So far, ligand steric effects of the α-diimine nickel catalysts on the polyolefin branching densities are not systematically investigated. Generally, in contrast to the α-diimine palladium systems, the branching densities of the polyethylene obtained by the α-diimine nickel catalysts increased when the more sterically encumbering substituent was employed. In this contribution, we described the synthesis and characterization of a series of α-diimine ligands and the corresponding nickel catalysts bearing the diarylmethyl moiety and varied steric ligands. In ethylene polymerization, the catalytic activities [(2.82–15.68) × 106 g/(mol Ni·h)], polymer molecular weights [M n: (0.37–131.51) × 104 g mol–1], branching densities [(28–81)/1000 C], and polymer melting temperatures (−4.7–122.9 °C) can be tuned over a very wide range. To our surprise, the polymer branching density first rose and then fell when we systematically increased the steric bulk of α-diimine nickel catalysts, like a downward parabola, not in line with previous conclusions. In ethylene-methyl 10-undecenoate (E-UA) copolymerization, the catalytic activities [(1.0 × 103) – (104.8 × 104) g/(mol Ni·h)], copolymer molecular weights [(1.2 × 103) – (242.4 × 103) g mol–1], branching densities [(42–70)/1000 C], and UA incorporation ratio (0.17–2.12%) can also be controlled over a very wide range. The tuning in steric ligands enables the tuning of the polymer microstructures such as molecular weight and branching density. In this way, the best polyethylene elastomer catalysts are screened out.

The investigation of the relationship between the structure of the catalyst and the microstructure of the obtained polymer has attracted much attention and broad interest in the field of transition metal-catalyzed olefin polymerization.

Thermoplastic elastomers (TPE) can be easily molded, extruded, and reused like plastics and exhibit typical elastic properties like rubbers. They are widely used in the automotive sector and household appliances sector. The synthesis of thermoplastic polyolefin elastomers using only ethylene as feedstock in a single step is a fascinating concept. The synthesis of polyethylene thermoplastic elastomers through α-diimine nickel catalyzed ethylene polymerization has been successfully achieved in recent years. However, the utilization of α-diimine palladium catalysts for this purpose is much more challenging due to their greater chain-walking tendency versus the nickel counterpart, thereby leading to the formation of highly branched polyolefins with poor mechanical properties. In this contribution, we report the direct synthesis of polyethylene thermoplastic elastomers through α-diimine palladium catalyzed ethylene polymerization. With exquisite catalyst design and polymerization regulation, polyethylene thermoplastic elastomers with great mechanical and elastic properties can be obtained. Most importantly, polar functionalized polyethylene thermoplastic elastomers bearing great elastic properties can be generated through copolymerization of ethylene with biomass-derived comonomers.

The bulky yet flexible substituent on the N-aryl moieties of α-diimine ligands may adopt different conformations and provide dynamic steric hindrance in the process of ethylene (co)polymerization.

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It is usually challenging to obtain high-molecular-weight polyethylene in the iminopyridyl Ni(II) and Pd(II) system due to the only one side axial steric structure of the iminopyridine ligand. In this study, the effectiveness of the dibenzosuberyl substituent in retarding the chain transfer in the iminopyridyl Ni(II) and Pd(II) catalysts has been demonstrated. The complexes Ni5 and Pd5 with phenyl and dibenzosuberyl substituents are noted to generate polyethylene and ethylene-MA copolymers with significantly higher molecular weight as compared to the complexes Ni1−4 and Pd1−4 with various diarylmethyl moieties. The synergistic effect of phenyl and dibenzosuberyl groups can further enhance the function of the catalyst to suppress the chain transfer. Further, polyethylene, ethylene oligomers, and ethylene-MA co-oligomers obtained in this study are observed to be highly branched. In addition, the iminopyridyl palladium catalysts can effectively promote the copolymerization of ethylene and MA to obtain the polar functionalized ethylene-MA co-oligomers with high incorporation ratios (up to 15.2 mol %).

A series of iminopyridyl Pd(II) catalysts containing bulky diarylmethyl substituents with various remote nonconjugated electron-withdrawing or -donating groups were synthesized and characterized. These catalysts possessing high catalytic activities and long...