8-(2-Cycloalkylphenylimino)-5,6,7-trihydro-quinolylnickel halides: polymerizing ethylene to highly branched and lower molecular weight polyethylenes

Abstract

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.

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Late transition metal complexes have been the hot topic in ethylene polymerization in the past two decades^1,2 since the discovery of α-diiminometal (nickel or palladium)³ and bis(imino)pyridylmetal (iron or cobalt) complexes⁴ as highly active pre-catalysts. The nickel complex pre-catalysts show the unique characteristic of producing highly branched polyethylenes.^1,3 Therefore, modifications of existing α-diiminonickel complexes have been focused towards higher activities of the catalytic systems and better properties of the obtained polyethylenes,⁵ meanwhile alternative models of nickel pre-catalysts have also been developed using N,N-bidentate 2-iminopyridine⁶ or other sophisticated derivatives.^7–11 Interestingly, 8-arylimino-5,6,7-trihydroquinolinylnickel halide pre-catalysts showed high activities towards ethylene reactivity,⁷ selectively catalyzing either the oligomerization^7a or polymerization^7b–e on the basis of the presence of 2-substituents within the 5,6,7-trihydroquinoline framework or not.

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

Scheme 1 Synthetic procedure.

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.

Ethylene polymerization

Similar to determining the catalytic performance of analogous nickel pre-catalysts,^7b–d complex Ni1 was used to select a suitable co-catalyst by employing MAO, MMAO, Et₂AlCl, and Me₂AlCl (Table 1). The promising co-catalysts turned out to be MAO, MMAO, and Et₂AlCl.

Table 1 Ethylene polymerization using Ni1 with various alkylaluminiums^a

Entry	Co-cat.	Al/Ni	Yield (g)	Act.^b	M w (kg mol^−1)	M w/Mn^c	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 10^6 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	—	—	—

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Ethylene polymerization in the presence of MAO

The molar ratio of MAO to Ni1 was changed from 1000 to 3500, and the highest activity of 4.70 × 10⁶ g(PE) mol(Ni)⁻¹ h⁻¹ was observed at 2500 (entry 4, Table 2). Along with increasing the Al/Ni ratio (entries 1–6, Table 2), the polyethylenes obtained show slightly decreasing molecular weights due to higher chain transfer rates;¹⁴ an impressively narrow polydispersity between 1.57 to 1.60 reflected the well-defined single site catalysis.

Table 2 Ethylene polymerization using Ni1–Ni6/MAO^a

Entry	Cat.	Al/Ni	T (°C)	t (min)	Yield (g)	Act.^b	M w (kg mol^−1)	M w/Mn^c	T m (°C)
a 3 μmol of Ni; 10 atm of ethylene; total volume 100 mL. b Values in units of 10^6 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

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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 (T_m) 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 T_m 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

Ethylene polymerization in the presence of Et₂AlCl

Similarly, complex Ni1 was extensively investigated with Et₂AlCl to optimize the polymerization parameters (entry 1–11, Table 3). Within the Al/Ni ratios from 200 to 600 (entries 1–5, Table 3), the best activity of 4.85 × 10⁶ g(PE) mol(Ni)⁻¹ h⁻¹ was observed at the Al/Ni ratio of 300 (entry 2, Table 3). Within the temperature range of 20 °C to 50 °C (entry 2 and 6–8, Table 3), the best activity was achieved at 30 °C (entry 2, Table 3). The obtained polyethylenes showed gradually lower molecular weights along with increasing the reaction temperature.

Table 3 Ethylene polymerization using Ni1–Ni6/Et₂AlCl^a

Entry	Cat.	Al/Ni	T (°C)	t (min)	Yield (g)	Act.^b	M w (kg mol^−1)	M w/Mn^c	T m (°C)
a 3 μmol of Ni; 10 atm of ethylene; total volume 100 mL. b Values in units of 10^6 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

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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).

Conclusions

In conclusion, cycloalkyl-substituted 8-(2,4,6-trimethylphenylimino)-5,6,7-trihydroquinolylnickel halides (Ni1–Ni4) showed higher and adaptable activities than their analogous 8-(2,4,6-trimethylphenylimino)-5,6,7-trihydroquinolylnickel halides.^7b Upon activation with either MAO or Et₂AlCl, all nickel complexes exhibited high activities (up to 5.33 × 10⁶ g(PE) mol(Ni)⁻¹ h⁻¹) with the single-site feature of the catalytic system. The resulting polyethylenes were found to be highly branched waxes with low molecular weights and narrow polydispersity, which indicates a high potential in their application as additives for lubricants and pour-point depressants.

Acknowledgements

This work is supported by the NSFC (No. 21374123, 51411130208 and U1362204).

Notes and references

1. (a) W.-H. Sun, Adv. Polym. Sci., 2013, 258, 163–178; (b) R. Gao, W.-H. Sun and C. Redshaw, Catal. Sci. Technol., 2013, 3, 1172–1179; (c) S. Wang, W.-H. Sun and C. Redshaw, J. Organomet. Chem., 2014, 751, 717–741; (d) C. Bianchini, G. Giambastiani, L. Luconi and A. Meli, Coord. Chem. Rev., 2010, 254, 431–455.
2. (a) W. Zhang, W.-H. Sun and C. Redshaw, Dalton Trans., 2013, 42, 8988–8997; (b) J. Ma, C. Feng, S. Wang, K.-Q. Zhao, W.-H. Sun, C. Redshaw and G. A. Solan, Inorg. Chem. Front., 2014, 1, 14–34.
3. L. K. Johnson, C. M. Killian and M. Brookhart, J. Am. Chem. Soc., 1995, 117, 6414–6415.
4. (a) B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am. Chem. Soc., 1998, 120, 4049–4050; (b) G. J. P. Britovsek, V. C. Gibson, B. S. Kimberley, P. J. Maddox, S. J. McTavish, G. A. Solan, A. J. P. White and D. J. Williams, Chem. Commun., 1998, 849–850.
5. (a) H. Liu, W. Zhao, X. Hao, C. Redshaw, W. Huang and W.-H. Sun, Organometallics, 2011, 30, 2418–2424; (b) H. Liu, W. Zhao, J. Yu, W. Yang, X. Hao, C. Redshaw, L. Chen and W.-H. Sun, Catal. Sci. Technol., 2012, 2, 415–422; (c) S. Kong, C.-Y. Guo, W. Yang, L. Wang, W.-H. Sun and R. Glaser, J. Organomet. Chem., 2013, 725, 37–45.
6. (a) T. V. Laine, M. Klinga and M. Leskelä, Eur. J. Inorg. Chem., 1999, 959–964; (b) T. V. Laine, K. Lappalainen, J. Liimatta, E. Aitola, B. Löfgren and M. Leskelä, Macromol. Rapid Commun., 1999, 20, 487–491; (c) T. V. Laine, U. Piironen, K. Lappalainen, M. Klinga, E. Aitola and M. Leskel, J. Organomet. Chem., 2000, 606, 112–124; (d) B. Y. Lee, X. Bu and G. C. Bazan, Organometallics, 2001, 20, 5425–5431; (e) J. M. Benito, E. De Jesús, F. J. de la Mata, J. C. Flores, R. Gómez and P. Gómez-Sal, Organometallics, 2006, 25, 3876–3887; (f) S. Jie, D. Zhang, T. Zhang, W.-H. Sun, J. Chen, Q. Ren, D. Liu, G. Zheng and W. Chen, J. Organomet. Chem., 2005, 690, 1739–1749; (g) S. Zai, F. Liu, H. Gao, C. Li, G. Zhou, S. Cheng, L. Guo, L. Zhang, F. Zhu and Q. Wu, Chem. Commun., 2010, 46, 4321–4323; (h) H. Gao, H. Hu, F. Zhu and Q. Wu, Chem. Commun., 2012, 48, 3312–3314; (i) H. Hu, L. Zhang, H. Gao, F. Zhu and Q. Wu, Chem. – Eur. J., 2014, 20, 3225–3233; (j) S. Zai, H. Gao, Z. Huang, H. Hu, H. Wu and Q. Wu, ACS Catal., 2012, 2, 433–440.
7. (a) J. Yu, X. Hu, Y. Zeng, L. Zhang, C. Ni, X. Hao and W.-H. Sun, New J. Chem., 2011, 35, 178–183; (b) J. Yu, Y. Zeng, W. Huang, X. Hao and W.-H. Sun, Dalton Trans., 2011, 40, 8436–8443; (c) L. Zhang, X. Hao, W.-H. Sun and C. Redshaw, ACS Catal., 2011, 1, 1213–1220; (d) X. Hou, Z. Cai, X. Chen, L. Wang, C. Redshaw and W.-H. Sun, Dalton Trans., 2012, 41, 1617–1623; (e) W. Chai, J. Yu, L. Wang, X. Hu, C. Redshaw and W.-H. Sun, Inorg. Chim. Acta, 2012, 385, 21–26.
8. C. Shao, W.-H. Sun, Z. Li, Y. Hu and L. Han, Catal. Commun., 2002, 3, 405–410.
9. (a) Z. Guan and W. J. Marshall, Organometallics, 2002, 21, 3580–3586; (b) F. Speiser, P. Braunstein, L. Saussine and R. Welter, Organometallics, 2004, 23, 2613–2624; (c) Z. Weng, S. Teo and T. S. A. Hor, Organometallics, 2006, 25, 4878–4882.
10. (a) F. Speiser, P. Braunstein, L. Saussine and R. Welter, Inorg. Chem., 2004, 43, 1649–1658; (b) X. Tang, D. Zhang, S. Jie, W.-H. Sun and J. Chen, J. Organomet. Chem., 2005, 690, 3918–3928; (c) W.-H. Sun, Z. Li, H. Hu, B. Wu, H. Yang, N. Zhu, X. Leng and H. Wang, New J. Chem., 2002, 26, 1474–1478.
11. (a) C. J. Stephenson, J. P. McInnis, C. Chen, M. P. Weberski, A. Motta, M. Delferro and T. J. Marks, ACS Catal., 2014, 4, 999–1003; (b) T. Wiedemann, G. Voit, A. Tchernook, P. Roesle, I. Göttker-Schnetmann and S. Mecking, J. Am. Chem. Soc., 2014, 136, 2078–2085.
12. (a) N. I. Ivancheva, V. K. Badaev, I. I. Oleinik, S. S. Ivanchev and G. A. Tolstikov, Dokl. Phys. Chem., 2000, 374, 203–205; (b) S. S. Ivanchev, G. A. Tolstikov, V. K. Badaev, N. I. Ivancheva, I. I. Oleinik, M. I. Serushkin and L. V. Oleinik, Polym. Sci., Ser. A, 2001, 43, 1189–1192; (c) I. I. Oleinik, I. V. Oleinik, I. B. Abdrakhmanov, S. S. Ivanchev and G. A. Tolstikov, Russ. J. Gen. Chem., 2004, 74, 1575–1578; (d) S. S. Ivanchev, G. A. Tolstikov, V. K. Badaev, I. I. Oleinik, N. I. Ivancheva, D. G. Rogozin, I. V. Oleinik and S. V. Myakin, Kinet. Catal., 2004, 45, 176–182; (e) I. Oleinik, I. V. Oleinik, I. B. Abdrakhmanov, S. S. Ivanchev and G. A. Tolstikov, Russ. J. Gen. Chem., 2004, 74, 1423–1427.
13. X. Hou, T. Liang, W.-H. Sun, C. Redshaw and X. Chen, J. Organomet. Chem., 2012, 708–709, 98–105.
14. (a) G. J. P. Britovsek, M. Bruce, V. C. Gibson, B. S. Kimberley, P. J. Maddox, S. Mastroianni, S. J. McTavish, C. Redshaw, G. A. Solan, S. Strolmberg, A. J. P. White and D. J. Williams, J. Am. Chem. Soc., 1999, 121, 8728–8740; (b) G. J. P. Britovsek, S. A. Cohen, V. C. Gibson and M. V. Meurs, J. Am. Chem. Soc., 2004, 126, 10701–10702; (c) M. V. Meurs, G. J. P. Britovsek, V. C. Gibson and S. A. Cohen, J. Am. Chem. Soc., 2005, 127, 9913–9923.
15. G. B. Galland, R. F. de Souza, R. S. Mauler and F. F. Nunes, Macromolecules, 1999, 32, 1620–1625.
16. M. M. Wegner, A. K. Ott and B. Rieger, Macromolecules, 2010, 43, 3624–3633.
17. Q. Xing, K. Song, T. Liang, Q. Liu, W.-H. Sun and C. Redshaw, Dalton Trans., 2014, 43, 7830–7837.
18. E. Yue, L. Zhang, Q. Xing, X.-P. Cao, X. Hao, C. Redshaw and W.-H. Sun, Dalton Trans., 2014, 43, 423–431.

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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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