Developments in Binuclear Late Transition Metal Catalysts for Ethylene Homo-and Copolymerization

—Recent advances in the research of late transition metal (Ni) salicylaldimine bimetallic catalysts in ethylene polymerization and copolymerization of ethylene with comonomers have been introduced. The effect of positioning two polymerization-active metal centers in close proximity on catalytic activity, molecular weight, molecular weight distribution, and levels of branching are thoroughly documented. The synergistic effect of these binuclear catalysts compared to their mononuclear analogs is the subject of this review.


Introduction
In 1997, Dr. Johnson from DuPont [1] and Professor Grubbs [2] from Caltech developed a highly effective catalyst, known as the late transition metal salicylaldimine, for olefin insertion polymerization. This sparked curiosity among researchers about the ligand structure of salicylaldimine, which is easily synthesized and modified to prepare various olefin polymerization catalysts with excellent performance using different metal centers. By changing the properties of the ligand's substituents, researchers delved into the polymerization mechanism. However, another approach to enhance catalytic performance is the development of binuclear metal complexes. Various structures with different metal-metal distances based on the salicylaldimine structure have been reported [3][4][5][6] . The appropriate distance between metals can exhibit strong synergistic effects, resulting in higher polymerization activity and copolymerization insertion rate than mononuclear catalysts. Binuclear catalysts can produce higher molecular weight polymers and may offer unique catalytic properties.

Synthesis and Structure of Late Transition Metal Catalysts
Compared to other mononuclear catalysts, binuclear salicylaldimine nickel catalysts demonstrate greater stability, activity, comonomer inclusion, and superior tolerance to polar monomers in olefin polymerization. The exceptional synergistic effects of these catalysts result in enhanced polymerization activity, increased methyl chain branching, and higher molecular weight.  In 2015, Marks and colleagues synthesized a series of bimetallic Ni complexes 1-5 [7] based on the previously reported planar aromatic bis(phenoxyimino)dinickel complex [8][9][10][11] . The complexes include monometallic 2 and 4, symmetrical bimetallic 3 and 5, and asymmetrical bimetallic 1. To obtain ligand 1a, 2,7-diformyl-1,8-dihydroxynaphthalene was first condensed with 2,6-(3,5ditrifluoromethylphenyl)aniline and then with 2-((4-( t butyl)phenyl-sulfonyl)aniline. Deprotonation of ligand 1a by reaction with NaH followed by reaction with trans-NiClMe(PMe 3 ) 2 furnished complex 1 in high yield ( Figure 1). All other complexes were prepared similarly. In 2016, Ma and colleagues reported a series of monometallic and bimetallic salicylaldehyde Ni catalysts 6-9 [12] . 1,8-diiodonaphthalene and Bpin-substituted salicylaldehyde were produced by Suzuki coupling reaction to generate compound 6a, and then combined with 2,6-diisopropylaniline is condensed to obtain the ligand 6b. The potassium salt obtained after the ligand reacted with KH in THF was reacted with trans-NiCl(1-Naphthyl)(PPh 3 ) to form complex 6 ( Figure 2). Other complexes were synthesized similarly. In 2017, Li and colleagues reported the synthesis of a series of bimetallic salicylaldehyde Ni catalysts 10-12 [13] , in which complexes 11 and 12 are bimetallic complexes. Using 9,9-Dimethylxanthene as a synthetic raw material, first react to synthesize 4-Hydroxy-5-trimethylsilyl-9,9dimethylxanthene, and then react through hydroxyl protection, formylation, and deprotection yields Ligand 10a. Ligands 10a combined with(pyridine) 2 Ni(Me) 2 in toluene solution gave nickel-methylpyridine complexes 10 in high yields ( Figure 3). Other complexes were synthesized similarly. By taking advantage of the functions of multimetallic enzymes in nature, metal centers with close spatial distances have played a huge role in the field of coordinated polymerization. The synergistic effect of adjacent metal active centers is closely related to the appropriate metal-metal distance, which can significantly improve polymer molecular weight, chain branch density, comonomer selectivity, etc., compared with the corresponding mononuclear analogues. Through the design of different bridging structures and ligand structures, the distance of the Ni-Ni active metal center can be adjusted to achieve synergistic distance. The statistics of the Ni-Ni metal center of the complex in the literature are shown in Table 1. Due to the presence of the aromatic bis(phenoxyimino) ligand structure, the metals are close to each other, and the asymmetry is the same as that of the metal complex 1. The intermetallic spacing is 5.926 Å, and the intermetallic spacing of the symmetrical homometallic complex 5 is 5.621 Å, both of which can have a synergistic effect. For dinuclear nickel complexes 6-8 with bridge chains of different lengths, due to the restraint of the rigid structure and the influence of bulky imine substituents, the two metal centers are forced to approach each other. From complex 6 to complex 8, as the length of the bridge chain increases, the Ni-Ni spatial distance gradually increases, and the synergistic effect will be weakened. For complexes 10 and 12 with similar rigid bridging ligands, the Ni-Ni distance is 6.92 Å, and a suitable spatial distance may produce synergistic effect, while complex 12 will promote Ligand dissociation.

Binuclear late transition metal catalysts for ethylene homopolymerization
From the data in Table 2, as seen in runs 1-5, asymmetric bimetallic complex 1 combines Ni metal centers (CF 3 /Ni) that can generate linear high molecular weight polyolefins and Ni metal centers that can generate high branching and low density (SO 2 /Ni), the synergistic effect between metals makes the complex synthesize polyethylene with high molecular weight, narrow distribution, and long chain branching. It can be seen from runs 6-9 that the synergistic effect is produced by the proximity of the two metal Ni centers, the activities of complexes 6 and 7 are increased by 3 times and 2 times respectively compared with the single metal analog 9, and the molecular weight of polyethylene is improved by 4 times and 3 times respectively, while the performance of complex 8 is closer to that of complex 9.
From run 10-12, the synergistic bimetallic Ni complex 10 exhibited high activity at 93 °C, while inactivation of mononuclear analogue 11 produced only trace amounts of polyethylene and complex 12 with large substituents produced polyethylene. The molecular weight of ethylene is smaller than that of complex 10.

Binuclear late transition metal catalysts for ethylene copolymerization
Based on the data presented in Table 3, a copolymerization reaction of ethylene/1-hexene using complex 6-9 in run1-4 shows a similar trend of both catalytic activity and polymer molecular weight. Notably, the polymerization activity decreases significantly while the branch density increases. The synergistic effect of complexes 6 and 7 leads to the production of ethylene/1hexene copolymers with higher molecular weight compared to polyethylene. Conversely, copolymers produced by complexes 8 and 9 show lower molecular weight than polyethylene. In run 5-8, ethylene copolymerization with 1,5-hexadiene (1,5-HD) and 1,7octadiene (1,7-OD) was conducted using complexes 10 and 11. Complex 10 exhibits moderate activity in ethylene and 1,5-HD copolymerization, while mononuclear complex 11 shows similar catalytic activity to complex 10. Interestingly, the ethylene polymerization product of mononuclear complex 11 in the copolymerization of ethylene with 1,5-HD was negligible. Furthermore, highly active dinuclear complex 10 forms pendant vinyl polymers.

Conclusion
Through the research presented in this paper, it is apparent that the structure of salicylaldimine ligands can be easily modified, allowing for the design of bimetallic salicylaldimine Ni catalysts with varying metal-metal distances using different bridging methods. Proximity between metals may lead to a synergistic effect, and binuclear catalysts often exhibit higher catalytic activity and produce unexpected variations in the polymer's microstructure, such as increased molecular weight, branching degree, and molecular weight distribution, when compared to their mononuclear counterparts. However, the synthesis of bimetallic complexes is typically expensive and crystal structure confirmation can be challenging, which limits their applications. Nevertheless, we are optimistic that there are still ample possibilities for designing bimetallic catalysts in the future. With an enhanced understanding of the mechanisms behind metal synergy, we anticipate the creation of more functionally diverse catalysts.