Reversible scission

In our model, this slowing down depends only on the distance from the polymerization point, not on the level of co-operativity. The reason is that the model presumes infinite co-operativity (or activation) for the fraction assembled material, which then behaves as if the polymerization transition were an actual phase transition. More elaborate calculations for reversible scission/recombination kinetics show that if Ka > 0 (i) the relaxation time f does not actually diverge at X = 1 albeit that it can become very large because Ka -C 1 and (ii) that f is also sensitive to the precise value of Ka (Nyrkova and Semenov, 2007). 

Because of the interaction of the two complicated and not well-understood fields, turbulent flow and non-Newtonian fluids, understanding of DR mechanism(s) is still quite limited. Cates and coworkers (for example, Refs. " ) and a number of other investigators have done theoretical studies of the dynamics of self-assemblies of worm-like micelles. Because these so-called living polymers are subject to reversible scission and recombination, their relaxation behavior differs from reptating polymer chains. An additional form of stress relaxation is provided by continuous breaking and repair of the micellar chains. Thus, stress relaxation in micellar networks occurs through a combination of reptation and breaking. For rapid scission kinetics, linear viscoelastic (Maxwell) behavior is predicted and is observed for some surfactant systems at low frequencies. In many cationic surfactant systems, however, the observed behavior in Cole-Cole plots does not fit the Maxwell model. 

Figure 7.4 Concentration dependence of the relative viscosity at different temperatures for C16E7/D2O system. The open symbols indicate the data reported previously [21]. The solid line indicates the slope predicted by Cates [24] for the reversible scission mechanism.

An early example of intramolecular addition of an alkoxyl radical had been reported for a steroid bearing an allylic nitrite ester group the possibility of intramolecular addition of an allyloxyl radical had been put forward to rationalize the formation of an epoxide.But if this is the real pathway, it must be a very special one. Indeed, it is now well known that the behavior of oxyranyl alkyl radicals parallels the behavior of methylcyclopropyl radicals (Section V.2.B) and that the reverse -scission pathway toward allyloxy radicals is very easy and very This is, in fact, faster than the other... 

Aldolase catalyzes the reversible scission of fructose 1,6-diphosphate into 3-phosphoglyceraldehyde and 3-phosphodihydroxyacetone. The enzyme is also capable of condensing a wide variety of aldehydes with 3-phosphodihydroxyacetone. Thus acetaldehyde and 3-phosphodihydroxyacetone react by aldol condensation as follows ... 

M. F. Roberts and S. A. Jenekhe, Sitc-spectfle reversible scission id hydrogen bonds in polymers. An investigation of polyamides and their Lewis acid-base complexes by infrared apectroacopy, Macramoleatl 24-J142 (1991V... 

The chemistry of both Ru-W and Os-W mixed-metal clusters was extensively investigated by Chi et al., and a number of exciting discoveries were made. These include the reversible scission of a coordinated acetylide ligand on the acetylide cluster 168, which can be prepared from the condensation of CpW(CO)3(C2Ph) and Ru3(GO)i2 in refluxing toluene.The carbide clusters 169 and 170 were formed when 168 was heated with Ru3(CO)i2 in heptane. Interestingly, the reaction sequence can be reversed to regenerate the acetylide cluster 168 in the presence of pressurized CO. This kind of reactivity is seldom observed for semi-encapsulated ruthenium carbide systems. 

The dynamics of highly entangled branched polymer motion in the presence of reversible cross-links is more complicated. In the case of entangled but unbranched polymers, it was recently shown how to include reversible scission effects within the framework of a tube model. With the results of Ref.6 to hand, a similar development is now possible in the branched case, and will be described in a future publication. ... 

The formation of cotar none from cotar nine methine methiodide by the action of potash (IX—X) led Roser to represent cotarnine and its salts by the following formulae, the loss of a molecule of water in the formation of cotarnine salts being explained by the production of a partially reduced pyridine ring, which is fully hydrogenated in the reduction of cotarnine to hydrocotarnine. In the reverse process, oxidation of liydrocotarnine to cotarnine, Roser assumed the scission of the ring at the point indicated, with the formation of a hydration product, and oxidation of the latter to cotarnine thus —... 

M. Cates. Reptation of living polymers Dynamics of entangled polymers in the presence of reversible chain-scission reactions. Macromolecules 20 2289-2296, 1987. 

Most ester-forming reactions are reversible. Depending on circumstances, these reactions may be either undesirable side reactions, for example hydrolytic chain scissions occurring during processing, or useful reactions when chemical modification or polymer recycling is considered. 

New sources of RaSn- radicals that have been developed include the reversible thermal dissociation of bis(trialkylstannyl)pinacols (290-292), the )3-scission of /3-stannylalkyl radicals (293), and the photolysis of cyclopentadienyltin compounds (294). 

The trapping of alkyl, alkoxyl and alkylthiyl radicals by trivalent phosphorus compounds, followed by either a-scission or p-scission of the ensuing phosphoranyl radical, is a powerful tool for preparation of new trivalent or pen-tavalent phosphorus compounds [59]. However, the products of these reactions strongly depend on the BDE of the bonds, which are either formed or cleaved. For example, the addition of phenyl radicals on a three-coordinate phosphorus molecule occurs irreversibly, while that of dimethylaminyl (Me2N ) or methyl radicals is reversible, the amount of subsequent P-scission (formation of compound C) depending on the nature of Z and R (Scheme 25). For tertiary alkyl radicals and stabilized alkyl radicals no addition is observed (Scheme 25) [63]. 

These structural findings have important implications for the mechanism of heat-induced AR. Heating formalin-treated proteins at elevated temperatures (>100°C) results in the reversal of formaldehyde adducts and cross-links regenerating the native protein chemical composition without causing a loss of primary structure (chain scission). However, this reversal is accompanied... 

Hydrocarbons with up to 16 carbon atoms are detected in a typical alkylate (82). With the liquid acids, it was found that the oligomerization rate is higher for isoalkenes than for linear alkenes (49). The same is true for solid acids (14,83). Because of their tertiary carbon atoms, isobutylene and isopentene obviously react more easily with carbenium ions. This point can be inferred from the reverse reaction, (3-scission (see below), which is fastest for reactions of tertiary cations to give tertiary cations. In oligomerization experiments, the following pattern of... 

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