The Chung group’s catalyst research focuses on 1) Organometallic reactions in neat water 2), Recyclable and removable catalysts development, and 3) ATRP with Ru benzylidene catalyst (Grubbs catalysts).

Ru benzylidene catalysts are important multitasking organometallic catalysts in organic and polymer chemistry because the catalyst leads to the formation of stable carbon-carbon covalent bonds. The most practically and commercially successful Ru benzylidene catalyst is Ru-based Grubbs 2nd generation catalyst which performs olefin metathesis reactions, ring opening metathesis polymerization (ROMP), and atom transfer radical polymerization (ATRP). The most iconic structural features of the Ru benzylidene catalyst are the benzylidene and N-heterocyclic carbene (NHC) ligand structures as shown in Figure 7. The benzylidene forms a metallacyclobutane intermediate according to the catalytic mechanism of olefin metathesis reactions. NHC and other ligand structures of the Ru catalyst controls electronic and steric environments of the catalyst that can yield various catalytic reaction performances. In particular, the NHC can be rationally modified to provide extraordinary features to the catalyst complex such as water solubility and removability.

The complete removal of Ru residue from the resulting products still presents a major challenge for their practical application because the metal residue causes undesirable electronic properties, colorization, and toxicity of pharmaceutical products. Specifically, the maximum amount of Ru permissible in pharmaceutical products is less than 10 ppm. The Chung group develops modified NHC ligands to efficiently remove / recycle the entire organometallic catalyst complexes after homogeneous reactions. 

The detailed mechanism of ATRP in the presence of Ru benzylidene catalyst has only recently been reported in 2016, despite the commercial and academic importance of ATRP. The ATRP of Ru benzylidene catalyst follows completely different mechanism (control between dormant species and active radical species) than olefin metathesis (formation of metallacyclobutane intermediate). Because direct ATRP of non-modified Ru benzylidene catalyst is new, a lot of further studies are needed with various reaction solvents, monomers, catalysts, temperatures, and other conditions. Especially, true multi-reactions (first ROMP then ATRP) can be performed to prepare sophisticated polymers (e.g., multi-functional bottle brush polymers). The Chung group has demonstrated that the modified NHC enables efficient metathesis reaction and ATRP in neat water. Well-defined polymers were obtained from water soluble polymers (acryl amide, zwitterionic monomers, and PEG methylether methacrylate) showing excellent control during radical polymerizations. Block copolymers were successfully prepared by chain extention experiments. The chain extension experiment results demonstrate living polymer chain end (halide) which is a representative feature of ATRP.