Scission degradation

Hoffman Degradation. Polyacrylamide reacts with alkaline sodium hypochlorite [7681-52-9], NaOCl, or calcium hypochlorite [7778-54-3], Ca(OCl)2, to form a polymer with primary amine groups (58). Optimum conditions for the reaction include a slight molar excess of sodium hypochlorite, a large excess of sodium hydroxide, and low temperature (59). Cross-linking sometimes occurs if the polymer concentration is high. High temperatures can result in chain scission. 

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

Because high temperatures are required to decompose diaLkyl peroxides at useful rates, P-scission of the resulting alkoxy radicals is more rapid and more extensive than for most other peroxide types. When methyl radicals are produced from alkoxy radicals, the diaLkyl peroxide precursors are very good initiators for cross-linking, grafting, and degradation reactions. When higher alkyl radicals such as ethyl radicals are produced, the diaLkyl peroxides are useful in vinyl monomer polymerizations. 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Norrish type I chemistry is claimed to be responsible for about 15% of the chain scission of ethylene—carbon monoxide polymers at room temperature, whereas at 120°C it promotes 59% of the degradation. Norrish I reactions are independent of temperature and oxygen concentration at temperatures above the T of the polymer (50). 

Radiation-induced degradation reactions are in direct opposition to cross-linking or curing processes, in that the average molecular weight of the preformed polymer decreases because of chain scission and without any subsequent... 

Coagents ate often used with peroxides to increase the state of cure. Some coagents, such as polybutadiene or multifimctional methacrylates, are used at high levels to form polymer grafts or interpenetrating networks. Other coagents such as triaHyl cyanurate, triaHyl trimelHtate, and y /i -phenjiene bismaleimide are used at low levels to reduce the tendency of the polymer to degrade by chain scission. 

It has already been shown that both the laevorotatory and dextrorotatory cinchona alkaloids on degradation yield scission products from the quinuclidine nucleus, which are structurally and optically identical, for example, meroquinenine, [a] -f- 27 6° d-/3-cincholoiponic acid. 

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