Sulfonium precursor route

The precursor polymer can be fabricated, e.g., by spin-coating, dipcoating or by Langmuir-Blodgett techniques into thin films. The pyro- 

Functionalized PPV precursor polymers with ester and carboxyl side groups have been prepared. These polymeric tetrahydrothiophenium salts can be converted via the sulfonium route into PPVs. After conversion, the pendent carboxyl functionalities can be further exchanged by others, such as nitro, amino, and aldehyde functionalities. 

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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. 

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... 

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... 

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. 

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). 

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). 

Sulfonium Precursor Route. The Wessling route has also been used to produce soluble derivatives from monomers containing solubilizing substituents on the phenyl ring. For example, dialkoxy-substituted monomers yield 11, which is soluble in organic solvents such as chloroform and chlorobenzene (26), as well as poly[2-(2-ethyIhexyl)oxy-5-methoxy-p-phenylene)vinylene], or MEH-PPV (12) (eq. 5) (27). The branched side chains in MEH-PPV improve the solubility of this derivative over imbranched analogs, and this polymer is one of the most popular for use in electroluminescence applications. 

The sulfonium precursor route has been used to prepare copolymers by using various proportions of different monomers in the S5mthesis. This method has yielded both partially conjugated (16) and fully conjugated polymers (17) (31,32), as well as copolsrmers containing other aromatic groups in the backbone in addition to phenylenevinylene moieties such as in copolymer 18 (33). 

In 1990, the discovery that PPV could be utilized as the emitter in an electroluminescent device was reported by Friend and co-workers at the Cavendish Laboratory in Cambridge (2). The devices consisted of a PPV film, prepared using the sulfonium precursor route, sandwiched between an ITO and an A1 electrode, and emitted green-yellow light under forward bias of 14 V, with a quantum efficiency of up to 0.05%. Shortly after this initial publication, Braun and Heeger demonstrated that MEH-PPV could also be used to fabricate EL devices in which the polymer was directly cast in the conjugated form from solution. They used both indium and calcium cathodes, and observed visible light at 4-V forward bias with the calcium cathode, with an efficiency of 1%. 

Scheme 1. Sulfonium precursor route to poly(p-phenylenevinylene).

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