Crucial to the development of polymeric semiconductors has been the establishment of structure-property correlations to achieve high-mobility and versatile devices. [2] Despite the continuing improvement of carrier mobilities, a clear understanding of charge transport mechanisms and the relationship between device function and molecular structure are yet to be fully explored. Thanks to the thirdgeneration donor-acceptor (D-A) alternating conjugated copolymers, tunable comonomers are able to be incorporated into the backbone, evolving in the diversity and complexity to deeply investigate the influencing factors. [3] Coplanarity, π-conjugation, linearity, symmetry, frontier molecular orbital (FMO) energy, heteroatom effect, molecular weight, and density of side chains, to name but a few, are usually deemed critical in the structural and conformational control for rational design of conjugated polymers. When attempting to gain insight into these sophisticated factors, one might select a state-of-the-art building block and adjust the other moiety to fine-tune electronic and solid-state structures. A delicate compromise between such factors is also needed to achieve the optimal device performance. [4] Exploring new structures to boost device performance has always been the motivation of synthetic scientists. [1c,h,5] During the past few years, bislactam or bisimide has emerged as electron-withdrawing building blocks for high-performance semiconductors. For example, field-effect transistors of diketopyrrolopyrrole-, isoindigo-, and naphthalenediimide-based polymeric semiconductors exhibit mobilities of 1-10 cm 2 V −1 s −1 , which are comparable to inorganic counterparts. [1c,d] The incorporation of bislactam or bisimide provides three major advantages: (a) the electron deficient nature of these functional groups lowers the FMO energy levels to stabilize injected charge carriers and enhances stacking interactions between π-orbitals by depleting electron density and subsequently relieving electrostatic repulsion; [6] (b) the presence of intramolecular throughspace interactions minimizes dihedral torsions and promotes backbone coplanarity; [1c] (c) readily introduced side chains can not only tune the polymer aggregation tendency but also ensure solution processability. [3b] Among these bislactam-orTo establish a structure-property relationship between polymer backbone structures and field-effect transistor performance has emerged as a new topic in organic electronics. The tunability and diversity of organic semiconductors provide the feasibility of controlling the electrical properties. Herein the characterization of thienothiophene-, dithiophenylethene-, biselenophene-, and diselenophenylethene-containing azaisoindigo copolymers is presented. As suggested by both theoretical calculations and experimental results, backbone electronic structure and linearity, density of side chains, aggregation, and thin film microstructure are involved in the differences in optical and electrical properties of these polymers. As...