Oxidative chain scission processes

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] ... 

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. 

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]. 

Oxidation. AH polyamides are susceptible to oxidation. This involves the initial formation of a free radical on the carbon alpha to the NH group, which reacts to form a peroxy radical with subsequent chain reactions leading to chain scission and yellowing. As soon as molten nylon is exposed to air it starts to discolor and continues to oxidize until it is cooled to below 60°C. It is important, therefore, to minimize the exposure of hot nylon to air to avoid discoloration or loss of molecular weight. Similarly, nylon parts exposed to high temperature in air lose their properties with time as a result of oxidation. This process can be minimized by using material containing stabilizer additives. 

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. 

The resulting polyol resembles the product that is hypothesized for the oligomerization of triglycerides via air oxidation, with the exception that there is a large increase in the hydroxyl content of the polyol product, and there is very little, if any, of the starting epoxide left unreacted. In addition, the epoxidation process does not produce low molecular weight chain scission products, which are a by-product of the blown oil process. The hydroxylation of epoxidized triglycerides is illustrated in Fig. 17. 

Alkyl lead compounds are extremely effective gasoline antiknock agents. By decomposing to form lead oxide compounds during the gasoline combustion process, lead alkyls interrupt the rapid chain scission reactions which lead to combustion knock. Also, lead alkyls help to prevent exhaust valve seat wear and may minimize octane requirement increase. However, unless utilized in conjunction with lead scavengers such as 1,2-dichloromethane, lead deposits can accumulate within the gasoline combustion chamber. 

---