Irreversible recombination with

As discussed already, termination of ring-opening polymerization may proceed by (a) irreversible recombination with counterion and (b) irreversible chain transfer to polymer. Other sources of termination are also possible, depending on the system (c) reaction with other components of the system, solvent or impurities and (d) different reactions of more reactive species existing in equilibrium with stable onium species. 

Termination by Irreversible Recombination with Counterion Irreversible collapse of ionic species via recombination with counterion is the most trivia] potential source of termination in cationic ring-opening polymerization. The relations discussed in Section II.B.6.C may also be applied to the present discussion. The termination will occur if counterion is nucleophilic enough (as compared with monomer) to compete successfully with monomer in Sn2 nucleophilic substitution and monomer is not nucleophilic enough to displace the counterion from the covalent termini ... 

As discussed in Section II.C, in any system in cationic ring-opening polymerization, a reaction of active species with polymer repeating unit may lead to chain transfer to polymer or termination (if the resulting branched or cyclic onium ions are not active), whereas recombination with counterion leads to termination in the case of irreversible reaction. The later reaction may be avoided by the proper choice of counterion. As the onium ions are generally inherently stable there is no other termination reaction, provided that impurities that may act as terminating agents are eliminated. 

Seventy-five percent of the ethoxy groups recombine with the surface hydrogen to reform ethanol as the reverse of reaction (32). The remaining twenty-five percent are stable to 500 K. In fact, these ethoxides were irreversibly formed when exposed to water at low temperature, they did not... 

A difficulty in the elution of antibody can arise if the antigen itself is oligomeric, since the conditions needed for the elution of antibody may also cause the elution of monomers of the antigen. If these are not irreversibly denatured by elution, they will merely recombine with antibody once the pH is restored to neutrality. One way of avoiding this problem (observed in the production of labeled antibodies to ferritin) is extreme care in the selection of pH for the elution of antibodies, but avoiding the... 

For the computer simulation, a two-dimensional matrix of 60 x 60 with a hexagonal packing of elements was randomly filled by 1 and 0 (corresponding to Dad and Had atoms formed at the sites where appropriate reactions took place) with a deuterium content equal to that detected experimentally. A numerical value of the sum of n-n elements (0-6) was used to calculate the probability of H2, HD, and D2 formation for each element. The content of H2, HD, and D2 was calculated in the central part of the matrix of 30 x 30 elements to avoid the edge effects during both simultaneous irreversible recombination of the elements into dimmers and rearrangements of the matrix (alternatively, periodic boundary conditions could be applied). The calculated ratio H2% HD% D2% for the matrix randomly filled by Had and Dad at D2 mol% = 50.17 was found to be 25.02 49.61 25.37 confirming a proper statistical distribution of atoms in the initial matrix. 

The first term describes the photogeneration of holes for illumination via the electrolyte, the second term represents reaction of holes with the reduced species and the third term describes irreversible trapping of holes by a distribution of trap states. The trapped holes may be consumed in the oxidation of the reducing agent or by recombination with electrons (see section 5.2). 

DNA catenanes were prepared by the addition of T4 DNA ligase to the mixture of linear DNA and nicked DNA as shown in Scheme 4.4. The linear and nicked DNAs were prepared by the addition of EcoRl or DNase I to a solution of pBR322 plasmid DNA, respectively. The advantage of using T4 DNA ligase is an irreversible recombination between the complementary ends of DNA, in contrast with the reversible one for topoisomerase I. Therefore we can expect an increase in the yield of DNA catenanes. Many slower migrated bands were observed in elec-... 

In the case of Colil(CN)5X3- such a photoredox decomposition is not expected to take place since the primary complex fragment Co KCN)53- does not decay but undergoes a recombination with X- (or X2) [94]. However, if the ligand radical reacts rapidly and irreversibly Co(CN)5X3- will also yield stable photoredox products, e.g. (X = benzyl) [96] ... 

In the presence of both the monomer and a fast-decomposing radical initiator, such stable radicals can serve to trap reactive free radical species with a rate constant comparable to that of irreversible recombination. 

Mechanism of Atom recombination. In order to explain the complex temperature dependence of y of atom on the surface of silica, a mechanism of atom recombination on surface was proposed (77). The surface is covered with a small fraction of active sites, denoted by, which hold chemisorbed atoms irreversibly. The chemisorbed atoms A recombine with atoms A arriving at the active sites. In addition to the direct arrival of atoms from the gas phase, atoms reversibly adsorbed on the rest of the surface diffuse along the surface to the active sites. Here is a proposed sequence of elementary steps for adsorption, desorption, surface diffusion,... 

The efficiency and specificity of this method depends on the irreversibility of the whole process due to a high rate constant and favorable thermodynamics of Reaction (10) [4] and a high rate of subsequent Reaction (11) (which is the recombination of a free radical anion and a free radical cation with the diffusion rate constant of about 109 1 mol-1 s ). 

It should be noted that the unfolding kinetics can sometimes involve quite complex unfolding schemes of different substates in equilibrium with the native state. Staphylococcal nuclease is an example of such behavior, known to unfold with three different substates that exhibit an equilibrium that does not appear to shift with temperature.49 Irreversible aggregation processes of proteins have been known to involve first- or second-order reactions.132141 The mechanism of recombinant human interferon-y aggregation is an example where thermodynamic and kinetic aspects of the reaction provided a powerful tool for understanding the pathway of instability and permitted a rationale for screening excipients that inhibited the process.141... 

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