Carbosilane side chain-equipped isoindigo-bithiophene semiconducting polymers (PII2T) have been designed and synthesized for stretchable electronics applications. Systematically tailoring the length and branch position of carbosilane side chains (C6 to C10) offers an effective route to optimize charge-transport behavior and improve the mechanical properties of semiconducting polymer thin films. The basic polymer properties, surface morphology, electrical characteristics, and strain-dependent performance of polymers with various lengths of carbosilane side chains were explored. The series of polymers exhibited a field-effect mobility over 2 cm(2) V-1 s(-1), and an odd-even effect was observed relating to the length of side chains. On the other hand, when the longer side chain was incorporated, a lower thin-film modulus was reached because the extended side chain can dilute the volume of the rigid polymer backbone and open up the space between polymer chains (i.e., larger lamellar spacing). Surprisingly, PII2T-C10 thin films possess desirable electrical and mechanical properties, achieving a mobility of 1 cm(2) V-1 s(-1) even when stretched under 100% strain, which is the best electrical performance among intrinsically stretchable conjugated polymers in the research community.