Fluoride-ion-induced oligomerisation reactions

Since fluoroatkenes are so susceptible to nucleophilic attack, we might have expected anionic polymerisation, initiated by fluoride ion, to occur readily. As we have noted, fluorocarbanions are readily generated by fluoride-ion attack on fluoroalkenes these 

By contrast, anionic polymerisation of hexafluoro-2-butyne (see Section IIB) proceeds rapidly because elimination of fluoride ion from the propagating anion is difficult, in that it would require the formation of allenes. 

Similar fluoride-ion-induced oligomerisations of hexafluoropropene [143, 144] (in which only dimers and trimers are formed, due to increased steric hindrance in the propagation step) and chlorotrifluoroethene [145] have been described. In each of these cases, highly branched fluoroaUcene systems are formed, due to the tendency of the intermediate carba-nion to lose fluoride ion, giving the most thermodynamically stable fluoroalkene. 

Oligomerisation of fluoroalkenes can also be initiated by tertiary amines, such as pyridine [147], trimethylamine [148] and tetrakis(dimethylamino)ethylene [149], via processes that involve either initial ylid formation or the generation in situ of an active source of fluoride ion. 

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Many useful, higher-molecular-weight fluoroalkenes can be conveniently prepared by fluoride-ion-induced oligomerisation reactions of smaller fluoroalkenes such as tetrafluoroethene and hexafluoropropene, and these methods are discussed in Section C. 

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