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Chromium-based complexes are among the most important catalysts in the field of ethylene polymerization and oligomerization. Heterogeneous Cr Phillips catalysts account for more than one-third of the commercialized high density polyethylene (HDPE). In this contribution, chromium complexes, LCrCl3 (Cr1-Cr4: L = 2,6-R1-4-R2-C6H2-N[double bond, length as m-dash]CH-C6H4-2-PPh2; Cr1: R1 = H, R2 = H; Cr2: R1 = Me, R2 = H; Cr3: R1 = iPr, R2 = H; Cr4: R1 = Ph2CH, R2 = iPr), have been synthesized and characterized by elemental analysis, ESI and IR spectroscopy. The molecular structures of Cr3 and Cr4 are defined by X-ray diffraction, revealing a distorted octahedral geometry around the chromium center in both structures. In the presence of an aluminum cocatalyst, complexes Cr1-Cr4 show moderate to high activities toward ethylene polymerization. The nature of the catalysts and various reaction conditions, such as the nature and the amount of cocatalyst, reaction time and temperature, are investigated in detail. The results show that the title complexes have good thermal stability and the substituents on the ligands significantly affect the catalytic properties. Particularly, complex Cr4 can produce HDPE with a high molecular weight up to 68.3 × 104 g mol-1 due to the suppression of the chain transfer/termination by the introduction of bulky Ph2CH groups.

The aluminum complexes LAlMe2 (Al1–Al5: LH = 2,6-(R1)2-4-R2-C6H2NHCH2C6H4-2-PPh2; Al1, R1 = H, R2 = H; Al2, R1 = Me, R2 = H; Al3, R1 = i Pr, R2 = H; Al4, R1 = Ph2CH, R2 = i Pr; Al5, R1 = Cl, R2 = H) have been synthesized and characterized by elemental analysis and 1H, 13C, and 31P NMR. NMR analysis in solution reveals an interesting hemilabile coordination of the soft P donor. The molecular structures of Al2–Al4 were defined by X-ray diffraction studies, showing a distorted-tetrahedral geometry around the aluminum center in all structures. Careful comparison of these crystal structures suggested that different substituents on the ligands could lead to unignorable changed coordination environments around the Al center, thus affecting their catalytic properties. In the presence of BnOH, complexes Al1–Al5 efficiently catalyzed the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) with high conversions in a controlled manner, and high molecular weights (M n up to 118.6 kg mol–1) of polycaprolactones (PCLs) were readily prepared. Immortal polymerizations by Al4 having bulky Ph2CH groups were also studied with up to 20 equiv of alcohols and 2000 equiv of monomers, without sacrificing polymerization control.

Dinuclear titanium complexes syn-Ti 2 and anti-Ti 2 bearing anthracene-bridged bisphenoxyimine ligands (syn-L and anti-L) have been prepared and characterized by NMR and elemental analyses. The molecular structure of syn-Ti 2 was defined by X-ray diffraction, revealing a separation of 7.183 Å between the two Ti centers. The mononuclear complex [(2,6-i Pr 2 -C 6 H 3 NCH-C 6 H 2 -3,5-t Bu-2-O)TiCl 3 (THF)] (Ti 1 ) was also prepared for control experiments. In the presence of MAO as a cocatalyst, syn-Ti 2 and anti-Ti 2 were active toward ethylene homopolymerization but had distinct catalytic properties. The complex syn-Ti 2 with a shorter metal−metal separation exhibited higher activity and produced polymers with higher molecular weights, in comparison to anti-Ti 2 , having a longer metal−metal separation, and the mononuclear catalyst control Ti 1 under identical conditions. More importantly, the complex syn-Ti 2 exhibited excellent thermal stability and thus the a greater activity can be obtained at 100 °C, which is a preferred temperature for industrial operations. For ethylene/1-hexene copolymerization, the activity, M w , and incorporation of 1-hexene by syn-Ti 2 were 2.3×, 3.2×, and 6.0× those of anti-Ti 2 under identical conditions.