Sulfonium precursors

The sulfonium precursor route may also be applied to alkoxy-substituted PPVs, but a dehydrohalogenation-condensation polymerization route, pioneered by Gilch, is favored 37]. The polymerization again proceeds via a quinomethide intermediate, but die syndicsis of the conjugated polymer requires only two steps and proceeds often in improved yields. The synthesis of the much-studied poly 2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylene vinylene], MEH-PPV 15 is outlined in Scheme 1-5 33, 35]. The solubility of MEH-PPV is believed to be enhanced by the branched nature of its side-chain. 

Several authors have recently adapted the sulfonium elimination route to prepare poly(2,5-thienylenevinylene) and reported high conductivities (200 S cm-1) on doping 244 245). In this case the sulfonium precursor is insoluble in water but can be cast from dimethylformamide solution. Jen et al. 246) report an alternative route involving a water-soluble precursor which eliminates HC1 and tetrahydrothiophene. 

Blends of polyphenylenevinylene with water-soluble polymers have been prepared by mixing solutions of the sulfonium precursor with polyethyleneoxide, hydroxy-propylcellulose and polyvinylmethylether317). Polyethyleneoxide forms spherulites which impose a spherulitic texture to the polyphenylenevinylene that is retained after transformation. As a result of this open network, high conductivities are reached at only 10% conducting polymer. 

PPV samples prepared by solution based sulfonium precursor routes were found to be polycrystalline. PPV thin films prepared by CVP, however, show a structure that is dependent... 

These are the most widely used routes to PAVs. The first to be developed was the Wessling-Zimmerman route to PPV as shown in Scheme 6.1 [5]. Here the starting material is a p-xylenyl bis(sulfonium salt) 6, which on treatment with 1 equivalent of base generates a quinodimethane 7, which then polymerizes to produce the sulfonium precursor polymer 8. This is water soluble and can be used to make thin films that are thermally converted to the final films of PPV by heating at 220-250 °C under vacuum. Alternatively the sulfonium groups can be displaced by methanol to give the more stable methoxy-precursor 9, which requires a combination of heat and hydrochloric acid vapor for efficient conversion to 1. 

The Wessling sulfonium precursor route has also been used to make a variety of other PAVs, including MEH-PPV (2) [15] but in view of the environmentally undesirable properties of the sulfur reagents used (toxicity, stench) other routes... 

Secondly, the polarity/solubility of the material can be changed. This approach has barely been followed. One prominent example, however, is the first luminescent polymer, poly(p-phenylenevinylene (PPV), which was obtained via a polar sulfonium precursor [5]. Here, the ionic precursor can be transformed thermally into a nonpolar polymer, which is insoluble in all organic solvents... 

The polymers have been used in different multi-layered devices using PPV as emissive layer. Typical devices were prepared on glass substrates precoated with patterned indium-tin oxide (ITO) electrodes (resistance < 20 Q/square). In a two layer LED, the oxadiazole polymethacrylates (10a or 10b) were spin-coated on top of poly(p-phenylene vinylene) (PPV), prepared on an ITO glass substrate by the sulfonium precursor route (22). Cdcium was used as the top metal contact. A comparable device, but without the polymethacrylate, was fabricated as a reference. Both devices emitted green yellow light under forward bias potential (15 V). 

Abstract In situ spectroscopy is an important tool to characterize polymers synthesized via a precursor route. Highly conjugated polymers such as po y(p-phenylene vinylene) (PPV) and PPV derivatives are commonly prepared from a precursor polymer because the final polymers are very insoluble and intractable. Preparation in the precursor form enables the polymer materials to be cast as films. The PPV polymers are obtained from the precursor forms using a thermal elimination reaction. The exact conditions of the reaction are important as they influence the properties of the resultant polymer. The details of this thermal elimination reaction have been analyzed using thermal gravimetric analysis (TGA) coupled with infrared analysis of the evolved gas products. In situ infrared spectroscopy of the precursor films during thermal conversion to the polymers has provided further details about the elimination reaction. We have characterized PPV synthesized from a tetrahydrothiophenium monomer (sulfonium precursor route) and via the xanthate precursor route. PPV derivatives under study include poly(2,5-dimethoxy-p-phenylene vinylene) and poly(phenoxy phenylene vinylene). 

PPV and PPV derivatives have been synthesized using precursor routes because the final highly conjugated product is insoluble and intractable. The advantage of the precursor route is that the precursor polymer is soluble and the material can be readily cast as a film. Subsequently, the precursor film is thermally converted to the final conjugated PPV product. The earliest precursor route to PPV is known as the Wessling precursor route and involves a sulfonium precursor (also referred to as the sulfonium precursor route (SPR)). Other routes can be used to prepare PPV and PPV derivatives. These include the xanthate precursor route (XPR) and the chlorine precursor route (CPR). ... 

The solid state thermal elimination reaction is a very important step in the formation of the final PPV or PPV derivative. In situ infrared spectroscopy therefore plays a critical role in the ability to monitor the reaction that converts the precursor polymer to the final product. We have characterized the mechanism of this conversion reaction in the formation of PPV synthesized by both the sulfonium precursor route (SPR) and the xanthate precursor route (XPR). The polymerization reaction of PPV from the tetrahydrothiophenium monomer is shown in Figure 1. After polymerization of the precursor polymer, the material is thermally converted to the final PPV product. This SPR method involves the thermal elimination of the tetrahydrothiophenium (THT) group and HCl as shown. 

Figure 1. Synthesis of poly(p-phenylene vinylene)(PPV) from the tetrahydrothiophenium monomer via the sulfonium precursor route...

We have previously reported the synthesis of PPV and the analysis of the thermal elimination reaction in the polymer prepared from the tetrahydrothiophenium (THT) monomer via the sulfonium precursor route. The reaction is shown in Figure 1. PPV prepared via the xanthate precursor route has been recently described.The reaction is shown in Figure 2. 

FIGURE 47.3. Mechanistic processes for the sulfonium precursor synthesis of poly(phenylene vinylene)s, showing the ylide, the xylylene, and the poly(p-xylylene-a-dialkylsulfonium halide) (PXD). Substituents X and Y can be alkyl, alkoxy, and aryl groups. Reprinted with permission from Synthesis, properties of poly(phenylene vinylene)s, related poly (arylene vinylene)s. 1998. p. 61. chapter 3. 1998 Marcel Dekker, Inc. 

The degree of polymerization was determined by H-NMR-spectroscopy and MALDI TOP mass spectrometry. Results of H-NMR-spectroscopic investigations prove the defect-free structure and dW-trans configuration. In contrary to the product obtained by Me Murry reaction [19] or by sulfonium precursor route [20] there is no sign for any cis bonding (Figure 3, at the top, marked regions). 

Conversion of the sulfonium precursor polymer 10 to PPV 1 is typically achieved by heating thin films to as high as 300 °C in high vacuum (10 mbar) for a period of several hours.The elimination conditions have a substantial influence on the properties of the thin film material particularly in device applications. Studies on the elimination process have shown a complex multistep mechanism but it is instructive to remember that the elimination products from a sulfonium chloride precursor polymer are HCl and thioethers. The size of the sulfonium group has been shown to influence both the intrachain and... 

The first decision in choosing a synthetic method for a PPV material is the way in which the material will be processed (Scheme 7.8). The precursor routes will enable the preparation of solvent-resistant and more durable thin films of PPV. This is particularly desirable if a multilayer device structure is required for the application. When choosing different precursor methods, it is important to assess the criteria of the application. Most precursor methods involve a thermal elimination step to convert the precursor polymer to the PPV material. Sul-fonium precursors require higher-temperature elimination compared to sulfinyl precursors. This makes the sulfinyl route compatible with deposition on plastic substrates. Another factor to consider in precursor methods is the nature of the elimination byproducts. Sulfonium precursors convert to PPV with elimination of acids, such as HCl or HBr, which has been shown to be detrimental to device performance. Xanthate and dithiocarbamate routes involve the elimination of amine and CO2 and CS2, respectively. 

Sulfonium Precursor Route. In this route, pol5mierization of the bis sulfonium salt 1 with base yields a soluble polyelectrolyte 2 (8,9). This intermediate may then be purified, processed, and finally thermally converted to PPV. Both the nature of the sulfide used in the sulfonium salt and the counterion affect the conditions required in the preparation, as well as the molecular weight and structure of the resulting polymer (9,10). When dodecylbenzenesulfonate is used as a counterion, conversion to PPV is achieved in 3 min at only 115°C (11). A modified sulfonium precursor route has also been developed, in which the soluble methoxy-substituted polymer 3 is converted to PPV in the presence of HCl gas (12). 

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