Stepwise kinetics

Researchers studying the stepwise kinetics of nitrogenase electron transfer using stopped-flow kinetic techniques have presented other scenarios. One hypothesis presents kinetic evidence that dissociation of Fe-protein from MoFe-protein is not necessary for re-reduction of Fe-protein by flavodoxins.13 These authors state that the possibility of ADP-ATP exchange while Fe-protein and MoFe-protein are complexed with each other cannot be excluded and that dissociation of the complex during catalysis may not be obligatory when flavodoxin is the Fe-protein reductant. This leads to the hypothesis that MgATP binds to the preformed Fe-protein/... 

In this chapter we will emphasize condensation polymers. Since most of these are formed from stepwise kinetics, we will also focus on this kinetic process. 

There is a large, but not total, overlap between the terms condensation polymers and stepwise kinetics and the terms addition (or vinyl) polymers and chain kinetics. In this section we will look at each of these four terms and illustrate the similarities and differences between them. 

The term stepwise kinetics, or step-growth kinetics, refers to polymerizations in which the polymer s molecular weight increases in a slow, stepwise manner as reaction time increases. 

FIGURE 4.2 System molecular weight for stepwise kinetics as a function of reaction time and reaction temperature T. 

The formation of polyesters from a dialcohol (diol) and a dicarboxylic acid (diacid) is used to illustrate the stepwise kinetic process. Polymer formation begins with one diol molecule reacting with one diacid, forming one repeat unit of the eventual polyester (structure 4.3) ... 

Thus, the reactants are consumed with few long chains formed until the reaction progresses toward total reaction of the chains with themselves. Thus, polymer formation occurs one step at a time, hence the name stepwise kinetics. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

The formation of PUs and polyureas typically occur through stepwise kinetics with the polymer backbones clearly containing noncarbon atoms. Yet, no by-product is... 

For stepwise kinetics what happens to chain length as time increases ... 

To date, the stepwise, kinetically controlled, classical synthesis is the most effective approach to highly annelated chiral Jt-systems. With significant improvements in asymmetric annelation methodologies, multi-step syntheses are likely to remain the main tool in the exploration of novel chiral structures. However, the development of novel synthetic methods will be essential for the preparation of polymers with extended helical-type, ladder-type connectivity of the Jt-systems. Important criteria are to minimize the density of defects in the ladder connectivity and to provide conjugation pathways circumventing at least some of the defects. 

Three typical types of foam column decay due to gravity can be distinguished linear, logarithmic and stepwise kinetics [32], Besides, the foam often decays as a whole after a certain induction period without any visible signs of volume reduction [41]. 

However, the kinetic dependence of foam column decay proposed by Schwarz appears to be better grounded [8]. No attempts to relate the constants characterising the rate of foam column destruction with both physicochemical characteristics of the foaming solution and foam structure have been reported. The stepwise kinetics of decay of the foam column is typical for aqueous as well as non-aqueous foams. Manegold [32] points out that coarse foams decay in larger steps than finely disperse foams. 

The reaction of A-methylimidazole with TPPFeCl proceeds rapidly according to the overall equation (34) to give the low-spin complex TPPFe(N-MeIm)2+ CH, but the stepwise kinetics could be studied in various solvents by stopped-flow kinetics. The rate... 

The reaction does not involve elimination of any small molecules, and thus according to Carothers could be classified as addition polymers. However, the polymers are structurally more similar to condensation polymers than to addition polymers. The repeating unit contains functional groups (or is heteroatomed). The formation of the two polymers also proceeds through stepwise kinetics. 

Ladder polymers produced from Diels-Alder reactions are formed through a stepwise kinetic process, yet no small molecules are eliminated. 

Although the overlap of terms is great, many exceptions exist. For example, the formation of polyurethanes typically occurs through stepwise kinetics. The polymers are classified as condensation polymers, and the backbone is heteratomed, yet no byproduct is released when the isocyanate and diol are condensed. The formation of nylon 6, a condensation polymer, from the corresponding internal lactam occurs through chain-growth kinetics. 

Operating a series of alternating additions of capsule to stop the oxidation reaction by encapsulation of [Ru(bpy)3] (OIT state) followed by additions of excess of tetraethylammonium competitive guest with concomitant release in solution of [Ru(bpy)3] (ON state), it was possible to operate a reversible switching of the photocatalytic activity of the complex, simply changing its solvation sphere without changing either the coordination on the metal or the experimental conditions (Fig. 7.5). Overall a stepwise kinetic profile for the... 

The relationship between degree of polymerization, DP, and reaction time varies according to the mechanism of reaction. For chain reaction kinetics, DP is approximately constant as the reaction time increases. Conversely, in stepwise kinetics DP increases steadily throughout the reaction. First order or pseudo-first order reactions give a linear relationship for plots of In(DP) vs. reaction time second order re-... 

These very interesting complexes of high-molecular-weight polymers have been the subject of many papers, and interest remains high today. In general, two types of complex formation can occur. These are described as stepwise complexation and one-to-one complexation (119), and the POE/PAA is of the latter type. A complex formed between poly(vinyl pyrrolidone) and a phenolic in an acetone/methanol mixture forms with stepwise kinetics, whereas with poly(ethylene oxide) and the phenolic in the same liquid system, it forms by one-to-one complexation. Formation of the latter complex can be followed with infrared spectroscopy because the ether oxygen absorption shifts from 1095 cm-i to 1070 cm i and the phenolic -C-O-absorption shifts from 1225 cm-i to 1250 cm T... 

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