Terminal groups cyano

A number of experiments performed since the late 1970s [11-13, 301 found a variety of smectic phases with a broken head-to-tail symmetry among mesogens possessing strongly polar cyano or nitro terminal groups (Fig. 6). 

Using other diazo compounds [di(3-hydroxybutyl)-2,2 -azobisisobutyrate, di(2-hydroxypropyl)-2,2 -azobisisobutyrate and di(2-hydroxyethyl)-2,2 -azobisisobuty-rate] hydroxytelechelic polybutadienes containing primary or secondary hydroxyl terminal groups were prepared 32). The decomposition kinetics of di(4-hydroxybutyl)-2,2 -azobisisobutyrate and di(3-hydroxybutyl)-2,2 -azobisisobutyrate is faster than that of 4,4 -azobis(4-cyano-n-pentanol). Usually, at the same temperature and in the same solvent, the decomposition of di(x-hydroxyalkyl)-2,2 -azobisisobutyrate is faster than that of AIBN 35). The solvent polarity has no effect on the decomposition kinetics. 

The terminal group primarily contributes to the dielectric anisotropy and the refractive indices, which in turn affects the threshold voltage and optical properties in display applications, respectively. Common terminal groups are alkyl, alkoxy, cyano, nitro, isocyanate (NCS), sulfide and halides such as F, Cl, CF3 and OCF3. 

Nevertheless, a more efficient approach is to attach an electron-accepting atom or radical directly to the chiral carbon atom. Chiral acids and alcohols containing chlorine at the chiral carbon atom can easily be produced from natural amino acids via the Sandmeyer reaction they are widely used in the synthesis of FLCPs [25,26,49,52-54]. A cyano group [55,56] or a perfluorinated alkyl chain [21,57] can also be attached to the chiral center. Lactic acid is a natural chiral product as well, and its derivatives are widely used as chiral terminal groups of FLCPs [27,58-61]. The search for new chiral structures with high transverse dipole moments for FLCPs has resulted in the successful application of heterocycles such as 1,3-dioxolanes [39,62] and oxiranes [63,64]. 

Like cyclohexyl systems, the bicyclo-[2.2.2]octanes are usually only prepared with an alkyl terminal group and the principal route to the central intermediate (l-alkylbicyclo[2.2.2]octane with a 4-hy-droxy-,4-methoxy- or 4-bromo-substituent) is shown in Scheme 3 [45, 46]. All three intermediates can provide the 4-alkylbi-cyclo[2.2.2]octane-l-carboxylic acids (although the hydroxy or methoxy route is more direct) and the bromo compound can be used in Friedel-Crafts alkylation of aromatic compounds. The main examples of bicyclo[2.2.2]octane systems are cyano-aryl esters (3.2, X = CN [47-49], alkyl- and alkoxy-aryl esters (3.2, X = R, OR) [48,... 

Many of the earlier schemes in this section show examples of the synthesis of alkyl, alkoxy and cyano terminal groups alkyl and alkoxy groups are also conveniently generated in alternative ways from halides and phenols as shown in Scheme 8. If the alky-nyl coupling shown in Scheme 8 for 8.1 is carried out with the phenolic triflate 8.2, then subsequent reduction of the product to the alkyl group gives a way of creating an alkyl unit at a phenolic site. 

In addition to these, a number of three ring esters which contained the more polar cyano group as a terminal group were prepared and evaluated (compounds 59 and 60) the thermodynamic properties of a selection of these materials is shown below [64]. 

The most extensively studied azo compound is azobisisobutyronitrile (AIBN). AIBN is used to synthesize telechelics with cyano end groups according to Scheme 24. For example, telechelic PE was synthesized in t-butanol to yield a functionality of 1.7. " Using gas chromatography (GC) and mass spectrometry (MS), the three products (31) 33) were identified. In other solvents such as methanol and benzene, some of the terminating groups were fragments of the solvent. 

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