Context <p>Due to the versatile properties, the hydrocarbon ladder polymers developed recently represent a class of promising materials for membrane separations of industrially relevant gas mixtures. The excellent thermomechanical properties and ultrahigh permselectivity are generally attributed to the chain rigidity of the three-dimensional (3D) ladder structures, which are not readily accessible via experiments. Therefore, this work takes advantage of molecular simulations to quantify the chain rigidity of such polymers, which exhibit quite different behaviors from the conventional polymers. It is first identified that the characteristic segments and repeating units behave like bows that are easy to press but difficult to pull. Furthermore, it is demonstrated that the subtle influences of side groups and temperature on the end-to-end distance (Re) can be well captured by these all-atomistic (AA) simulations. As suggested by the fluctuation in Re, the configuration provides an effective way to tune overall chain morphology and thus its rigidity, resulting from both the rigidity and shape of characteristic segments. These results highlight the unique rigidity of such polymers, which should inspire further development of new robust polymer membranes of gas separations.</p> Methods <p>The software named Materials Studio 4.0 was employed to construct the all-atomistic structures of model systems and to perform all the energy minimizations and molecular dynamics simulations with the Deriding force field and QEq charges, and to analyze the structural and energetic properties.</p>