Unsubstituted conjugated polymers

FIGURE 8.9 Linear unsubstituted conjugated polymers poly(p-phenylene), poly(para-phenylene vinylene), polythiophenes, and polyfluorenes. 

It should be pointed out that, despite this attention, 17 years after Shirakawa s discovery polyacetylene is not yet a commercial polymer, and many early research efforts at industrial laboratories have been discontinued. In part, progress has been hampered by the fact that polyacetylene, and most other unsubstituted conjugated polymers, can be neither dissolved nor melted. In addition, polyacetylene is unstable in air, complicating its incorporation into products. However, the diversity of potential uses for conjugated polymers, combined with the experimental challenges of preparing more tractable materials, has kept the field vibrant. 

FIGURE 17.1 Sketches of various well-known linearly unsubstituted conjugated polymers. 

These substituted conjugated polymers were prepared to develop processible electroactive materials because most unsubstituted conjugated polymers are not melt- or solution-processible owing to strong interchain interactions and chain stiffness. However, as mentioned above, the presence of bulky substituents in polyacety-... 

Unsubstituted conjugated polymers such as polythiophene and poly(para-phenylene) cannot be processed in the form of thin films. One means to overcome this difficulty is to use oligomers since they share frequently the properties of their higher molecular weight counterparts (e.g., the electroluminescence of para-sexiphenyl [PSP] and poly(para-phenylene)). The oligomers are easier to process and, in the context of this chapter, can be oriented by epitaxial crystallization. Deposition techniques are very varied. For PSP, various processing conditions are available molecular beam epitaxy, hot wall... 

The UV-visible absorption maxima and edges of the various PPV derivatives are summarized in Table 1. PFPV [7] with electron-withdrawing substituent shows a blue-shifted absorption at 410 nm and perfluorinated biphenyl substituted PPFPV [20] shows more blue-shifted absorption at 400 nm. Benjamin et al. [21] have reported that a copolymer composed of the PPV part and the 2,3,5,6-tetrafluoro-PPV part showed a blue-shifted absorption compared with that of unsubstituted PPV. These shifts are explained as being a result of the inductive electron withdrawing properties of the fluorine substitution leading to reduced electron density in the conjugated polymers and leading to an increase in the HOMO-LUMO band gap. 

So it is worth beginning a study of the physical properties of conjugated polymers with a discussion of structural problems. Although there are several excellent review discussions of electronic properties (in a broad sense) of CPs, data on structures are more scattered. In conjugated polymers, unsubstituted materials, such as poly acetylene, and substituted ones, such as the poly alky lthiophenes, pose different structural problems, so the two classes will be discussed separately, with more emphasis on the first class of polymers. 

As an alternative, copolymerization of alkynes bearing bulky substituents with TCDTF6 (7,8-bis(trifluoromethyl)tricyclo [4.2.2.0 ]deca-3,7,9-triene) was carried out. In the course of this copolymerization, usually referred to as the Durham Route [86-89], the Feast-monomer was introduced into the polymer main chain and subsequently converted into three unsubstituted, conjugated double bonds via a thermally-induced retro-Diels Alder reaction (Scheme 3) [53]. 

As mentioned in the introduction, the electrical conductivity upon doping is one of the most important physical properties of conjugated polymers. The conductivity ranges from lOOOOOS/cm for iodine-doped polyacetylene [41], 1000 S/cm for doped and stretched polypyrrole [42], to 500 S/cm for doped PPP [43], 150 S/cm for hydrochloric acid doped and stretched polyaniline [44], and 100 S/cm for sulfuric acid doped PPV [45] to 50 S/cm for iodine-doped poly thiophene [46]. The above listed conductivities refer to the unsubstituted polymers other substitution patterns can lead to different film morphologies and thus to a different electrical conductivity for the same class of conjugated polymer in the doped state. 

PPV derivatives and block copolymers have probably drawn more attention than any other class of r-conjugated polymers. Several surveys of PPVs have been published recently see, e.g., the recent reiew by Friend et al.34 or the chapter by Greenham and Friend in this volume.52 The most commonly used PPV is the unsubstituted, which is typically deposited by spincoating a precursor polymer, followed by thermal conversion of the precursor to PPV, and various derivatives such as 2,5-dioctoxy PPV (DOO-PPV) or MEH-PPV. Similar to Alq3, PPVs have also been used as hosts for lower-gap emitters. 

In contrast to unsubstituted acetylene, the polymerization of differently substituted 1-alkynes and di-1-alkynes may be carried out conveniently using Schrock-type catalysts. Dipropargylmalonate and derivatives thereof may be cyclopolymerized in a living manner using Mo(A/ 2,6-i-Pr2-C6H3)(CH-t-Bu)-(OCMe(CF3)2)2. The resulting conjugated polymer... 

FIGURE 17.2 Views of the prototypical herringbone packing in poly(p-phenylene vinylene) and this motif is often seen in crystalline unsubstituted ir-conjugated polymers. (Reprinted from Winokur, M.J. and Chunwachira-siri, W., /. Polym. Sci., 41, 2630, 2003. With permission.)... 

Polyaniline is one of the earliest studied conjugated polymers and remains as one of the most researched, especially in the field of electrochromics. While potentially quite useful as a stable and cost-effective conductor, it is a bit unfortunate that substituted polyanilines exhibit little change in the electrochromic properties as a function of substituent identity and position. Substituted and unsubstituted polyanilines have the potentially useful property of being polyelectrochromic in thin film form, but structural modification has not been shown to increase the range of colors available. 

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