Hyperbranched Polyethylenes Encapsulating Self-Supported Palladium(II) Species as Efficient and Recyclable Catalysts for Heck Reaction

We demonstrate in this paper a unique self-supporting strategy for the synthesis of hyperbranched polyethylenes (HBPEs) encapsulating Pd(II) species as efficient and recyclable catalysts for Heck reaction. This strategy combines remarkably the synthesis of HBPE polymer support and the immobilization of Pd(II) species in situ in one pot. It is achieved by chain walking copolymerization of ethylene with an acrylate comonomer (2) containing a disulfide functionality with a cationic Pd(II)-diimine catalyst (1). The copolymerization renders successfully HBPEs containing pendant disulfide groups at a controllable content, while with no poisoning effect from the disulfide functionality. Meanwhile, the Pd(II) catalyst in situ immobilizes itself onto the polymers by coordinative binding with the pendant disulfide groups, giving rise to the homogeneous self-supported Pd(II) catalysts. The resulting Pd-containing HBPEs having a low content of 2 (ca. < 1 mol %) have been found to be efficient and recyclable catalysts for the Heck reaction of iodobenzene and n-butyl acrylate while with a low/minimum Pd leaching (to form catalytically active species) during the reaction/recycling. In particular, the convenient recovery and reuse of the catalysts while at maintained high catalytic performance has been demonstrated with the use of a biphasic solvent system comprised of N,N-dimethylformamide and n-heptane in the Heck reaction.