Rational Design and Construction of Helical Secondary Structures in Synthetic Polymers Based on Ladderization

Ladder polymers are a distinct class of macromolecules with double-strand backbone structures that impart certain features, such as exceptional rigidity, structural stability, intrinsic microporosity, and highly restricted conformational freedom, inaccessible to conventional single-strand polymers. While their mechanical, electronic, and separation-related functions have been widely evaluated, their potential for rational control of secondary structures has only recently begun to emerge. In this Perspective, we review recent advances in ladder polymer chemistry, focusing particularly on the rational construction of well-defined helical secondary structures. After outlining the historical development and synthetic strategies of ladder polymers, we highlight the crucial role of quantitative and chemoselective intramolecular cyclization, particularly modified alkyne benzannulation, in the construction of defect-free ladder frameworks. This precise ladderization enables the predictable control of helical geometry when combined with chiral molecular design, establishing a direct link between the monomer design and the resulting secondary structure. We also discuss recent examples demonstrating how the rational secondary structure control of ladder polymers gives rise to emergent functionalities including chiral recognition and circularly polarized luminescence. Finally, we outline future challenges and opportunities regarding the construction of more complex higher-order architectures, emphasizing the importance of integrating molecular design, advanced synthetic methodologies, and theoretical insights to bridge the gap between biological and synthetic macromolecular systems.