Zipper mechanism

The observation of large amounts of polyfluorinated compounds dissolved in the HF-phase was claimed not to be peculiar to these reactions but was said to be a normal part of all ECF reactions, and as such, said to contradict the zipper mechanism production of perfluorinated compounds, a pathway regarded as a support of the ECbECN process [62,172]. 

In the argument against the ECbECN concept, much, perhaps too much, store has been placed by critics on observations which seem to undermine the zipper mechanism in the apparent belief that this mechanism is axiomatic with the ECbECN as a whole. However, as has been shown, changing physico-chemical properties of progressively fluorinated products inevitably result in different adsorptive forces for these compounds, with possible desorption from the electrode as a consequence this situation could apply equally well with products formed via a carbocation mechanism as with radically generated compounds, even though, as originally stated, the implication is otherwise. 

The complex formation of PECs and PE-surfs is closely linked to self-assembly processes. A major difference between PECs and PE-surfs can be found in their solid-state structures. PE-surfs show typically highly ordered mesophases in the solid state [15] which is in contrast to the ladder and scrambled-egg structures of PECs [2]. Reasons for the high ordering of PE-surfs are i) cooperative binding phenomena of the surfactant molecules onto the polyelectrolyte chains [16-18] and ii) the amphiphilicity of the surfactant molecules. A further result of the cooperative zipper mechanism between a polyelectrolyte and oppositely charged surfactant molecules is a 1 1 stoichiometry. The amphiphilicity of surfactants favors a microphase separation in PE-surfs that results in periodic nanostructures with repeat units of 1 to 10 nm. By contrast, structures of PECs normally display no such periodic nanostructures. 

Another laboratory invoked the role of water in bread staling involving a zipper mechanism that results in the formation of interchain crosslinks 492 Moisture redistribution from crumb to crust played a significant role in crumb firming and amylopectin recrystallization for breads stored for more than seven days 493 According to Piazza and Masi494 to inhibit staling it is more important to slow the dehydration phenomena than it is to increase the initial moisture content in the bread. 

Marvel and Horning (6) suggested an allyl (zipper) mechanism. More is known about the mechanism of PVC stabilization with tin compounds as Kenyon (5) has shown. In this case, it could be proved with Cu-labelled dibutyltin diacetate, that only the butyl groups react with the radicals, and the rest adds on the HC1. 

A number of studies have been designed to determine the nature of the mechanism of proteolysis with a specific enzyme and substrate. Ginsberg and Schachman (1960a,b) concluded that chymotryptic hydrolysis of insulin probably proceeds by the all or none mechanism, whereas peptic hydrolysis of ribonuclease follows a zipper mechanism. 

Computer programs have been used to correlate the degradation kinetics of poly(vinyl halides) assuming a kinetic model that is based on the zipper mechanism. Best fit values of the parameters of the kinetic model allow reproduction of a degradation with an error that is usually less than 0.6% per point. 

The two kinetic models that effectively correlate data for the degradation of vinyl polymers were developed on the basis of the zipper mechanism and differ only in the approximations used to account for the premature termination of zip chains. Although the equations are complex in appearance (3), they are based on the relatively simple assumptions of the zipper mechanism. Chains are initiated as a certain fraction of chains per second, k., and unzip at a certain fraction of a started chain per second, R.. 

Thus, first order kinetics and zero order kinetics appear as special cases of the more general kinetics which have been derived on the basis of the zipper mechanism. The zipper model does not invoke the steady state assumption but allows the number of producing sites to increase during the acceleratory period as more zip chains start production and to decrease during the de-celeratory period as more chains are terminating than are starting. 

Before the implications of the zipper mechanism in terms of reproducible data were fully appreciated, efforts to attain reproducibility commensurate with the quality of the apparatus and the method of operation led to hundreds of degradation runs. Some of these runs... 

The "zipper" mechanism is confirmed by the work of Baum and Wartman (27) who found that ozonolysis followed by hydrolysis of partially degraded PVC did not appreciably reduce its molecular weight. If the HCl loss had occurred in a random fashion within the polymer chain, a considerable drop in molecular weight would have been expected. In addition, when the resin was mildly chlorinated... 

The role of contact angles in LB film deposition was outlined by Bikerman (1939) who recognized the need of hydrophobic surfaces, i.e. contact angles 4>s > 90° for successful immersion deposition (X-type), and of hydrophilic surfaces, i.e. contact angles < s 90° for successful removal deposition (Y- or Z-type). Bikerman (1939) used a simple geometrical argument, essentially the zipper mechanism that has been accepted as the basic deposition mechanism (Roberts, 1990). 

The dehydrohalogenation of poly(vinylhalides) is important because of its presumed relationship to thermal stability. Most reports agree that PVC and PVDC dehydro-chlorinate by a Zipper Mechanism but kinetic studies have not followed the implications of that mechanism. The rate of PVC dehydrochlorination has been reported to increase, decrease, and remain constant with time and the catalytic effect of hydrogen chloride has not always been observed. PVDC has been reported to follow first order kinetics, but the acceleratory phase of the dehydrochlorination was not adequately accounted for. 

PVC, PVDC, and PVF follow acceleratory dehydrohalo-genations and a kinetic model based on the Zipper Mechanism gives excellent correlation of data over the entire ranges of decomposition for all three polymers. The time for a PVC chain to unzip depended upon the length of the chain and an initiation mechanism at a site at or near a chain end seemed reasonable. 

Vinylidene terminal groups can be formed by disproportioning they cause degradation by a zipper mechanism that reaches its maximum degradation rate at 280 °C [620]. Small amounts of statistically incorporated methyl acrylate disrupt this mechanism and increase thermal stability. 

At elevated temperatures, statistical chain cieavage is the dominant reaction its radicais initiate secondary reactions. Intramoiecuiar radical-hydrogen transfer that leads to depolymerization ( zipper mechanism ) releases monomers, dimers, and trimers. Figure 5.172. Intermolecular hydrogen transfer causes degradation in another polymer chain. Figure 5.173. 

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