Wessling-Zimmerman route,

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. 

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. 

Table I. Some Examples of PPVs Prepared by the Wessling-Zimmerman Route...

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 firsX(lO) and the second/"//J methods are known to be Wessling-Zimmerman method and Gilch-Wheelwright polymerization, respectively. The last reaction route is practically the same as that is utilized in the synthesis of poly(/ -xylylene) from /7-xylene via the CVD polymerization method. The first two methods have been widely used not only in the synthesis of PPV but also PPV derivatives. Since the methods produce soluble precursor polymers, the final polymer can be obtained in various shapes. 

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

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 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. 

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