Randi Zhang

Polyolefin elastomers (POEs) produced by ethylene/α-olefin copolymerization are indispensable for high-end applications such as photovoltaic encapsulation, precision microelectronic protection, advanced foamed footwear, and automotive lightweight components. α-Diimine nickel catalysts that operate through a chain-walking mechanism convert ethylene as the sole feedstock into highly branched polyethylene elastomers, eliminating the need for expensive comonomers. This review systematically analyzes α-diimine nickel complexes developed in recent years via modulation of ligand steric hindrance, electronic effects, and backbone rigidity, and provides a quantitative comparison of their catalytic performance. The compiled data reveal that the catalytic activities span 10⁴–10⁷ g PE (mol Ni)⁻¹ h⁻¹, the molecular weights range from 10⁴ to 10⁶ g mol⁻¹, and the polydispersity indices (PDIs) can be tuned between 1.2 and 29.3, affording polyethylenes with branching densities from 2 to over 186 branches per 1000 carbons. These structural parameters directly govern the mechanical flexibility, elastic recovery, thermal properties, and processability in injection molding, foam extrusion, and film blowing, thus dictating the materials’ suitability for the aforementioned high-value applications. By establishing clear structure–performance relationships, this review offers forward-looking guidance for the industrial scale-up and catalyst design of polyethylene elastomers produced exclusively from ethylene.

Binuclear α-diimine nickel catalysts have attracted significant interest in ethylene polymerization due to their distinctive metal–metal cooperativity, which enables catalytic properties not achievable with mononuclear systems. This review summarizes the structural design principles and recent advances of these binuclear catalysts, with an emphasis on how different bridging architectures such as rigid aryl, flexible chain, N-heterocyclic, and specialty bridges regulate intermetallic distance, steric environment, and electronic effects. The impact of these structural factors on catalytic activity, polymer molecular weight, branching density, and mechanical properties is discussed. By establishing structure–performance relationships, this review provides a foundation for the rational design of binuclear nickel catalysts for advanced polyethylene synthesis.

This work provides a comprehensive exploration of α-olefin polymerization characteristics catalyzed by the classic α-diimine Ni catalyst. The polymerization process exhibited quasi-living behaviour, and a reaction kinetic model for the monomer coordination–insertion process was established. It was observed that the reaction exhibits living polymerization features during the first 10 min, and the coordination–insertion rate constant was determined to be 1.08 L·mol⁻¹·s⁻¹ at 30 °C. The regulation rules for factors including co-catalyst amount, monomer concentration, polymerization temperature, monomer type on the molecular weight, molecular weight distribution and chain structure of poly(α-olefin)s were clarified. The co-catalyst (methylaluminoxane) primarily served to activate the catalyst without inducing a chain transfer effect, suggesting that chain stagnation is likely the primary cause of the deviation from typical living polymerization behaviour. Based on temperature-controlled experiments, the activation energy for the coordination–insertion reaction was calculated to be 28.40 kJ·mol⁻¹ through GPC curve analysis. The kinetic model established in this study, along with the revealed chain branching rules, provides a theoretical foundation for the design of poly(α-olefin)s with novel structures and functions.

The depicted benzhydryl-containing Fe catalysts appended with ortho-C_nH_2n−1 (n = 5, 6, 8) and ortho-halide groups, showed high activity/thermostability for C₂H₄ polymerization.

The direct copolymerization of ethylene with polar monomers to produce functional polyolefins continues to be highly appealing due to its simple operation process and controllable product microstructure. Low-cost nickel catalysts have been extensively utilized in academia for the synthesis of polar polyethylenes. However, the development of high-temperature copolymerization catalysts suitable for industrial production conditions remains a significant challenge. Classified by the resultant copolymers, this review provides a comprehensive summary of the research progress in nickel complex catalyzed ethylene-polar monomer copolymerization at elevated temperatures in the past five years. The polymerization results of ethylene–methyl acrylate copolymers, ethylene-tert–butyl acrylate copolymers, ethylene–other fundamental polar monomer copolymers, and ethylene–special polar monomer copolymers are thoroughly summarized. The involved nickel catalysts include the phosphine-phenolate type, bisphosphine-monoxide type, phosphine-carbonyl type, phosphine-benzenamine type, and the phosphine-enolate type. The effective modulation of catalytic activity, molecular weight, molecular weight distribution, melting point, and polar monomer incorporation ratio by these catalysts is concluded and discussed. It reveals that the optimization of the catalyst system is mainly achieved through the methods of catalyst structure rational design, extra additive introduction, and single-site catalyst heterogenization. As a result, some outstanding catalysts are capable of producing polar polyethylenes that closely resemble commercial products. To achieve industrialization, it is essential to further emphasize the fundamental science of high-temperature copolymerization systems and the application performance of resultant polar polyethylenes.

The less-expensive and earth-abundant nickel catalyst is highly promising in the copolymerization of ethylene with polar monomers and has thus attracted increasing attention in both industry and academia. Herein, we have summarized the recent advancements made in the state-of-the-art nickel catalysts with different types of ligands for ethylene copolymerization and how these modifications influence the catalyst performance, as well as new polymerization modulation strategies. With regard to α-diimine, salicylaldimine/ketoiminato, phosphino-phenolate, phosphine-sulfonate, bisphospnine monoxide, N-heterocyclic carbene and other unclassified chelates, the properties of each catalyst and fine modulation of key copolymerization parameters (activity, molecular weight, comonomer incorporation rate, etc.) are revealed in detail. Despite significant achievements, many opportunities and possibilities are yet to be fully addressed, and a brief outlook on the future development and long-standing challenges is provided.

Precipitation polymerization (PP) is a powerful tool to prepare various types of uniform polymer particles owing to its outstanding advantages of easy operation and the absence of any surfactant. Several PP approaches have been developed up to now, including traditional thermo-induced precipitation polymerization (TRPP), distillation precipitation polymerization (DPP), reflux precipitation polymerization (RPP), photoinduced precipitation polymerization (PPP), solvothermal precipitation polymerization (SPP), controlled/‘‘living’’ radical precipitation polymerization (CRPP) and self-stabilized precipitation polymerization (2SPP). In this review, a general introduction to the categories, mechanisms, and applications of precipitation polymerization and the recent developments are presented, proving that PP has great potential to become one of the most attractive polymerization techniques in materials science and bio-medical areas.

Combined condensation and complexation of 2‐acetyl‐5,6,7‐trihydroquinolin‐8‐one, cobalt(II) chloride and the corresponding aniline has been employed as an effective one‐pot route to the 2‐(1‐(arylimino)ethyl)‐8‐arylimino‐5,6,7‐trihydroquinoline‐cobalt(II) chlorides (aryl=2‐Me‐4‐(CHPh₂)‐6‐(C₅H₉)C₆H₂ Co1, 2‐Me‐4‐(C₅H₉)‐6‐(CHPh₂)C₆H₂ Co2, 2‐(C₅H₉)‐4‐Me‐6‐(CHPh₂)C₆H₂ Co3, 2‐(C₅H₉)‐4,6‐(CHPh₂)₂C₆H₂ Co4, 2‐(C₆H₁₁)‐4,6‐(CHPh₂)₂C₆H₂ Co5, 2‐(C₈H₁₅)‐4,6‐(CHPh₂)₂C₆H₂ Co6, 2‐F‐4,6‐(CHPh₂)₂C₆H₂ Co7, 2‐Cl‐4,6‐(CHPh₂)₂C₆H₂ Co8, 2‐Me‐4,6‐(CHPh₂)₂C₆H₂ Co9). All complexes have been well characterized including by single crystal X‐ray diffraction for Co3. On activation with either MAO or MMAO, Co1 was the most active for ethylene polymerization with the MAO‐activated precatalyst reaching a peak level at 60 °C (up to 13.66×10⁶ g (PE) mol⁻¹(Co) h⁻¹). By varying the electronic and steric properties of the ortho‐ and/or the para‐substituents of the N‐aryl groups, excellent control over molecular weight could be achieved with values falling in the range 0.97–101.63 kg mol⁻¹ obtainable. Notably, with a benzhydryl group positioned on the ortho‐position of the N‐aryl group, a significant enhancement of the molecular weight was observed, while the least bulky ortho‐cyclopentyl substituent led to increased catalytic activity. As a final point, distinctive microstructural characteristics of the polyethylenes were observed with Co1/MAO producing near quantitative levels of vinyl end groups.

Six types of ferrous chloride complex, [2‐(ArN=CMe)‐8‐(ArN)‐7,7‐Me₂C₉H₆N]FeCl₂ (Ar=2‐(C₅H₉)‐6‐MeC₆H₃ Fe1, 2‐(C₆H₁₁)‐6‐MeC₆H₃ Fe2, 2‐(C₈H₁₅)‐6‐MeC₆H₃ Fe3, 2‐(C₅H₉)‐4,6‐Me₂C₆H₂ Fe4, 2‐(C₆H₁₁)‐4,6‐Me₂C₆H₂ Fe5, 2‐(C₈H₁₅)‐4,6‐Me₂C₆H₂ Fe6), each bearing a gem‐dimethyl‐substituted N,N,N‐chelating ligand that differs in the ortho‐cycloalkyl ring size and para‐substituent of the N‐aryl groups, are disclosed. All complexes have been prepared by a straightforward one‐flask route and characterized by FT‐IR spectroscopy and elemental analysis. In the case of Fe1, oxidation to form the oxo‐bridged diferric species [2‐(2‐(C₅H₉)‐6‐MeC₆H₃N=CMe)‐8‐(2‐(C₅H₉)‐6‐MeC₆H₃N)‐7,7‐Me₂C₉H₆N]FeCl(μ‐O)FeCl₃ (Fe1') has been demonstrated and its structural identity confirmed by single crystal X‐ray diffraction. Following treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), Fe1–Fe6 exhibited optimal catalytic activities at 60°C for ethylene polymerization [up to 2.35 × 10⁷ g of PE (mol of Fe)⁻¹ h⁻¹ for Fe1/MMAO] producing linear polyethylene; even over prolonged run times, high activities were maintained. Moreover, this family of iron catalysts exhibited remarkable thermostability by displaying appreciable activity at temperatures as high as 100°C [activity up to 7.8 × 10⁶ g of PE (mol of Fe)⁻¹ h⁻¹]. By varying the ring size of the ortho‐cycloalkyl and para‐R² group (R² = H, Me), excellent control over molecular weight of the polyethylenes could be achieved with values in the range 1.4–67.8 kg mol⁻¹ obtainable. In addition, end‐group analysis of the polymers generated with Fe/MAO or Fe/MMAO revealed that both β‐H elimination and chain transfer to aluminum are competitive processes during chain termination.