Transition linking groups

Three strategies for the stabilization of these highly reactive heterocarbonyl species have proven to be successful (a) electronic stabilization by 7T interaction with heteroatoms such as nitrogen, oxygen, and sulfur via unsaturated linking groups, (b) kinetic stabilization by bulky substituents, and (c) coordination to transition metals. 

Squarate Complexes of Heavier Transition Metals and Lanthanides The Squarate Ion as a Linking Group Spectroscopy of Squarate Complexes Luminescence Studies Electrochemistry... 

The linking group makes an important contribution to the phase transition and physical properties such as the birefringence. The following linking groups have been well studied... 

The experimental results suggest that, for these polylmides, the most flexible system had the lowest wear rate. Of the three polylmides tested, PIB should have the most flexible chain, as indicated by the glass transition temperature, because of the presence of the oxygen linking group. Chain rigidity is approximated... 

FIGURE 7.6 Effect of the linking group on the odd-even character of the PM-type spacer. The transition entropy, 45ni/R, oscillates with the numher of methylene units (n) of the spacer. The upper three curves represent the dimer, and the lower two are for the monomer LCs. The filled symbols indicate the carbonate-type dimer, a ,ester-type dimer, a,< -bis[(4,4 -cyanobiphenyl)carbonyloxy]alkane (CB-n triangles), and the ether-type monomer LC, 4 -n-aUcoxy-4-cyanobiphenyl (nOCB squares). 

The urethane hard phase has a melting point of approximately 150 °C. The linking groups are selected to have glass transition temperatures which are below room temperature. The polyether/polyester rich phase is often referred to as the soft phase, reflecting the fact that at room temperature it has mobility, giving it... 

Table 24. Transition temperatures of compounds containing different linking groups (from Praefcke etal. [115]).

Terminal bond twins have been synthesized by Vorlander [223] in his studies on the influence of central linking groups on the mesomorphic properties of molecules. Figure 15 shows the pronounced alternation in the transition temperature in a series of compounds. This is due to the alternation of the molecules between the most elongated and the bent shape. 

Increasing the length of the molecule is a major contributor to increasing transition temperatures, and to aid this effect a linking group is often placed between the rings ... 

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