Synthesis of Various Branched Ultra-High-Molecular-Weight

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Synthesis of Various Branched Ultra-High-Molecular-Weight Polyethylenes Using Sterically Hindered Acenaphthene-Based α‑Diimine Ni(II) Catalysts Lihua Guo,†,# Kunbo Lian,‡,# Wenyu Kong,† Shuai Xu,† Guorun Jiang,‡ and Shengyu Dai*,‡ †

School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China Chinese Academy of Science Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China

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‡

S Supporting Information *

ABSTRACT: A series of highly sterically hindered acenaphthene-based α-diimine nickel complexes with the remote R group in 4-position of diarylmethyl moiety have been synthesized and characterized. Activated with Et2AlCl, ethylene polymerization by these nickel complexes is investigated in detail, involving the remote substituent effect and influence of polymerization temperature on catalyst activity, thermal stability, polymer molecular weight, and branching density. These thermostable nickel catalysts are very active (up to 5.1 × 106 g·mol−1·h−1) for ethylene polymerization and capable of producing various moderate to highly branched (26−71/1000 C) ultra-high-molecular-weight polyethylenes (UHMWPEs, Mw up to 4.5 × 106 g·mol−1). These polymeric materials with such unique structure show properties characteristic of thermoplastic elastomers, i.e., good elastomeric recovery and high strain at break.

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nium dichloride showed high activity (1.04 × 106 g·mol−1·h−1) in ethylene polymerization and generated UHMWPEs at high temperature (70 °C) with number-average molecular weight (Mn) up to 2 × 106 g·mol−1 (Scheme 1, I).48 The titanium based bis(phenoxy-imine) catalysts with ortho-fluorine substituents (Scheme 1, II) initially reported by Fujita et al. can lead to the formation of UHMWPEs in a controlled manner with Mn exceeding 1 × 106 g·mol−1.49,50 Mecking et al. also showed that the ortho-fluorinate bis(β-ketoiminato) titanium catalysts (Scheme 1, III) afforded UHMWPEs in a living fashion with Mn ca. 1 × 106 g·mol−1 and molecular weight distribution of 1.17.51 All of these UHMWPEs catalyzed by early transition metal catalysts are linear polymers. In contrast, late transition metal catalysts offer great potential to achieve the branched UHMWPEs due to their strong “chain walking” ability in ethylene polymerization. These branched UHMWPEs may show some new mechanical properties and be expected to have the improved processability. However, late transition metal catalysts suitable for the synthesis of UHMWPEs remain scarce.52−56 Brookhart et al. showed that the neutral nickel complexes incorporating 2,8-diarylnaphthyl groups (Scheme 1, IV) were highly active for ethylene polymerization and generated branched UHMWPEs (Mn up to 1.3 × 106 g·mol−1) with low to moderate branch density in the range of 4−31 branches per 1000 carbon atoms which can

INTRODUCTION A huge amount of late transition metal catalysts have been designed and used for olefin polymerization since Brookhart’s discovery of α-diimine type nickel and palladium catalysts in 1995.1−23 The key to generate the high polymerization activity and high polymer molecular weight using these catalysts is the presence of bulky ortho-aryl substituents in the ligand which could enhance the catalyst stability and retard the chain transfer process.1,2,5 Generally, Brookhart catalysts have a high tendency toward β-hydride elimination and reinsertion, which results in the movement of the metal along the alkyl chain and production of branched polyethylenes.24,25 Moreover, the polymers with various types of branching can also be obtained by tuning the ligand structures and polymerization conditions.26−38 Ultra-high-molecular-weight polyethylenes (UHMWPEs), molecular weight values of which are in the range of 106− 107 g mol−1, have attracted much attention because these unique polymers have outstanding physical and mechanical properties.39 Most notable are their chemical, photochemical, and biological stability, lubricity, abrasion resistance, and high impact toughness.40 For example, they can be used in total joint replacement and medical devices.40 However, processing of UHMWPEs suffers from difficulty due to the extremely high viscosity of UHMWPEs. So far, some well-defined early transition metal based metallocene or nonmetallocene catalysts have been reported to generate linear UHMWPEs.41−51 The covalently bridged ethylene-bis(4,7-dimethyl-1-indenyl) zirco© XXXX American Chemical Society

Received: April 29, 2018

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DOI: 10.1021/acs.organomet.8b00275 Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Selected Examples of Transition Metal Catalyst That Can Catalyze Ethylene Polymerization and Generate UHMWPEs

Scheme 2. Synthesis of Nickel Complexes

be tuned by the polymerization conditions.52 Chen’s group reported that the α-diimine palladium catalysts containing a naphthalene or benzothiophene substituent (Scheme 1, V) perform extremely well in ethylene polymerization and copolymerization with polar monomers. In some cases, UHMWPEs with Mn above 4 × 106 g·mol−1 and low to moderate branch density (6−23/1000 C) were generated.53 Daugulis and Brookhart also prepared a series of “sandwich” αdiimine nickel catalysts incorporating 8-arylnaphthyl groups (Scheme 1, VI), which were shown to generate highly branched UHMWPEs (43−149/1000 C) with Mn values up to 1.8 × 106 g·mol−1.54 These studies have shown that the branch density of UHMWPEs can be controlled by tailoring the catalyst structures. In this work, a series of novel highly sterically hindered acenaphthene-based α-diimine nickel complexes bearing bulky diarylmethyl moiety were designed and used for the synthesis of moderate to highly branched UHMWPEs. Very interestingly, these novel UHMWPEs displayed properties characteristic of thermoplastic elastomers.

reported strategies usually yield unexpected monoimine products.57,58 Recently, a modified synthetic procedure using trimetylaluminum as catalysts for acenaphthene-based αdiimines bearing the bulky benzhydryl moiety has been reported. However, only ∼10% yield is achieved, and the column separation is also required.59 In this work, multigram highly sterically hindered acenaphthene-based α-diimine ligands with R group in 4-position of diarylmethyl moiety (L-OMe, L-Me, L-t-Bu, and L-Ph) were prepared with no chromatography involved in 68−80% yields by an efficient method we previously reported.60 With an easy access to the required diimine, nickel complexes Ni−OMe, Ni−Me, Ni−tBu, and Ni−Ph were readily prepared in 80−92% yields from the reaction of the corresponding ligands with 1 equiv of (DME)NiBr2 (DME = 1,2-dimethoxyethane) (Scheme 2). For comparison, structurally similar hydrogen substituted ligand L−H and corresponding nickel complex Ni−H were also prepared using literature procedures.59 As a result of the paramagnetic nature, these nickel complexes could not be characterized by NMR spectroscopic analysis. Instead, they were characterized by mass spectrometric and elemental analysis as well as X-ray crystallography. The molecular structure of complex Ni−t-Bu is shown in Figure 1. In solid state, the nickel center adopts distorted tetrahedron geometry with a N1−Ni1−N1 angle of 84.5(3)o and a Br1−Ni1−Br2

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RESULTS AND DISCUSSIONS Catalysts Synthesis. The effort to synthesize highly sterically hindered acenaphthene-based α-diimines bearing bulky diarylmethyl moiety is usually unsuccessful since the B

DOI: 10.1021/acs.organomet.8b00275 Organometallics XXXX, XXX, XXX−XXX

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2.5 × 106 g·mol−1. Furthermore, polymerizations conducted at 100 °C showed a slightly decrease with time of activity (from 5.3 × 106 to 2.9 × 106 g·mol−1·h−1) and molecular weight (from 1.5 × 106 to 1.2 × 106 g·mol−1), thereby signifying decomposition of catalyst at this temperature. These results indicated that the chain transfer and catalyst deactivation in this system were suppressed by the bulky ligands. In addition, using hexane as the solvent in place of toluene, these catalysts gave the similar polymerization results as those obtained in toluene. The most notable feature of these complexes is the production of moderate to highly branched UHMWPEs. Under almost all of the polymerization conditions, the molecular weight (Mw) of the polyethylene is higher than one million (Tables 1 and 2). It should be noted that the molecular weight obtained in this system is relative to polystyrene standards which may lead to inaccurate estimate of actual polyethylene molecular weights. The reported acenaphthene-based α-diimine nickel complex, Ni−H, produced the polyethylene with the maximum molecular weight ca. 22.0 × 105 g·mol−1 at 20 °C (Table 1, entry 1), which is lower than the molecular weight of polyethylene obtained by other catalysts under the same reaction condition (Table 1, entries 6, 11, 16, and 21). As a result, the presence of remote substituent (OMe, Me, t-Bu, and Ph) in 4-position of diarylmethyl moiety in these complexes seems to block the axial position more efficiently than H substituent, thus enhancing the molecular weight of generated polyethylenes. Although the decreased molecular weight of polyethylenes was observed at elevated temperatures (80−100 °C), polyethylenes with the molecular weight close to or higher than one million can still be obtained (Tables 1 and 2). A few late transition metal catalysts as mentioned in the introduction have been reported to generate UHMWPEs at a temperature below 60 °C.52−54 However, elevated polymerization temperatures (typically 60−90 °C or even higher) are preferred for economic reasons in industrial processes. As a result, the synthesis of UHMWPEs at elevated temperature represents a big advantage of this catalyst system. The structure of ligands had a significant influence on the degree of branching of the generated UHMWPEs. For example, the polymerization at 20 °C revealed the following trends in branching density: Ni−t-Bu (50/1000 C)> Ni− OCH3 (37/1000 C) > Ni−Me (27/1000 C) > Ni−Ph (26/ 1000 C). Specially, Ni−t-Bu led to polyethylene with much higher branching density (50−71/1000 C) than other catalysts (e.g., 26−54/1000 C for Ni−Ph) at the same temperature. Considering the higher steric hindrance of t-Bu than other R substituents, the ligand steric effect arising from the remote substituents could be operative in this system, which can slow the trapping of the intermediate alkyl cation by ethylene more than the rate of chain walking.27 Polyethylenes generated by Ni−Ph showed the fewest branches. This might originate from the plane structure of phenyl substituent, which exhibits only a single orientation and provides a little lower steric bulkiness than those of other three-dimensional R substituents. The difference of the branches in the polymer chain is translated into the greater difference in polymer melting points determined by differential scanning calorimetry (DSC). Compared to the polyethylenes obtained by Ni−OMe, Ni− Me, and Ni−Ph, lower melting points of the UHMWPEs generated by Ni−t-Bu at the same temperature were observed. In addition, the polymer branching density was increased with

Figure 1. Molecular structures of Ni−tBu with 50% probability level and H atoms have been omitted for clarity. Solvent molecule (CH2Cl2) was also omitted. Selected bond lengths (Å) and angles (deg): Ni(1)−N(1), 2.001(5); Ni(1)−Br(2), 2.2739(18); Ni(1)− Br(1), 2.294(2); N(1)−Ni(1)−N(1), 84.5(3); Br(2)−Ni(1)−Br(1), 124.83(8).

angle of 124.83(8)°. The observed bond lengths (Ni1−Br1 = 2.294(2) Å, Ni1−Br2 = 2.2739(18) Å, and Ni1−N1 = 2.001(5) Å) are similar to the reported 2,3-butadione derived nickel α-diimine complexes bearing the bulky diarylmethyl moiety.61 Both phenyl rings attached on imine groups are nearly perpendicular to the coordination plane, with the dihedral angles being 88.6°. The effective blockage of the axial positions of the metal center from N-aryl moiety can be observed from this structure. In addition, it is noteworthy that the remote tert-butyl substituent in 4-position of diarylmethyl moiety can also enhance the steric hindrance of the metal center. Ethylene Polymerization. All of these nickel complexes were screened for ethylene polymerization with Et2AlCl as cocatalyst, and the polymerization results are summarized in Table 1. Overall, these catalysts showed high polymerization activities (0.84−5.1 × 106 g·mol−1·h−1) and generated UHMWPEs (Mw up to 4.5 × 106 g·mol−1) with the branching density and melting point in a wide range of 26−71/1000 C and 40.4−110.3 °C, respectively. The substituents in 4position of diarylmethyl moiety had a significant influence on the catalytic activities of the complexes at low temperature. For example, at 20 °C, complex Ni−Ph showed the highest activity of 5.1 × 106 g·mol−1·h−1, which was almost twice that of Ni−tBu. However, at higher polymerization temperatures (40−100 °C), the difference in catalytic activity became smaller for these complexes. Specifically, all of these nickel complexes showed great thermal stability and maintained high activities up to 80 °C. To gain further insight into catalyst stability at elevated temperatures, polymerizations with the different reaction time were conducted at 80 and 100 °C (Tables S1 and 2). The plot of activities and molecular weight versus time for Ni−Ph was shown in Figure 2. At 80 °C, almost constant activities (3.5− 4.7 × 106 g·mol−1·h−1) were found over 30 min. The molecular weight increased with polymerization time and reached over C

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Organometallics Table 1. Effect of Catalyst and Temperature on Ethylene Polymerizationa ent.

cat.

T (°C)

yield (g)

act.b

Mwc

PDIc

Bd

Tme

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26f 27f 28f 29f

Ni−H Ni−H Ni−H Ni−H Ni−H Ni−OMe Ni−OMe Ni−OMe Ni−OMe Ni−OMe Ni−Me Ni−Me Ni−Me Ni−Me Ni−Me Ni−t-Bu Ni−t-Bu Ni−t-Bu Ni−t-Bu Ni−t-Bu Ni−Ph Ni−Ph Ni−Ph Ni−Ph Ni−Ph Ni−OMe Ni−Me Ni−t-Bu Ni−Ph

20 40 60 80 100 20 40 60 80 100 20 40 60 80 100 20 40 60 80 100 20 40 60 80 100 60 60 60 60

2.83 2.91 2.50 2.32 2.13 2.39 2.44 2.65 2.67 2.04 3.20 2.47 2.38 2.49 2.77 2.36 2.64 3.62 2.14 0.84 5.10 2.42 3.51 2.99 2.42 2.36 2.12 1.77 2.60

2.83 2.91 2.50 2.32 2.13 2.39 2.44 2.65 2.67 2.04 3.20 2.47 2.38 2.49 2.77 2.36 2.64 3.62 2.14 0.84 5.10 2.42 3.51 2.99 2.42 2.36 2.12 1.77 2.60

220.0 176.7 141.5 124.3 106.4 304.2 260.8 290.3 190.3 73.2 325.3 223.8 241.0 213.0 121.6 450.5 293.3 277.5 178.6 81.3 295.7 288.0 244.5 249.5 133.0 179.0 182.8 292.7 170.0

2.13 2.30 2.77 2.54 3.51 2.34 2.27 2.13 2.76 2.64 1.82 2.66 2.64 2.72 2.73 1.68 1.93 1.96 2.19 2.58 2.24 2.26 2.77 2.67 2.90 2.90 3.18 2.08 3.21

g

81.9 69.7 62.4 54.4 54.4 82.1 74.1 71.3 66.6 56.0 110.3 78.1 73.1 67.0 53.1 71.7 57.5 46.1 46.0 45.4 107.3 78.7 68.8 63.4 54.2 60.5 65.5 40.4 65.2

37 39 45 47 51 27 36 42 47 52 50 56 68 69 71 26 38 46 49 54 52 48 70 49

Conditions: 2 μmol of precatalyst, 200 equiv of Et2AlCl, 1 mL of CH2Cl2, 40 mL toluene, 7 atm, 30 min. bActivity (act.) = 106 g/(mol Ni·h). cMw: 104 g mol−1; Mw and PDI (Mw/Mn) were determined by GPC in trichlorobenzene at 150 °C using polystyrene standards. dB = branches per 1000 carbons, determined by 1H NMR spectroscopy. eDetermined by differential scanning calorimetry (DSC). fUsing hexanes as solvent. gNot determined. a

lightly branched (4−31/1000 C) or the extremely highly branched (43−149/1000 C) UHMWPEs,52−54 the nickel catalysts in this system produced the moderate to highly branched (26−71/1000 C) UHMWPEs. These UHMWPEs appear to be unique and can bridge the gap between the lightly branched and the extremely highly branched UHMWPEs as mentioned above. To examine the mechanical properties of these branched UHMWPEs, tensile tests were carried out for all the polymer samples prepared using Ni−OMe, Ni−Me, Ni−t-Bu, and Ni− Ph catalysts at different polymerization temperatures (Figure 3 and Table S2). These samples showed stress at break values in the range from 8.3 to 25.9 MPa and high strain at break values ranging from 413 to 844% with typical feature of elastomers (i.e., low modulus (stress/strain), high strain at break and good elastomeric recovery (see details below, Figure 4)). Basically, the higher reaction temperature resulted in the polymers with the higher strain at break values. Furthermore, polymers obtained by Ni−t-Bu catalyst displayed lower stress at break values (8.3−13.4 MPa) than that obtained by other nickel catalysts (11.7−25.9 MPa) under the same polymerization conditions. Clearly, the polymer microstructures including molecular weights and branching densities, which can be modulated by catalyst structure and polymerization temperature, have a significant influence on the mechanical properties of these polymers.

Table 2. Effect of Reaction Time on Ethylene Polymerizations Using Catalyst Ni−Ph at Elevated Temperaturesa ent.

T (°C)

time (min)

yield (g)b

act.b

Mwc

PDIc

Tmd

1 2 3 4 5 6 7 8

80 80 80 80 100 100 100 100

5 10 20 30 5 10 20 30

0.64 1.56 2.89 3.48 0.82 1.78 2.38 2.88

3.84 4.68 4.33 3.48 4.92 5.34 3.58 2.88

54.7 142.6 174.6 249.5 154.4 150.4 120.8 133.0

2.56 3.32 2.85 2.67 2.88 2.70 2.69 2.90

65.8 61.6 63.6 63.4 56.6 58.5 57.4 54.4

a Conditions: 2 μmol of precatalyst, 200 equiv of Et2AlCl, 1 mL of CH2Cl2, 40 mL of toluene, 7 atm, 30 min. bActivity (act.) = 106 g/ (mol Ni·h). cMw: 104 g/mol; Mw and PDI (Mw/Mn) were determined by GPC in trichlorobenzene at 150 °C using polystyrene standards. d Determined by differential scanning calorimetry (DSC).

polymerization rised, following the same trend as other αdiimine Ni(II) catalyst.2,62 However, the branching density of polyethylene produced by these catalysts is not sensitive to the reaction temperature. The 13C NMR spectra indicate that mainly methyl (C1) and long-chain (C ≥ 6) branches are present in these UHMWPEs (Figure S1). This can be explained by the reported mechanism.63 Very interestingly, in contrast to reported catalysts IV−VI, which generated the D

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Figure 2. Plots of (a) activity versus time and (b) Mw versus time for Ni−Ph at 80 °C (black) and 100 °C (red).

Figure 3. Stress−strain curves for polymers generated by (a) Ni−OMe, (b) Ni−Me, (c) Ni−t-Bu, and (d) Ni−Ph at 100, 80, 60, 40, and 20 °C.

Figure 4. Plots of hysteresis experiments of ten cycles at a strain of 300% for samples generated by Ni−OMe at 100 °C (a), Ni−Me at 100 °C (b), Ni−t-Bu at 100 °C (c), Ni−Ph at 100 °C (d), Ni−t-Bu at 20 °C (e), Ni−t-Bu at 40 °C (f), Ni−t-Bu at 60 °C (g), and Ni−t-Bu at 80 °C (h).

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To investigate the elastic recovery (i.e., the capability to return to the initial state once the force is removed), polymer samples were also subjected to hysteresis testing where each sample was extended to 300% strain over 10 cycles (Figure 4 and Table S1). The strain recovery values (SR) can be calculated by SR = 100 (εa − εr)/εa, where εa is the applied strain and εr is the strain in the cycle at zero load after cycle 10. These polymer samples exhibit a certain amount of unrecovered strain after the first cycle, followed by minimal deformation on each subsequent cycle. A permanent structural change happens during the first cycle, after which better elastomeric properties are created. Overall, these samples exhibit SR values up to 62%, which are comparable with those of previously reported elastic polyolefin materials obtained by late transition metal catalysts64−67 and commercial polyolefin elastomers (POE).68 The very high molecular weight and highly branched microstructure of the polymers appear to together contribute their unique elastic properties. Hence, the current catalytic system provides a rare route to the synthesis of thermoplastic elastomers in one step using only ethylene as the feedstock.66,67 The catalyst structure plays an important role on the elastic properties of these polymer samples. The unrecovered strain from 300% after 10 cycles is 134% for the polymer sample produced using Ni−t-Bu at 100 °C (SR = 55%), while higher unrecovered strain (189−233%) is observed for the polymer samples obtained by other nickel catalyst (SR: 22−37%). This result can be attributed to the higher branching density of polymer samples produced using Ni−t-Bu. In addition, the polymer sample generated by Ni−tBu at 80 °C showed an SR value (SR = 62%) higher than that generated at 100 °C. Since these two samples have the similar branching densities, the higher molecular weight of the polymer sample generated by Ni−t-Bu at 80 °C (1.8 × 106 g·mol−1) than that at 100 °C (8.1 × 105 g·mol−1) seems to enhance the elastic property.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00275. Full experimental details for the synthesis of nickel complexes and polymers, CIF files for complexes Ni−tBu and Ni−OMe (PDF) Accession Codes

CCDC 1816927 and 1839356 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lihua Guo: 0000-0002-0842-9958 Shengyu Dai: 0000-0003-4110-7691 Author Contributions #

L.G. and K.L. are equal first authors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant No. 51703215), Shandong Provincial Natural Science Foundation (ZR2018MB023), the Fundamental Research Funds for the Central Universities (WK2060200024), China Postdoctoral Science Foundation (2017M612077), the Key Laboratory of Polymeric Composite & Functional Materials of Ministry of Education (PCFM2017-01), and Foundation of Qufu Normal University (xkJ201603).

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CONCLUSIONS In summary, a series of highly sterically hindered acenaphthene-based α-diimine nickel complexes bearing bulky diarylmethyl moiety have been synthesized and characterized. Activated by Et2AlCl, these nickel catalysts showed high activities (up to 5.1 × 106 g·mol−1·h−1), great thermal stability (quite stable at up to 100 °C), and generated branched UHMWPEs (Mw up to 4.5 × 106 g·mol−1) with branching density in a wide range of 26−71/1000 C. Such branched UHMWPEs with moderate to high branching density appear to be unique and can bridge the gap between the lightly branched and the extremely highly branched UHMWPEs reported with late transition metal catalysts.52−54 The presence of remote substituent (OMe, Me, t-Bu, and Ph) in 4-position of diarylmethyl moiety in these complexes and polymerization temperature exert great influences on the catalytic properties of these nickel catalyst as well as the mechanical properties of the generated UHMWPEs. Specifically, steric t-Bu-substituted complex Ni−t-Bu led to UHMWPEs with much higher branching density and elastic strain recovery value than other catalysts at the same polymerization temperature. These branched UHMWPE materials displayed properties characteristic of thermoplastic elastomers. As a result, this work may provide an alternative and effective strategy to synthesize thermoplastic elastomers in one step using only ethylene as the feedstock.

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REFERENCES

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DOI: 10.1021/acs.organomet.8b00275 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00275 Organometallics XXXX, XXX, XXX−XXX