
α-Diimide catalysts have attracted widespread attention due to their unique chain walking characteristics. A series of α-diimide nickel/palladium catalysts with different electronic effects and steric hindrances were designed and synthesized for olefin polymerization. In this work, we synthesized a series of asymmetric α-diimide nickel complexes with different steric hindrances and used them for ethylene polymerization. These nickel catalysts have high ethylene polymerization activity, up to 6.51×106 g·mol−1·h−1, and the prepared polyethylene has a moderate melting point and high molecular weight (up to 38.2 × 104 g·mol−1), with a branching density distribution between 7 and 94 branches per 1000 carbons. More importantly, the polyethylene prepared by these catalysts exhibits excellent tensile properties, with strain and stress reaching 800% and 30 MPa, respectively.
Synthesis of branched polyethylene by ethylene polymerization catalyzed by nickel catalysts with asymmetric steric hindrance.
Figure 2. Crystal structure of Ni1. The selected bond lengths (Å) and angles (°) were as follows: Br2–Ni1 = 2.3442(13), Br1–Ni1 = 2.3577(13), Ni1–N1 = 2.043(5), Ni1–N2 = 0.055(6), Br2–Ni1–Br1 = 110.57(5), N1–Ni1–N2 = 83.0(2), N1–Ni1–Br2 = 97.94(17), N1–Ni1–Br1 = 134.85(18), and N2–Ni1–Br1 = 99.18(16).
Figure 3. Crystal structure of Ni2. The selected bond lengths (Å) and angles (°) are as follows: Br1–Ni1 = 2.3451(9), Ni1–Br2 = 2.3256(9), Ni1–N2 = 2.020(4), Ni1–N1 = 2.020(4), Br2–Ni1–Br1 = 122.07(4), N2–Ni1–Br1 = 115.23(10), N2–Ni1–Br2 = 107.94(10), N1–Ni1–Br1 = 101.99(10), N1–Ni1–Br2 = 120.38(10), and N1–Ni1–N2 = 82.98(15).
Figure 4. The polymerization performance of catalysts. (a) Polymer weight comparisons of generated polyethylene at 0 °C, 30 °C, 60 °C and 90 °C. (b) Molecular weight comparisons of generated polyethylene at 0 °C, 30 °C, 60 °C and 90 °C. (c) Branching density comparisons of generated polyethylene at 0 °C, 30 °C, 60 °C and 90 °C. (d) Time-dependent studies (polymer yields vs polymerization time) at 90 °C.
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Entry | Catalyst |
T(°C) | Yieldb (g) | Activityb (106) | Mnc(104) | PDIc |
Branchd |
Tme(°C) |
1 | Ni1 | 0 | 2.65 | 3.18 | 38.2 | 1.8 | 7 | 128.2 |
2 | Ni1 | 30 | 5.43 | 6.51 | 33.6 | 2.1 | 21 | 118.0 |
3 | Ni1 | 60 | 2.52 | 3.02 | 21.9 | 2.0 | 40 | 114.4 |
4 | Ni1 | 90 | 2.01 | 2.41 | 16.6 | 2.1 | 41 | 114.0 |
5 | Ni2 | 0 | 2.35 | 2.82 | 37.6 | 1.8 | 15 | 120.2 |
6 | Ni2 | 30 | 4.11 | 4.93 | 29.7 | 1.9 | 26 | 117.0 |
7 | Ni2 | 60 | 2.76 | 3.31 | 17.3 | 2.1 | 46 | 113.6 |
8 | Ni2 | 90 | 0.90 | 1.08 | 14.8 | 2.1 | 61 | 80.9 |
9 | Ni3 | 0 | 1.10 | 1.32 | 16.2 | 2.3 | 36 | 115.1 |
10 | Ni3 | 30 | 1.94 | 2.33 | 12.8 | 2.6 | 51 | 106.1 |
11 | Ni3 | 60 | 0.80 | 0.96 | 11.9 | 3.1 | 72 | 69.1 |
12 | Ni3 | 90 | 0.02 | 0.02 | 11.5 | 3.2 | 94 | – |
a 1 μmol of catalyst in CH2Cl2 (2 mL), [Al]/[Ni] = 500. Vn-heptane = 20 mL, tpolymerization = 10 min, Pethylene = 8 atm. b Activity is in units of 106 g·mol−1·h−1. c Determined by Gel Permeation Chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C. d Branches per 1000 carbons, determined by 1H NMR. e Determined by differential scanning calorimetry. |