Synthesis and Persistence Length Study of Defect-Free and Non-Aggregated Conjugated Ladder Polymers

A fundamental understanding of the solution properties of conjugated ladder polymers (CLPs) is essential for advancing their design, synthesis, and solution processing toward high-performance optoelectronic applications. Nevertheless, elucidating the solution conformation of CLPs remains a significant challenge in the field of polymer physics, owing to the difficulty of synthesizing defect-free samples, their intrinsically low solubility that results in weak signals and limited analytical accuracy, the pronounced tendency of CLPs to aggregate even when dissolved, and the absence of reliable theoretical models. Here, these fundamental challenges are addressed by the synthesis, neutron scattering measurements, and computational simulations of two model CLPs, LP1 and LP2. Owing to their bulky three-dimensional side chains, LP1 and LP2 exhibit a non-aggregated character and high dispersibility as single polymer chains. Small-angle neutron scattering revealed unexpectedly low persistence lengths (L _p) of 3-5 nm. The L _p being similar to those of non-ladder conjugated polymers such as P3HT indicates the long-range conformational semiflexibility of CLPs despite them possessing a ladder-type constitution. Machine learning-based molecular dynamics simulations further showed that the semiflexibility of these CLP chains mainly results from the pronounced out-of-plane deformations, which is synergistically influenced by the steric congestion of the side chains. Overall, a comprehensive experimental and computational approach demonstrates that CLPs, despite their fused-ring polyaromatic backbones, are best described as ribbon-like semiflexible chains, in contrast to the common belief that they are rigid-rod polymers.