Ladder polymers are a unique, synthetically challenging class of polymers with two continuous junctions along the backbone. Non-conjugated ladder polymers are typically synthesized through a mechanism that allows simultaneous formation of both bonds during polymerization, such as Diels–Alder cycloadditions. The non-planar nature of the resulting ladder polymers results in microporous materials, enabling applications in gas-phase separation and water purification. However, current syntheses require either hyperbaric, high-temperature conditions or the use of expensive transition-metal catalysts. Moreover, ladder polymerizations proceed via a step-growth mechanism that does not offer control of sequence, molecular weight, or architecture. A chain-growth polymerization will allow better synthetic access to ladder polymers and increase their utility and scope in separation and purification applications. To access these ladder polymers through a chain-growth mechanism, we were inspired by a recently reported visible-light-mediated photoredox Diels–Alder cycloaddition. Light serves as an ideal stimulus by lowering the amount of energy required for the polymerization; furthermore, organic photoredox catalysts eliminate the need for transition metals. In this polymerization, a dienophile initiator is oxidized by an excited photoredox catalyst and subsequently undergoes a radical cation [4+2] cycloaddition with a suitable diene. We hypothesized that the aryl group on the diene will stabilize the resulting radical cation towards propagation. We have identified conditions that promote the radical cation [4+2] cycloaddition between dienophile initiators and 2-aryl butadiene monomers. Increasing the ratio of diene relative to initiating dienophile yielded an increase in molecular weight. Characterization of these materials indicate microporosity, which is not observed when the same 2-aryl butadienes are polymerized under typical cationic conditions. Analysis of oligomeric products shows exclusive <i>endo</i> [4+2] cycloaddition adducts to form ladder structures.

Photoredox ladder polymerization

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.