Thermally Robust Heterobimetallic Palladium–Alkali Catalysts for Ethylene and Alkyl Acrylate Copolymerization

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.

One-Step Green Synthesis of Isoeugenol Methyl Ether from Eugenol by Dimethyl Carbonate and Phase-Transfer Catalysts

In this paper, the green synthesis of isoeugenol methyl ether (IEME) from eugenol by O-methylation and isomerization is completed using a one-step green process. In the methylation reaction, dimethyl carbonate (DMC) was used as a green chemistry reagent instead of the traditional harmful methylation reagents, in accordance with the current concept of green chemistry. The phase transfer catalyst (PTC) polyethylene glycol 800 (PEG-800) was introduced into the isomerization reaction to break the barrier of difficult contact between solid and liquid phases and drastically reduce the reaction conditions by shortening the reaction time and reducing the alkalinity of the reaction system. The catalytic systems for the one-step green synthesis of IEME were screened, and it was shown that the catalytic system "K2CO3 + PEG-800" was the most effective. The effects of reaction temperature, n(DMC):n(eugenol) ratio, n(PEG-800):n(eugenol) ratio, and n(K2CO3):n(eugenol) ratio on eugenol conversion, IEME yield, and IEME selectivity were investigated. The results showed that the best reaction was achieved at a reaction temperature of 140 °C, a reaction time of 3 h, a DMC drip rate of 0.09 mL/min, and n(eugenol):n(DMC):n(K2CO3):n(PEG-800) = 1:3:0.09:0.08. As a result of the conversion of 93.1% of eugenol to IEME, a yield of 86.1% IEME as well as 91.6% IEME selectivity were obtained.

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.

Rigidifying Cation-Tunable Nickel Catalysts Increases Activity and Polar Monomer Incorporation in Ethylene and Methyl Acrylate Copolymerization

In this study, we synthesized and characterized two nickel complexes featuring conformationally rigid bisphosphine mono-oxide ligands, where one has an o-methoxyphenyl (Ni2) and the other has an o-(2-methoxyethoxy)phenyl (Ni3) substituent on the P═O moiety. We performed metal binding studies using Ni3 and found that its reaction with Li⁺ and Na⁺ most likely produced 1:1 and 1:1/2:1 nickel:alkali species in solution, respectively. The nickel complexes were competent catalysts for ethylene homopolymerization and copolymerization, with activities up to 3.8 × 10³ and 8.1 × 10 kg mol^-1 h^-1, respectively. In reactions of ethylene with methyl acrylate (1.0 M), the addition of Li⁺ to Ni3 led to a 5.4-fold enhancement in catalyst activity and a 1.9-fold increase in polar monomer incorporation in comparison to those by Ni3 alone under optimized conditions. A comparison with other nickel catalysts reported for ethylene and methyl acrylate copolymerization revealed that our nickel-alkali catalysts are competitive with some of the most efficient Ni-based systems developed thus far.

Ligand–metal secondary interactions in phosphine–sulfonate palladium and nickel catalyzed ethylene (co)polymerization

Ligand secondary interactions and Lewis acid modulation are simultaneously achieved in palladium and nickel catalyzed ethylene polymerization and copolymerization.

Development of highly productive nickel–sodium phenoxyphosphine ethylene polymerization catalysts and their reaction temperature profiles

Heterobimetallic nickel–sodium phenoxyphosphine complexes were found to be among one of the most efficient late metal catalysts for ethylene polymerization.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

Here we will summarize some of the recent advances in α-diimine ligand developments, as well as some new and challenging monomers that this class of catalysts can address through these ligand improvements.

Cation-controlled catalysis with crown ether-containing transition metal complexes

This Feature Article reviews the structural motifs and catalytic applications of crown ether-containing catalysts and details the development of “pincer-crown ether” ligands for applications in controlled catalysis.

Accelerating ethylene polymerization using secondary metal ions in tetrahydrofuran

A variety of metal cations are capable of enhancing the ethylene polymerization rates of nickel phosphine phosphonate-polyethylene glycol catalysts.