Thermal clearing temperature

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

A closer look at the thermal behavior variation upon introduction of a second aryl ring (see Figure 8.5 for the behavior of the derivatives with a w-decyloxy chain) reveals very interesting features for the phenyl isocyanide complexes the melting and clearing temperatures decrease in the order Cl > Br > I. This is also the trend of the clearing points for biphenyl isocyanide complexes, but their melting temperatures follow the opposite trend that is, I > Br > Cl. 

In summary, [Au(alkynyl)(CNR)j complexes also show mesomorphic behavior but, in contrast to the halide-isocyanide derivatives, they are thermally less stable and usually decompose before or at the clearing temperatures. This thermal instability is thought to be associated with the gold-(alkynyl) bond and makes these compounds... 

Let us consider some aspects of thermal behaviour of LC polymers 45> (Fig. 3). In case of crystallizable polymers, which are mainly those containing mesogenic groups in the main chain, the LC state is observed from above the melting temperature (Tm) and up to the clearing temperature (Tcl), the melt displays anisotropy and may flow. The polymer thus behaves alike low molecular liquid crystals (Fig. 3 a), the viscosity of the former being, however, essentially higher. 

The difficulty with these polymers is that they usually do not melt without heavy thermal degradation and can hardly be dissolved. If melting or dissolving would be successful an extreme viscosity of the melt or solution above the clearing temperature had to be expected. 

The effect of substituents on the thermal properties of a series of aromatic azoxy polymers is shown in Table 3, Polymers 10-12, in which it is seen that the introduction of methyl groups into the mesogenic units resulted in a depression in both the melting point and the clearing temperature of the polymer. As can be seen from these data, the substitution position in the mesogenic unit is also important, but it will be necessary to have much more experimental results before the positional effect may be evaluated systematically. 

The polaron model was also used by Waragai and Hotta [121] to analyze temperature-dependent data on methyl-substituted oligothiophenes. While their data did not cover a sufficient temperature range to discern a clear temperature dependence of the mobility, they noticed that [j. represented a drain voltage dependence, which they attributed to the dependence given by Eq. (14.69). Accordingly, the temperature-dependent mobility could be fitted to a thermally activated law of the form where Etot = fc/2 - ys/F, y being... 

Several patterns, similar to that shown in Figure 6.9, were produced for different thermal treatment temperatures. These patterns were modeled according to the method we have just described, and we extracted, in particular, the size distribution functions of the tetragonal zirconia crystals. The resulting curves ate shown in Figure 6.17. Clearly, increasing the thermal treatment temperature significantly widens the crystal size distribution. 

The curves clearly illustrate the presence of an increasingly large exothermic reaction after the 244°C lattice transition of the ammonium perchlorate. In the 0.1% KC103 region of impurity, the heat evolved after ihe lattice transition was great enough to initiate complete thermal decomposition of the sample. This represents an effective 15(7" lowering of the thermal decomposition temperature of the pure material, which normally decomposes... 

A selection of monosubstituted and disub-stituted poly-(/ -phenylene-terephthalate)s is compared in Table 2. Poly(p-phenylene-terephthalate)s with methyl, methoxy, chlo-ro, or bromo substituents on either the hy-droquinone or the terephthalic acid moiety exhibit melting temperatures of 350 °C or higher. Thermotropic liquid crystalline behavior is observed in these samples, although it is in the range of thermal decomposition. A comparison of the mono- and diphenyl substituted polyesters reveals an important trend. The monosubstituted poly-(p-phenylene-terephthalate) with the phenyl substituent in the hydroquinone moiety melts at 346 °C, also forming a nematic melt up to a clearing temperature of... 

For these samples, the melting temperatures decrease on lengthening the spacer,as expected. However, the clearing temperatures appear to be more influenced and are drastically depressed (Figure 2). As a result the thermal persistence of the mesophase reduces and eventually disappears for the sample with a long alkyl chain (m= 12), in which dilution effects must play a role in inhibiting an ordered arrangement in the melt in addition to the increased overall chain flexibility. 

By contrast, no marked effect on clearing temperatures is observed. Consequently, sample 62 (n= 2) possesses a liquid crystal state expanding over an extremely wide thermal interval ( 200 C). 

Thermal transition, thermal behavior, clearing temperatures... 

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