Poly l,4-phenylene s

Gray (1961) has had a good account for the structure and properties of low molecular mass liquid crystals. In this section we discuss only a few structural elements having the most impact on the properties of liquid crystals. The structural elements include those imposing on molecules, the side-steps , the kinks, the flexible spacers, the bulky substituents, and others. The discussion starts with poly(l,4-phenylene)s as the parent or reference molecules. 

if not all, the liquid crystal compounds have the 1,4-phenylene unit in the molecules. This structural brick has all the most desirable characteristics for liquid crystals stiffness, polarizability, and linear orientation. The simplest, although very difficult at the same time, construction of a liquid crystal with this moiety is the synthesis of poly(l,4-phenylene)s, (3.1). In Table 3.1 the number of the phenylene rings and the length-to-diameter ratio are related with the liquid crystallinity of these compounds. 

As shown in Table 3.1, poly( 1,4-phenylene) becomes liquid-crystalforming when five rings are present and the molecular rod is sufficiently long. The property of this polymer is most predictable by the Flory theory because of its rod-like nature. 

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Table 3.1. The Axial ratio and Phase Transition Temperature of Poly(l,4-phenylene)s. ...

Poly(l,4-phenylene vinylcne) and its Derivatives 2 The Basic Polymer LED Device Architecture 4 Substituted Poly(phcnylene vinylcne)s 6 Poly(anthrylenevinylcne)s 10 Step-Growth Routes to PPV Derivatives 10 PPV Copolymers 11... 

Polyfarylene vinylene)s form an important class of conducting polymers. Two representative examples of this class of materials will be discussed in some detail here. There are poly(l,4-phenylene vinylcne) (PPV) 1, poly(l,4-thienylene viny-lenc) (PTV) 2 and their derivatives. The polymers are conceptually similar PTV may be considered as a heterocyclic analog of PPV, but has a considerably lowci band gap and exhibits higher conductivities in both its doped and undoped stales. The semiconducting properties of PPV have been shown to be useful in the manufacture of electroluminescent devices, whereas the potential utility of PTV has yet to be fully exploited. This account will provide a review of synthetic approaches to arylene vinylene derivatives and will give details an how the structure of the materials relate to their performance in real devices. 

Poly(phenyleneethynylene)s, 482 optically active, 516-517 synthesis of, 496-500, 502 Poly(m-phenylene isophthalamide), 136 synthesis of, 185-186 Poly(l,4-phenylene terephthalate) liquid crystalline polymers, 51 Poly(para-phenylene)s, 472. See also Poly (p -pheny lene)... 

A second, very fruitful synthetic principle for structurally homogeneous, processable PPP derivatives involves the preparation of soluble PPPs by the introduction of solubilizing side groups. The pioneering work here was carried out at the end of the 80s by Schliiter, Wegner et al. [16,17l who prepared soluble poly(2,5-dialkyl-l,4-phenylene)s (6) for the first time. 

H.D. Burrows, Y.M.M. Lobo, J. Pina, M.L. Ramos, J.S. de Melo, A.J.M. Yelente, M.J. Tapia, S. Pradhan, and U. Scherf, Fluorescence enhancement of the water-soluble poly l,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate) ]fluorene-2,7-diyl copolymer in M-dodecylpentaoxyethylene glycol ether micelles, Macromolecules, 37 7425-7427, 2004. 

The molecular orbital (MO) calculations within the PM3 method, using a MOP AC package, provided an explanation of the advantages of a new redox system, poly(l,4-phenylene-l,2,4-dithiazolium-3, 5 -yl) (PPDTA), as a cathode material for high-capacity lithium secondary batteries in comparison with three typical polymer conductors (poly-/>-phenylene, polypyrrole, and polythiophene). The MO calculation revealed that the S-S bond in the 1,2,4-dithiazo-lium moiety of PPDTA caused gap narrowing and a downshift of HOMO and LUMO levels, which is consistent with the electrochemical experiment (HOMO = highest occupied molecular orbital LUMO = lowest unoccupied molecular orbital) <2001MI2305>. 

It is worthwhile pointing out that the critical axial ratio 6.4 given by Flory s theory is based on assumptions that include zero free volume and zero net interaction energy between the rods (Chapter 2). By increasing the free volume, for example, the ratio will be increased accordingly. It is thus also understandable that the critical axial ratio will become larger when the temperature is increased. On the other hand, no actual system will meet these assumptions to any perfection. In addition, the flexibility of an actual molecule such as poly(l,4-phenylene) and PHA will increase with increasing temperature. In other words, the axial ratio of a molecule is not a constant,... 

Ballauff M (1986) Rigid rod pdymers having flexible side chains. I. Thermotropic poly(l,4-phenylene-2,5-dialkoxyterephthalate) s. Makromol. Chem. Rapid Commun. 7 407... 

Ballauff M, Schmidt GF (1987) Rigid rod polymers having flexible side chains. 3. Structural investigations on a novel layered mesophase formed by thermotropic poly(l,4-phenylene-2,5-dialkoxyterephthalate)s. Mol. Liq. Cryst. 147 163... 

These monolayers (SAMs) are characterised by UV-Vis, fluorescence and Raman spectroscopy. This is made easy because of the presence of benzene cores within the acetylene and linker chains, these being made of poly(l,4-phenylene-l,3-butadiynylene) polymers. AFM then provides the topology, i.e. shapes (contours), thicknesses and aspect ratios (length and width), of the 2D S.AMs. This is done at all stages including those described below. 

Poly(2,5-dialkoxy-l,4-phenylene)s can also be made by oxidation of para-dialkoxybenzenes (Scheme 11). Thus, 1,4-dimethoxybenzene (23) can be polymerised with aluminium chloride and copper(II) chloride or iron(III) chloride. The polymer 24 is only soluble in sulfuric acid, however, and so not usable in LEDs [7]. Oxidation of 1,4-dibutoxybenzene (25) with iron(III) chloride by contrast gives a polymer 26 which is soluble in organic solvents [70,71]. 

Wang, Y, and Quirk, R. P., Synthesis and characterization of poly(benzoyl-l,4-phenylene)s. 2. Catalyst coligand effects on polymer properties. Macromolecules, 28, 3495 (1995). 

Oxidative polymerization of bis(l-naphthoxy) monomers is known as the Shell Reaction (eq. 11) (228). 1,4-Dialkoxybenzenes have been pol5mierized nsing FeCls (229,230) and oxovanadium catalyst with dioxygen (231) to give poly(2,5-dialkoxy-l,4-phenylene)s (eq. 12). Poly(4,6-di-n-butyl-l,3-phenylene) was also obtained (232). 

H.-K. Shim, S.-K. Kim, J.-I. Jin, K.-H. Kim, and Y.-W. Park, Electrical properties of poly(l,4-phenylene vinylene-co-2,5-dimethoxy-l,4-phenylene vinylene)s and poly(l,4-phenylene vinylene-ro-2,5-thienylene vinylene)s. Bull. Korean Chem. Soc. 11 11 (1990). 

In principle, all methods used for the generation of poly(l,4-phenylene-ethynylene)s (PPE) can be also used for the OPE synthesis however, it is often difficult to stop the polycondensation at an early stage and it is always difficult to separate uniform oligomers from oligodisperse mixtures. 

Yamamoto T, Osakada K, Wakabayashi T, Yamamoto A (1985) Nickel and palladium catalyzed dehalogenating polycondensation of dihaloanunatic compounds with zinc. A new route to poly(2,5-thienylene)s and poly(l,4-phenylene). Makromol Chem Rapid Commun 6 671-674... 

Figure 2. Chemical structures of the aromatic polyamides [aramids] poly-(1,4-benzamide) (I) and poly(l,4-phenylene terephthalamide) (II). The latter macromolecule constitutes Du Font s high-performance Kevlar fibers.

Poly(l,4-benzamide) (I, PBA) is most commonly prepared by low-temperature solution polycondensation of 4-aminobenzoyl chloride hydrochloride. Poly(l,4-phenylene terephthalamide) (II, PPTA), which is the aramid that has become of prime commercial importance, t5 ically is s)mthesized by solution polycondensation of terephthaloyl chloride and 1,4-phenylene diamine (1-3 see also references 4 and 5 for excellent reviews of the polymerization and processing of aramids). 

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