Enhancing Poly(4-vinylpyridine)-Based Catalysts for CO ₂ Conversion via Quaternization

Amine-functionalized polymers are attractive platforms for converting CO2 into value-added cyclic carbonates, yet limited understanding of how polymer structure governs catalytic performance and stability has hindered their rational design. Here, we synthesize and systematically evaluate a series of alkyl-halide-quaternized poly(4-vinylpyridine) (P4VP) materials as a polymer-based, tunable catalyst platform for CO2 cycloaddition. By independently varying alkyl chain length and halide counterion identity within a common polymer backbone, we elucidate how polymer functionalization modulates catalytic activity and recoverability. Catalytic performance was evaluated for the cycloaddition of CO2 to epichlorohydrin, used as a model substrate, at 57 °C. Increasing alkyl chain length enhances activity through a combination of weakened halide-pyridinium ion pairing and increased polymer solubility in the hydrophobic reaction medium, which together improve intrinsic active-site reactivity and accessibility. However, increased solubility also leads to reduced catalyst recovery, highlighting an inherent trade-off between activity and physical recoverability. Counterion identity further governs performance, with iodide-based catalysts outperforming bromide analogs due to weaker electrostatic interactions with the pyridinium cation and reduced susceptibility to deactivation by hydrogen-bond donors. Studies conducted in the presence of catalytic amounts of water and ethanol reveal that hydrogen-bond donor strength modulates activity through a balance between epoxide activation and anion sequestration. Density functional theory calculations corroborate these trends by showing stronger hydrogen bonding between water and halide anions, particularly bromide, which limits anion availability for catalysis. Together, these results establish polymer-specific structure–property-performance relationships for quaternized P4VP catalysts and provide mechanistic design principles for developing functional polymeric materials for CO2 cycloaddition under mild conditions.