SCLCPs polymers

In general terms, high molar mass liquid crystals are classified according to the location of the mesogenic unit in the polymer. Thus, they are either incorporated into the main chain (main-chain liquid crystal polymers - MCLCP Fig. 5A) or they are pendant from the main chain (side-chain liquid crystal polymers - SCLCP Fig. 5B). 

All Liquid Crystal Polymers are characterised by the fact that they contain stiff meso-genic groups, often inserted in flexible chain systems (so called "spacers") and connected to them by linking functional groups the mesogenic unit is inserted either in the main chain or in the side chains or (in exceptional cases) in both. We shall discuss MCLCPs and SCLCPs. A schematic representation of common structures of LCPs is displayed in Fig. 6.14 (Jansen, 1996). An example of a SCLCP with disc-like mesogens is displayed in Fig. 6.15 (Franse et al., 2002, 2004). 

Polymers with mesogen groups in the side chain (SCLCPs)... 

The situation looks more complicated for polymers with mesogenic groups in the side chain, but in fact it is rather simple too. All these polymers can be represented by a general structural formula structure I in Fig. 6.17. This structure can immediately be derived from the general structure of comb-polymers (structure II in Fig. 6.17) the SCLCP is a comb-polymer with an... 

Lyotropic SCLCP are far less well studied by rheology than lyotropic MCLCPs. An example is the discotic SCLCP, as mentioned in Chap. 6 (Fig. 6.15), by Franse et al. (2002-2004). Fig. 16.37 gives a schematic representation of side chain discotic polymers (Franse, 2002). In solution the polymers have a tendency to form networks due to interaction between the discotic side chains. From viscoelastic measurements (G and G" as functions of angular frequency) it appeared that the networks formed in a 13% solution in 1,1,2-trichloroethane are very fragile, with a rubber modulus of not more than IN/m2. 

Side chain liquid crystalline polymers (SCLCPs). 47... 

ROMP Ring-opening metathesis polymerization ADMET Acyclic diene metathesis polymerization ALTMET Alternating diene metathesis polycondensation MCLCP Main chain liquid crystalline polymer SCLCP Side chain liquid crystalline polymer mru molecular repeating unit... 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

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...

Komiya et al. described the living ROMP synthesis of AB-type block copolymers that contain side chain liquid crystalline polymer blocks and amorphous blocks [62]. Norbornene (NBE), 5-cyano-2-norbornene (NBCN) and methyl-tetracyclododecene (MTD) were used for the amorphous polymer blocks, while I-n (n=3,6) were used for the SCLCP block (see Fig. 9). Initiator 1 was used for the ROMP. Block copolymers with monomer ratios from 75/25 to 20/80 (amor-... 

The homopolymer showed an enantiotropic nematic mesophase, whereas the diblock copolymer generated microphase-separated lamellae, in which the SCLCP block possessed a nematic-isotropization transition similar to the homopolymer (Table 17). Upon heating, the nematic microphase decreased continuously in the nematic phase from 38.5 nm to 27 nm and showed a constant value of about 26 nm after the nematic-isotropization transition. Therefore, materials in which these block copolymers are macroscopically aligned are expected to show reversible contraction in one dimension, making this polymer system an interesting candidates for an artificial muscle or actuator. 

Discotic SCLCPs were synthesized by the group of Grubbs [82]. For this purpose, norbornenes XXVIII-n (n=5, 10) and cyclobutenes XXIX-n (n=5, 10) with alkoxy-substituted triphenylenes as mesogenic units (see Fig. 18) were prepared. Polymerization was carried out with initiator 6. The resulting polymers had a narrow PDI between 1.09 and 1.17. Physico-chemical data for poly-XXVIII and poly-XXIX are listed in Table 18. 

Fig. 17 Discotic polymer architectures. A MCLCP B SCLCP C discotic network...

Norbornene-based and oxa-norbornene-based monomers bearing dendritic side chains, XXX and XXXI (Fig. 19), were synthesized and polymerized via ROMP with initiator 6 [83]. Based on size exclusion chromatography data, the polymerization shows hving-like character up to DP=70. H- and C-NMR-spectroscopy revealed 35% cis and 65% tram sequences. These polymers displayed enantiotropic nematic and smectic mesophases, except for DP=5. In contrast to other classes of SCLCPs, the dependence of the DP on the transition temperatirre of the polymer was very weak. Glass transition and isotropization temperatures became independent of molecular weight above a degree of polymerization of about 10. 

In contrast to ROMP, ADMET offers the possibility of synthesizing both side-chain and main-chain liquid crystalline polymers. The scope and limitations of ADMET are discussed in detail by Wagener et al. in this issue. We herein focus on a few contributions that used step growth polymerization methods to prepare MCLCPs and SCLCPs. 

Beside classical SCLCPs, attaching dendritic side chains to poly(norborn-enes) and poly(7-oxanorbornenes) leads to highly-ordered columnar mesophases (Sect. 2.5). In these polymers, the dendritic side chains force the polymer to adopt a rod-like structure. 

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. 

TABLE 5.5 Schematic Representation of the Organization of a Side-Chain Liquid Crystal Polymer (SCLCP) [4]... 

Unil recently, most side chain liquid crystalline polymers (SCLCPs) were prepared by either hydrosilations of mesogenic olefins with poly(methylsiloxane)s or by free radical polymerizations of acrylates, methacrylates and chloroacrylates [1,2]. Both routes involve chain polymerizations, either directly or prior to a polymer analogous reaction. Chain polymerizations involve the four elementary reactions shown in Scheme 1 [3], In contrast to step polymerizations,... 

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-... 

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