Abnormal N-Heterocyclic Carbene-Palladium Complexes for the Copolymerization of Ethylene and Polar Monomers

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 (M-n = 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 H-1 and C-13 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.