Polyolefins, particularly polyethylene (PE), are crucial polymers with diverse architectures and applications. Branching significantly influences PE properties. Traditionally, branches are introduced via comonomers, an approach which adds cost and complexity. Chain walking polymerization, utilizing late transition metal catalysts such as nickel(II) and palladium(II), offers an alternative route by generating branches from ethylene alone. However, prior chain walking methods have resulted in uncontrolled branch patterns, including a mixture of methyl and longer branches, hindering precise control over microstructure. This lack of control is particularly pronounced at high temperatures and branch densities. This study addresses this challenge by demonstrating a novel nickel-based catalyst system enabling the highly selective generation of methyl branches with unprecedented control over branch pattern and distribution, even at industrially relevant temperatures.

Brookhart's pioneering work in 1995 established α-diimine late transition metal catalysts for olefin polymerization, introducing the concept of chain walking. This mechanism involves successive β-H elimination and re-insertion events, creating branches from ethylene monomers. However, previous studies using these catalysts have shown limitations in controlling branch pattern and distribution, resulting in a complex mixture of methyl and longer branches. The lack of control is particularly challenging at higher temperatures and higher branch densities, rendering the production of well-defined PE structures problematic, especially ethylene-propylene copolymers from ethylene alone at industrial temperatures (40-70 °C). This study aims to improve upon this limitation by exploring a new catalyst system.

A novel nickel(II) α-diimine catalyst, ipty-Ni, was synthesized from NiBr2(DME) and a rigid, sterically constrained ligand. The catalyst's structure was confirmed via 1H NMR spectroscopy, elemental analysis, and X-ray diffraction. Ethylene polymerization was performed using modified methylaluminoxane (MMAO) as an activator. The impact of temperature (30–90 °C) and pressure (2–8 bar) on the polymerization reaction was studied. The resulting polyethylenes were characterized using 1H and 13C NMR spectroscopy to determine the degree of branching, branch pattern, and branch distribution. Density Functional Theory (DFT) calculations were employed to probe the reaction mechanism and rationalize the observed selectivity. The 13C NMR spectra were analyzed in depth to identify the various branch patterns. A statistical model based on probability was used to predict the branch distribution.

The ipty-Ni catalyst exhibited high activity for ethylene polymerization, producing high-to-ultrahigh molecular weight polyethylenes. The degree of branching was tunable with temperature and pressure, reaching ultrahigh levels of >200 branches per 1000 carbons. Crucially, the catalyst demonstrated unprecedented selectivity toward the formation of methyl branches. Across all reaction conditions tested (30–90 °C, 2–8 bar), the methyl branch pattern was exclusive (99%), even at ultrahigh branching levels. This selective methyl branching is in sharp contrast to previous chain walking polymerization attempts which produced mixtures of different branch types. 13C NMR analysis revealed a highly selective and predictable branch distribution, which could be computed accurately using a statistical probability model that predicts branch placement based on the probability (p) of branch formation at any given point along the polymer chain. The model accurately predicted the percentage of 1,4-, 1,6-, 1,8- and longer methyl branch placements with even intervals. The model suggests that longer branch units like 1,3-Me/Et are inaccessible due to higher energy barriers during chain walking. DFT calculations support the experimental observations by revealing that the formation of methyl branches is kinetically and thermodynamically favored over longer branch formation. The calculations show the energy barrier for methyl branch formation is significantly lower compared to those of longer chain branches.

The results demonstrate a significant advancement in controlling polyethylene microstructure via chain walking polymerization. The ipty-Ni catalyst provides a remarkably simple and effective method for producing polyethylenes with well-defined properties. The ability to exclusively generate methyl branches at high densities and across a wide temperature range overcomes a long-standing limitation in chain walking polymerization. The high degree of control over branching is attributable to the specific design of the nickel catalyst and its unique interaction with ethylene during the chain walking mechanism. The strong agreement between experimental results and DFT calculations provides robust support for the proposed mechanism. The ability to predict branch distribution using a statistical model opens avenues for designing polymers with tailored properties for specific applications. The findings have direct implications for industrial ethylene polymerization, enabling the production of ethylene-propylene copolymers using ethylene as the sole feedstock and avoiding the use of expensive comonomers, thus simplifying the industrial process.

This study demonstrates highly selective methyl branch formation in ethylene polymerization using a low-cost nickel(II) catalyst (ipty-Ni). This catalyst enables the production of ultrahigh molecular weight polyethylenes with exclusive methyl branching and predictable distribution. This advance addresses a long-standing challenge in chain walking polymerization, offering a new pathway for efficient synthesis of tailored polyethylene materials with significant implications for industrial polymer production. Future work could explore other late transition metal catalysts and ligand modifications to enhance control and explore novel polymer architectures.

The study focuses on ethylene polymerization. Further research is needed to evaluate the catalyst’s performance with other α-olefins or explore its application in other polymerization processes. While the DFT calculations provide valuable insights into the mechanism, they are limited by inherent approximations within the computational methods. The statistical model, while accurate for this system, might need adjustments for other catalysts or monomers. Long-term stability of the catalyst under industrial conditions requires further investigation.