Photoredox Diels-Alder ladder polymerization

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. Ladder polymers with highly saturated backbones are prepared using a photoredox-catalyzed radical cation Diels-Alder polymerization.