The diverse properties and applications of polyethylenes depend on their molecular weights, molecular weight distributions, and chain topology. Considering the importance of chromium complexes in catalytic ethylene polymerization and oligomerization, we have synthesized a series of Cr complexes (Cr1−Cr6) bearing a SNN-tridentate ligand. In the presence of MAO as cocatalyst, complexes Cr1−Cr6 exhibited moderate to extremely high activity (up to 2.4 × 10 7 g(PE) mol −1 (Cr) h −1 ) toward ethylene polymerization. The influences of the ligand and of various reaction parameters, including the nature and amount of the cocatalyst and the reaction temperature and pressure, were systematically investigated with Cr1. It was found that lower reaction temperatures (50 °C) and ethylene pressure (5 atm) and larger MAO/Cr ratios (1500) favored bimodal distributions with the dominant high-M w fraction. In contrast, higher reaction temperatures (≥80 °C) and ethylene pressure (40 atm) and lower MAO/Cr ratios (≤500) almost exclusively led to the production of low-M w polyethylene waxes with monomodal and narrow distributions. Based on DFT calculations and UV−vis−NIR spectroscopy, two types of active species generated by Cr1 and MAO were proposed to be responsible for the production of bimodal polyethylene. By tuning the structures of the Cr complexes in the Cr1−Cr6/MAO systems and the reaction conditions, polyethylenes with molecular weights ranging from low-M w waxes to UHMWPE and monomodal or bimodal distributions were readily synthesized.

In order to examine the possibility to promote cooperative effects on catalytic activity and selectivity in ethylene dimerization through ligand design, the bisphosphino-iminato ligands syn- L and anti- L were prepared to support the dinuclear nickel complexes syn -Ni 2 and anti- Ni 2 . The Ni centers are successfully locked in relatively close proximity in syn- Ni 2 (6.433(5) Å) but are much farther apart in anti- Ni 2 because of the rigid anthracene skeleton. The mononuclear complex [NiBr2(C14H9-NCH-C6H4-2-PPh2)] (Ni 1 ) was also prepared for control experiments. In the presence of EtAlCl2, syn -Ni 2 showed a remarkably high activity for ethylene dimerization (>90%) (up to 9.10 × 106 g (mol of Ni)−1 h–1), which is approximately 1.5- and 3.3-fold higher, respectively, than those of anti- Ni 2 or of mononuclear Ni 1 . The redox properties of dinuclear complexes were studied by cyclic voltammetry (CV) and their comparison with those of the mononuclear complex indicates the possible existence of cooperativity between the two metal centers in the dinuclear structures. Although a detailed mechanism has not been elucidated, cooperative effects favor the isomerization of 1-butene, and dinuclear syn- Ni 2 and anti- Ni 2 exhibited higher selectivity for 2-butene in comparison to mononuclear Ni 1 under otherwise identical reaction conditions.

The development of efficient catalysts is an ongoing pursuit in the field of olefin polymerization in order to synthesize desired polyolefins and to advance the present polymerization processes. In this contribution, the synthesis and characterization of a class of Hf and Zr complexes bearing SNN-tridentate ligands were described. The Hf (Hf1 and Hf2) and Zr (Zr1 and Zr2) trimethyl complexes were synthesized by one-pot reactions of thio-imino-quinoline ligands with in situ formed MMe4 (M = Hf, Zr). Both NMR analysis and X-ray diffraction study suggest the formation of thio-amido-quinoline metal complexes via methyl migration from Hf (or Zr) to the carbon of imine. These Hf and Zr complexes exhibited high activity and excellent thermal stability toward ethylene/1-octene copolymerization and an activity as high as 2.57 × 106 g(PE)·mol–1(cat)·h–1 was obtained by Hf1 even at 150 °C. The resultant polymers had moderate to high molecular weights (6.1–41.3 × 104 g·mol–1) with very broad distributions (D̵ = 6.8–34.7). NMR study revealed that multiple active species were formed when Hf1 was activated by 1 equiv. of [Ph3C][B(C6F5)4].