Effect of cross-linker on morphology, catalytic activity, and recyclability of immobilized palladium chloride

摘要

Two kinds of immobilized palladium (Pd) catalysts were prepared by reversible addition fragmentation chain transfer polymerization of pyridine-containing monomer followed by immobilizing palladium chloride (PdCl_2) on block copolymers. Namely, one of them includes the cross-linking structure of maleic anhydride with 1,6-diaminohexane (cross-linker), polystyrene-block-poly(4-(4-vinylbenzyloxy)butylpicolinate-alt- maleic anhydride)-Pd (PS-b-P(VBP-alt-MAn)-Pd), and the other is its non-cross-linking counterpart, polystyrene-block-poly(4-(4-vinylbenzyloxy) butylpicolinate)-Pd (PS-b-PVBP-Pd). From transmission electron microscopy images, it could be observed that they both assembled into micelles in the selective solvents. The Pd of PS-b-P(VBP-alt-MAn)-Pd located in the core of micelles, whereas the Pd of PS-b-PVBP-Pd was on the shell of the micelles. The PS-b-P(VBP-alt-MAn)-Pd can be continuously used for five times without any appreciable loss of activity in the aqueous Suzuki-coupling reaction. However, the catalytic activities of the PS-b-PVBP-Pd decreased sharply with the increase in the recycle times. Thus, this promising cross-linking strategy not only greatly restrained the loss of Pd in the catalytic cycles, but also effectively maintained the immobilized Pd catalyst's high activity.