Degradation radical-based

Figure 8.1 shows the GPC analysis of polysilane 2 and its radical-based degradation [12]. Line A shows a typical bimodal distribution... 

Although there is still debate as to whether hydroxyl radicals or ferryl species are the key oxidants in Fenton systems, most literature reports on the mechanisms of degradation of organic compounds invoke the hydroxyl radical. Based on the reports discussed above, it seems likely that hydroxyl radical is a major oxidant during Fenton degradations. Although ferryl ions or other highly oxidized forms of iron may occur, either to a limited extent or more abundantly under specific conditions, this section will deal with documented reaction pathways and kinetics for hydroxyl radical or species assumed to be hydroxyl radical. The reader should keep in mind that ferryl pathways may need to be considered under certain conditions. 

Radical-based dephosphorylation via a phosphonyl radical constitutes one formulation for the mechanism of C-P bond cleavage during microbial degradation of organophospho-nates.80 Frost has demonstrated that anhydrides 117 derived from phosphonic acid and thiohydroxamic acid react in a chain sequence with thiols, Bu3SnH and CC14 to give dephosphoryl-... 

It has been noted that initial work on the thermal degradation of PET suggested that homolytic (i.e., free radical-based) scission did not have a role, but this was not confirmed and, in any case, certain conditions may exist where such reactions might come into play. At very high temperatures (or under certain conditions in processing machinery where formation of mechanoradicals may occur), homolytic scission of the polyester chain may occur [7-10]. The weakest link in the PET chain would appear to be the sequence carbonyl-oxygen-methylene, which may be expected to homolytically cleave in two possible ways ... 

Agarwal, S., Kuma, R., Kissel, X, and Reul, R. (2009) Synthesis of degradable materials based on caprolactone and vinyl acetate units using radical chemistry. Polym. J., 41 (8), 650. 

Decarboxylation. - Barton s group has further extended their studies of radical-based decarboxylations in a number of ways (Scheme 10). One-carbon degradations of acids, RCO2H, to alkanes, RH, can be simply achieved by conversion into the thiohydroxamic esters derived from N-hydroxypyridine-2-thione followed by heating in benzene or toluene in the presence of t-butyl thiol.This avoids the use of more expensive tin hydrides which produce organostannane by-products which can be difficult to remove. By changing the traps available to the... 

The basic objective in the selection of antioxidants is to obtain the best stabilization with no effect on optimum values of technical properties of polymers. The stabilizing effect on oxidative degradation is based on protection against the formation of active free radicals and on nonradical decomposition of hydroperoxides. Optimum stabilization is obtained by the combination of different compounds that function with different mechanisms as metallic ion deactivating agents, UV absorbers, and so on. 

TABLE 3. - Present level of technical development of oxidative degradation techniques based on hydroxyl radical generation [120]. 

The theoretical treatments of controlled degradation are based on its radical character [189, 191, 192, 202] and chemically (peroxide) initiated degradation, or pure chain random scission, where Saito s integral is valid [202a]. 

Molecular oxygen, O2, readily reacts with free radicals, and since free radicals play a dominant role in the radiolysis of polymers, O2 can significantly affect radiation-induced chemical alterations. For instance, it enhances the radiation-induced degradation of most polymers. Linear polymers, including polyethylene, polypropylene, polystyrene and poly(vinyl chloride), that crosshnk in the absence of oxygen undergo predominantly main-chain scission in its presence. As a typical example, a free-radical-based reaction mechanism proposed for the oxidative degradation of polyethylene is shown in Scheme 5.16. 

As formulated for general radical-based degradation pathways, this process can be regarded as proceeding in three distinct steps chain initiation, chain propagation including chain branching, and chain termirration (Scheme 3). ... 

Additionally, organotin mercaptides can act as antioxidants, as they can sequester free-radical degradation mechanisms (48). The one drawback of mercaptide-based tin stabilizers is the discoloration of the sulfur after exposure to uv-radiation. Special precautions or formulations need to be developed for outdoor apphcations. 

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