Abnormal N-Heterocyclic Carbene–Palladium Complexes for the Copolymerization of Ethylene and Polar Monomers
Da-Ae Park, Seunghwan Byun, Ji Yeon Ryu, Jin‐Young Lee, Junseong Lee, Sukwon Hong
Abstract
Palladium complexes bearing abnormal imidazo[1,5-a]pyridine (aImPy)-based N-heterocyclic carbene ligands were developed for the homopolymerization of olefins and the copolymerization of olefins and polar monomers. The highly electron-donating nature of these abnormal N-heterocyclic carbenes (aNHCs) embedded in the carbene–phenolate chelating ligand scaffold resulted in good catalytic activity, generating linear polyethylene with a high molecular weight (Mn = 237 000). aImPy–Pd complexes efficiently catalyzed propylene polymerization to afford polypropylene free of regio defects. These catalytic systems exhibit good tolerance of various polar monomers and afford the desired copolymers. Up to 3.0% of the polar monomers were incorporated into the main chain, as determined by 1H and 13C NMR analysis. Furthermore, methyl methacrylate (MMA), as a 1,1-disubstituted ethylene derivative, was successfully incorporated into the main chain of polyethylene. Computational studies indicated that ethylene insertion into a Pd-alkyl bond by aImPy–Pd catalyst 6a is faster than that by the Pd catalyst bearing normal NHC analogues, which could be attributed to the high linearity and high molecular weight of polyethylene.