MCLCPs

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). 

Chemical group used to disrupt the linearity of the backbone of molecules of MCLCPs. Note Examples are (a, b) rigid-kink or (c) crankshaft units. 

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 mesogenic groups in the main chain (MCLCPs)... 

Estimate the characteristic temperatures of the MCLCP with the following structural unit... 

Transient behaviour of lyotropic MCLCPs is similar to that of thermotropic MCLCPs as discussed in Sect. 15.7 (Fig. 15.47). An example is given in Fig. 16.35 for PpPTA in sulphuric acid (Doppert and Picken, 1987). It shows damped oscillating behaviour, which is in contradistinction to conventional polymers, but which is also found as a result of simple rheological models, like the Jeffreys model (see Chap. 15). This behaviour turns out to be a general feature of lyotropic MCLCPs. The oscillatory behaviour is easier to measure for lyotropic than for thermotropic systems, where it is less pronounced. 

As a resume the rheological responses of MCLCP solutions (in comparison with conventional polymer solutions) to the most important variables are shown (qualitatively) in the comprehensive Fig. 16.36. 

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. 

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. 

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

The bulkiness of dendritic side chains can force the polymer backbone to adopt special geometries [84]. The overall contour of the polymer can then be spherical or rod-hke so that the polymers adopt Uquid crystalUne phases. Even when the driving force of this behavior is caused by the bulkiness of the side chain, these polymers show MCLCP-Uke behavior. 

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. 

Jung-Ii Jin et al. prepared a series of ester-based MCLCPs and realized a combination of MCLCP and SCLCP by copolymerizing the monomers XXXVIII and XXXIX shown in Fig. 22. The Grubbs catalyst 6 was used for this... 

The same group reported on the synthesis and characterization of all-hydrocarbon MCLCPs, poly-(XXXX), and their hydrogenated derivatives poly-(XXXXI), based on 4,4 -bis(a-co-alkenyl)-l,r-biphenyl derivatives, see Fig. 23. Monomers with different a-co-alkenyl chain lengths were used to prepare not only the corresponding homopolymers but also statistical copolymers. Crystallinity, thermal transition properties and LC properties were studied. 

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). 

MCLCPs prepared by ADMET and ALTMET have recently gained much attention. In particular, ALTMET allows the synthesis of perfectly alternating copolymers, opening new possibilities for the design of materials with specific properties. 

M = Rh, Ir dip = 2,5-diiminopyrazine ligand) in comparison with the related radicals [Cp ClM(/i-dip)MClCp ] + hyperfine information (which would settle the question of localized or delocalized valences) could not, however, be obtained because of the unfavorable isotopic situation [55b, cj. 

It is of interest that reaction of (MeNC(H)NMe H with MClCp(CO)3 afforded a carbamoyl complex M(MeNC(H)NMe C(0)(C0)2 (M = Mo, W), which on irradiation was converted into the complex with the chelated amidino group, i.e. MCp MeNC(H)NMe (CO)2. The Me substituents on the amidino N atoms clearly favor intramolecular attack on a CO group, which may be caused by the strongly electron donating properties of the Me groups. 

Semiflexible MCLCPs were prepared following two basic conceptual procedures. The first one (melt transesterification) involves the chemical modification of pre-... 

Preparation of Main Chain Liquid Crystalline Polymers (MCLCPs) Table 1. Representation of some typical structures of polyesters. 

The effect of structural disclinations upon texture and the origins of these in local deformations of or discontinuities in the arrangement of the molecules have been studied only in the case of nematic MCLCPs. The primacy aim of this chapter is therefore to present to the reader defects and textures in nematic MCLCPs. Some of consequences of constructing a nematic phase from long... 

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