CO₂-Triggered Recoverable Metal Catalyst Nanoreactors using Unimolecular Core–Shell Star Copolymers as Carriers

Metallic nanocatalysts are highly active in a variety of organic transformations, but their durable and facile reuse without comprising activity remains a great challenge. Herein, we describe the use of a CO2-triggered unimolecular core–shell star copolymer as carrier for immobilizing gold (Au) and silver (Ag) nanocatalysts for preserving high activity, while achieving facile recoverability. The core–shell star polymers comprising an inert hyperbranched polyethylene (HBPE) core and multiple CO2-responsive arms with tertiary amine groups were synthesized by palladium-catalyzed chain walking polymerization of ethylene and 2-(2-bromoisobutyryloxy)ethyl acrylate, followed by atom-transfer radical polymerization of dimethylaminoethyl methacrylate (DMAEMA) and diethylaminoethyl methacrylate (DEAEMA) using HBPE as a macroinitiator. The star copolymers were synergistically engineered via chain topology, composition, and functionality control. They form well-dispersed unimolecular micelles in aqueous solution with CO2 treatment and immobile Au or Ag nanoparticles on the arms via electrostatic interaction. The supported Au nanoparticle nanoreactors were shown to possess high activity in catalyzing the reduction of 4-nitrophenol. They could be readily precipitated with N2 bubbling after reaction and redispersed into the aqueous solution upon purging CO2 for successive reactions. The catalytic reaction process could be paused or resumed by controlling CO2/N2 bubbling steps without scarifying the catalytic activity. The catalysts had an average catalytic activity of apparent reaction rate constant (kapp) of 8.4 × 10–2 s–1 in 15 cycles of reduction of 4-nitrophenol, in comparison with the reported highest kapp = 1.2 × 10–2 s–1 in 10 cycles previously reported. Further experimental and model studies indicate that larger Au/star polymer ratio, higher grafting density, and longer arm length provide for higher catalytic activity. It is demonstrated here that CO2-triggered star copolymer carrier is a promising approach for providing metallic nanocatalysts with high activity and durability.