Highly Active and Thermally Robust Nickel Enolate Catalysts for the Synthesis of Ethylene-Acrylate Copolymers

Ultrahigh Molecular Weight Ethylene-Acrylate Copolymers Synthesized with Highly Active Neutral Nickel Catalysts

Molecular weight is a crucial parameter in catalytic olefin polymerization to determine the thermomechanical properties of polymers. Elevating the molecular weight of ethylene-acrylate copolymer, a typical polar polyolefin has been a persistent challenge. This study presents the direct synthesis of ultrahigh molecular weight (UHMW) ethylene-acrylate copolymers using a highly active, neutral, single-component phosphine-phenolate nickel catalyst under mild conditions. The coordination-insertion copolymerization of ethylene and various acrylates, such as methyl acrylate, n-butyl acrylate, and t-butyl acrylate, can yield ethylene-acrylate copolymers with UHMWs (Mn) ranging from 1000 to 2700 kDa, with exceptional activities of up to 4.03 × 106 g mol-1 h-1. A relatively large-scale synthesis (30-50 g) of UHMW ethylene-acrylate copolymers was notably achieved. The properties of these UHMW ethylene-acrylate copolymers were comprehensively evaluated, demonstrating that these copolymers retained the bulk properties of ultrahigh molecular weight polyethylene (UHMWPE) while exhibiting improved surface and dyeing properties. In particular, the tensile strength of the ethylene-acrylate copolymers significantly increased with higher molecular weights, reaching 44 MPa (147 MPa after a predrawing ratio of 3.1), which is comparable to that of UHMWPE.

Catalyst Editing via Post-Synthetic Functionalization by Phosphonium Generation and Anion Exchange for Nickel-Catalyzed Ethylene/Acrylate Copolymerization

Rapid, efficient development of homogeneous catalysts featuring desired performance is critical to numerous catalytic transformations but remains a key challenge. Typically, this task relies heavily on ligand design that is often based on trial and error. Herein, we demonstrate a "catalyst editing" strategy in Ni-catalyzed ethylene/acrylate copolymerization. Specifically, alkylation of a pendant phosphine followed by anion exchange provides a high yield strategy for a large number of cationic Ni phosphonium catalysts with varying electronic and steric profiles. These catalysts are highly active in ethylene/acrylate copolymerization, and their behaviors are correlated with the electrophile and the anion used in late-stage functionalization.

Recent Advances in Nickel Catalysts with Industrial Exploitability for Copolymerization of Ethylene with Polar Monomers

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.

Aqueous Coordination-Insertion Copolymerization for Producing High Molecular Weight Polar Polyolefins

Hydrocarbons, when used as the medium for transition metal catalyzed organic reactions and olefin (co-)polymerization, are ubiquitous. Environmentally friendly water is highly attractive and long-sought, but is greatly challenging as coordination-insertion copolymerization reaction medium of olefin and polar monomers. Unfavorable interactions from both water and polar monomer usually lead to either catalyst deactivation or the formation of low-molecular-weight polymers. Herein, we develop well-behaved neutral phosphinophenolato nickel catalysts, which enable aqueous copolymerization of ethylene and diverse polar monomers to produce significantly high-molecular-weight linear polar polyolefins (219-549 kDa, 0.13-1.29 mol %) in a single-component fashion under mild conditions for the first time. These copolymerization reactions occur better in water than in hydrocarbons such as toluene. The dual characteristics of high molecular weight and the incorporation of a small amount of functional group result in improved surface properties while retain the desirable intrinsic properties of high-density polyethylene (HDPE).

Trivalent Cations Slow Electron Transfer to Macrocyclic Heterobimetallic Complexes

Incorporation of secondary redox-inactive cations into heterobimetallic complexes is an attractive strategy for modulation of metal-centered redox chemistry, but quantification of the consequences of incorporating strongly Lewis acidic trivalent cations has received little attention. Here, a family of seven heterobimetallic complexes that pair a redox-active nickel center with La3+, Y3+, Lu3+, Sr2+, Ca2+, K+, and Na+ (in the form of their triflate salts) have been prepared on a heteroditopic ligand platform to understand how chemical behavior varies across the comprehensive series. Structural data from X-ray diffraction analysis demonstrate that the positions adopted by the secondary cations in the crown-ether-like site of the ligand relative to nickel are dependent primarily on the secondary cations' ionic radii and that the triflate counteranions are bound to the cations in all cases. Electrochemical data, in concert with electron paramagnetic resonance studies, show that nickel(II)/nickel(I) redox is modulated by the secondary metals; the heterogeneous electron-transfer rate is diminished for the derivatives incorporating trivalent metals, an effect that is dependent on steric crowding about the nickel metal center and that was quantified here with a topographical free-volume analysis. As related analyses carried out here on previously reported systems bear out similar relationships, we conclude that the placement and identity of both the secondary metal cations and their associated counteranions can afford unique changes in the (electro)chemical behavior of heterobimetallic species.

Origin of Suppressed Chain Transfer in Phosphinephenolato Ni(II)-Catalyzed Ethylene Polymerization

Recent breakthroughs in the generation of polar-functionalized and more sustainable degradable polyethylenes have been enabled by advanced phosphinephenolato Ni(II) catalysts. A key has been to overcome this type of catalysts' propensity for extensive chain transfer to enable formation of high-molecular-weight polyethylene chains. We elucidate the mechanistic origin of this paradigm shift by a combined experimental and theoretical study. Single-crystal X-ray structural analysis and cyclic voltammetry of a set of six different catalysts with variable electronics and sterics, combined with extensive pressure reactor polymerization studies, suggest that an attractive Ni-aryl interaction of a P-[2-(aryl)phenyl] is responsible for the suppression of chain transfer. This differs from the established picture of steric shielding found for other prominent late transition metal catalysts. Extensive density functional theory studies identify the relevant pathways of chain growth and chain transfer and show how this attractive interaction suppresses chain transfer.

Acrylate-Induced β-H Elimination in Coordination Insertion Copolymerizaton Catalyzed by Nickel

Polar monomer-induced β-H elimination is a key elementary step in polar polyolefin synthesis by coordination polymerization but remains underexplored. Herein, we show that a bulky neutral Ni catalyst, 1Ph, is not only a high-performance catalyst in ethylene/acrylate copolymerization (activity up to ∼37,000 kg/(mol·h) at 130 °C in a batch reactor, mol % tBA ∼ 0.3) but also a suitable platform for investigation of acrylate-induced β-H elimination. 4Ph-tBu, a novel Ni alkyl complex generated after acrylate-induced β-H elimination and subsequent acrylate insertion, was identified and characterized by crystallography. A combination of catalysis and mechanistic studies reveals effects of the acrylate monomer, bidentate ligand, and the labile ligand (e.g., pyridine) on the kinetics of β-H elimination, the role of β-H elimination in copolymerization catalysis as a chain-termination pathway, and its potential in controlling the polymer microstructure in polar polyolefin synthesis.

A co-anchoring strategy for the synthesis of polar bimodal polyethylene

Since polar groups can poison the metal centers in catalysts, the incorporation of polar comonomers usually comes at the expense of catalytic activity and polymer molecular weight. In this contribution, we demonstrate polar bimodal polyethylene as a potential solution to this trade-off. The more-polar/more-branched low-molecular-weight fraction provides polarity and processability, while the less-polar/less-branched high-molecular-weight fraction provides mechanical and melt properties. To achieve high miscibility between these two fractions, three synthetic routes are investigated: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy (CAS) to heterogenize different homogeneous catalysts on one solid support. The CAS route is the only viable strategy for the synthesis of polar bimodal polyethylene with good molecular level entanglement and minimal phase separation. This produces polyolefin materials with excellent mechanical properties, surface/dyeing properties, gas barrier properties, as well as extrudability and 3D-printability.

Recent Advances in Transition Metal-Based Catalysts for Ethylene Copolymerization with Polar Comonomer

The synthesis of polar functionalized polyolefin (PFP) offers improvement in mixing properties, polymer surface, and rheological properties with the potential of upgraded polyolefins for modern and ingenious applications. The synthesis of PFP from metal-based catalyzed olefin (non-polar in nature) copolymerization with polar comonomers embodies energy-efficient, atom-efficient, and apparently an upfront methodology. Despite their outstanding success during conventional polymerization of olefin, 3^rd and 4^th group (early transition metal)-based catalysts, owing to their electrophilic nature, face challenges mainly due to Lewis basic sites of the polar monomers. On the contrary, late transition metal-based catalysts have also made progress, in recent years, for PFP synthesis. The recent past has also witnessed several advancements in the development of dominating palladium-based catalysts while their lower resistance towards ligand functional groups has limited the practical application of abundant and cheaper nickel-based catalysts. However, the relentless efforts of the scientific community, during the past half-decade, have indicated rigorous progress in the development of nickel-based catalysts for PFP synthesis. In this review, we have abridged the recent research trends in both early as well as late transition metal-based catalyst development. Furthermore, we have highlighted the role of transition metal-based catalysts in influencing the polymer properties.

Customizing Polymers by Controlling Cation Switching Dynamics in Non-Living Polymerization

Controlling the chain growth process in non-living polymerization reactions is difficult because chain termination typically occurs faster than the time it takes to apply an external trigger. To overcome this limitation, we have developed a strategy to regulate non-living polymerizations by exploiting the chemical equilibria between a metal catalyst and secondary metal cations. We have prepared two nickel phenoxyphosphine-polyethylene glycol variants, one with 2-methoxyphenyl (Ni1) and another with 2,6-dimethoxyphenyl (Ni2) phosphine substituents. Ethylene polymerization studies using these complexes in the presence of alkali salts revealed that chain growth is strongly dependent on electronic effects, whereas chain termination is dependent on both steric and electronic effects. By adjusting the solvent polarity, we can favor polymerizations via non-switching or dynamic switching modes. For example, in a 100:0.2 mixture of toluene/diethyl ether, reactions of Ni1 and both Li⁺ and Na⁺ cations in the presence of ethylene yielded bimodal polymers with different relative fractions depending on the Li⁺/Na⁺ ratio used. In a 98:2 mixture of toluene/diethyl ether, reactions of Ni2 and Cs⁺ in the presence of ethylene generated monomodal polyethylene with dispersity <2.0 and increasing molecular weight as the amount of Cs⁺ added increased. Solution studies by NMR spectroscopy showed that cation exchange between the nickel complexes and alkali cations in 98:2 toluene/diethyl ether is fast on the NMR time scale, which supports our proposed dynamic switching mechanism.