Zelin
Sun
ab,
Erlin
Yue
b,
Mengnan
Qu
*a,
Irina V.
Oleynik
c,
Ivan I.
Oleynik
c,
Kanshe
Li
a,
Tongling
Liang
b,
Wenjuan
Zhang
b and
Wen-Hua
Sun
*b
aCollege of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, China
bKey laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: whsun@iccas.ac.cn
cN.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Pr. Lavrentjeva 9, Novosibirsk 630090, Russia
First published on 12th January 2015
Cycloalkyl-modified 8-arylimino-5,6,7-trihydroquinolylnickel pre-catalysts, activated with either MAO or Et2AlCl, are highly active for the polymerization of ethylene into branched polyethylene waxes with narrow polydispersity.
To verify the suitability of aniline derivatives, cycloalkyl-substituted anilines have been used to synthesize bis(iminoalkyl)pyridyliron complexes as pre-catalysts in ethylene polymerization.12 Subsequently, the cycloalkyl-substituted 8-arylimino-5,6,7-trihydroquinolylnickel halides were synthesized and are presented herein. Activated with MAO or Et2AlCl, all these nickel complexes show high activities towards ethylene polymerization, producing highly branched polyethylenes with low molecular weights and narrow polydispersity.
The 8-(2-cycloalkylphenylimino)-5,6,7-trihydroquinoline derivatives (L1–L3 and their enamine forms L1′–L3′, Scheme 1) were synthesized as yellowish oil compounds in moderate yields according to the literature.7b These compounds individually reacted with (DME)NiBr2 or NiCl2 to afford the corresponding nickel halide complexes (Scheme 1). The compositions of these nickel complexes were confirmed with elemental analysis and FT-IR spectroscopy. The structure of Ni1 was further confirmed through a single-crystal X-ray diffraction study and is shown in Fig. 1. The nickel atom is penta-coordinated with an N,N-bidentate ligand, two bromides and also an additional water molecule, and the complex adopts a distorted trigonal bipyramidal geometry, consistent with its analogs.13
Fig. 1 ORTEP drawing of the molecular structure of Ni1·H2O. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity. |
Entry | Co-cat. | Al/Ni | Yield (g) | Act.b | M w (kg mol−1) | M w/Mnc | T m (°C) |
---|---|---|---|---|---|---|---|
a 3 μmol of Ni1; 30 min; 30 °C; 10 atm of ethylene; total volume 100 mL. b Values in units of 106 g(PE) mol(Ni)−1 h−1. c Determined using GPC. d Determined using DSC. | |||||||
1 | MAO | 1000 | 5.43 | 3.62 | 2.55 | 1.62 | 66.52 |
2 | MMAO | 1000 | 3.52 | 2.35 | 2.49 | 1.63 | 67.72 |
3 | Et2AlCl | 200 | 4.46 | 3.00 | 1.83 | 1.49 | 55.98 |
4 | Me2AlCl | 200 | Trace | Trace | — | — | — |
Entry | Cat. | Al/Ni | T (°C) | t (min) | Yield (g) | Act.b | M w (kg mol−1) | M w/Mnc | T m (°C) |
---|---|---|---|---|---|---|---|---|---|
a 3 μmol of Ni; 10 atm of ethylene; total volume 100 mL. b Values in units of 106 g(PE) mol(Ni)−1 h−1. c Determined using GPC. d Determined using DSC. | |||||||||
1 | Ni1 | 1000 | 30 | 30 | 5.43 | 3.62 | 2.55 | 1.62 | 66.52 |
2 | Ni1 | 1500 | 30 | 30 | 5.62 | 3.75 | 2.39 | 1.60 | 65.79 |
3 | Ni1 | 2000 | 30 | 30 | 5.84 | 3.89 | 2.32 | 1.60 | 62.65 |
4 | Ni1 | 2500 | 30 | 30 | 7.05 | 4.70 | 2.28 | 1.59 | 60.24 |
5 | Ni1 | 3000 | 30 | 30 | 6.17 | 4.11 | 2.15 | 1.59 | 59.41 |
6 | Ni1 | 3500 | 30 | 30 | 6.07 | 4.05 | 2.06 | 1.57 | 56.66 |
7 | Ni1 | 2500 | 20 | 30 | 6.70 | 4.47 | 2.92 | 1.72 | 75.08 |
8 | Ni1 | 2500 | 40 | 30 | 3.45 | 2.30 | 1.81 | 1.50 | 51.00 |
9 | Ni1 | 2500 | 50 | 30 | 1.55 | 1.03 | 1.53 | 1.43 | 10.12 |
10 | Ni1 | 2500 | 30 | 15 | 2.00 | 2.67 | 2.26 | 1.56 | 61.57 |
11 | Ni1 | 2500 | 30 | 45 | 8.04 | 3.57 | 2.32 | 1.59 | 61.66 |
12 | Ni1 | 2500 | 30 | 60 | 9.93 | 3.31 | 2.43 | 1.68 | 63.49 |
13 | Ni2 | 2500 | 30 | 30 | 3.80 | 2.53 | 3.33 | 1.51 | 67.94 |
14 | Ni3 | 2500 | 30 | 30 | 8.00 | 5.33 | 2.65 | 1.72 | 70.37 |
15 | Ni4 | 2500 | 30 | 30 | 5.96 | 3.97 | 3.09 | 1.70 | 78.84 |
16 | Ni5 | 2500 | 30 | 30 | 7.47 | 4.98 | 3.42 | 2.32 | 70.26 |
17 | Ni6 | 2500 | 30 | 30 | 5.54 | 3.69 | 2.83 | 1.72 | 77.90 |
The ethylene polymerization was conducted at temperatures from 20 to 50 °C (entries 4 and 7–9, Table 2), and the maximum activity was observed at 30 °C (entry 4, Table 2). In addition, the higher temperature resulted in a lower activity and produced the polyethylene with a lower molecular weight (entries 4, 8, and 9, Table 2). This means that both the deactivation of active species and fast chain transfer take place at elevated temperatures,4,14 which was consistent with the analogous pre-catalysts.7 The narrow polydispersity was observed for all polyethylenes obtained, as confirmed by their GPC curves (Fig. 2).
Fig. 2 GPC curves of PEs using Ni1/MAO at different temperatures (entries 4, 7–9, Table 2). |
More interestingly, the lower melting points (Tm) of the resulting polyethylenes were confirmed using differential scanning calorimetry (DSC); a low melting point is commonly caused by the low molecular weight and/or high branching. It was surprising to observe a quite low Tm value of 10.12 °C for the polyethylene with a molecular weight of 1.53 kg mol−1 and a PDI of 1.43 (entry 9 in Table 2). It is worth mentioning the possibility of inaccurate data regarding measuring molecular weights of such polyethylene waxes because the measurement range of our equipment is set for 10 K to one million g mol−1. The 13C NMR measurement was conducted for the polymer at ambient temperature in deuterated 1,2-dichlorobenzene with TMS as an internal standard. Interpreted according to the literature,15 the spectrum (Fig. 3) indicated a branching number of 177 per 1000 carbons; the resonances of the carbons illustrate that the majority of the carbons are either tertiary or close to tertiary carbons, which are attributed to hydrogen migration happening on the active nickel species.16 The polyethylenes with lower molecular weights and high branching are in high demand as additives for lubricants and pour-point depressants in industry.
Fig. 3 13C NMR spectrum of the polyethylene formed using Ni1/MAO at 50 °C (entry 9, Table 2). |
The polymerization tests were also conducted for different time periods from 15 to 60 min (entries 4 and 10–12, Table 2); the activities were well maintained along with prolonging the reaction time, indicating the long lifetime of the active species.17 Moreover, higher molecular weights and larger polydispersity indices of the polyethylenes were observed for longer reaction times.
Other nickel analogs (Ni2–Ni4) were extensively investigated at the Al/Ni ratio of 2500 and the temperature of 30 °C. For the purpose of comparison, the 8-(2,4,6-trimethylphenylimino)-5,6,7-trihydroquinolylnickel halides (Ni5 and Ni6),7b without cyclohexyl-substituents, were also investigated. The low molecular weight waxes were obtained with narrow polydispersity, indicating single-site active species. Better catalytic activities were observed for the nickel complexes bearing cyclohexyl-modified ligands (Ni1 and Ni3) (entries 4 and 13–17, Table 2); moreover, the bromo-nickel pre-catalysts showed higher activities than their chloro-nickel analogs (entries 14 to 15, and 16 to 17, Table 2), which was attributed to the better solubility of the bromide complexes.18
Entry | Cat. | Al/Ni | T (°C) | t (min) | Yield (g) | Act.b | M w (kg mol−1) | M w/Mnc | T m (°C) |
---|---|---|---|---|---|---|---|---|---|
a 3 μmol of Ni; 10 atm of ethylene; total volume 100 mL. b Values in units of 106 g(PE) mol(Ni)−1 h−1. c Determined using GPC. d Determined using DSC. | |||||||||
1 | Ni1 | 200 | 30 | 30 | 4.46 | 3.00 | 1.83 | 1.49 | 55.98 |
2 | Ni1 | 300 | 30 | 30 | 7.27 | 4.85 | 1.84 | 1.56 | 55.15 |
3 | Ni1 | 400 | 30 | 30 | 6.83 | 4.55 | 1.86 | 1.58 | 55.80 |
4 | Ni1 | 500 | 30 | 30 | 6.38 | 4.25 | 1.90 | 1.59 | 56.49 |
5 | Ni1 | 600 | 30 | 30 | 5.89 | 3.93 | 2.02 | 1.59 | 59.18 |
6 | Ni1 | 300 | 20 | 30 | 4.94 | 3.29 | 2.13 | 1.62 | 65.05 |
7 | Ni1 | 300 | 40 | 30 | 3.70 | 2.47 | 1.51 | 1.46 | 16.42 |
8 | Ni1 | 300 | 50 | 30 | 0.73 | 0.49 | 1.43 | 1.38 | 9.11 |
9 | Ni1 | 300 | 30 | 15 | 2.12 | 2.92 | 1.81 | 1.54 | 55.19 |
10 | Ni1 | 300 | 30 | 45 | 7.88 | 3.50 | 1.88 | 1.56 | 56.07 |
11 | Ni1 | 300 | 30 | 60 | 8.27 | 2.76 | 1.88 | 1.92 | 56.21 |
12 | Ni2 | 300 | 30 | 30 | 3.60 | 2.40 | 3.21 | 1.79 | 63.77 |
13 | Ni3 | 300 | 30 | 30 | 6.05 | 4.03 | 1.72 | 1.54 | 57.02 |
14 | Ni4 | 300 | 30 | 30 | 5.08 | 3.39 | 2.19 | 1.87 | 60.11 |
15 | Ni5 | 300 | 30 | 30 | 6.10 | 4.10 | 2.24 | 1.42 | 68.25 |
16 | Ni6 | 300 | 30 | 30 | 4.47 | 2.98 | 1.98 | 1.71 | 60.61 |
Regarding the catalytic lifetime with Et2AlCl (entry 2 and 9–11, Table 3), the tendency observed is similar to that with MAO (entries 4 and 10–12, Table 2). With Et2AlCl, however, a significant change illustrated that the active species were rather formed with an initiating period because of the lower activity observed after 15 min than that after 30 min. Employing the Al/Ni ratio of 300 at 30 °C for 30 min (entry 2, Table 3), all other complexes also show high activities (entry 12–16, Table 3), producing polyethylenes with low molecular weights and narrow polydispersity. In general, compared with the systems using MAO, the resulting polyethylenes with Et2AlCl show somewhat lower molecular weights. Therefore, the current catalytic systems have a high potential for producing massive amounts of additives for lubricants and pour-point depressants. Again, the bromo-nickel pre-catalysts showed higher activities than their chloro-analogs (Table 3, entries 13 and 14 for complexes Ni3 and Ni4; entries 15 and 16 for complexes Ni5 and Ni6).
Footnote |
† Electronic supplementary information (ESI) available: Synthesis and characterization of all organic compounds and nickel complexes. The ethylene polymerization using nickel complex pre-catalysts as well as the characteristics of the obtained polyethylenes. CCDC 1029135 for Ni1. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00162a |
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