AB Block Copoly(3-alkylthiophenes): Synthesis and Chiroptical Behavior

In a first part of this article, the synthesis of AB type block copoly(3-alkylthiophene)s initiated by Ni(dppp)Cl-2, the most commonly used initiator for these polymers, is investigated. For this study the respective H-1 NMR resonances of all possible end-groups are identified. This result confirms the hypothesis that the Ni(dppp) species can walk back to the beginning of the polymer chain and that propagation can occur at both chain ends. The next part of the article studies the chiroptical behavior of AB-type block copoly(3-alkylthiophene)s with one chiral block and compares the results with those of the corresponding random copolymers. In order to obtain exclusively AB-type block copolymers, the polymers were prepared from a modified Ni initiator. They all have the same degree of polymerization but vary in the length of the respective blocks. The chiroptical behavior was studied by changing the ratio solvent/nonsolvent, meanwhile monitoring the UV-vis and circular dichroism (CD) spectra. Three series were investigated: one in which both blocks aggregate simultaneously, one in which the achiral block stacks before the chiral block, and one in which the chiral block stacks first followed by the achiral block. It was found that when the blocks stack independently, the (chiral/achiral) stacking of the latter is significantly influenced by the former. If both blocks of the polymer chains aggregate simultaneously, Cotton effects which are significantly larger than those of the chiral homopolymer are found.