Processes bond scission

We examined the role of vector percolation in the fracture of model nets at constant strain and subjected to random bond scission, as shown in Fig. 11 [1,2]. In this experiment, a metal net of modulus Eo containing No = 10" bonds was stressed and held at constant strain (ca. 2%) on a tensile tester. A computer randomly selected a bond, which was manually cut, and the relaxation of the net modulus was measured. The initial relaxation process as a function of the number of bonds cut N, could be well described by the effective medium theory (EMT) via... 

These observations were the basis for the proposal that polymers, like ionic crystals, exhibit shock-induced polarization due to mechanically induced defects which are forced into polar configurations with the large acceleration forces within the loading portion of the shock pulse. Such a process was termed a mechanically induced, bond-scission model [79G01] and is somewhat supported by independent observations of the propensity of polymers to be damaged by more conventional mechanical deformation processes. As in the ionic crystals, the mechanically induced, bond-scission model is an example of a catastrophic shock compression model. 

In the absence of solvation mechanisms, the process of homolytic bond scission in organic compounds requires much less energy than heterolytic bond scission... 

The finite transit time (xr) in the high strain-rate region has important consequences on the scission yield curve. Since bond scission is a first-order process (Eq. 96), the degradation yield in a single pass experiment is given approximately by ... 

The decomposition of Ba(C104)2 has been the subject of several studies [858]. It is believed that during the initial acceleratory process, reactions are analogous to those cited above for Mg(C104)2. After a series of several successive bond scissions, the accumulating concentration of chloride product participates in the equilibrium... 

Since the suggested conversion process does not include a thermally promoted bond-scission step, the question arises of how the addition of hydrogen results in the bond breaking necessary for significant reduction in molecular weight. We have already noted that the nucleophilic action of the basic methanol system was sufficient to cleave diphenyl ether, and a similar route is available in the basic i-PrOH and C0/H20 systems. On the other hand, we showed in control experiments that strongly basic conditions alone were not sufficient for significant conversion of coal. 

Some polymers degrade readily when heated to the processing temperature by bond scission. In contrast, if the polymer has a completely aromatic structure (e.g., PES, PEEK) the thermal stability is high (greater than 400°C for short... 

At negative potentials EG can compete successfully with glycolate for adsorption sites, unless the EG concentration is too low, and this accounts for the lack of glycolate at the lowest EG concentration. The simultaneous appearance of glycolate and carbonate was taken by the authors as strong evidence for the existence of a common intermediate, identified as A in the scheme. It seems that A may desorb oxidatively or the second carbon may become bound to the surface if a suitable neighbouring adsorption site is available. Once this last process has taken place, carbon carbon bond scission occurs with complete oxidation to CO3". 

One of the most observed degradation pathways of non-ionic surfactants of ethoxylate type in the biochemical wastewater treatment process is the bond scission between the lipophilic alkyl chain and the hydrophilic ethoxylate moieties. The resulting ethoxylate compounds, PEG or PPG, are highly polar and are not quite easy to degrade, therefore often they can be observed in wastewater discharges. So, APCI— FIA-MS(+) product ion spectra of selected [M + NH4]+ ions, which were under suspicion as PEG (general formula HO—(CH2—CH2—0) H)... 

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

The process was shown to be more efficient when performed in the presence of some bromide salts whose oxidation allows lowering of the working potential and favors the S—S bond scission. Bromide mediated oxidation of bis(2-cyclopent-l-enylethane) and bis (2 -cyclohex-1 -enylethane) disulfide in AN/Et4NCl04/Et4NBr [130] undergoes the S—S bond cleavage followed by an intramolecular attack of the sulfenium cation on the double bond to give [3.3.0] and [4.4.0] bicyclic products. The anion Br also serves in the process as an... 

Unlike oxygen derivatives, thioesters of phosphor(III) acids (S-ethyldiphenylphos-phinite, S,S-diethylphenylphosphonite and triethylthiophosphite) undergo electroreduction atDME (dropping mercury electrode) in DME or AN [215]. An irreversible two-electron process results in the P—S bond scission. 

Since both of these reactions proceed through similar transition states, their rate constants have similar pre-exponential factors, and the AH, corresponds to activation energies. Consequently, at process temperatures, i.e., T X 1000°C or less, the ratio of the rate coefficient of reaction (20) to that of (21) will be the order 10 or more. Therefore, C-Cl bond scission, i.e., reaction (20), will always dominate the unimolecular decomposition of CH3CI. 

That is, the CH2 biradical formation process is far too endothermic to be of any significance compared to reactions (62) and (63). On the other hand, 2-C3H7 decomposes primarily by the following CH3CHCH2-H bond scission ... 

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