Random chain dissociation

To account for the physical and chemical processes in the model proposed by Westerhout et al. (1997), termed the random chain dissociation (RCD) model, a polymer chain is represented as a chain of carbon atoms with side groups. A certain rate constant, fc can be associated with each bond type / between the carbon atoms. The number of bonds of type i is represented by /V and the rate of change of the number of each bond type is described by a simple flrst-order model ... 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

Previous theoretical treatments of the transition between the helicel and random forms of the desoxyribose nucleic acid (DNA) molecule are extended to Include formally the explicit consideration of the dissociation into two separate chains and the consideration of the effects of the.ends of the chains, An approximate form for the fraction of the base pairs that are bonded is obtained in terms of two parameters, a stability constant for base pairing and a constant representing the interaction of adjacent base pairs. The matrix method of statistical mechanics proves to be adaptable to this problem. Some numerical examples are worked out for very long molecules, for which case it is found that the effect of concentration is small. 

Enzyme-catalyzed polymerization reactions have an important characteristic that is not found elsewhere. Once the enzyme has added a monomeric unit to the growing chain, it can either dissociate and recombine at random with other growing termini, or it can remain attached to the same chain and add further residues. Enzymes that dissociate between each addition and distribute themselves among all the termini are termed distributive. Those that process along the same chain without dissociating are termed processive. These terms apply also to degradative enzymes such as exonucleases. 

As we shall see, the components of the chains are such that the strands can be held together efficiently by hydrogen bonds. In agreement with this structure, it has been found that, when DNA is heated to about 80° under proper conditions, the strands of the helix unwind and dissociate into two randomly coiled fragments. Furthermore, when the dissociated material is allowed to cool slowly under the proper conditions, the fragments recombine and regenerate the helical structure. 

In water, SPG exists as a thermodynamically stable triple-helix (Fig. 2) held together by hydrogen bonds. Under special conditions these hydrogen bonds can be broken for further discussion, it is important to know that, in DMSO, schizophyllan dissociates and exists as a single randomly-coiled chain, but it will re-associate into triple helices upon dilution with water. 

The conformation of the macroion strongly depends on the extent of dissociation and the added amount of small ions as well as on the temperature. A macroion having a flexible linear chain may show random coil, rod,... 

The breaking of chemical bonds under the influence of heat is the result of overcoming bond dissociation energies. Organic substances such as polymers are highly heat sensitive due to the limited strength of the covalent bonds that make np their structures. Scission can occur either randomly or by a chain-end process, often referred to as an unzipping reaction. 

These different mechanisms and prodnct distribntions, to some extent, are related to the bond dissociation energies, the chain defects of the polymers, and the degree of aromaticity, as well as the presence of halogen and other heteroatoms in the polymer chains. Large amount of styrene monomers can be obtained by pyrolysis of PS, while a wide range of hydrocarbons are prodnced by random degradation of PE and PP [3, 59, 60]. 

Transcription cycle of E. coli RNA polymerase showing dissociation of the ct subunit shortly after chain elongation begins, dissociation of the core enzyme during termination, and re-formation of the holoenzyme from the core enzyme and the a subunit. A previously joined core enzyme and a subunit will rarely become rejoined instead, reassociation occurs at random. 

---