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 -Bu 3 PPd 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 M n 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. The Suzuki-Miyaura coupling polymerization of PinB-F8T(3)-Br was accompanied by disproportionation, whereas that of PinB-F8T(4)-Br proceeded in a chain-growth polymerization manner to afford a well-defined fluorene-thiophene alternating copolymer.