in irradiated ultra-high molecular weight polyethylene. Polym. Degrad. Stab. 92, 1498-1503 (2007)... 

It was noted above that the thermally induced processes occurring during the implantation are coupled with radiation-induced ones that involve the excited polymer units, cation radicals, and trapped electrons. In the case of polyethylene (which is the simplest one of polyolefins), the relaxation of the excited polymer units results in formation of alkyl macroradicals and H radicals the latter can recombine or interact with the polymer yielding molecular hydrogen [21,22] ... 

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