Controlled living ring expansion polymerization for the precision synthesis of cyclic poly(aryl isocyanides)

Cyclic polymers exhibit physical properties distinct from those of their linear polymer counterparts. Although cyclic polymer materials with advanced designs can be synthesized using methods that control their molecular weight, dispersity, and topology, synthetic limitations have thus far restricted structurally defined cyclic polymers to non-conjugated structures with sp³-hybridized carbon backbones. Here we report a living ring expansion polymerization that enables the precise synthesis of cyclic polymers composed of sp²-hybridized carbon backbones, that is, cyclic poly(aryl isocyanides). Polymerization is initiated from a diarylpalladacycle complex in the presence of iodobenzene dichloride and proceeds via the successive insertion of aryl isocyanides into the palladacycle. This living polymerization process yields structurally controlled cyclic poly(aryl isocyanides) with low dispersity and predefined, customizable, high or low-molecular weights. The cyclic topology was unambiguously characterized by multiple analytical techniques, including direct visualization by high-resolution atomic force microscopy. Furthermore, the high fidelity of the palladium center facilitates the controlled synthesis of cyclic block copolymers. This advancement opens avenues for the development of topologically conjugated polymer materials.