Bond dissociation energy, depolymerization

In addition to depolymerization through p-scission, intra-and intermolecular hydrogen transfer occur. So, primary carbon-centered radicals are expected to isomerize by intramolecular hydrogen abstraction (backbiting) and to form secondary radicals, which are more stable. On the basis of activation and bond dissociation energies, Kuroki et al claimed that backbiting reactions and intermolecular radical transfer reactions are much more likely to occur than depolymerization reactions. ... 

From the value of hond-dissociation energy, it is predictable which bond, in preference to others, should be more susceptible to chain scission. However in practice, depolymerization is a also combination of other reactions, such as hydrolysis, dehydration, hydrogen abstraction, and cross-linking. 

The first law of photochemistry [the Grotthus-Drapper principle (30)] states that for a photochemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry [the Stark-Einstein principle (3J)j states that a molecule can only absorb one quantum of radiation. The absorbed energy causes the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to formation of chromophoric groups such as carbonyls, carboxyls, qui-nones, peroxides, hydroperoxides, and conjugated double bonds. 

In high-intendty ultrasonics in liquids, the local absorption of elastic energy is closely related to the phenomenon of cavitation. In depolymerization reactions free radicals are usuaUy formed. In this care the shear stresses not only dissociate the bonds in the main chain, but also separate the radicals formed, preventing in this way their recombination. An extensive review of the experimental woik on the mechanical degradation of polymers has been given by Porter and Casale (77). 

---