A series of alkyl-substituted indacenodithiophene (alkyl-IDT) semiconducting donor–acceptor polymers were designed by DFT to have varying degrees of backbone planarity and synthesized via direct arylation polymerization (DArP). These polymers exhibit weak intermolecular interactions, a glass transition temperature (T g) below room temperature, and low degrees of crystallinity from XRD measurements. Despite this, the field-effect mobilities (μ) of these polymers are relatively high (0.06–0.20 cm2 V–1 s–1) with mobility increasing with increasing backbone planarity. Because of the weak intermolecular interactions, the polymers exhibit low elastic moduli (E f) of less than 450 MPa. The polymer with the most twisted backbone exhibits high ductility with a crack-onset strain (CoS) over 100%. These structure–property relationship studies provide useful guidelines for designing semiconducting polymers with high mobility, low stiffness, and high ductility enabling applications in stretchable electronics.