Dissociation energy, depolymerization

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. 

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. ... 

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 the case of polymerization of G-ATP-actin, the ratio of the rates of association and dissociation of monomers is different at two ends. The critical concentration of G-ATP-actin is defined by the condition that the sum of the growth rates at the two ends and the sum of the depolymerization rates are equal. At one of the two ends, the depolymerization may be faster than the growth and at the other end, the growth may be faster. Then a cycling of actin monomers takes place from one end of F-actin to the free monomer to the other end. This phenomenon was named tread-milling [27]. As a result, the polymer translates. Consumption of the free energy of ATP hydrolysis is required for such unidirectional translation of F-actin. The coupling of the polymerization with an irreversible chemical reaction produces a dynamic function of the polymer. 

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). 

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