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 Et₃N, 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 Et₃N 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.