Photoredox Diels–Alder ladder polymerization

Abstract

Ladder polymers are synthetically challenging targets that comprise a sequence of rings in which each repeat unit shares at least two atoms with the adjacent one. Ladder polymers with sp³-hybridized backbones feature kinked structures with restricted bond rotation. Such ladder polymers are typically synthesized through a mechanism that allows simultaneous formation of both bonds during polymerization, such as the Diels–Alder cycloaddition. Prior Diels–Alder polymerizations required elevated temperature and/or pressure to achieve the desired reactivity, and the resulting products include aromatic linkages within the backbone. Here, we show that photoredox catalysis provides access to unique ladder polymers with sp³ backbones under mild reaction conditions. We design 2-arylbutadiene monomers that enable propagation of the cyclohexene formed by each successive cycloaddition by stabilizing the required radical cation. The polymerization achieves molecular weights up to 4400 g mol⁻¹ with various electron-rich 2-aryl butadiene monomers. The resulting products may also be employed as macromonomers to form ladder bottlebrush polymers through a cationic polymerization. This report represents the first example of applying photoredox catalysis to the synthesis of ladder polymers and yields a novel sp³-rich ladder polymer structure.