Quaternization-induced catalyst-free synthesis of viologen-linked ionic polyacetylenes towards heterogeneous catalytic CO2 fixation

In this work, we reported a facile catalyst-free and initiator-free strategy to fabricate a new class of viologen-linked ionic polyacetylenes (denoted as VIPAs) that can achieve simultaneous ionization and polymerization in one-pot and one-step. To our surprise, the ionic polymers VIPA-X (X = Br or Cl) were directly synthesized by quaternization-induced in situ polymerization from commercially available 4,4′-bipyridine with propargyl bromide or propargyl chloride. It was found that reaction temperatures have a remarkable influence on the synthesis of VIPAs, achieving the transformation from soluble oligomers to insoluble highly cross-linked polymers by adjusting the temperatures from 30 °C to 100 °C. At a higher temperature of 100 °C, the obtained highly cross-linked ionic polymers VIPA-Br and VIPA-Cl were insoluble in both hot water and even dimethyl sulfoxide (DMSO). By virtue of abundant halogen anions and the absorbed H-bonded water, both VIPA-Br and VIPA-Cl were employed as efficient heterogeneous catalysts for the conversion of CO2 with various epoxides into cyclic carbonates at atmospheric pressure and low temperatures. In particular, the catalytic activities of VIPA-Br for the conversion of various epoxides are much better than those of VIPA-Cl at low temperatures, which should be attributed to the higher nucleophilicity and better leaving ability of the Br− anion than those of the Cl− anion. The present work affords a succinct catalyst-free strategy to construct more task-specific ionic polyacetylenes by quaternization-induced polymerization of multi-N-heterocycle monomers and propargyl halides towards diverse applications.