Synthesis of Ladder Polymers: Developments, Challenges, and Opportunities.

Ladder polymers are unique in that their backbones consist of fused rings with adjacent rings having two or more atoms in common. The restriction of bond rotations in rigid ladder polymers greatly limits their conformational freedom, leading to many intriguing and unique properties. As a non-traditional type of polymers, rigid ladder polymers are of great fundamental interest and technical importance as advanced materials for applications such as membrane gas separation and organic electronics. Ladder polymers can be divided into non-conjugated (with kinked conformations) and conjugated (with planar conformations) structures. Their synthesis can be broadly classified into two general strategies: direct ladder polymerization, and zipping of a linear precursor polymer. This Concept article outlines the historical development of ladder polymers and the chemical strategies used for their synthesis; highlights the challenges associated with their synthesis and characterization, and presents opportunities and outlooks for this unique and intriguing type of polymers.