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 (mu) of these polymers are relatively high (0.06-0.20 cm(2) 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.