Synthesis, Morphology, and Optical and Electrochemical Properties of Poly(3-hexylthiophene)-b-poly(3-thiophene hexylacetate)

A series of all-conjugated diblock copolythiophenes of poly(3-hexylthiophene)-b-poly(3-thiophene hexylacetate) (P3HT-b-P3THA) were synthesized via modified sequential Grignard metathesis polymerization. The living P3HT was formed first, then reacting with the monomer of P3THA. By using 2-bromo-3-hexyloxycarbonylmethylene-5-iodothiophene instead of dibromo monomer in metal exchange reaction and by controlling the polymerization temperature relatively low at 16-20 degrees C, the reaction between carboxylate group and Grignard reagent can be minimized and the polymerization can be controlled; low PDI (<1.3), high regioregularity (>95%), and well-controlled block ratios of block copolymer were obtained. The introduction of carboxylate group in the side chain of one of the monomers, and controlling the side-chain length difference by only three atoms between two monomers, there are profound effects on the optical and electrochemical properties and morphologies of the block copolymers. The electron-withdrawing carboxylate causes the absorption maximum of copolymer in solution to be blue-shifted from that of pristine P3HT, and the extent of blue shift is increased monotonically with increasing the molar ratio of P3THA. However, in thin film, the intermolecular pi-pi stacking plays a role in the absorption behavior of copolymer which decreases the extent of blue shift. The HOMO level of the copolymer is lowered by 0.38 eV from that of P3HT due to the presence of P3THA block. The crystalline structure of the copolymer can be controlled according to the molar ratio of each block. Crystalline-amorphous, crystalline-crystalline, and cocrystalline structures are observed in the bulk samples when the block molar percentage of P3THA is increased from 22, 40, to SO and higher, respectively. Microphase separation is clearly present in the thin film fabricated from the copolymer containing crystalline-amorphous and crystalline-crystalline structures. The observation of various crystalline structures in a single type of all-conjugated diblock copolymer is very significant and provides a new approach to simultaneously manipulate the optical and electronic properties and nanostructures of conducting polymers by simply changing their compositions.