Some Special Effects

In the final analysis, the DCB mechanism has passed every test so far applied and is now the accepted mechanism for these reactions. The results leading to these conclusions are summarized in several articles.  

It is well documented that the ligand in the position trans to the leaving group has a significant influence on the rate of substitution reactions. This effect has been most thoroughly studied in Pt(II) chemistry, where the ordmiig of trans labilization is 

The sources of the trans effect are conunonly separated into two factors. The weakening of the bond to the leaving group by the bans ligand in the ground state of the reactant is now called the trans influence, or the thermodynamic trans effect. It can be assessed by strur ural, spectroscopic and thermodynamic properties of the reactant. The kinetic factor relates to the stabilization of the transition state by the trans ligand and is referred to here as the kinetic trans effect. It is determined from the kinetic properties of the system. 

The metal a orbital is empty and both T and X donate electrons into it. If the T ligand is a sbonger o donor, then the a bond to X will be weaker. The large trans effects of CH, and H are directly attributed to this situation. 

In classic studies to separate the o and rt effects, Parshall studied the F NMR chemical shifts for a series of T ligands in the following systems  

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Some Special Effects in the Flow of Filled Polymers... 

The endo isomer was the major product, but the proportion of the endo isomer decreased with temperature, from approximately 90% at 0 °C to 85 % at 60 °C [69]. Ge-dye et al. [71] performed the reaction in methanol solution under MW heating in a closed Teflon container and found that the product contained 79% of the endo isomer at an estimated temperature of 110 °C. A plot of temperature versus percent endo isomer is effectively linear between 0 °C and 60 °C and assuming it remains linear to 110°C, the product should contain 80% endo isomer at this temperature. Thus it was concluded that the change in product composition was due to the change in temperature rather than to some special effect of MW. At higher temperatures there is an increase in the proportion of the exo isomer, which is thermodynamically more stable than the endo isomer. 

The concentrations of the small-molecule reactant inside and outside are the same for soluble polymers unless there is some special effect responsible for attracting or repulsing the reactant from the polymer coils. Such situations are described in the remainder of Sec. 

This chapter is organized as follows. First a general discussion will be given on the basis of the configurational coordinate diagram (see Chapter 2). Then we will consider a number of different classes of luminescent ions. At the end some special effects, like afterglow and stimulated emission, are treated. 

The concentrations of the small-molecule reactant inside and outside are the same for soluble polymers unless there is some special effect responsible for attracting or repulsing the reactant from the polymer coils. Such situations are described in the remainder of Sec. 9-1. The concentration of a small molecule reactant inside the polymer coils can be lower than outside when one uses a poor solvent for the polymer. This results in lower local and overall reaction rates. In the extreme, a poor solvent results in reaction occurring only on the surfaces of a polymer. Surface reactions are advantageous for applications requiring modification of surface properties without affecting the bulk physical properties of a polymer, such as modification of surface dyeability, biocompatibility, adhesive and frictional behavior, and coatability [Ward and McCarthy, 1989]. 

In the study of swollen networks, two problems are of major importance The dependence of the stress-strain properties on the solvent or polymer fraction and the mking contributions to the free energy of the network or the elastic contribution to the chemical potential. Latest research seems to provide an improved insight into some special effects which are typical for swollen and completely crosslinked networks, and for unswollen (and swollen) incompletely crosslinked networks. The relaxation on the deformation dependence of topological constraints, which leads to a constraint release, is one of them. 

Examples of effects of substrate structure on the rates of nucleophilic substitution reactions have appeared in the preceding sections of this chapter. Additionally, some special effects will be covered in detail in succeeding sections. This section will emphasize the role steric effects can play in nucleophilic substitution reactions. 

The general effects observed in mixed polymer/surfactant systems have already been described in Chapter 4. Here only brief summaries and some special effects will be presented. 

Furthermore, to understand the results of LLE experiments in polymer solutions, one has to take into account the strong influence of polymer distribution functions on LLE, because fractionation occurs during demixing (1968KON, 1972KON, 200 IKON). Fractionation takes place with respect to molar mass distribution as well as to chemical distribution if copolymers are involved. Fractionation during demixing leads to some special effects by which the LLE phase behavior differs from that of an ordinary, strictly binary mixture, because a conunon polymer solution is a multicomponent system. 

Alkyl-substituted pyridines show some special effects in sulfonation thus, 2-, 3-, and 4-picoline always produce the corresponding 5-sulfonic acid ... 

In Fig. 2a are shown selected results for the density profile, p(x), at temperatures close to and above the critical temperature for different values of the chemical potential. Except for some special effects at the edges, p x) is as expected a linear function. 

In monochromatic two-photon photoemission, only one photon energy is used (Hva = hvi), which leads to some special effects. The most simple reahzation of the experiment is the use of only one laser pulse. In this case, the optimum temporal and spatial overlap is intrinsically ensured. If the laser beam is spHt and overlapped with a controllable time delay, the cross correlation (= autocorrelation) is symmetric and the distinction between pump and probe pulses becomes meaningless. For identical polarization of the two photon beams, the autocorrelation is dominated by interference effects, which permits a very precise determination of the time delay [29]. 

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