Chain dissociation

The small and weakly time-dependent CPG that persisLs at longer delays can be explained by the slower diffusion of excitons approaching the localization edge [15]. An alternative and intriguing explanation is, however, field-induced on-chain dissociation, a process that does not depend on the local environment but on the nature of the intrachain state. The one-dimensional Wannier exciton model describes the excited state [44]. Dissociation occurs because the electric field reduces the Coulomb barrier, thus enhancing the escape probability. This picture is interesting, but so far we do not have any clear proof of its validity. 

During polymer chain growth, a back-biting process can lead to cyclic carbonate formation. In general, this process is more facile for aliphatic epoxides than for alicyclic epoxides and when the growing polymer chain dissociates from the metal center (Scheme 3). 

In the case of platelet-derived factor XIII, the resultant product (a )2, is the activated form. Thrombin action on plasma-derived factor XIII generates an a 2jfi2 dimer, which is devoid of transglutaminase activity. However, in the presence of Ca +, the a P chains dissociate, yielding the biologically active a 2. 

Ionizable groups incorporated in the chain dissociate into fixed charges and counterions, whose existence causes osmotic swelling pressure, give rise to the swelling of the gel. We assumed the following equation based on the van t Hoff equation ... 

Side chain dissociates to -C00 at physiologic pH Side chain is protonated and generally has a positive charge at physiologic pH... 

Fibrin polymers are responsible for the fibrin-dependent enhancement of Factor XIII activation (Greenberg et al, 2003). The mechanism for this effect involves the formation of a tight ternary complex between fibrin, Factor XIII, and thrombin, accompanied by a conformational change of Factor XIII that exposes the active site, after which Factor XHIa remains bound to fibrin. However, the B chains dissociate, which is necessary to expose the active site cysteine of plasma Factor XIII. Platelet Factor XIII without the B chains, is more rapidly activated by thrombin than plasma Factor XIII because of the time that it takes for the B chains to dissociate. 

The location of the oxime group in the product in Scheme 4 depends upon where the methyl group is located, but the yields can be high (79%). Irradiation of 2-methoxyfuran also induces side-chain dissociation and the formation of lactones (i.e., methylbutenolides).21 Here the abundant formation of ethane testifies to the participation by methyl radicals. Such reactions are too complex for synthetic purposes. This is true also of 2-acetoxyfuran photochemistry in which the photo-Fries reaction is not observed.21 Curiously, furan itself sensitizes the photo-Fries reaction in benzenoid esters.22... 

Our group previously reported that radiation induced crosslinking reactions in polysilane derivatives are mainly promoted by side-chain dissociated silyl radicals, and that the predominant reaction is determined by the radical concentration in the ion tracks [14,20,21]. Thus, the distribution of crosslinks in an ion track is expected to obey the equations (2) and (3) given in the previous section, and the crosslinking reaction also gives cylindrical poly(methylphenylsilane) (PMPS)-based nanowires with fairly controlled sizes as shown in Fig. 2. Using the reported value of G(x) = 0.12 derived from... 

In tellurium a boundary in the liquid state has been indicated by Deaton and Blum (22) which extends from the temperature at which tellurium begins to show metallic conduction at 1 atm (943°K) (41) to the neighborhood of the melting curve maximum (22) and is postulated to represent chain dissociation. Above this boundary metallic type behavior appears to dominate the conductivity (22). Controversy over this boundary is apparent also as Stishov (44) reports only semiconducting behavior, rather than metallic, in the liquid tellurium high pressure fields and states that the boundary intersects the liquidus beyond the maximum. 

According to (5-134), to permit the long-chain dissociative attachment process, the ionization degree should be veiy high ... 

If one end of an active chain dissociates from a junction due to thermal motion, or a tension caused by the external force, the chain becomes dangling and relaxes to an equilibrium state after the single-chain relaxation time r, which is of the order of the Rouse relaxation time tr = /()7t k T)n in the unentangled regime. We... 

The initial distribution is therefore not Gaussian if the chain dissociation rate p, or recombination rate a, depends on the end-to-end distance. 

The first term Vg (t) gives the number of chains that were initially active and remain active until time r. It is therefore a steadily decreasing function of the time it goes down to 0 at r = oo in most cases. It can reach a finite value at t = oo when the chain dissociation rate P(r) vanishes in a certain finite region of r. 

Let us next consider the special case where the chain dissociation rate (r) is a constant. This limit is called the Green-Tobolsky limit (GT limit). Since r)= o iP ir) = 0), we And from (9.70a,b)... 

Because the reactive group is attached on the chain end, it is pulled by the tension / from the chain. Hence, the potential barrier for the associative group to dissociate is effectively reduced to A — /a. The chain dissociation rate is therefore enhanced to... 

Fig. 9.4 (a) Modulus-frequency master curve for the chain dissociation rate p r) = The... 

Fig. 9.17 Nonlinear stress relaxation of the transient network model with a quadratic chain dissociation rate under a constant shear deformation for y = 0.5. The decay rate is fixed as (a) /3q = 0 and (b) /3q = 1. The total number Ve of active chains and the number Vg of chains that remain active from the initial state are shown on a logarithmic scale. These are normalized by the stationary value of Ve. The shear stress hxy, the first normal stress difference N, and the second normal stress difference N2 are shown in the unit of Ve B T. (Reprinted with permission from Ref. [19].)...

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