Wessling-Zimmermann

Scheme 11 Synthesis of PPV 17 according to the Wessling-Zimmermann route.

While the Wessling-Zimmermann route is a typical method of polymer synthesis, both PPVs and their corresponding oligomers can be synthesized by the extension of methods used for the synthesis of the easiest building block, stilbene 70 (scheme 13). Conceptionally, this is possible by (i) carbon-carbon double bond formation, for whieh synthetic organic chemistry provides a great number of methods [116], and (ii) by aryl-vinyl coupling [117] some examples of both methods are outlined in scheme 13. 

PPV (62a) has been prepared by a number of different methods which were studied in detail by Horhold and Opfermann [390]. 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 A jiV-dimethylformamide and with potassium amide in liquid ammonia. The most popular route to PPV used today is the precursor route, first described by Wessling and Zimmermann [391-394] and Kanabe and Okawara [395], starting from the monomers p-xylylene-bis(di-methylsulphonium tetrafluoroborate) [395] or chloride (Scheme 11.17) [391-394]. The latter is polymerized to yield a water soluble sulphonium salt polyelectrolyte (63d) which is then purified by dialysis [396]. The precursor polymer is converted to PPV (63e) by the thermal elimination of dimethyl sulphide and HCl. The method was later developed by Horhold et al. [397], Lenz and co-workers [398,399], Murase et al. [400] and Bradley [401], One of the major improvements within the last years has been the use of tetrahydrothio-phene instead of dimethyl sulphide in the synthesis of the precursor polymer [402]. The use of the cyclic leaving group facilitates the elimination when the precursor polymers is heated at 230-300°C and leads... 

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