Synthetic Attempts towards Polymers with Pentafulvene Structural Units

Synthetic attempts towards fully conjugated polymers 9 with pentafulvene‐diyl structural units are described. Cationic polymerization of pentafulvenes 1a (R = X = Me) and 1b (R = X = MeS) nearly quantitatively gives polymers 8a and 8b with typical M n and M w values of 38800 and 53750, respectively, for 8a , and 12000 and 35900, respectively, for 8b . Key step of the conversion 8a → 9a ( Scheme 6 ) is a quantitative bromination 8a → 32a , the structure of 32a being confirmed by analytical data as well as by spectroscopic comparison with model compound 23 . Best results in view of two‐fold the HBr elimination 32a → 9a are obtained with Et 3 N, but so far elimination has not been complete. Synthetic sequences are optimized with model compound 21 ( Scheme 4 ). Here again, bromination 21 → 23 is quantitative, while two‐fold HBr elimination 23 → 22 with Et 3 N proceeds in 51% yield. Dibromide 23 easily undergoes HBr elimination followed by a Br shift to give bromide 29 . Contrary to cationic polymerization, anionic polymerization of simple pentafulvenes 1 to 2 (which would be attractive in view of the conjugated polymers 3 ) is not successful: For pentafulvene 1b (R = X = MeS), the main reaction is Diels ‐ Alder ‐type dimerization 1b → 15b ( Scheme 2 ), even under anionic conditions.