The demand of stretchability for a semiconducting polymer has increased to realize wearable devices and sensors. However, studies involving intrinsically stretchable pi-conjugated polymers are still limited. Here, we develop a soft-polythiophene derivative, P3SiHT, with a trisiloxane unit in the side chains via a hexylene spacer unit. In addition, diblock (P3HT-b-P3SiHT) and triblock (P3HT-b-P3SiHT-b-P3HT) copolymers could be synthesized based on Kumada catalyst-transfer polycondensation. The results of atomic force microscopy and grazing incidence small-angle X-ray scattering indicate that the block copolymer thin films form a phase-separated structure between the P3HT and P3SiHT domains. The organic thin film transistor devices were prepared to assess the electrical properties of the block polymers. As a result, the block copolymers showed comparable or even higher hole mobility than that of P3HT homopolymer, thus due to the enhanced phase-separation and thereby charge transportation. The mechanical test of the bulk films indicates that P3HT-b-P3SiHT-b-P3HT shows lower tensile modulus and longer elongation at break than P3HT homopolymer and other diblock copolymers. (C) 2018 Wiley Periodicals, Inc.