Polymers degradation disproportionation

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

The cured and the liquid polymers degrade essentially by the same mechanism (see Equation 6.1). The kinetic analysis of the isothermal and dynamic thermogravimetric data of the liquid polysulfide polymer cured with ammonium dichromate is explained by a kinetic model based on random initiation, followed by rapid termination by disproportionation. The average overall activation energy obtained by different methods for the decomposition is 145.3 kj/mole ... 

Figure 6.4 Polymer degradation by free-radical formation followed by disproportionation/chain scissions and termination to non-radical by-products.

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

In the free-radical kinetic Scheme for the thermal degradation of polymers, it was considered that the termination reaction resulted in disproportionation so that there was no increase in molar mass of the polymer in the termination step. At moderate temperatures, particularly in polymers such as PE and PVC for which unsaturation plays an important role... 

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