Wessling route

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). 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

Scheme 1-19. Wessling route to PPyV 49 a) NCS, CCU b) tetrahydrothiophene, MeOH, 50 °C c) aq. NaOH, MeOH d) heat e) 0.98 eq. KOtBu, THF f) heat, MeOH.

Polyfpyridine vinylene) 55 may also be prepared by the Wessling route (Scheme 1 -19) [95]. Fully conjugated material may be obtained by thermal elimination of the sulfonium salt 61 or the halide 62. The method, however, does not... 

Scheme 1-24. Wessling route to PTV 2 a) MeOH, tetrahydrothiophene b) NaOH, H2O c) heat...

Louwet, R, Vanderzande, D., Gelan, J., 1992. The synthesis of poly(l,4-phenylene-l,2-ethanediyl) derivatives—an adaptation of the Wessling route. Synth. Met. 52,125-130. 

Scheme 29.1 Key steps of the Wessling route. The lateral substituents, R, which are usually attached to the PPVs for ensuring their solubility and tailoring their properties, are omitted here (and also in the other schemes and figures) for reasons of clarity.

In contrast to the Wessling route, uncharged starting materials 6 are used in the sulfinyl and sulfonyl routes, which are substituted nonsymmetricaUy in their a and a positions the halogen substituent X acts as a leaving group while the sulfinyl or sulfonyl group P polarizes the benzylic C-H bond (Scheme 29.2). 

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. 

Method of synthesis polymer In final form cannot be processed, therefore precursor polymer Is synthesized first and then converted into film or the final forms. Precursor polymer can be obtained by one of the following methods Wessling route, ring opening polymerization, chemical vapor deposition, electropolymerization, condensation, phase transfer catalysis, or anionic polymerization Rnk, J K, High Performance Polymers, William Andrew, 2008. 

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