Chiral combined polymers

Figure 4. Schematic representation of a network prepared from chiral combined polymers [5].

FIGURE 12 Typical structures of combined smectic C polymers (a) and cross-linkable chiral combined polymers (b). 

Chiral lc-polymers can be prepared by a proper functionalization of lc-polymers with chiral and reactive groups. These elastomers are interesting, because they combine the mechanical orientability of achiral lc-elastomers with the properties of chiral lc-phases, e.g. the ferroelectric properties of the chiral smectic C phase. The synthesis of these elastomers was very complicated so far, but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable chiral lc-polymers can be prepared via this route. 

The first chiral combined lc polymers prepared for this purpose showed the desired cholesteric and chiral smectic C phases only at high temperatures (8) (the melting point was always above 100°C). By using lateral substituents (see Figure 3) it is possible however to suppress the melting temperature and to obtain polymers with a glass transition temperature of about room temperature, without losing the cholesteric and chiral smectic C phases (9). 

Due to the interesting LC properties of combined LC polymers (i.e. broad LC phases, and the occurrence of different smectic phases and a nematic phases at different temperatures) and their intermediate nature between that of side-chain and that of main-chain polymers, a lot of research has been undertaken on these materials. Most of the research has been directed towards the preparation of cross-linkable polymers and LC elastomers [3-11] and of chiral combined LC polymers [4, 6, 7, 9, 12-16]. 

Because of the interest in chiral LC phases in general, which give rise to selective reflection of light (cholesteric phase) or fer-roelectricity (chiral smectic C phase), chiral combined LC polymers were prepared quite early on [4]. Polymers with cholesteric and chiral smectic C phases could be prepared easily. As these polymers were synthesized using to the polycondensation process shown in Scheme 1, the chiral groups had to be selected carefully in order to prevent racemization during polycondensation [4, 7, 12, 13]. 

Scheme 2. Synthesis of chiral combined LC polymers by esterification of chiral acids with a preformed polyphenolic polymer. DCC, dicyclohexylcarbodi-imide.

Table 1. Phase assignment of chiral combined main-chain/side-chain polymers.

Since the synthesis of the first chiral smectic C side chain LCP by Shibaev et al. [6], chemists over the last ten years have considerably extended that field. Now, the SmC mesophase can be exhibited by a variety of polymeric materials including homopolymers, copolymers and terpolymers, oligomers, combined polymers, and cross-linked polymers. 

The investigation of combined FLCPs was initiated by Zentel et al. [91-93] as a part of their approach to ferroelectric LC elastomers [94]. Figure 12 shows typical structures of combined FLCPs and cross-linkable chiral combined LC polymers. Poths et al. [67] used that approach to synthesize combined polymers with axially chiral mesogenic side groups (similar to the acrylic side-chain polymer above). The smectic C structure of polymers has been identified by optical microscopy and x-ray data, but no ferroelectric properties of the polymers have been reported yet. 

So far, three types of homochiral metal-organic polymeric assembhes have been developed for this purpose by Ding and coworkers. In the most straightforward method, an enantiopure ditopic (or polytopic) chiral ligand is copolymerized with a catalytically active metal ion to give a homo-combination polymer that may... 

A mixture of 1.4 g (10 mmol) of 4-chlorobenzaldehyde and 0.71 g (5 mol %) of the chiral polymer E is stirred in 10 mL of dry toluene for 15 h, under a dry nitrogen atmosphere, to form the Schiff base. After cooling to 0lC, 15 mL (15 mmol) of 1 M diethyl/inc in hexane is added and the mixture is stirred for a further 24 h at O C. 1 N HC1 is then added dropwise at O C, and the chiral polymer is removed by filtration. The polymer is washed several times with 11,0 and Et,0. The aqueous layer is separated and extracted with Et20. The combined organic layer is dried over MgS04 and concentrated under reduced pressure. The crude product is purified by column chromatography (silica gel, CHC1,) yield 1.61 g (95 %) 99 % ee [a]2,0 —23.9 (r = 4.93, benzene). 

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). 

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