Recyclable Palladium-Loaded Hyperbranched Polytriazoles as Efficient Polymer Catalysts for Heck Reaction

Two polytriazole-based hyperbranched polymers (HBPs) with different molecular weights were prepared by chain-growth Cu-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization of an AB(2) monomer. Subsequent chain-end-capping reactions with alkynyl-containing ligands, such as 2-ethynylpyridine, phenylacetylene and alkyne-terminated polyethylene glycol (ay-PEG), produced a small library of HBPs that were applied to complex with Pd(OAc)(2) and produce Pd-loaded HBP catalysts for efficient Heck reaction of iodobenzene and styrene. The effects of HBP terminal ligand groups and polymer molecular weights were explored on the Heck reaction performance. At the same molecular weight, the HBP-based catalyst with PEG or pyridyl terminal groups showed faster Heck reaction kinetics as comparing to those with phenyl terminal groups. Meanwhile, higher-molecular-weight polymer catalysts exhibited faster catalytic kinetics than lower-molecular-weight ones. Comparing to the small-molecule monotriazole ligands, these polytriazole-based HBPs demonstrated higher catalytic efficiency due to multigroup cooperative effect, confirming that the polytriazole cluster was a boosting ligand for the catalytic Heck reaction. These HBP catalysts exhibited good recyclability as the catalyst after five-time recycling still achieved over 90% conversion of the reaction substrate.