Direct Synthesis of Thermoplastic Polyolefin Elastomers from Nickel

Article pubs.acs.org/Macromolecules

Direct Synthesis of Thermoplastic Polyolefin Elastomers from NickelCatalyzed Ethylene Polymerization Kunbo Lian,† Yun Zhu,‡ Weimin Li,† Shengyu Dai,*,†,‡ and Changle Chen*,‡ †

Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, School of Pertrochemical Engineering, Changzhou University, Changzhou 213164, China ‡ CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: As a promising alternative to thermoset elastomers, thermoplastic elastomers (TPEs) have attracted much attention because of their unique properties, including processability, reusability and recyclability. The synthesis of TPEs based on olefinic building blocks usually requires the use of long chain α-olefins, multiple steps, and/or multiple catalysts. The concept of using only ethylene as feedstock in a single step is fascinating but also very challenging. In this contribution, we report the synthesis of polyethylene-based TPEs through α-diimine nickel-catalyzed ethylene polymerization. The stress-at-break and strain-at-break values of these polyethylene products could be tuned over a very wide range using different nickel catalysts and different polymerization conditions. Most importantly, products with excellent elastic properties could be generated in the screening process.

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shown to be enhanced by using long-chain α-olefins or by the development of highly selective catalysts.31−34 Thermoplastic elastomers (TPEs) have attracted much attention because they can be recycled and reprocessed, in addition to their superior mechanical properties.35 These materials are usually based on multiblock copolymers containing both hard segments and soft segments. The majority of polyolefin-based TPEs are generated using group IV metallocene-type catalysts.36−39 By taking advantage of the chain walking properties using α-diimine palladium and nickel catalysts, a huge number of polymers and copolymers with various microstructures have been designed and synthesized. However, very few efforts have been directed to preparing polyolefin materials with good mechanical properties and potential practical applications. By adjusting the catalyst structures or the polymerization conditions, ethylene or propylene polymerization could lead to branched/amorphous segments, while α-olefin polymerization could lead to relatively linear/semicrystalline segments. As such, multiblock copolymers could be generated by alternating the polymerization temperatures or by the sequential addition of different olefin monomers.40−42 Recently, Ricci et al. employed this approach and prepared triblock copolymers with high tensile strength (18 MPa) and strain-at-break values (ca. 1000%).43 However, significant permanent deformations after stretching were observed for the copolymers (∼35% recovery after 10 cycles at 300% strain). Furthermore, Coates et al. developed a one-pot

INTRODUCTION

Among the numerous spectator ligands used in olefin polymerization catalysis, α-diimines are one of the most extensively studied systems.1−5 Palladium α-diimine catalysts are able to mediate copolymerization reactions of ethylene with a variety of polar monomers.6−18 Nickel α-diimine catalysts possess activities that are comparable with many early transition metal catalysts.19−28 This class of catalyst has unique chain walking properties during olefin polymerization, which is a process of β-hydride elimination followed by reinsertion with opposite regiochemistry into the olefin hydride. Because of this unique feature, ethylene polymerization usually leads to highly branched (and even dentritic) polyethylene microstructures (Scheme 1).29,30 In α-olefin polymerization, chain straightening through 1,ω-enchainment can occur, leading to relatively linear polyolefin products. This chain-straightening effect has been Scheme 1. Mechanism for α-Diimine Palladium- and NickelCatalyzed Ethylene and α-Olefin Polymerization

Received: May 24, 2017 Revised: July 16, 2017

© XXXX American Chemical Society

A

DOI: 10.1021/acs.macromol.7b01087 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 2. Synthesis of the α-Diimine Nickel Complexes

Figure 1. Molecular structure of complex Ni-CF3 in the solid state. Hydrogen atoms have been omitted for clarity. Atoms are drawn at 30% probability level. Selected bond lengths (Å) and angles (deg): Br1−Ni1 2.3274(11), Br2−Ni1 2.3302(11), Ni1−N1 2.044(4), Ni1−N2 2.052(4), N1−Ni1−N2 82.91(15), N1−Ni1−Br1 107.50(11), N2−Ni1−Br1 119.56(11), N1−Ni1−Br2 107.18(11), N2−Ni1 Br2 99.59(11), Br1−Ni1−Br2 129.91(4).

similar to each other, thereby eliminating the possible influence of ligand electronic effects.46 Our initial objective was to explore the possible steric or ligand second coordination sphere effects of this ligand framework on the properties of their corresponding nickel complexes. The nickel complexes (Ni-Ph, Ni-CF3, Ni-NO2, and Ni-OMe) were prepared from the reactions of the ligands (L-Ph, L-CF3, L-NO2, and L-OMe, respectively) with (DME)NiBr2 (DME = ethylene glycol dimethyl ether) in 80−92% yields (Scheme 2). These nickel complexes were characterized by MALDI-TOF-MS and elemental analysis. The classic αdiimine Ni(II) complex [(2,6-iPr2C6H3)NC(An)−(An)C N(2,6-iPr2C6H3)]NiBr2 (Ni-iPr, An = acenaphthyl) was prepared and studied for comparison. The molecular structure of complex Ni-CF3 was determined by X-ray diffraction (Figure 1). The nickel center adopts a distorted tetrahedron geometry. As can be seen from the structure, the aryl substituent on the imine nitrogen is also distorted, suggesting a possible steric influence of the remote substituents. The Ni1−F6 distance is 3.596 Å, which is longer than the sum of the van der Waals radii of the atoms (3.1 Å). However, during polymerization and after the generation of the cationic nickel species, such interactions could be enhanced. Moreover, the CF3 group or NO2 group could interact with the β-hydrogen atom on the growing polymer chain and influence the chain transfer process.47−55 Complex Ni-CF3 can exist as a rac (with both −C6H4−CF3 substituents on the opposite side of the N−Ni−N plane) or a meso (with both −C6H4−CF3 substituents on the same side of the N−Ni−N plane) structure. The X-ray analysis indicates a meso structure in the solid state.

strategy for the synthesis of multiblock copolymers with excellent mechanical properties (strain-at-break values of 630− 710% and elastic recovery of 59−85% after 10 cycles at 300% strain).44 For these strategies to work, multiple steps and long chain α-olefins (1-decene or 1-dodecene) are required. Recently, Sun et al. reported the synthesis of some polyolefin materials using α-diimine nickel catalysts.45 These polyolefins displayed properties characteristic of thermoplastic elastomers (tensile strength in the range 0.3−13 MPa and strain-at-break values in the range 100−645%), although the elastic recovery properties were only moderate (ca. 67% recovery after 10 cycles at 200% strain). In this contribution, we wish to demonstrate that polyolefin materials with excellent mechanical and elastic recovery properties can be generated in a single step using only ethylene as the feedstock.

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RESULTS AND DISCUSSION Recently, our group reported the synthesis and olefin polymerization studies of some α-diimine palladium catalysts bearing different substituents on the remote positions of the ligand’s second coordination sphere.46 These palladium catalysts showed enhanced thermal stability and higher polyethylene molecular weights than the conventional αdiimine palladium catalysts. In that system, the electronic effects from the remote substituents are minimal because of the nonconjugating nature of the bridging moiety as well as the large covalent distance to the metal center. In that study, the corresponding palladium carbonyl complexes were prepared, and the CO stretching frequencies were found to be very B


Article

Macromolecules Table 1. Ethylene Polymerization with Nickel Complexesa entry

precat.

T/°C

yield/g

act.b

Mnc

Mw/Mnc

Bd

Tme

Tgg

SRh

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-CF3 Ni-CF3 Ni-CF3 Ni-CF3 Ni-NO2 Ni-NO2 Ni-NO2 Ni-NO2 Ni-OMe Ni-OMe Ni-OMe Ni-OMe Ni-iPr Ni-iPr Ni-iPr Ni-iPr

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

3.72 3.45 3.01 2.48 3.03 2.16 3.42 3.43 2.41 2.95 2.56 2.16 8.33 7.10 6.62 4.21 4.22 3.14 2.15 0.75

3.10 2.90 2.51 2.07 2.53 1.80 2.85 2.86 2.01 2.46 2.13 1.80 6.94 5.92 5.52 3.51 3.52 2.62 1.79 0.63

133.1 132.8 28.0 28.6 153.5 97.1 94.7 52.1 66.5 52.0 74.1 31.6 10.5 9.2 8.9 7.0 6.4 4.8 3.3 2.1

2.23 1.89 1.64 1.71 1.94 2.19 2.05 2.48 2.61 3.16 2.13 2.50 2.21 3.62 3.72 3.63 2.75 3.33 3.08 2.83

78 82 86 94 41 56 70 81 28 42 59 86 61 64 70 75 55 70 83 101

47.4 45.6 41.2 f 114.1 101.1 f f 117.8 114.8 101.1 f 50.1 38.4 30.4 25.8 50.2 f f f

−52.4 −51.7 −57.4 −59.7 −41.2 −48.8 −52.5 −57.9 −24.7 −36.7 −33.6 −50.0 −46.5 −41.7 −42.5 −50.3 −48.1 −49.6 −56.8 −63.6

82 84 71 44 54 57 69 20 33 31 53 35 41 36 40 76 66 13

General conditions: Ni = 2.4 μmol, Al/Ni = 500, CH2Cl2 = 2 mL, toluene = 48 mL, ethylene = 9 atm, time = 30 min. b106 g of PE (mol of Ni)−1 h−1. cMn: 104 g mol−1; Mn and Mw/Mn determined by GPC. dBranching numbers per 1000C were determined by 1H NMR. eMelting temperature determined by DSC. fNot observed at the temperature range of −50 to 120 °C. gGlass transition temperature was determined by dynamic mechanical analysis (DMA). hThe strain recovery (SR) was determined at 300% strain using the equation SR = 100(εa − εr)/εa, where εa is the applied strain and εr is the strain in the cycle at zero load after the applied strain of 300%. a

Figure 2. Stress−strain curves for polymers generated by (a) Ni-Ph, (b) Ni-CF3, (c) Ni-NO2, (d) Ni-OMe, and (e) Ni-iPr at 20, 40, 60, and 80 °C.

It is highly possible that a mixture of rac and meso isomers exists in solution and during polymerization. With MAO as cocatalyst, these nickel complexes are all highly active in ethylene polymerization (Table 1). Complexes Ni-Ph, Ni-CF3, and Ni-NO2 showed similar activities and high thermal stabilities, with activities well above 106 g of PE (mol of Ni)−1 h−1 even at 80 °C (Table 1, entries 1−12). Specifically, time dependence studies showed that Ni-Ph remained active within 40 min at 80 °C (Figure S1). Moreover, the molecular weight of the produced polyethylene was observed to reach one million at 20 °C and half a million at 80 °C. Interestingly, NiCF3 and Ni-NO2 led to polyethylene with much lower

branching density (41 and 28/1000C) than Ni-Ph (78/ 1000C) at 20 °C. Considering the similar electronic environment of these complexes, steric effect or an interaction of CF3/ NO2 unit with β-H on the growing polymer chain could be operative in this system, which can inhibit chain transfer and lead to higher molecular weight.47−55 At higher polymerization temperatures, the difference became smaller. This could be due to the weakening of such interactions at high temperatures, which is consistent with previous reports.47−55 This difference in branching density translates into greater differences in polymer melting points (114.1 and 117.8 °C versus 47.4 °C) and glass transition temperatures (−41.2 and −24.7 °C versus C


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Macromolecules

Figure 3. (a, b) Stress−strain curves during step-cycle tensile deformation at different strains of the polyethylene samples generated using catalysts Ni-Ph and Ni-OMe, respectively, at 60 °C. (c) Elastic recovery percentages for the samples generated by Ni-Ph, Ni-CF3, Ni-NO2, Ni-OMe, and Ni-iPr at 60 °C. (c) Elastic recovery percentages for the samples generated by Ni-Ph at 20, 40, 60, and 80 °C. (e, f) Plots of hysteresis experiments of ten cycles at a strain of 300% for samples generated using catalyst Ni-Ph at 40 and 60 °C.

−52.4 °C). Interestingly, catalyst Ni-OMe is much more active than the above three catalysts (Table 1, entries 13−16), which may originate from the high initiation efficiency for this catalyst. However, the polyethylene molecular weights are greatly reduced. As a comparison, the conventional catalyst Ni-iPr showed similar activity but much lower polyethylene molecular weights (Table 1, entries 17−20). The most interesting results came from the studies of the mechanical properties of these polyethylene products. The polymers were melt-pressed at 30−35 °C above their melting temperatures to obtain dog-bone-shaped tensile test specimens. The test specimens each had a 28 mm gauge length, a 3 mm width, and a thickness of 1 mm. Tensile strength was measured for all of the polyethylene samples prepared using these nickel catalysts at different temperatures (Figure 2). The properties of the resulting polyethylene products could be modulated over a very wide range. These samples exhibited stress-at-break values ranging from 3 to 28 MPa and strain-at-break values ranging from 300% to 1800% (instrument detection limit). For comparison, the product generated from the classical catalyst Ni-iPr showed very low stress-at-break values, and only oily products could be generated at 60 and 80 °C. Clearly, the mechanical properties of the polyethylene products are influenced by their microstructures, including their molecular weights and branching densities. To investigate the elastic properties, all of the samples from Table 1 were extended step by step up to different strains (Figures S25−S34). Two representative stress−strain curves from samples generated by Ni-Ph at 60 °C and Ni-OMe at 60 °C are shown in Figures 3a and 3b. Based on the curves, the strain recovery values (SR) can be obtained as SR = 100(εa − εr)/εa, where εa is the applied strain and εr is the strain in the cycle at zero load after the applied strain. The SR values are plotted as a function of the applied strain for the samples obtained at 60 °C (Figure 3c, see Figures S25−S34 for all the samples). The sample from Ni-Ph clearly stands out, with excellent elastic properties, maintaining SR values of above 80%

throughout the cyclic tensile deformation. For the samples generated by the other nickel catalysts, the SR values decrease rapidly with increasing applied strains. In addition, the samples generated by Ni-Ph at different temperatures (40, 60, and 80 °C) all maintained high SR values in this cyclic tensile deformation test (Figure 3d). In another set of cyclic loading experiments (hysteresis testing), the samples obtained from Ni-Ph were cyclically loaded and unloaded ten times to 300% strain. These samples exhibit a certain amount of unrecovered strain after the first cycle and minimum deformation on each subsequent cycle. A permanent structural change occurs during the first cycle, after which better elastomeric properties are induced. The unrecovered strain from 300% is 83% for the sample produced using Ni-Ph at 60 °C (elastic strain recovery value of 72%). Interestingly, a similar unrecovered strain value from 300% was observed for the sample produced using Ni-Ph at 40 °C. These two samples exhibited dramatically different strain-at-break values (565% versus 1605%). However, they possess similar stress-at-break values and similar elastic properties (elastic strain recovery value of 70% for Ni-Ph at 40 °C). Overall, the polyethylene samples generated by Ni-Ph demonstrated similar or even better tensile and elastic properties than those of block or random ethylene/α-olefin copolymers.56−58 The elastic properties of these polyethylene materials are comparable with the multiblock polyolefin materials reported by Coates et al.44 and much better than those reported by Ricci et al.43 The tensile properties of these polyethylene samples could be modulated over a very wide range using these nickel catalysts. However, only catalyst Ni-Ph is able to generate polyethylene products with excellent elastic properties. The exact operative mechanism that leads to the unique properties of catalyst Ni-Ph is currently unknown. The highly branched microstructures and the presence of both methyl and long chain branches in these samples probably contribute to their unique properties. 13C NMR analysis of the polyethylene samples prepared by Ni-Ph, Ni-CF3, Ni-NO2, Ni-OMe, and D


Article

Macromolecules Ni-iPr showed the presence of methyl, ethyl, n-propyl, n-butyl, and long chain branches (Figures S2−S4 and Table S1). Interestingly, much higher percentages of long chain branches were observed for the cases of Ni-Ph and Ni-iPr. Considering the much higher polymer molecular weight for the polyethylene generated by Ni-Ph versus Ni-iPr, a combination of high molecular weight and high amount of long chain branches could be the key to achieve great elastic properties. Alternatively, the excellent elastic properties of these polyethylene samples suggest that they may possess multiblock structures with both hard segments and soft segments. Coates and Waymouth demonstrated the synthesis of multiblock copolymers consisting of alternating isotactic and atactic polypropylene segments using an oscillating zirconocene catalyst that changes its geometry during polymerization (Scheme 3a).59 It is possible that a similar oscillating scenario

of only one catalyst, and the avoidance of the need for expensive long-chain α-olefin feedstocks.66 Further work is needed to understand the exact mechanism and to explore the full potential of this type of catalytic system.

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CONCLUSIONS To conclude, we report herein the synthesis and characterization of a series of α-diimine nickel catalysts. In ethylene polymerization, these α-diimine Ni(II) complexes showed high activities (up to 6.9 × 106 g of PE (mol of Ni)−1 h−1) and good thermal stability (stable at up to 80 °C) and generated polyethylene with very high molecular weight (Mn up to 1.53 × 106) and narrow molecular weight distributions. Although at a remote position on the ligand framework, the installed substituents (Ph, CF3, NO2, and OMe) exert a dramatic influence on the properties of the corresponding nickel catalysts as well as the properties of the resulting polyethylene products. Polyethylene samples with a wide range of mechanical properties could be produced using different nickel catalysts under different conditions. Specifically, the nickel catalyst bearing a Ph substituent (Ni-Ph) led to the formation of polyethylenes with exceptional elastic properties. This work demonstrates the great potential of generating thermoplastic elastomers in a single step using only ethylene as the feedstock. Mechanistic studies and the design of new catalysts systems are currently in progress.

Scheme 3. (a) The Oscillating Zirconocene Catalyst System; (b) Isomerization of rac and meso α-Diimine Nickel Catalysts; (c) Properties of rac and meso α-Diimine Nickel Catalysts in Ethylene Polymerization

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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.macromol.7b01087. Experimental procedures, characterization of metal complexes and polyethylene (PDF) X-ray crystallographic data of C80H70Br2F6N2NiO2 (CIF)

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exists in this catalytic system. As discussed above, catalyst NiPh could exist as a mixture of the rac and meso isomers. Previously, our group and several other groups showed that rac and meso α-diimine nickel catalysts generate polyethylene or polypropylene with different molecular weights and different microstructures (Scheme 3b).60−63 Specifically, Wu et al. and Jordan et al. showed that polyethylenes with dramatically different molecular weights and branching densities could be generated using preisolated rac and meso α-diimine nickel catalysts (Scheme 3c).64,65 The rac isomer led to the formation of polyethylenes with low branching densities (hard segment), while the meso isomer led to the formation of polyethylenes with high branching densities (soft segment). If the chain transfer rate is slower than the exchange rate between the rac and meso isomers under polymerization conditions, multiblock copolymers could be generated. This is highly speculative at this point, and we are designing new catalysts to test this hypothesis. It is possible that polyethylenes without multiblock structures could demonstrate great elastic properties when they possess high molecular weight and high percentage of long chain branches. In this contribution, we simply wish to demonstrate the possibility of generating thermoplastic polyolefin elastomers with superior properties using only ethylene as feedstock and in a single step. This strategy is fascinating due to its simple procedure, the use

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.D.). *E-mail: [email protected] (C.C.). ORCID

Changle Chen: 0000-0002-4497-4398 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC, 51522306, 21374108, 21690071), the Fundamental Research Funds for the Central Universities (WK2060200024, WK3450000001), Advanced Catalysis and Green Manufacturing Collaborative Innovation Center (ACGM2016-06-01), Yixing Taodu Ying Cai Program, and the Recruitment Program of Global Experts.

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