Conjugated polymers preparation methods

Polymers. The Tt-conjugated polymers used in semiconducting appHcations are usually insulating, with semiconducting or metallic properties induced by doping (see Flectrically conductive polymers). Most of the polymers of this type can be prepared by standard methods. The increasing use of polymers in devices in the last decade has led to a great deal of study to improve the processabiUty of thin films of commonly used polymers. 

This idea was realized impressively in 1991 with the first synthesis of a soluble, conjugated ladder polymer of the PPP-type [41]. This PPP ladder polymer, LPPP 26, was prepared according to a so-called classical route, in which an open-chain, single-stranded precursor polymer was closed to give a double-stranded ladder polymer. The synthetic potential of the so-called classical multi-step sequence has been in doubt for a long time in the 1980s synchronous routes were strongly favoured as preparative method for ladder polymers. 

In the via precursor method, however, it is difficult to prepare the ji-conjugated polymers with ideally developed -conjugation system the -conjugated polymer chains contain many conformational defects because the jc-conjugated chains are caused to develop from disordered precursor polymer, which form random coil conformation, in solid state. For the preparation of polymers with well-developed jc-conjugation system by the via precursor method, accordingly, it is necessary to introduce orientational and conformational orderliness of the precursor polymers in the films. 

Besides the polymerization techniques discussed above, other polymerization methods have been used for the preparation of surface grafts. Recently, ring-opening metathesis polymerization (ROMP) became popular. This polymerization type will be discussed by Buchmeiser in Chapter 8. Recently, interesting accounts have appeared on solventiess polymerization techniques applying (living) ROMP on surfaces to prepare structured brush surfaces of conjugated polymers [331, 332]. 

In the search for potentially conducting conjugated polymers all of these methods have been explored. In the following sections we survey the main types of conjugated structure and briefly outline the strategies which have been adopted to prepare them. 

Molecular imprinting has been used to devise a chemosensor for L-nicotine (Table 6) [178]. For that, poly(methacrylic acid) (PMA) beads, imprinted with the L-nicotine template in chloroform, were incorporated in a film of the conjugated polymer, OCiC10-PPV. EIS has then been utilized for the L-nicotine determination in the 1-10 nM concentration range. This MIP chemosensor showed predominant affinity towards L-nicotine over a structurally related L-nicotine metabolite, L-cotinine. Similarly, the polydopamine-imprinted film prepared by electropolymerization in the phosphate buffer (pH = 7.4) has been used to devise a chemosensor for L-nicotine with LOD of 0.5 pM (Table 6) [106]. This LOD is still much higher than that reported for other L-nicotine determination methods based on MIPs, such as SPE combined with differential pulsed elution, which was 6 nM [31]. 

As utilized in the synthesis of PPV-based polymers, soluble precursor routes have been developed for the synthesis of various heterocyclic Jt-conjugated polymers. The two most widely employed of these methods for heterocycle formation, shown in Scheme 66, center around ring closure of pre-polymers containing diacetylene 65 or 1,4-diketone units 66 [335-339]. The synthesis of heterocyclic structures from 1,4-diketones has been a known transformation in organic chemistry for decades. While once mainly used for monomer preparation through various cyclizations, it is now being employed to make heterocycle-containing polymers and copolymers [340-342]. 

Molecular self-organization in solution depends critically on molecular structural features and on concentration. Molecular self-organization or aggregation in solution occurs at the critical saturation concentration when the solvency of the medium is reduced. This can be achieved by solvent evaporation, reduced temperature, addition of a nonsolvent, or a combination of all these factors. Solvato-chromism and thermochromism of conjugated polymers such as regioregular polythiophenes are two illustrative examples, respectively, of solubility and temperature effects [43-45]. It should therefore be possible to use these solution phenomena to pre-establish desirable molecular organization in the semiconductor materials before deposition. Our studies of the molecular self-assembly behavior of PQT-12, which leads to the preparation of structurally ordered semiconductor nanopartides [46], will be described. These PQT-12 nanopartides have consistently provided excellent FETcharacteristics for solution-processed OTFTs, irrespective of deposition methods. 

Scheme 8 shows the synthetic routes of the oxadiazole unit containing phospho-nium salt monomers and Scheme 9 outlines the preparation of Jt-conjugated polymers composed of oxadiazole units [67]. The synthetic pathways to the para-linked salt monomer (A) and meta-linked salt monomer (B) are of multisteps, but the synthetic method for each step is not difficult and the product yields of intermediate compounds are very high (75-95%). Treatment of bis(4-methylphenyl)hydrazine with POCl3 results in the formation of the 2,5-bis(4-methylphenyl)-1,3,4-oxadiazole. 

Several conjugated polymers such as polyaniline, polypyrrole and poly thiophene have been prepared by chemical methods using doping. The electrical conductivity of these polymers has been controlled by (i) the type of dopant, (ii) the concentration of doping, (iii) the conditions of doping (the current density, temperature of reaction, etc.)... 

A self-assembly DNA-conjugated polymer-based DNA detection method was previously mentioned. This study showed specific immobilization to a gold substrate with the self-assembly technique, not physical adsorption and great selectivity to fully matched DNA instead of nonspecific adsorption due to self-assembly DNA-conjugated polyallylamine and PAA coating [67,68]. However, a major problem was that we needed to prepare many kinds of DNA-conjugated polymer to analyze many kinds of target DNA due to direct modification of probe DNA to the polymer side chain. 

Beginning with tetra-A -ethyl-terephthalamide a five-step method for preparing poly [2-(dimethyloctylsilyl)-5-(dimethyldecylsilyl)-1,4-phenylene vinylene] is described. The conjugated polymers are useful in electric, electronic, optical, and optoelectronic... 

Nanoparticles of the conjugated polymers Me-LPPP and MEH-PPV were produced with the miniemulsion method. Monolayer of the particles on glass with ITO coating was prepared by spin-coating. The content of components in Me-LPPP MEH PPV blend is 2 1. 

Hsu et al. [75,76] reported a new method for incorporating metal complexes into polyfluorenes to prepare phosphorescent polymers (polymer 47 and 48). A pyridine end-capped polyfluorene has been synthesized. The pyridine was used to form a polymer metal complex with 2,2-bipyridyl(tri-carbonyl)rhenium(I) chloride. Using the end-capping approach not only can control the molecular weight of polymer, but also avoid the interference of the metal complex and conjugated polymer in energy transfer. They can... 

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