Living Chain-Walking (Co)Polymerization of Propylene and 1-Decene by Nickel α-Diimine Catalysts

Abstract

Homo- and copolymers of propylene and 1-decene were synthesized by controlled chain-walking (co)polymerization using phenyl substituted α-diimine nickel complexes activated with modified methylaluminoxane (MMAO). This catalytic system was found to polymerize propylene in a living fashion to furnish high molecular weight ethylene-propylene (EP) copolymers. The copolymerizations proceeded to give high molecular weight P/1-decene copolymers with narrow molecular weight distribution (M_w/M_n ≈ 1.2), which indicated a living nature of copolymerization at room temperature. The random copolymerization results indicated the possibility of precise branched structure control, depending on the polymerization temperature and time.

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1. Introduction

Cationic late-transition metal catalyzed homo- and copolymerization of alkenes such as ethylene and propylene have received substantial attention in polymer science and materials chemistry [1,2,3,4,5]. Specifically, polypropylene (PP) is a very popular plastic material used for many applications, namely, in electrical devices, automotive parts, food packaging, household equipment, and many others [6]. At present, due to the epidemic situation of novel coronavirus pneumonia in the world, epidemic prevention materials in many countries are in short supply, especially for medical grade polypropylene raw materials for the production of medical masks.

In the field of polyolefin research [7,8,9,10,11,12,13,14,15,16,17,18,19,20], many studies have reported on the homopolymerization of ethylene [21,22,23,24], different higher 1-alkene [25,26,27,28,29,30,31], and linear internal alkenes [32,33,34] using Brookhart-type α-diimine nickel and palladium catalysts [35,36,37,38,39,40,41] because of the chain-walking process [42,43,44,45]. The interest in highly branched polyolefins such as dendrimer and hyper branched polymers has increased obviously in recent years owing to these unique physical properties, chemical properties, and their potential applications as adhesives, lubricants, and paints [7]. Compared with a commercial linear polyethylene (LPE), these branched polyolefin-based materials with a chain topology possess unique physical properties stemming from their architectures [2,7], and provide the potential to increase polymeric melt fracture resistance, elasticity, paintability, and compatibility with other materials. However, propylene polymerization by imine-based nickel catalysts has been reported only in a limited number of cases [46,47,48], and no example for propylene copolymerization with higher 1-alkene. Polymerization of propylene with α-diimine nickel complexes produced PPs with different structures depending on the ortho-substituent groups of the catalysts and the polymerization temperature [44]. Suitable catalysts polymerize propylene to a syndiotactic polymer at a low temperature [46] but to regioregular polymers at higher temperature.

Chain-walking alkene polymerization [44] can give polymers unique structures, which cannot be obtained by common vinyl polymerization. Chain-branching formation in the polymerization of linear 1-alkene using cationic nickel α-diimine catalysts is shown in Scheme 1 [49]. This mechanism involves 1,2- and 2,1-insertion followed by chain-walking behavior [44], in which the active metal undergoes chain-walking to the terminal or internal carbon followed by monomer insertion [49]. A 2,1-insertion followed by complete chain-walking installs a long methylene sequence [44], and 1,2-insertion gives the methyl branch in 2,ω-enchainment and the n-alkyl branch without chain-walking behavior. While some methyl and alkyl branches are also derived from a small amount of the partial chain-walking to the 1-alkene internal carbon [48], the product properties strongly depend on the microstructure of different branched polyolefins [1]. Generally, the highly branched polyolefins containing methyl and alkyl branches are amorphous, and the chain-straightened linear polymeric products are semicrystalline. For instance, Coates et al. demonstrated that 1-butene polymerization via stereoretentive chain-walking using cationic nickel α-diimine catalysts to produce semicrystalline isotactic 4,2-poly(1-butene) [50]. Cationic “sandwich” α-diimine nickel conducted accurate chain-walking polymerization of higher 1-alkenes to generate semicrystalline “polyethylene” [51].

Our group reported recently that the introduction of phenyl groups in α-diimine nickel system can efficiently improve catalytic performances with a fast chain-walking process [52,53,54,55]. Consequently, these catalysts conducted polymerizations of ethylene [52,53], 4-methyl-1-pentene [54] and linear 1-alkenes [55] to gain high molecular weight polymers and proved living behavior at low temperature. In this work, therefore, the propylene homopolymerization and copolymerization with 1-decene were conducted by phenyl substituted nickel α-diimine complexes activated by MMAO.

2. Experimental Section 2.1. General Considerations

All manipulations, unless otherwise mentioned, were carried out using standard Schlenk or in glovebox. Nuclear magnetic resonance (¹H, ¹³C NMR) spectra were recorded on a Varian 400 NMR instrument (Varian, Inc., Palo Alto, CA) at room temperature and 50 °C, respectively, using CDCl₃ as a solvent and tetramethylsilane (TMS) as internal standard for the compounds. Gel permeation chromatography (GPC) analyses of the molecular weight and molecular weight distribution of the polymers were performed on a Tosoh HLC-8320GPC chromatograph (Tosoh Asia Pte. Ltd., Tokyo, Japan) at 40 °C using THF as an eluent. Differential scanning calorimetry (DSC) was performed by a SII EXSTAR6000 system (Hitachi, Tokyo, Japan). Research-grade propylene was purified by passing it through dehydration column of ZHD-20 and deoxidation column of ZHD-20A. MMAO (6.5 wt % Al, 2.17 M in toluene) was donated by Tosoh-Finechem Co (Tokyo, Japan). Toluene was dried with sodium/benzophenone under nitrogen atmosphere and distilled before use. n-Hexane, CDCl₃ and CH₂Cl₂ were purified over 4 Å molecular sieves. 1-Decene was purchased from Kanto Chemical Co on Aldrich Chemical Company (Tokyo, Japan) were dried over CaH₂ and distilled before use. Nickel complexes 1–5 were synthesized according to the literature [55]. Other chemicals were commercially obtained and purified with common procedures.

2.2. Polymerization Procedure The propylene polymerization/copolymerization experiments were performed in a 100-mL glass reactor equipped with a temperature controller and a magnetic stirrer. A 30 mL amount of toluene was added to the dried reactor reactor under N₂ atmosphere. Propylene polymerization: Propylene (1.2 bar) was introduced to the reactor kept at polymerization temperature after the toluene was saturated with propylene; MMAO was added and the solution was stirred for 10 min. Then, the catalyst solution (10 μmol) in toluene was injected into the polymerization system with a syringe to start polymerization. After the desired amount of polymerization time, the pressure vessel was vented and the reaction was quenched by addition of 5% acidic methanol (HCl/MeOH). Propylene/1-decene copolymerization: 1-Decene was added to the toluene via a syringe, and the resulting solution was saturated with propylene (1.2 bar) under vigorous stirring for 10 min. Then MMAO was added and the solution was stirred for 10 min. Then, the catalyst solution (10 μmol) in toluene was injected into the polymerization system to start polymerization. After the desired amount of polymerization time, the pressure vessel was vented and the reaction was quenched by addition of 5% acidic methanol (HCl/MeOH). The PPs and copolymers obtained were filtered from solution, washed with methanol, and dried in a vacuum oven to constant weight. 2.3. Time-Course of Propylene and 1-Decene Copolymerization Copolymerization of propylene and 1-decene was performed in a 100-mL glass reactor equipped with a magnetic stirrer. A 30 mL amount of toluene was added to the fully dried reactor under N₂ atmosphere. After the reactor was set at polymerization temperature, 1-decene (12 mmol) was added to the toluene via a syringe, and the resulting solution was saturated with propylene (1.2 bar) under vigorous stirring for 10 min. Then MMAO was added and the mixture was stirred for 10 min. The catalyst 2 (12 μmol) solution in toluene was added to the reactor (total volume = 40 mL) to start polymerization. The reaction solution (6.6 mL) was sampled six times via a syringe at different polymerization time (5 min, 10 min, 30 min, 50 min, 70 min and 90 min), and the reaction mixture was terminated with 40 mL of a 3% HCl/MeOH solution. The copolymers obtained were filtered from solution, washed with methanol, and dried in a vacuum oven to constant weight. 3. Results and Discussion 3.1. Synthesis and Characterization of the Nickel Complexes

Our previous results have shown that fast chain-walking process can be efficiently modulated using phenyl substituted Brookhart-type nickel α-diimine catalysts [52,53,54,55]. Complex 2 activated with MMAO exhibited high activity for ethylene, linear, and branched 1-alkenes polymerizations in a living manner at room temperature. Four phenyl substituted α-diimine complexes were therefore synthesized as a precursor for propylene polymerization with modified MMAO, and these nickel complexes 1–4 [55] are summarized in Scheme 2. The corresponding nickel catalyst {[(2,4,6-Me₃C₆H₂N=C)₂Nap]NiBr₂ (5)} [56] was also synthesized for comparison.

Single-crystal of 3 was obtained by slowly diffusing of n-hexane into the CH₂Cl₂ solution of complex at room temperature, and its molecular structure was confirmed by X-ray diffraction (Figure 1). Crystal data, data collection, and refinement parameters are listed in Table S1 (see the Supplementary Materials). The geometry at the Ni center is pseudo-tetrahedral, showing pseudo-C₂-symmetry. The bond length of N1−C20 [1.298(8) Å] and N2−C31 [1.264(8) Å] have typical imine double bonds character, respectively. In the solid state, the Ni1−N1 and Ni1−N2 bond distances of 2.036(6) and 2.017(6) Å in 3 are slightly narrower than the value of 2.041(5) Å for analogue 2 [56]. In addition, the N1–Ni1–Br3 angles of 117.99(7)° for 3 is also approximate to those for complex 2 (112.96°). The conjugation effect of phenyl substituents in the para- and ortho-aryl position of α-diimine ligand can be clearly observed in these molecular structures, expected to improve the catalyst performance for propylene (co)polymerization.

3.2. Catalytic Polymerization of Propylene

Upon activation with MMAO, polymerization of propylene was examined by complexes 1–5 with the [Al]/[Ni] ratio of 300 at 1.2 bar of propene for 30 min, and the results are listed in Table 1. The effect of reaction temperature was examined by varying the temperature from 0 to 50 °C (Table 1, entries 1–3). Both the highest activity and the highest molecular weight of 13.4 × 10⁴ g mol⁻¹ were observed with 2-MMAO at 25 °C (Table 1, entry 2). The narrowest molecular weight distribution of 1.06 was observed at 0 °C (Table 1, entry 1). Increasing the polymerization temperature further to 50 °C (Table 1, entry 3), the molecular weight decreased dramatically, which was accompanied by a slight broadening of the M_w/M_n value.

The effect of different catalyst precursors 1–5 was also studied at 25 °C (Table 1, entries 2 and 4–7). Phenyl substituted 1–4 produced the PPs in moderate yields with high molecular weights of (11.9–19.1) × 10⁴ g mol⁻¹ and narrow molecular weight distribution (M_w/M_n = 1.16–1.48, Table 1, entries 2 and 4–6). The catalytic activities decreased in the order, 2 ≥ 1 ≥ 3 ≥ 4, and the M_n values increased with increasing the ligand bulkiness (4 ≥ 3 ≥ 2 ≥ 1). Complex 2 produced higher molecular weight PP in highest yield than those of one methyl or phenyl groups (1, 3 and 4, Table 1, entries 4–6). The highest molecular weight PP obtained by 4 (Table 1, entry 6), indicating the rate of chain propagation was greatly promoted by the bulky ortho-phenyl substituent on the N-aryl moiety, which should retard chain-transfer reactions [44]. Complex 2 with the conjugation ability of the para-phenyl substituent exhibited higher activity than the corresponding methyl-substituted 5 (Table 1, entries 2 vs. 7), as already observed in polymerization of ethylene and higher 1-alkenes [55]. The higher molecular weight PP with narrower molecular weight distribution was obtained by 2 suggests that phenyl substituent should suppress chain-transfer reactions [44].

The effect of polymerization time in propylene polymerization was also studied by 2-MMAO at 25 °C (Table 1, entries 2 and 8–11). The yields and the M_n value increased with the prolongation of polymerization time. Figure 2a shows the plot of M_n increased linearly polymerization time accompanied with keeping narrow M_w/M_n = 1.10–1.25 (Figure S8 from Supplementary Materials), and the N value was almost constant. This polymerization result verified that the propylene polymerization by 2-MMAO proceeded in a living manner at room temperature within a certain period of time.

¹H NMR spectroscopy analyses [57,58] showed that the obtained PPs proceeded only methyl group, while the total branching densities (245–267/1000 C, Table 2) were less than the expected value (333/1000 C), indicating the 1,3-enchainment of propylene via 2,1-insertion followed by chain-straightening. The 1,3-enchainment increased slightly with increasing reaction time and temperature (Table 2). Upon comparison with common polypropylene (PP, T_g = −10~−5 °C), the produced PPs had low T_g of approximately −29.4~−24.1 °C and no T_m (Table 2), indicating that the PPs were amorphous.

The microstructure analyses of the polypropylenes were further studied by ¹³C NMR spectroscopy [59,60]. ¹³C NMR spectra of polypropylenes obtained by 2-MMAO at 0 and 25 °C are shown in Figure 3 (Table 1, entries 1 and 2), where only normal methyl branch was observed at 0 °C (Figure 3a), while 2-methylhexyl structure was observed at room temperature (Figure 3b). When nickel metal migrates forward along the newly 2,1-added propylene unit, ethylene-type unit coming from 1,3-enchainment can be produced, and backward along the polymer chain, longer branched chains can be formed [46]. Consequently, the microstructures of the polypropylenes were similar to the ethylene-propylene (EP) copolymers. Therefore, 2-MMAO was found to polymerize propylene in a living fashion to furnish EP copolymers.

3.3. Copolymerization of Propylene and 1-Decene

Copolymerization of propylene and 1-decene (D) was catalyzed by 2-MMAO at 0 and 25 °C with the [Al]/[Ni] ratio of 300, and the results are listed in Table 3. In comparison with the 1-decene homopolymerization under the same conditions [55], P/1-decene copolymerization exhibited higher activity and produced copolymers with higher molecular weight and narrow M_w/M_n values. In the P/1-alkene copolymerization study, similar trends were observed to those for propylene and 1-decene homopolymerization. For instance, 2-MMAO exhibited higher activity and higher molecular weight than the corresponding methyl-substituted 5 (Table 3, entries 1 vs. 3).

The P/1-decene copolymer obtained with 2-MMAO at 25 °C showed narrow molecular weight distribution of 1.14 (Table 3, entry 1). Time-course of polymerization was investigated with 2-MMAO at 25 °C under the same conditions by sampling method, and the results are summarized in Table 4. The plot of M_n against the P-D copolymer yield shows a good linear relationship accompanied by a very narrow polydispersity (M_w/M_n = 1.10–1.12, Figure 2b), keeping the number of polymer chains (N) constant. This result verified that the P/1-decene copolymerization with 2-MMAO proceeded in a living/controlled manner at 25 °C within a certain period of time. To the best of our knowledge, this is the first example of late transition metal catalyzed living copolymerization of propylene and 1-decene at room temperature.

The copolymerization results indicated the possibility of precise microstructure control, depending on the polymerization temperature, which in turn strongly affects the physical polymer properties. The branching densities of the obtained P/1-decene copolymers thus obtained are shown in Table 3. P/1-decene copolymers possessed highly B values in comparison with the corresponding poly(1-decene)s [55]. The branching densities of the P/1-decene copolymer obtained at 0 °C (187/1000 C, Table 3, entry 2) was much higher than those of at room temperature (169/1000 C, Table 3, entry 1). Therefore, the branching density can be controlled by the polymerization temperature. The produced copolymers had T_g of approximately −49 °C no T_m (Table 3), indicating that the P/1-decene copolymers were amorphous.

The methyl branches were formed by predominant P/1-alkene 1,2-insertion and precise chain-walking to form a ω,2-enchaninment (Scheme 1), while the long branches were formed by 1,2-insertion of higher 1-decene. P/1-decene copolymers have almost exclusively methyl and longer branches (Figure 4), because the 1-decene give octyl (C₈) branch separately in 1,2-insertion. For example, the ¹³C NMR spectra of the copolymers obtained by 2-MMAO, and showed that the poly(P-co-D) produced at 25 °C possessed 170 branches/1000 carbons including 133 methyl and 37 longer chains (C₈) (Figure 4), whereas poly(1-decene) possessed 83 branches/1000 carbons including 40 methyl, 4 propyl, and 39 longer chains. There were no carbon atoms of adjacent methyl branches (14.5–17.5 ppm) and ethyl branch in the ¹³C NMR spectra. Therefore, the 2,1-insertion of propylene and 1-decene always evolves into a 1,3- and 1,8-enchainment to give methylene sequences (Scheme 1). The branch-type distribution depended on the reaction temperature, and poly(P-co-D) produced at 0 °C possessed 186 branches/1000 carbons including 146 methyl and 40 longer chains (Figure 4b).

4. Conclusions In summary, we conducted the living chain-walking polymerization of propylene using phenyl substituted α-diimine nickel catalysts in combination with MMAO to generate high molecular weights amorphous “ethylene-propylene (EP) copolymers” at room temperature. Copolymerization of propylene and 1-decene was successfully achieved using 2-MMAO, produced highly branched P/1-decene copolymers with high molecular weight and narrow molecular weight distribution. At room temperature, living copolymerization of propylene and 1-decene was observed. The ¹³C NMR spectra of the P/1-decene copolymers obtained showed a microstructure almost exclusively composed of methyl and octyl (C₈) branches.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4360/12/9/1988/s1. Experimental details including synthesis and characterization of the complexes (Scheme S1) and (co)polymers (Figures S1-S11), (co)polymerization data, microstructure analysis of the (co)polymers (Equations (S1)-(S3)), and X-ray crystallographic data (Table S1). CCDC 1851985 contains the supplementary crystallographic data for this paper. Copies of the data can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: (internat.) +44-1223/336-033; E-mail: [email protected]].

Author Contributions

F.W. conceived and designed the experiments; P.L., X.L., M.X., F.Y. and F.W. performed the experiments; F.W. and G.X. analyzed the data and wrote the paper. S.B., X.L. and G.X. checked and revised the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 21801002), the Natural Science Foundation of Anhui Province (No. 1808085MB47) and Open Project of the State Key Laboratory of Physical Chemistry of the Solid Surface of Xiamen University (Grant No. 201923).

Conflicts of Interest

The authors declare no conflict of interest.

References

1. Takeuchi, D.; Osakada, K. Controlled isomerization polymerization of olefins, cycloolefins, and dienes. Polymer 2016, 82, 392-405.

2. Wang, Z.; Liu, Q.; Solan, G.A.; Sun, W.H. Recent advances in Ni-mediated ethylene chain growth: Nimine-donor ligand effects on catalytic activity, thermal stability and oligo-polymer structure. Coord. Chem. Rev. 2017, 350, 68-83.

3. Chen, Z.; Brookhart, M. Exploring ethylene/polar vinyl monomer copolymerizations using Ni and Pd α-diimine catalysts. Acc. Chem. Res. 2018, 51, 1831-1839.

4. Chen, C.L. Designing catalysts for olefin polymerization and copolymerization: Beyond electronic and steric tuning. Nat. Rev. Chem. 2018, 2, 6-14.

5. Tan, C.; Chen, C.L. Emerging palladium and nickel catalysts for copolymerization of olefins with polar monomers. Angew. Chem. Int. Ed. 2019, 58, 7192-7200.

6. Gahleitner, M.; Paulik, C. Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2014.

7. Chen, Y.; Wang, L.; Yu, H.; Zhao, Y.; Sun, R.; Jing, G.; Huang, J.; Khalid, H.; Abbasi, N.M.; Akram, M. Synthesis and application of polyethylenebased functionalized hyperbranched polymers. Prog. Polym. Sci. 2015, 45, 23-43.

8. Chen, M.; Chen, C.L. Direct and Tandem Routes for the Copolymerization of Ethylene with Polar Functionalized Internal Olefins. Angew. Chem. Int. Ed. 2020, 59, 1206-1210.

9. Dai, S.; Li, S.; Xu, G.; Chen, C.L. Direct Synthesis of Polar Functionalized Polyethylene Thermoplastic Elastomer. Macromolecules 2020, 53, 2539-2546.

10. Na, Y.N.; Chen, C.L. Catechol Functionalized Polyolefins. Angew. Chem. Int. Ed. 2020, 59, 7953-7959.

11. Zhang, Y.; Jian, Z. 2-Phosphine-pyridine-N-oxide palladium and nickel catalysts for ethylene polymerization and copolymerization with polar monomers. Polymer 2020, 194, 122410.

12. Tan, C.; Qasim, M.; Pang, W.M.; Chen, C.L. Ligand-Metal Secondary Interaction in Phosphine-Sulfonate Palladium and Nickel Catalyzed Ethylene (Co)Polymerization. Polym. Chem. 2020, 11, 411-416.

13. Xu, M.L.; Yu, F.; Li, P.; Xu, G.; Zhang, S.J.; Wang, F.Z. Enhancing Chain Initiation Efficiency in the Cationic Allyl-Nickel Catalyzed (Co)Polymerization of Ethylene and Methyl Acrylate. Inorg. Chem. 2020, 59, 4475-4482.

14. Zou, C.; Chen, C.L. Polar-Functionalized, Crosslinkable, Self-Healing and Photoresponsive Polyolefins. Angew. Chem. Int. Ed. 2020, 59, 395-402.

15. Liang, T.; Goudari, S.; Chen, C.L. A Simple and Versatile Nickel Platform for the Generation of Branched High Molecular Weight Polyolefins. Nat. Commun. 2020, 11, 372.

16. Muhammad, Q.; Tan, C.; Chen, C.L. Concerted steric and electronic effects on α-diimine nickel- and palladium-catalyzed ethylene polymerization and copolymerization. Sci. Bull. 2020, 65, 300-307.

17. Li, Y.; Cheng, H.; Xiao, R.; Cai, Z. Rational design of nickel catalysts containing N-acylated imidazolin-2-imine ligand for ethylene copolymerization with polar monomer. J. Catal. 2020, 383, 117-123.

18. Chen, M.; Chen, C.L. A versatile ligand platform for palladium- and nickel-catalyzed ethylene copolymerization with polar monomers. Angew. Chem. Int. Ed. 2018, 57, 3094-3098.

19. Dai, S.Y.; Chen, C.L. A Self-Supporting Strategy for Gas-Phase and Slurry-Phase Ethylene Polymerization using Late-Transition-Metal Catalysts. Angew. Chem. Int. Ed. 2020, 59, 2-9.

20. Zhou, S.X.; Chen, C.L. Synthesis of silicon-functionalized polyolefins by subsequent cobalt-catalyzed dehydrogenative silylation and nickel-catalyzed copolymerization. Sci. Bull. 2018, 63, 441-445.

21. Kocen, A.L.; Brookhart, M.; Daugulis, O. A highly active Ni(II)-triadamantylphosphine catalyst for ultrahigh-molecular-weight polyethylene synthesis. Nat. Chem. 2019, 10, 438.

22. Na, Y.N.; Dai, S.Y.; Chen, C.L. Direct synthesis of polar-functionalized linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE). Macromolecules 2018, 51, 4040-4048.

23. Figueira, C.A.; Lopes, P.S.; Gomes, C.S.B.; Gomes, J.C.S.; Veiros, L.F.; Lemos, F.; Gomes, P.T. Neutral mono(5-aryl-2-iminopyrrolyl)nickel(II) complexes as precatalysts for the synthesis of highly branched ethylene oligomers: Preparation, molecular characterization, and catalytic studies. Organometallics 2019, 38, 614-625.

24. Brown, L.A.; Anderson, W.C.; Mitchell, N.E.; Gmernicki, K.R.; Long, B.K. High Temperature, Living polymerization of ethylene by a sterically-demanding nickel(II) α-diimine catalyst. Polymers 2018, 10, 41.

25. Zhang, D.; Chen, C.L. Influence of polyethylene glycol unit on palladium- and nickel-catalyzed ethylene polymerization and copolymerization. Angew. Chem. Int. Ed. 2017, 56, 14672-14676.

26. Hu, H.; Gao, H.; Chen, D.; Li, G.; Tan, Y.; Liang, G.; Zhu, F.; Qing, W. Ligand-directed regioselectivity in amine-imine nickel-catalyzed 1-hexene polymerization. ACS Catal. 2015, 5, 122-128.

27. Ma, X.; Hu, X.; Zhang, Y.; Mu, H.; Cui, L.; Jian, Z.B. Preparation and in situ chain-end-functionalization of branched ethylene oligomers by monosubstituted α-diimine nickel catalysts. Polym. Chem. 2019, 10, 2596-2607.

28. Wang, F.Z.; Tian, S.S.; Li, R.P.; Li, W.M.; Chen, C.L. Ligand steric effects on naphthyl-α-diimine nickel catalyzed α-olefin polymerization. Chinese J. Polym. Sci. 2018, 36, 157-162.

29. Gao, H.; Liu, X.; Tang, Y.; Pan, J.; Wu, Q. Living/controlled polymerization of 4-methyl-1-pentene with α-diimine nickel-diethylaluminium chloride: Effect of alkylaluminium cocatalysts. Polym. Chem. 2011, 2, 1398-1403.

30. Gao, J.; Zhang, L.; Zhong, L.; Du, C.; Liao, H.; Gao, H.; Wu, Q. Living isomerization polymerizations of alkenylcyclohexane with camphyl α-diimine nickel catalysts. Polymer 2019, 164, 26-32.

31. Wang, F.Z.; Tanaka, R.; Cai, Z.G.; Nakayama, Y.; Shiono, T. Precision chain-walking polymerization of trans-4-octene catalyzed by α-diimine nickel(II) catalysts bearing ortho-sec-phenethyl groups. Macromol. Rapid Commun. 2016, 37, 1375-1381.

32. Wang, F.Z.; Tanaka, R.; Cai, Z.G.; Nakayama, Y.; Shiono, T. Living polymerization of higher 2-alkene with α-diimine nickel catalysts: Synthesis and characterization of high molecular weight poly(2-alkene)s. Polymer 2017, 127, 88-100.

33. Wang, F.Z.; Tanaka, R.; Li, Q.S.; Nakayama, Y.; Shiono, T. Chain-walking polymerization of linear internal octenes catalyzed by α-diimine nickel complexes. Organometallics 2018, 37, 1358-1367.

34. Wang, F.Z.; Xu, G.Y.; Li, Q.S.; Tanaka, R.; Nakayama, Y.; Shiono, T. Chain-walking polymerization of 3-heptene with phenyl substituted α-diimine nickel catalysts. Polymer 2019, 181, 121801.

35. Zhang, D.F.; Nadres, E.T.; Brookhart, M.; Daugulis, O. Synthesis of highly branched polyethylene using "sandwich" (8-p-tolyl naphthyl α-diimine) nickel(II) catalysts. Organometallics 2013, 32, 5136-5143.

36. Wang, X.X.; Fan, L.L.; Ma, Y.P.; Guo, C.Y.; Solan, G.A.; Sun, Y.; Sun, W.H. Elastomeric polyethylenes accessible via ethylene homo-polymerization using an unsymmetrical α-diimino-nickel catalyst. Polym. Chem. 2017, 8, 2785-2795.

37. Li, M.; Wang, X.B.; Luo, Y.; Chen, C.L. A second-coordination-sphere strategy to modulate nickel- and palladium-catalyzed olefin polymerization and copolymerization. Angew. Chem. Int. Ed. 2017, 56, 11604-11609.

38. Lian, K.B.; Zhu, Y.; Li, W.M.; Dai, S.Y.; Chen, C.L. Direct synthesis of thermoplastic polyolefin elastomers from nickel-catalyzed ethylene polymerization. Macromolecules 2017, 50, 6074-6080.

39. Pei, L.; Liu, F.; Liao, H.; Gao, J.; Zhong, L.; Gao, H.; Wu, Q. Synthesis of polyethylenes with controlled branching with α-diimine nickel catalysts and revisiting formation of long-chain branching. ACS Catal. 2018, 8, 1104-1113.

40. Fang, J.; Sui, X.L.; Li, Y.G.; Chen, C.L. Synthesis of polyolefin elastomers from unsymmetrical α-diimine nickel catalyzed olefin polymerization. Polym. Chem. 2018, 9, 4143-4149.

41. Liao, Y.; Zhang, Y.; Cui, L.; Mu, H.; Jian, Z.B. Pentiptycenyl substituents in insertion polymerization with α-diimine nickel and palladium species. Organometallics 2019, 389, 2075-2083.

42. Tan, C.; Pang, W.; Chen, C.L. A phenol-containing α-diimine ligand for nickel- and palladium-catalyzed ethylene polymerization. Chinese J. Polym. Sci. 2019, 37, 974-980.

43. Chen, C.L. Redox-controlled polymerization and copolymerization. ACS Catal. 2018, 8, 5506-5514.

44. Wang, F.Z.; Chen, C.L. A continuing legend: The Brookhart-type α-diimine nickel and palladium catalysts. Polym. Chem. 2019, 10, 2354-2369.

45. Leone, G.; Canetti, M.; Pierro, I.; Zanchin, G.; Rosa, C.D.; Ricci, G.; Bertini, F. (Micro)structure, thermal behavior and mechanical properties of ethylene-propylene-1-octadecene terpolymers from chain-walking polymerization of 1-octadecene. Polymer 2019, 166, 27-37.

46. Zambelli, A.; Sessa, I.; Grisi, F.; Fusco, R.; Accomazzi, P. Syndiotactic polymerization of propylene: Single-site vanadium catalysts in comparison with zirconium and nickel. Macromol. Rapid Commun. 2001, 22, 297-310.

47. Liu, J.; Chen, D.R.; Wu, H.; Xiao, Z.F.; Gao, H.Y.; Zhu, F.M.; Wu, Q. Polymerization of α-olefins using a camphyl α-diimine nickel catalyst at elevated temperature. Macromolecules 2014, 47, 3325-3331.

48. Chen, Z.; Zhao, X.; Gong, X.; Xu, D.; Ma, Y. Macrocyclic trinuclear nickel phenoxyimine catalysts for high-temperature polymerization of ethylene and isospecific polymerization of propylene. Macromolecules 2017, 50, 6561-6568.

49. O'Connor, K.S.; Lamb, J.R.; Vaidya, T.; Keresztes, I.; Klimovica, K.; LaPointe, A.M.; Daugulis, O.; Coates, G.W. Understanding the insertion pathways and chain walking mechanisms of α-diimine nickel catalysts for α-olefin polymerization: A 13C NMR spectroscopic investigation. Macromolecules 2017, 50, 7010-7027.

50. Vaccarello, D.N.; O'Connor, K.S.; Iacono, P.; Rose, J.M.; Cherian, A.E.; Coates, G.W. Synthesis of semicrystalline polyolefin materials: Precision methyl branching via stereoretentive chain walking. J. Am. Chem. Soc. 2018, 140, 6208-6211.

51. Vaidya, T.; Klimovica, K.; LaPointe, A.M.; Keresztes, I.; Lobkovsky, E.B.; Daugulis, O.; Coates, G.W. Secondary alkene insertion and precision chain-walking: A new route to semicrystalline "polyethylene" from α-olefins by combining two rare catalytic events. J. Am. Chem. Soc. 2014, 136, 7213-7216.

52. Wang, F.Z.; Tanaka, R.; Li, Q.S.; Yuan, J.C.; Nakayama, Y.; Shiono, T. Synthesis and application of α-diimine Ni(II) and Pd(II) complexes with bulky steric groups to polymerization of ethylene and methyl methacrylate. J. Mol. Catal. A Chem. 2015, 398, 231-240.

53. Wang, F.Z.; Li, R.P.; Tian, S.S.; Lian, K.B.; Guo, D.F.; Li, W.M. Synthesis of highly branched polyethylene using para-phenyl-substituted α-diimine nickel catalysts. Appl. Organomet. Chem. 2018, 32, 4298.

54. Zhang, M.; Wu, Q.; Xu, G.; Li, Y.F.; Wang, Z. Cationic para-phenyl-substituted α-diimine nickel catalyzed ethylene and 4-methyl-1-pentene (co)polymerizations via living/controlled chain-walking. Appl. Organomet. Chem. 2019, 33, e4911.

55. Wang, F.Z.; Tanaka, R.; Cai, Z.G.; Nakayama, Y.; Shiono, T. Synthesis of highly branched polyolefins using phenyl substituted α-diimine Ni(II) catalysts. Polymers 2016, 8, 160.

56. Liu, J.Y.; Li, Y.G.; Li, Y.S.; Hu, N.H. Ethylene polymerization by (α-diimine) nickel(II) complexes bearing different substituents on para-position of imines activated with MMAO. J. Appl. Polymer. Sci. 2008, 109, 700-707.

57. McCord, E.F.; McLain, S.J.L.; Nelson, T.J.; Ittel, S.D.; Tempel, D.; Killian, C.M.; Johnson, L.K.; Brookhart, M. 13C NMR Analysis of α-olefin enchainment in poly(α-olefins) produced with nickel and palladium α-diimine catalysts. Macromolecules 2007, 40, 410-420.

58. Azoulay, J.D.; Bazan, G.C.; Galland, G.B. Microstructural characterization of poly(1-hexene) obtained using a nickel α-keto-β-diimine initiator. Macromolecules 2010, 43, 2794-2800.

59. Jeon, M.; Han, C.J.; Kim, S.Y. Polymerizations of propylene with unsymmetrical (α-diimine)nickel(2) catalysts. Macromol. Res. 2006, 14, 306-311.

60. Pellecchia, C.; Zambelli, A. Syndiotactic-specific polymerization of propene with a Ni-based catalyst. Macromol. Rapid Commun. 1996, 17, 333-338.