Backbones SCLCP

SCLCPs combine liquid crystalline properties and polymeric behavior in one material. If the mesogenic unit is fixed directly to the polymer main chain, the motion of the liquid crystalline side chain is coupled with the motion of the polymer backbone, preventing the formation of a LC mesophase. Therefore, Finkelmann and Ringsdorf proposed that the introduction of a flexible spacer between the main chain and the mesogenic unit would decouple their motions, allowing the mesogenic moiety to build up an orientational order [29,30]. 

Cho et al. described the synthesis and polymerization of 4,8-cyclododeca-dien-l-yl-(4 -methoxy-4-biphenyl) terephthalate VIII [54,55]. Polymerization was carried out with WCl4(OAr)2/PbEt4. The double bonds in the polymer backbone were subsequently hydrogenated with H2/Pd(C), leading to a SCLCP with a fully saturated hydrocarbon backbone. This polymer system had a very flexible polymer backbone but a stiff connection between the main chain and the mesogenic unit. The distance between two adjacent side chains was about 12 methylene units. This very flexible main chain allowed the polymer to organize into a LC mesophase. Both polymers - the unsaturated and the saturated -showed smectic liquid crystalline mesophases with almost the same transition temperatures (see Table 5). 

In this chapter we want to discuss the correlation of the mesophase behavior of a cyanobiphenyl-based SCLCP with its backbone structure. As shown before, the backbone structure, the spacer lengths, and the mesogen density per repeat unit have great influence on the LC mesophase evolved. Ligure 8 shows some examples of backbone structures bearing the cyanobiphenyl-moiety that have been reported in literature. The above-mentioned ROMP-derived polymers poly-(II-n) [39],poly-(IV-n) [42,47],poly-(VI-n) [41],andpoly-(VII-n) [53] will be compared with each other and with acrylate-based [56-59], siloxane-based [60] and vinylcyclopropane-based systems [61]. The detected mesophases and their transition temperatures are summarized in Table 6. 

Fig. 8 Polymer backbone structures of SCLCPs based on cyanobiphenyl mesogens...

Based on previous work which showed that backbone rigidity correlates with mesophase behavior for SCLCPs bearing calamitic mesogens [64], the double bonds of the main chain in polybutadienes were hydrogenated, leading to pure saturated alkane main chains, poly-(XXIX-5)a and poly-(XXIX-10)a in Table 18. But in contrast to the studies of calamitic SCLCPs, no dependency of backbone rigidity on mesophase behavior could be estabhshed. 

Zhang and coworkers chose a base polymer alternative to poly(styrenesulfonate) and prepared SCLCP with a polysiloxane backbone [102]. Differently from the previous cases, side groups were linked to the main chain by covalent bonding. 

The CF3S03H/SM62 initiating system has also been used to cyclopolymerize ll-[(4 -cyanophenyl-4"-phenoxy)alkyl]undecanyl-3,4-bis(ethenyloxyethoxy)benzoate to form SCLCPs with crown ethers in the polymer backbone, although the polymerization is accompanied by termination [143]. Polymerization of the chiral vinyl ethers shown in Scheme 10 were reported to give only oligomers with DPn = 5 under the same condi-... 

Many reviews [1-3] of SCLCPs describe the structure of the backbone (main chain), the spacer (flexible linkage), and the side group (mesogenic unit) of the SCLCP. For example, the most widely used backbones include polyacrylates or polymethacrylates, polysiloxanes, and polyphosphazenes po-ly-ct-chloroacrylates, itaconates, and ethylene oxides have also been reported. 

The backbones that have been most commonly employed are those of the acrylate [177, 190, 194, 196], methacrylate [152, 171,196,198-200], and siloxane [152,177, 197] types. Polyethers [207-209], polyesters [182,191, 192], and polystyrenes [177, 189, 195] have also been reported. Typical spacer groups consist of between 3 and 12 methylene units. The phase transitions of a number of SCLCPs containing NLO meso-genic groups are collected in Table 19. Unfortunately, the molecular masses of many of these polymers have not been determined, and the influence of the polymer structure on the phase transitions can not therefore be quantitatively discussed. However, the general points to emerge from these data are as follows ... 

Side chain liquid crystal polymers (SCLCPs) consist of mesogenic structural moieties appended from a polymer backbone (Figure 5.2). The mesogenic units that have been used parallel those previously used for low molar mass liquid crystals, and the stmctural nature of the polymer backbone is widely variable. The mesogenic units (usually calamitic but many discotic types exist) are invariably separated from the polymer backbone by fairly long spacer units which are usually several methylene (-CH -) units,... 

A great maity SCLCPs have been synthesised because of the combination of the vast number of mesogenic core nnits available from stmctuie-property relationship evaluation of low molar mass liquid crystals and the maity different backbone (BB) types possible e.g., siloxanes, acrylates, methacrylates, ethylenes, epoxides). Additionally, as shown in Figure 5.4, homopolymers (5.4a), SC copolymers (5.4b), BB copolymers (5.4c), and SC/BB copolymers (5.4d) considerably extend the scope of known and potential liquid crystal polymers. 

The reasons for employing a flexible spacer that links the mesogenic nnit to the polymer backbone are discussed above. The influence of the flexible spacer that is normally essential for the generation of mesophases in SCLCP is of great interest Figure 5.5 shows some examples of simple flexible spacers where the actual size of the spacer may be smaller or larger. 

However, there is an enormous number of possible polymer backbones that can be used for generating SCLCPs, but only a small selection have been widely used in the synthesis and evaluation of SCLCPs. In general, poly(siloxanes) and poly(aciylates) constitute the vast majority of SCLCPs. The reported role of the spacer group is to decouple the effects of the backbone from those of the mesogenic side chains. Accordingly, the backbone should not influence the mesomorphic properties of SCLCPs. However, decoupling is... 

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