Shapes of mesogens

These are liquid-crystalline polymers consisting of an alternate sequence of rigid and flexible molecular sections. Virtually aU segmented-chain liquid-crystalline polymers are polycondensated. 

The shapes of mesogens are classified into three types rodlike, disclike, and boardlike. Among the three, the first two are more common than the third. 

Rodlike mesogenes have an elongated anisotropic geometry which allows for preferential ahgnment along one spatial direction. They are the core of liquid crystals. At either end are terminal groups, one of which is usually a long alkyl, acyl, or alkoxy chain. 

Discotics are flat disclike mesogens consisting of a core of adjacent aromatic rings. This allows for mesophases based on columnar ordering (see next section). 

Boardlike molecules do not show rotational symmetry about any axis at all, nor about the long axis as in rods or about the short axis as in discs. 

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In contrast to older models of the classical molecular shape of mesogens, some meso-gens are star-like oligomers with flexible central units. 

Since such molecules either lack shape anisotropy or high polarisability and constitute molecules of unusual shape with voids between their arms, this family of mesogens is considered to be non-conventional. In the last decades several reviews summarised most of the unconventional mesogens including the small subgroup of... 

Closing this section, a new metathesis-based method for the preparation of strictly alternating copolymers should be mentioned. Alternating diene metathesis polycondensation (ALTMET) was used to prepare a MCLCP consisting of an alternating architecture of a calamitic (linear rod-like core) and a banana-shaped (bend-shape core) mesogen (Fig. 24). 

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