Effect of the Nature and the Position of Defects on the Chiral Expression in Poly(3-alkylthiophene)s

Abstract

Polymers produced with the most used controlled polymerization method with the simplest initiators, namely the Kumada catalyst transfer condensation polymerization (KCTCP) with Ni2+ salts, all have one regiodefect distributed along the polymer backbones. The irregularity created by these defects can be split up into three components, i.e., the nature of the defect, the position of the defect, and the additional inhomogeneity created by mixing polymers with the defect at different positions. In this research, the influence of these three parameters on the chiral expression of poly(3-alkylthiophene)s (P3ATs) is investigated separately for the first time. A series of chiral P3ATs with one tail-to-tail (TT) or head-to-head (HH) defect in different positions is synthesized by means of the Pd(RuPhos) protocol, and their chiral expression and that of their mixtures are studied via UV–vis and circular dichroism (CD) spectroscopy. For the polymers with an HH defect, it is found that the position at the very beginning of the polymer chain is the most optimal to obtain the largest chiral expression, and the obtained signal is even the highest ever reported for PTs in chloroform and MeOH solutions. When a TT defect is incorporated into the polymer, the chiral expression is much weaker, and the most optimal position of the defect is a little shifted toward the middle of the chain compared to their HH analogues. For the mixtures of the polymers with an HH defect, a weighted average of the signals of its constituents is obtained, but for the mixtures of the polymers with a TT defect different results are obtained. When 25% of each polymer is mixed, no chiral response is obtained, and when more of the polymers with the defect more toward the chain ends are incorporated, a chiral response higher than that of any of the component polymers is recorded. This shows that the chiral expression is not only influenced by the nature or the position of a defect, but also by the mixing of polymers with a defect in different positions.