Wessling precursor route

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). ... 

Xanthate Precursor Route. Son et al. [994] published a modification of the Wessling precursor route. The sulfonium groups were replaced by xanthate groups in order to avoid typical side reactions of the Wessling approach (Fig. 81). The precursor is stable at room temperature and soluble in most common organic solvents. Elimination with formation of the polyconjugated system takes place between 160 and 250° C. 

Precursor methods have been developed for polyacetylene and poly(p-phenylene). The precursor approach has been intensively stimulated by developments in the field of poly(p-phenylene viny-lene) and the use of this polymer in light-emitting devices. The Wessling precursor route is one of the most used procedures to prepare this material. The precursor route of Louwet and Van-derzande has overcome many of the limitations inherent in the Wessling precursor route and will most probably allow further interesting developments for the application of poly(p-phenylene vinylene) in polymer electronics. 

This polymerization strategy is extensively used for the synthesis of PPV and its substituted derivatives. Wessling and Zimmerman [38] developed a method for the synthesis of PPV via thermo-conversion of sulfonium intermediate (prepolymer) into PPV to yield its film. In the Wessling precursor route l,4-xylylene-bis-(dialkyl sulfonium)-dichloride is used which upon elimination of tetrahydrothiophene and HCl yields PPV polymer. Fig. 5. The thermo-conversion mechanism yields pin-hole free thin films of conjugated polymers applicable for PLED fabrication. The thermo-conversion temperatures can be lowered to as low as 100 °C using bromide derivatives which can help in the fabrication of flexible PLED devices. 

The scope of Wessling route has been extended by Mullen and co-workers to develop a soluble precursor route to poly(anthrylene vinyiene)s (PAVs) [51]. It was anticipated that the energy differences between the quinoid and aromatic resonance structures would be diminished in PAV relative to PPV itself. An optical band gap of 2.12 eV was determined for 1,4-PAV 29, some 0.3 eV lower than the value observed in PPV. Interestingly, the 9, lO-b/.v-sulfonium salt does not polymerize, possibly due to stcric effects (Scheme 1-9). 

SCHEME 2.1 The Wessling-Zimmerman precursor route to PPY. (From Wessling, R.A. and Zimmerman, R.G., Polyelectrolytes from Bis Sulfonium Salts, U.S. Patent 3,401,152, 1968.)... 

The Wessling and Zimmerman aqueous precursor route is illustrated in Scheme 38 [156]. Here, a bis(halomethyl)monomer is reacted with dimethyl-sulfide and subsequent treatment with base affords the high molecular weight precursor polyelectrolyte 31. Due to the instability of 31, polymerization must be carried out at low temperatures (<4 °C) to avoid thermal elimination of the polyelectrolyte. Precursor polymer 31 can be stored in solution with refrigeration, and its shelf life can be increased by the addition of a small amount of pyridine. Precursor polymer 31 can be processed into highly oriented, free-standing films or fibers that can subsequently be converted to PPV with the elimination of gaseous dimethylsulfide and HCl at 200 °C. 

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... 

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. 

PPV (27a) has been prepared by a number of different methods which were studied in detail by Horhold and Opfermann [129]. It can be synthesized by bifunctional carbonyl olefination of terephthalaldehyde according to Wittig s reaction and from /)-xylylene-bis-(diethyl phosphonate) as well as by dehydrochlorination of p-xylylene dichloride with sodium hydride in N,N-dimethylformamide and with potassium amide in liquid ammonia. Another route to PPV used today is the precursor route, first described by Wessling [130 133] and Kanabe [134], starting from the monomers /)-xylylene-bis(dimethylsulfo-nium tetrafluoroborate) [134] or chloride (Scheme 28) [130-133],... 

As the Wessling and Gilch precursor routes are characterized by the polymerization behavior ofp-quinodimethane tems, Louwet et al. [996-998] were aiming to develop a more generalized scheme to study the boundary conditions for such polymerizations. In this more general scheme (Fig. 82), a distinction is made among three phases first, the in situ formation of the p-quinodimethane system, which acts as the monomer second, the polymerization reaction, and, finally, the conversion to the fully conjugated tem. 

A modification of the Wessling-Zimmerman route consists, as mentioned before, of the conversion of sulfonium polyelectrolytes to methoxy-substituted polymers, which are soluble in organic solvents such as chloroform. These precursor polymers have the advantage of being more stable than the sulfonium ones and can also be further stabilized by weak bases such as pyridine [30]. The conversion to PPV can be carried out by heating, by acid catalysis, or both, as shown in Scheme 9. Some examples of PPVs obtained by this methodology are shown in Tkble II. 

While not necessarily a precursor route, a closely related polymerization method to the Wessling-Zimmerman... 

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