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 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.

Microstructural, Thermal and Electrical Properties of Methyl Methacrylate and 1‐Hexene Copolymers Made by Dinuclear Ni‐Based Catalysts

“…The polymerization at different temperatures for both catalysts revealed the highest productivity at 40 °C, (Table 1 and Figure 1). Increasing the kinetic energy of the polymerization along with the stability of the catalyst might be the reasons for these observations [33] . At elevated temperature (i. e., higher than 40 °C), the catalyst undergoes irreversible deactivation.…”

“…Despite a slew of literature on multinuclear LTM catalysts used to homopolymerization olefins, there are few reports on the copolymerization of olefins and polar monomers [2,28–43] . As an example, Khoshsefat and his coworkers synthesized polyolefins using dinuclear α‐diimine Ni‐, Co‐, and Fe‐based catalysts bearing different backbones, ortho‐substituents, and spacers between the active centers and investigated the steric effect, nuclearity and electronic features of ligands and their effect on the catalyst activity [2,28,29,33,34,41–43] . Cheng and coworkers employed a dinuclear Ni‐based catalyst with greater polar monomer incorporation than its mononuclear counterpart [44] .…”

“…Hong and his coworkers introduced Palladium complexes bearing abnormal imidazo‐ [1,5‐a]pyridine (aImPy)‐based N‐heterocyclic carbene ligands for the homopolymerization of olefins and the copolymerization of olefins and polar monomers. Up to 3.0 % of the polar monomers were incorporated into the main chain, as determined by 1 H and 13 C NMR [40] . Besides, it is known that in copolymerization, the copolymer composition has a dependency on the reactivity ratios (r 1 and r 2 ) of the monomers.…”

Synthesis of imidazo[1,5-a]pyridines via cyclocondensation of 2-(aminomethyl)pyridines with electrophilically activated nitroalkanes

“…The imidazo [1,5-a]pyridine core is also considered to be one of the privileged pharmacophoric scaffolds and can be found in many biologically active compounds, for example, the potent antitumor agent C 1311 inhibiting topoisomerase II [4][5][6][7][8][9] or pirmagrel, a cytotoxic immunosuppressant and DNA synthesis inhibitor ( Figure 1) [10][11][12]. In addition, compounds with this struc-ture were investigated as photoluminescent sensors [13] and have been employed to generate pincer and heterocyclic carbene ligands for transition metal catalysis [14,15]. A lot of efforts have been dedicated to the development of efficient synthetic methods to access imidazo [1,5-a]pyridines, with more than 120,000 individual compounds prepared to date.…”

“…1 ) [ 10 – 12 ]. In addition, compounds with this structure were investigated as photoluminescent sensors [ 13 ] and have been employed to generate pincer and heterocyclic carbene ligands for transition metal catalysis [ 14 – 15 ]. A lot of efforts have been dedicated to the development of efficient synthetic methods to access imidazo[1,5- a ]pyridines, with more than 120,000 individual compounds prepared to date.…”