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 (<b>3a</b>) (that has bulky substituent adjacent to the chloromagnesium group), was investigated. LiCl promotes the polymerization of <b>3a</b>, and the polymerization also exhibits living characteristics similar to those of the “normal” monomer, 2-bromo-5-chloromagnesio-3-hexylthiophene (<b>3b</b>). 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 (<i>M</i><sub>n</sub>) than the theoretical value was obtained. Because of the slow initiation with <b>3a</b>, the polymerization of <b>3a</b> occurs primarily after the consumption of <b>3b</b> in the copolymerization of <b>3a</b> and <b>3b</b>, and P3HT with high regioregularity was still obtained.