Chiral liquid crystal polymers

He XZ, Zhang BY, Meng FB, Tian M, Mu Q. Synthesis and characterizahon of chiral liquid crystal polymers containing donor-acceptor group. J Mater Sci 2010 45(l) 201-8. 

Chiellini, E., Galli, G., and Cioni, F., Chiral liquid crystal polymers with potential ferroelectric properties synthesis and characterization, Ferroelectrics, 114, 223-228 (1991). 

Liquid crystal polymers are usually made chiral in the same manner as for low molar mass liquid crystals, i.e., by incorporating a chiral moiety within the structure (usually as part of the terminal chain because of simplicity). Chiral liquid crystal polymers are commonly designed and synthesised to exhibit the chiral smectic C (S ) phase because... 

A particular position among such chiral liquid crystal polymers is held by the systems based on cellulose derivatives." They can be classified as cellulose ethers, namely hydroxypropyl cellulose, with some adjunctive ester functions localized normally on the ether branches. The idealized substituted cellobiose repeating unit is represented as follows ... 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

The synthesis of technologically interesting ROMP materials using (233) includes the preparation of molecular wires,701 liquid-crystal polymers,702,703 chiral supports for catalysis,704 redox-active macromolecules,705 photochromic materials706 and embedded clusters of CdSe.707 Polymers... 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

Another saturated tetrahydrofuryl core has found application as a component of liquid crystals. Cholesteric liquid crystal polymers are useful as photostable UV filters in cosmetic and pharmaceutical preparations for the protection of human epidermis and hair against UV radiation, especially in the range 280-450nm <2000DEP19848130>. Fused bifuran 81 is a suitable monomer for the preparation of these desired polymers as it contains the requisite characteristics of having more than one chiral, bifunctional subunit type which is capable of forming a cholesteric liquid crystal phase with a pitch of <450 nm. It also contains an achiral aromatic or cycloaliphatic hydroxyl or amino carboxylic acid subunit, achiral aromatic or cycloaliphatic dicarboxylic acids, and/or achiral aromatic or cycloaliphatic diols or diamines. Polymers prepared from suitable monomers, such as diol 81, can also be used as UV reflectors, UV stabilizers, and multilayer pigments. 

It is also known that in side-chain LC polymers the copolymerization of optically active monomers with mesogenic monomers, in the same manner as the mixing of optically active compounds with nematic low molecular weight compounds, can induce the formation of a cholesteric mesophase. Therefore, it is expected that inclusion of chiral spacers in main chain liquid crystal polymers, which would be nematic... 

De Feyter S, De Schryver FC (2003) Two-dimensional supramolecular self-assembly probed by scanning tunneling microscopy. Chem Soc Rev 32 393 Ernst K-H (2006) Supramolecular surface chirahty. Top Curr Chem 265 209 Perez-Garcia L, Amabrlino DB (2007) Spontaneous resolution, whence and wither from enantiomorphic solids to chiral liquid crystals, monolayers and macro- and supramolecular polymers and assemblies. Chem Soc Rev 36 941... 

In this chapter we will review the recent advances of supramolecular photochirogenesis in various confined media, excluding micelles, chiral solvents, liquid crystals, metal complexes, polymer matrices, clays, and crystals. Micelles are a typical supramolecular assembly with an internal hydrophobic core which shows a unique boundary effect, e.g., enhanced radical recombination of geminate radical pairs produced by ketone photolysis [26], but essentially no asymmetric photoreaction has hitherto been reported in micelles. Photochemical asymmetric induction in chiral solvents [27,28] and chiral liquid crystals [29,30] have been known... 

Fig. 30. The normal layered structure of the smectic A phase of a side chain liquid crystal polymer, and the proposed twisted structure of the layered smectic phase of a chiral side...

As noted earlier form the work of Freidzon [41], side chain liquid crystal polymers derived from cholesterol can also apparently exhibit TGB phases. However, work on non-steroid systems also reveals that TGB phases can also be formed in typical chiral polymers. For example the polymethacrylate, structure 16, exhibits chiral nematic, TGBA , smectic A , and ferroelectric smectic phases. 

Another approach is to build up a polymer network aroimd a complex. Pavlov et al. studied a chiral liquid crystal matrix, in which the Wilkinson catalyst, [RhCl(PPh3)3] was embedded in cholesteryltridecanoate and catalyzed the enantioseleetive hydrogenation of 2-acetamidocinnamic acid into A-acetylphenylalanine with an ee of 60% (see details in Chapter 3). 

Similar to the other liquid crystal polymers described in Section 11.16, these materials offer the possibiUty of locking the chiral nematic phase into the glassy state by rapid supercooUng to temperatures below T. This leads to a preservation of the structure and, of course, the reflected color, thereby leading to the formation of stable, lightfast, monochromatic films, when suitable systems are used. 

To our knowledge, this is the first example of the coexistence of both twisted smectic and cholesteric phases in thermotropic liquid crystal polymers. Previous preparations of thermotropic polymers by the use of chiral derivatives both incorporated in the macromolecular backbone and pendant to it as side chain substituents (comb-like polymers) resulted in either cholesteric or smectic " polymeric products. 

Similar through-space asymmetric polymerization from achiral mono-, di-, or tri-thiophenes and pyrrole monomers was also achieved by the use of cholesteric liquid crystals as an asymmetric reaction solvent [19]. As no reaction occurred between the molecules of the liquid crystal and the monomers, the chiral morphology of the polymers (which have no chiral substituent) is considered to derive from the asymmetry produced by the chiral liquid crystal medium during polymerization. Heat treatment of the polymer causes disaggregation and a loss of chirality, and polymers prepared in this way exhibit an exiton splitting signal in the circular dichroism spectra in the absorbance region of the polymeric backbone they also display a circular polarized luminescence. A representative example is shown in Scheme 8.2 [19]. 

Le Bamy, P, and Dubois, J. C., The chiral smectic C liquid crystal side chain polymers. in Side Chain Liquid Crystal Polymers (C. B. McArdle, ed.), Blackie, Glasgow, 1989, pp. 130-158. 

Polymer 21 is a typical ferroelectric liquid crystal poly(acrylate) with a rather simple chiral terminal chain commonly seen in chiral liquid crystals. A range of chiral liquid crystal phases are exhibited by polymer 21, complicated by the reported presence of two phases. The recent interest in the antiferroelectric phase (see above) has been... 

Chirality in side chain liquid crystal polymers can also be introduced by constructing a chiral centre as part of the spacer moiety or as part of the polymer backbone, but these types of liquid crystal polymer are relatively rare. 

Chiral main chain liquid crystal polymers have also been prepared. These polymers have largely been designed as pigment-less coatings that change colour with changing temperature. 

The characteristic functionalities of naturally occurring polymers are, in most cases, related to their specific chiral structure. In nature, proteins, nucleic acids, and polysaccharides are constructed of readily available chiral monomers such as sugars and amino acids. Both natural and synthetic chiral polymers are finding application as chromatographic supports, polymeric reagents and catalysts, chiral membranes, and materials for preparation of cholesteric liquid crystal polymers (471,472). 

The reasons for this preference are still under debate and the process is not yet predictable. But spontaneous resolution can also afford chiral liquid crystals, monolayers, and supramolecular polymers and is likely controlled by subtle noncovalent interactions including crystal packing forces and crystallization kinetics. Obviously, it would be both fascinating and exhemely useful to be able to predict and control this process. 

Optically active polymers are important functional materials for several industrial and bio-m ical applications and are extensively used as chiral catalysts for asymmetric synthesis, packing materials of chromatographic columns and chiral materials for the preparation of liquid crystal polymers (7). Polymers such as poly hydroxy alkanoates (PHAs), naturally occurring microbial optically active polyesters, are important materials in biomedical applications owing to their biodegradability (2). In synthetic polymer chemistry, synthesis of optically active polymers has been one of the most challenging tasks. Most synthetic chiral polymers are prepared from optically pure starting materials which are, except when isolated from nature, in limited supply and difficult to prepare (7, 3). 

In a polymer, the rod-like structures can be attached as side groups—side-chain liquid crystal polymers or with the skeletal backbone—main chain liquid crystal polymers (Donald et al. 2006). The latter usually exhibit liquid crystal characteristics at elevated temperatures, while some side-chain liquid crystal polymers exhibit liquid crystalline order at room temperature. A number of more ordered smectic phases can be observed as well as chiral, nematic, and smectic phases (Donald et al. 2006). 

An interesting area, which involves chiral liquid crystal properties, is that of temperature sensors, used, for example, in the diagnosis of skin cancer, as well as in peripheral blood circulation problems. Another application in the medical field is represented by nematic elastomer films or fibers of liquid crystalline polymers with mesogene in the side chain, that can be used in the manufacture of muscle prosthesis [28]. To optimize their performance for different applications, the current knowledge on the relationship between the structure and properties of liquid crystals should be extended. In addition to further development of liquid crystals and their applications, the liquid crystal theories represent a sound basis for other areas of interest. For example, liquid crystals can be used as model compounds for the study of molecular interactions and of their effects on self-organization in supramolecular chemistry. 

Some Aspects of Polymer Dispersed and Polymer Stabilized Chiral Liquid Crystals... 

Polymer Dispersed and Stabilized Chiral Liquid Crystals 377... 

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