Lateral substituent

Discotic Phases. Molecules which are disk-shaped rather than elongated also form thermotropic Hquid crystal phases. Usually these molecules have aromatic cores and six lateral substituents, although the predominance of six lateral substituents is solely historical molecules with four lateral substituents also can form Hquid crystal phases. Although the flatness of these molecules creates a steric effect promoting alignment of the normal to the disks, the fact that disordered side chains are also necessary for the formation of these phases (as is often the case for Hquid crystallinity in elongated molecules) should not be ignored. 

Aromatic polyesters constitute an important class of main-chain liquid-crystalline polymers, but present the inconvenience of their reduced solubility and very high transition temperatures (sometimes not detected before the degradation of the sample). Their processability can be improved in several ways [2,3], e.g., reduction of the rigidity of the mesogen, lengthening of the spacer, or introduction of lateral substituents. 

The disruption of chain regularity by the introduction of lateral substituents or kinks on repeating units is a supplementary means to decrease the melting temperature of aromatic polyesters.72 This is illustrated in Table 2.9, where the melting temperatures of unsubstituted and methyl-substituted aromatic-aliphatic and aliphatic acids are reported. Regularity disruptions often cause significant... 

Introduction of bulky lateral substituents on monomer units to increase interchain distance and prevent close packing in polymer crystal. The use of unsymmetrically substituted monomers, resulting in a random distribution of head-to-head and head-to-tail structures in polymer chains, further helps in disrupting regularity. Some examples of substituent effects are given in Table 2.16. 

The combined results of kinetic studies on condensation polymerization reactions and on the degradation of various polymers by reactions which bring about chain scission demonstrate quite clearly that the chemical reactivity of a functional group does not ordinarily depend on the size of the molecule to which it is attached. Exceptions occur only when the chain is so short as to allow the specific effect of one end group on the reactivity of the other to be appreciable. Evidence from a third type of polymer reaction, namely, that in which the lateral substituents of the polymer chain undergo reaction without alteration in the degree of polymerization, also support this conclusion. The velocity of saponification of polyvinyl acetate, for example, is very nearly the same as that for ethyl acetate under the same conditions. ... 

Note See Fig. 35. The mesogenic side-groups can be attached either as lateral substituents to the backbone mesogenic moieties that are connected to each other either (a) directly or (b) by spacers or (c) they can be attached to the spacer incorporated into the main-chain. 

IR spectroscopy is not only useful for determining the chemical constitution of polymers. It additionally provides profound information on chain orientation and on the orientation of attached lateral substituents of polymers. In this case, polarized IR radiation is applied which is only absorbed by an IR-active bond if the plane in which the electrical field vector E of the IR beam oscillates is parallel to the transition dipole moment p of the vibration to be excited. If, on the other hand, the transition dipole moment p is perpendicular to the electrical field vector E of the IR beam no absorption is observed. Using this effect, the degree of orientation of a polymer sample (film, fiber) can be estimated by comparing the intensity at maximum /(11) and at minimum I ) absorption, i.e., the dichroic ratio. 

Chen Y-Y, Lin H-C. Synthesis and characterization of Ught-emitting main-chain metaUo-polymers containing bis-terpyridyl ligands with various lateral substituents. J Polym Sci Part A Polym Chem 2007 45 3243-3255. 

Complexes resulting from an aromatic isonitrile and an aliphatic acetylide (Figure 7.8) are less stable, but the use of lateral substituents allows the formation of SmA and N phases at low temperatures [9]. 

The helical structure is largely caused by crowding of the bulky lateral substituents, pendant on each backbone carbon atom. In addition, if not relieved by tautomeric structural changes, the accumulation of vicinal dipoles, as are known to destabilize vicinal triketones, should cause a tendency for a molecule to assume a progressively twisted conformation that would allow an orthogonal rather than a parallel situation of neighboring dipoles. The core, then, of the cylinder is made up... 

Fig. 32 Selected examples of rod-like mesogens with semiperfluorinated chains at both ends and additional bulky lateral substituents (77° C) [237-239]...

A variety of phase geometries (p6mm, p2mg, p2gg, c2mm) was observed depending on lateral substituent and complexed salt. Details can be found in [55, 118-120],... 

When the lateral substituents were changed from ether to gallic ester groups, a columnar hexagonal phase was observed for 112, while complexation with KI destroyed the mesomorphic behavior [121] (Scheme 58). The presence of ion pairs in the complex presumably leads to unfavorable interactions between neighboring molecules leading to non-mesogenity. 

FIGURE 18. Schematic comparison of dioxin, a polychlorobiphenyl (PCB), and thyroxine, emphasizing the common features of an accessible planar face for binding to the Ah receptor, and lateral substituents to facilitate binding to a nuclear receptor... 

These problems were eliminated by the use of modified PPV derivatives ", such as poly[2-methoxy-5-(2-ethylhexyloxy)-4-phenylene vinylene] (MEH-PPV) compound 2, see Table 6.1, which are soluble in organic solvents. The presence of lateral substituents in the polymers 3-6 shown in Table 6.1 induces a lower... 

The PPV derivatives (70-74) collated in Table 6.14 incorporate an electron-withdrawing group on the aromatic ring, either as a lateral substituent on the ring or as an heteroatom (N) within the ring, instead of on the vinyl linkage. All of these polymers exhibit a higher electron affinity than... 

Monomers decorated with chiral side-chains can induce a transfer of chirality at the supramolecular level. Indeed, several examples show that disklike molecules, designed to pile up into long reversible columns, form in fact helical columns, driven by the favorable packing of the chiral lateral substituents [30,32,93,94,99]. This chiral packing effect is strong enough to induce a chiral amplification a low amount of chiral monomer mixed with a non-chiral monomer can still drive the formation of helical columns. 

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