Scission processes, oxidative

The formation of cotar none from cotar nine methine methiodide by the action of potash (IX—X) led Roser to represent cotarnine and its salts by the following formulae, the loss of a molecule of water in the formation of cotarnine salts being explained by the production of a partially reduced pyridine ring, which is fully hydrogenated in the reduction of cotarnine to hydrocotarnine. In the reverse process, oxidation of liydrocotarnine to cotarnine, Roser assumed the scission of the ring at the point indicated, with the formation of a hydration product, and oxidation of the latter to cotarnine thus —... 

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

One can consider that the kinetics carbonyl build-up is representative of the overall oxidation kinetics, at least when considered at the molecular scale (or monomer unit). It remains to establish a relationship between structural changes at this scale and molar mass changes. For the PE polymer understudy, random chain scission is predominant. It will be assumed that the main scission process is the rearrangement of alkoxyl radical (p scission). Then, every elementary reaction generating alkoxyl radicals will induce chain scission. In the chosen mechanistic scheme, both hydroperoxide decomposition processes and the nonterminating bimolecular peroxyl combination are alkoxyl sources. Thus, the number of moles of chain scissions per mass unit (s) is given by ... 

Phosphoranyl radical intermediates (Z4P e.g., Z = alkyl, aryl, dialkylamino, aryloxy and combinations thereof) have been well-studied over the past two decades. The subject has been reviewed several times (1-8). The formation of these radical intermediates by oxidative addition processes and their subsequent scission processes to yield the products of oxidation and substitution are shown in Scheme I (R = alkyl, benzyl). Somewhat less well characterized are bimolecular processes in which intact phosphoranyl radicals (Z4P = a bicyclic or spiro species) are trapped, as illustrated in Scheme I by the reaction with a disulfide (9). 

As a result of these oxidation reactions polymeric materials gradually lose their useful mechanical properties. During the oxidation degradation (mainly by beta-scission processes) and crosslinking reactions occur. 

Oxidative chain scission processes in polyethylene have been described in detail (6). During the course of exhaustive oxidation, the reaction rate subsides as accessible... 

The primary oxidation and chain scission process in polystyrene at room temperature is as follows [527] ... 

Thermal degradation of PP starts at about 230 °C by a random scission process which yields virtually no monomer up to about 300 °C. Similar to PE, the degradation products of PP span a range of unsaturated hydrocarbons up to C70 and higher. PP is much more susceptible than PE to oxidation because PP has branch points on alternate carbon atoms. The greater availability of reactive tertiary H atoms explains why the temperature at which degradation initiates is lower for PP (230 °C) than for PE (290 °C). 

Hydrolysis without chain scission occurs only in acrylic and vinylic polymers with ester side groups. These polymers are not frequently used as composite matrices. Oxidation leads to a predominating chain scission process in the majority of cases, and to a predominating cross-linking in few cases such as polybutadiene (Coquillat et al., 2007). An important quantity is the yield of chain scission or cross-linking expressed as the number of broken chains or cross-links formed per oxygen molecule absorbed. There is, to our knowledge, no case of industrial polymer for which this quantity is null. 

Alkoxy radical addition followed by rapid 3-scission (process a) leads to overall oxidation Alternatively, a rapid 3 scission reaction (path h) of 5 yields the product of substitution. Addition of X (path c) followed by 3 scission is a third overall reaction, a free radical Arbuzov (reaction 2) (20). 

In the presence of oxygen, polymer alkyl radical (3.41) is oxidized to polymer oxy radical (PO ), which finally, by the ) -scission process gives a chain scission ... 

The natural aging of UHMWPE leads to oxidation and chain scission provoking embrittlement and degradation of its mechanical properties. The chain scission process allows crystal growth to occur, increasing the degree of crystallinity of aged UHMWPE [90]. 

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