LiCl-Promoted Chain Growth Kumada Catalyst-Transfer Polycondensation of the “Reversed” Thiophene Monomer

The effect of LiCl on the chain growth Kumada catalyst-transfer polycondensation (KCTP) of the “reversed” thiophene monomer, 5-bromo-2-chloromagnesio-3-hexylthiophene (3a) (that has bulky substituent adjacent to the chloromagnesium group), was investigated. LiCl promotes the polymerization of 3a, and the polymerization also exhibits living characteristics similar to those of the “normal” monomer, 2-bromo-5-chloromagnesio-3-hexylthiophene (3b). However, initiation is much slower than chain propagation due to the steric hindrance between hexyl groups in the transmetalation (TM) step that leads to the formation of the initiator via head-to-head (HH) coupling. Consequently, regioregular poly(3-hexylthiophene) (P3HT) with large polydispersity (PDI) and higher number-average molecular weight (M_n) than the theoretical value was obtained. Because of the slow initiation with 3a, the polymerization of 3a occurs primarily after the consumption of 3b in the copolymerization of 3a and 3b, and P3HT with high regioregularity was still obtained.