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Abstract π‐conjugated polymers (CPs) that are concurrently soft and stretchable are needed for deformable electronics. Molecular‐level modification of indacenodithiophene (IDT) copolymers, a class of CPs that exhibit high hole mobilities (_hole), is an approach that can help realize intrinsically soft and stretchable CPs. Numerous examples of design strategies to adjust the stretchability of CPs exist, but imparting softness is comparatively less studied. In this study, a systematic molecular weight (MW) series is constructed on a promising candidate for soft CPs, poly(indacenodithiophene‐co‐thienopyrroledione) (p(IDT_C16‐TPD_C8)), by optimizing direct arylation polymerization conditions in hopes of improving stretchability andμ_holewithout significantly impacting softness. We found p(IDT_C16‐TPD_C8) at a degree of polymerization of 32 shows high stretchability (crack onset strain,CoS> 100%) without significantly impacting softness (elastic modulus,E= 32 MPa), which to the best of our knowledge outperforms previously reported stretchable and soft CPs. To further study how molecular‐level modifications impact polymer properties, a MW series of a new extended donor unit polymer, poly(indacenodithienothiophene‐co‐thienopyrroledione) (p(IDTT_C16‐TPD_C8)), was synthesized. The IDTT_C16copolymers did not result in a greater averageμ_holewhen comparing between p(IDTT_C16‐TPD_C8) and p(IDT_C16‐TPD_C8) despite their higher crystallinity observed by GIWAXS. While these findings warrant further investigation, this study points toward unique charge transport properties of IDT‐based polymers. 

Abstract For wearable and implantable electronics applications, developing intrinsically stretchable polymer semiconductor is advantageous, especially in the manufacturing of large‐area and high‐density devices. A major challenge is to simultaneously achieve good electrical and mechanical properties for these semiconductor devices. While crystalline domains are generally needed to achieve high mobility, amorphous domains are necessary to impart stretchability. Recent progresses in the design of high‐performance donor–acceptor polymers that exhibit low degrees of energetic disorder, while having a high fraction of amorphous domains, appear promising for polymer semiconductors. Here, a low crystalline, i.e., near‐amorphous, indacenodithiophene‐co‐benzothiadiazole (IDTBT) polymer and a semicrystalline thieno[3,2‐b]thiophene‐diketopyrrolopyrrole (DPPTT) are compared, for mechanical properties and electrical performance under strain. It is observed that IDTBT is able to achieve both a high modulus and high fracture strain, and to preserve electrical functionality under high strain. Next, fully stretchable transistors are fabricated using the IDTBT polymer and observed mobility ≈0.6 cm²V⁻¹s⁻¹at 100% strain along stretching direction. In addition, the morphological evolution of the stretched IDTBT films is investigated by polarized UV–vis and grazing‐incidence X‐ray diffraction to elucidate the molecular origins of high ductility. In summary, the near‐amorphous IDTBT polymer signifies a promising direction regarding molecular design principles toward intrinsically stretchable high‐performance polymer semiconductor.