Precision synthesis of a fluorene-thiophene alternating copolymer by means of the Suzuki–Miyaura catalyst-transfer condensation polymerization: the importance of the position of an alkyl substituent on thiophene of the biaryl monomer to suppress disproportionation

The Suzuki–Miyaura catalyst-transfer condensation polymerization (CTCP) of fluorene-thiophene biaryl monomers was investigated for the synthesis of well-defined poly(fluorene-alt-thiophene). Model reactions of α,ω-dibromo(fluorene-thiophene) with arylboronic acid esters showed that t-Bu3PPd and di-tert-butyl(4-dimethylaminophenyl)phosphine (AmPhos) Pd catalysts undergo intramolecular catalyst transfer from fluorene to thiophene, irrespective of the use of phenyl- or thiopheneboronate. Based on the results of the model reactions, PinB-fluorene-thiophene-Br (PinB = pinacol boronate) monomers were synthesized. The polymerization of the monomer containing the 3-octyl-5-bromothiophene-2-yl unit was accompanied by disproportionation, whereas the polymerization of the monomer containing the 4-octyl-5-bromothiophene-2-yl unit with an AmPhos Pd initiator proceeded according to the CTCP mechanism: the Mn values of the obtained polymers increased in proportion to monomer conversion and to the feed ratio of the monomer to initiator. Moreover, successive CTCP using the fluorene-thiophene monomer, dioctylfluorene monomer, and 3-hexylthiophene monomer with the AmPhos Pd initiator yielded a variety of all-conjugated di- and triblock copolymers.