Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Ethylene Polymerization and Copolymerization Using Nickel 2‑Iminopyridine‑N‑oxide Catalysts: Modulation of Polymer Molecular Weights and Molecular-Weight Distributions Chen Zou, Shengyu Dai, and Changle Chen* 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: Molecular weight and molecular-weight distribution are two critical parameters that determine the properties of a polyolefin material. In this contribution, we report the synthesis and characterization of a series of 2iminopyridine-N-oxide nickel complexes; these catalysts demonstrate very high activities (up to 107 gPE molNi−1 h−1) at very low methylaluminoxane loadings (80 equiv) during ethylene polymerization. By tuning the structures of the catalysts and the polymerization conditions, we show that it is possible to tune the polyethylene molecular weight (Mw: (0.3−301.6) × 104), molecular-weight distribution (polydispersity index (PDI): 1.9−59.7), melting temperature (62.4−132.4 °C), and branching density (9−104/1000 C) over very wide ranges. This translates into the ability to tune the mechanical properties of these polymers as well as their complex viscosities. Equilibria between bis-ligated and mono-ligated nickel species are proposed to play important roles in this system. These nickel catalysts also mediate the efficient copolymerization of ethylene with methyl 10-undecenoate.
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INTRODUCTION Transition-metal catalysts based on a variety of ligands have played critical roles in the field of olefin polymerization, and imines are among the most common structural motifs found in spectator ligands. Notable examples include pyrrole-imines (Chart 1, I),1 pyridine-imines (II),2 pyridine-diimines (III),3 βdiimines (IV),4 salicylaldimines (V),5 and α-diimines (VI),6−17 among many others. These imine ligands are usually prepared by the condensations of aldehydes or ketones with various anilines; hence, the development of novel anilines facilitates the generation of new imine-based ligands and, as a consequence, new olefin polymerization catalysts. For example, the Long and Sun groups used a novel dibenzhydryl-based aniline to prepare several high-performance α-diimine nickel catalysts (Chart 1, VII).18−21 Our group subsequently designed some dibenzhydryl-based anilines bearing different substituents (Me, MeO, Cl, CF3) at the para-position to those previously reported. The corresponding α-diimine palladium catalysts (Chart 1, VIII) exhibited excellent ethylene-polymerization and ethylene/ methyl acrylate-copolymerization properties.22−24 We also prepared some salicylaldimine nickel catalysts using these anilines (Chart 1, IX).25 Significantly improved polymerization performance, in relation to all parameters, was observed with VII, VIII, and IX, compared to the classic Brookhart-type αdiimine nickel and palladium catalysts or classic Grubbs-type salicylaldimine nickel catalysts derived from 2,6-diisopropylaniline. Recently, we developed two novel α-diimine ligands bearing dinaphthylhydryl and dibenzothienylhydryl moieties and showed that the properties of the corresponding α-diimine © XXXX American Chemical Society
palladium catalysts (Chart 1, X) could be further improved for olefin polymerization and copolymerization.26 Brookhart, Coates, and Daugulis et al. investigated the olefin polymerization properties of some arylnaphthylamine-derived salicylaldimine and α-diimine Ni(II) catalysts (Chart 1, XI),13,27−29 while we combined the arylnaphthylamine and dibenzhydryl moieties and studied the properties of the corresponding pyridine-imine nickel catalysts (Chart 1, XII).30 Campora et al. reported studies into some 2-iminopyridineN-oxide nickel complexes (Chart 1, XIII), which are the cationic counterparts of the Grubbs-type salicylaldimine nickel catalysts.31 These nickel catalysts exhibited very interesting properties during ethylene polymerization. First, very high activities could be achieved (up to 1.3 × 106 g molNi−1 h−1 when R = H and 3.0 × 106 g molNi−1 h−1 when R = Ph) at very low cocatalyst loadings; MMAO (modified methylaluminoxane) as low as 50 equiv relative to Ni could be used. This is in stark contrast to most early and late transition-metal catalysts; hundreds and even thousands of equivalents of cocatalysts are required to achieve high activities with these catalysts. Second, both oligomerization and polymerization occurred during catalysis with XIIIa, and very low polyethylene molecular weights could be achieved (Mn up to 1.5 × 103), suggestive of the presence of two active species during polymerization. Third, this type of catalyst is thermally unstable, resulting in very low Received: October 6, 2017 Revised: November 24, 2017
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DOI: 10.1021/acs.macromol.7b02156 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Chart 1. Olefin-Polymerization Ligands Based on Imines and Catalysts with Sterically Bulky Aryl Substituents
activities at 50 °C. As part of our ongoing efforts to develop novel anilines and to apply the corresponding imines as ligands for olefin polymerization catalysis, we decided to study the 2iminopyridine-N-oxide nickel catalysts with the anilines from catalysts VIII, X, and XII. Catalytic activities, stabilities, and polymer molecular weights are greatly enhanced by the introduction of these novel aryl substituents.
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RESULTS AND DISCUSSION 2-Pyridinecarboxaldehyde-N-oxide was easily prepared following a literature procedure.31 Condensation reactions with the required anilines subsequently afforded the desired imines (LPh, L-Np, L-Bz, L-Ph-Ph, and L-An) in yields of 56−86%. The reactions of each of the first three imines with 1 equiv of the (DME)NiBr2 nickel precursor (DME = ethylene glycol dimethyl ether) led to the formation of the corresponding bis-ligated nickel complex (Ni-Ph, Ni-Np, and Ni-Bz), while the remaining imines gave the desired monoligated nickel complexes (Ni-Ph-Ph and Ni-An); the formation of each complex was confirmed by elemental analysis and X-ray diffraction. These results are unexpected since the 2,6diisopropylaniline-derived imine afforded the monoligated nickel complex.31 Eventually, the bis-ligated nickel complexes (Ni-Ph, Ni-Np, and Ni-Bz) were prepared in yields of 78−92% by the reactions of the imines with 0.5 equiv of (DME)NiBr2. Clearly, bis-ligation is sensitive to the steric nature of the ligand, which also plays a critical role in determining the properties of these nickel complexes for ethylene polymerization (vide inf ra). The nickel center in the molecular structure of Ni-Ph adopts an octahedral geometry, with the two bromine atoms oriented cis to each other (Figure 1a). This geometry is very important for the migratory insertion of the ethylene monomer during polymerization. If bis-ligation results in trans-configured bromine atoms, the bis-ligated nickel complex would be incapable of catalyzing ethylene polymerization. The molecular structure of Ni-Ph-Ph reveals a bridged-bromide structure (Figure 1b), which is expected to easily break down during polymerization. With the addition of a very small amount of methylaluminoxane (MAO) as a cocatalyst, these nickel complexes
Figure 1. (a) Molecular structure of complex Ni-Ph. Selected bond lengths (Å) and angles (deg): Ni1−Br1 2.5239(14), Ni1−Br2 2.5220(14), Ni1−O1 2.044(5), Ni1−O2 2.066(5), Ni1−N2 2.158(6), Ni1−N4 2.151(6); Br2−Ni1−Br1 91.78(5), O1−Ni1−O2 86.69(19), N4−Ni1−N2 161.9(2). (b) Molecular structure of complex Ni-Ph-Ph. Selected bond lengths (Å) and angles (deg): Ni1−Br1 2.461(2), Ni1−Br2 2.405(3), Ni1−O1 1.998(9), Ni1−N1 2.046(12); Br2−Ni1−Br1 105.24(9), O1−Ni1−N1 84.9(4). Hydrogen atoms have been omitted for clarity. Atoms are drawn at the 30% probability level.
exhibited high activities for ethylene polymerization (Table 1). The Ni-Bz complex is the only exception; it showed no activity toward ethylene polymerization in the presence of MAO (Table 1, entry 22). The inactivity of Ni-Bz is most likely due to the poisoning effect of the benzothienyl unit toward the nickel center. For Ni-Ph in the presence of 80 equiv of MAO (Table 1, entries 1−5), very high activities (up to 107 gPE molNi−1 h−1) were observed over a wide range of polymerization temperatures (0−80 °C). Most intriguingly, the polyethylene branching density could be tuned over a very wide range (12−104/1000 C) through simple control of the polymerization temperature. Ni-Ph exhibited high activity with the addition of as low as 40 equiv of MAO; its catalytic activity increased with increasing amounts of MAO (Table 1, entries 6−8). This represents a significant advantage, since most previously reported dibromo nickel catalysts required the addition of large amounts of the MAO cocatalyst for ethylenepolymerization activity. B
DOI: 10.1021/acs.macromol.7b02156 Macromolecules XXXX, XXX, XXX−XXX
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MAO improved its catalytic activity (Table 1, entries 15 and 16). Ni-An was observed to be the least active catalyst under these conditions (Table 1, entries 17−21), while Ni-Ph-Ph exhibited comparable activities to Ni-Ph and Ni-Np under these conditions (Table 1, entries 23−27). The Ni-Np and Ni-Ph-Ph complexes are more thermally stable than the other catalysts, remaining active for 60 min at 60 °C (Figure S1). In all cases, the polyethylene branching densities and melting points were tunable over very wide ranges. Ultrahigh-molecular-weight polyethylene (UHMWPE) represents a specialty class of polyethylene with molecular weights in the 106−107 range. UHMWPE is of great industrial importance as it has multiple applications.32−34 The literature reports very few late transition-metal-based catalysts capable of producing UHMWPE.228,29,35,36 Here, the Ni-Ph, Ni-Np, NiAn, and Ni-Ph-Ph complexes are all capable of generating polyethylene with molecular weights above 1 million. In addition to their abilities to produce UHMWPE, the ability to tune both molecular-weight distributions and branching densities over wide ranges is unprecedented and provides a system with highly advantageous properties. The bis-ligated Ni-Ph and Ni-Np complexes produced polyethylenes with bimodal GPC distributions, while the mono-ligated Ni-An and Ni-Ph-Ph complexes produced monomodal polyethylenes. Previously, Campora et al. reported the formation of both oligomers and polymers when XIII was
Scheme 1. Syntheses of the Ligands and Metal Complexes
Compared to Ni-Ph, the Ni-Np complex showed lower activities at low temperatures but higher activities at high temperatures, suggesting that Ni-Np is more thermally stable (Table 1, entries 9−13). In a similar fashion, larger amounts of Table 1. Ethylene Polymerization with Nickel Complexesa entry
Cat.
MAO:Ni
T (°C)
yield (g)
Act.b
Tmc (°C)
Bd
Mwe
Mw/Mne
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 26 27 28f 29f
Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-Np Ni-Np Ni-Np Ni-Np Ni-Np Ni-Np Ni-Np Ni-Np Ni-An Ni-An Ni-An Ni-An Ni-An Ni-Bz Ni-Ph-Ph Ni-Ph-Ph Ni-Ph-Ph Ni-Ph-Ph Ni-Ph-Ph Ni-Ph Ni-Np
80 80 80 80 80 40 500 800 80 80 80 80 80 40 500 800 80 80 80 80 80 80 80 80 80 80 80 80 80
0 20 40 60 80 20 20 20 0 20 40 60 80 20 20 20 0 20 40 60 80 20 0 20 40 60 80 20 20
5.28 3.20 1.73 1.11 0.42 0.24 5.20 5.83 0.36 1.13 0.92 1.12 0.93 0.42 1.27 1.35 0.52 0.34 0.50 0.28 0.21 trace 0.99 3.14 1.92 1.56 0.40 1.69 0.96
10.6 6.4 3.5 2.2 0.8 0.5 10.4 11.7 0.7 2.3 1.8 2.2 1.9 0.8 2.5 2.7 1.0 0.7 1.0 0.6 0.4
127.6 126.7 122.1 96.1/118.5 93.3/120.5 132.4 86.1/116.0 70.8 129.3 125.8 123.8 97.4/118.3 93.4/117.6 130.1 123.9 101.5/120.3 121.7 118.9 116.0 78.8 62.4
12 17 36 74 104
145.6 75.8 33.5 1.5 0.3 257.2 5.0 0.4 192.7 100.7 65.2 20.0 9.0 282.6 5.8 2.0 225.8 269.8 217.7 29.0 16.1
33.1 59.7 37.3 5.7 1.9 2.9 21.1 2.2 7.8 22.8 22.5 44.4 21.7 2.5 4.9 2.7 2.6 3.1 2.6 2.2 2.1
293.8 301.6 116.6 28.8 16.0 74.8 110.5
2.3 2.5 2.8 2.5 2.5 19.6 7.1
2.0 6.3 3.8 3.1 0.8 3.4 1.9
128.6 126.0 111.6 106.1 103.4 127.2 126.1
10 36 47 63 65
12 25 26 54 65 9 10 55 67 95
General conditions: Ni = 1 μmol, CH2Cl2 = 2 mL, toluene = 18 mL, ethylene = 8 atm, time = 30 min. bActivity (Act.) = 106 gPE molNi−1 h−1. Melting temperature determined by DSC. dBranching numbers per 1000C were determined by 1H NMR. eMw: 104 g mol−1, Mw and Mw/Mn determined by GPC. f1 μmol of precatalyst plus 1 equiv of ligand. a c
C
DOI: 10.1021/acs.macromol.7b02156 Macromolecules XXXX, XXX, XXX−XXX
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bulky bis-ligated complex is thermally stable, and the monoligated complex loses activity at high temperatures. Second, the proportion of the low-molecular-weight fraction increases with increasing MAO loading for both Ni-Ph and Ni-Np (Figures 2c and 2d), which is possibly due to chain transfer to aluminum species. In addition, the aluminum species in MAO is capable of extracting the Lewis basic ligand, thereby shifting the equilibrium shown in Scheme 2 toward the right. Third, the addition of an extra equivalent of ligand to Ni-Ph or Ni-Np shifts the equilibrium in Scheme 2 toward the left, which was indeed observed to lead to an increase in the proportion of the high-molecular-weight fraction (Table 1, entries 28 and 29, and the orange profiles in Figures 2c and 2d). Fractionation of the bimodal polyethylene sample revealed that the high-molecularweight fraction is much more linear than the low-molecularweight fraction (Scheme 2b, Table S1, and Figure S4). It is interesting to compare the mechanical properties of the polyethylene products generated by these nickel complexes (Figure 3). Excellent tunable mechanical properties were
used as the catalyst; however the mechanism was not reported. Here, we hypothesize that this is due to equilibria between bisligated and mono-ligated complexes for Ni-Ph and Ni-Np. The bis-ligated complex leads to a high molecular-weight fraction, while the mono-ligated complex leads to a low molecularweight fraction (Scheme 2a). In the case of the sterically bulky Scheme 2. (a) Equilibrium between the Bis-Ligated and Mono-Ligated Nickel Species; (b) Fractionation of Bimodal Polyethylene Sample (Table 1, Entry 2)
mono-ligated complexes, Ni-An and Ni-Ph-Ph, bis-ligation does not occur; consequently, only one active species is present during polymerization. In the 1H NMR analysis of Ni-Ph plus 2 equiv of L-Ph, the chemical resonances from Ni-Ph were shifted and the resonances from L-Ph were not observed at all, suggesting fast equilibrium among the ligand, bis-ligated complex, and mono-ligated complex (Figure S2). In contrast, in the 1H NMR analysis of Ni-Ph-Ph plus 2 equiv of L-Ph-Ph, the chemical resonances from both Ni-Ph-Ph and L-Ph-Ph were not affected (Figure S3), indicating the absence of such equilibrium in the sterically bulky system. This hypothesis is supported by control experiments. First, the proportions of the low-molecular-weight fraction increase with increasing polymerization temperature for both Ni-Ph and Ni-Np (Figures 2a and 2b). We suggest that the equilibrium
Figure 3. Stress−strain curves for the polymer products generated using Ni-Ph and Ni-Ph-Ph (Table 1, entries 1, 2, 23, and 24).
observed, with stress-at-break values ranging from 400 to 1040% and strain-at-break values ranging from 16.5 to 40.2 MPa. The samples formed by Ni-Ph catalysis exhibited slightly lower stress-at-break values but much higher strain-at-break values compared to those formed by Ni-Ph-Ph catalysis. Considering that these samples possess similar branching densities, the improvements in the strain-at-break values of the former polymers may originate from their broad molecularweight distributions. The complex viscosities of four polyethylene samples generated by Ni-Ph (Table 1, entries 1, 2, 3, and 6) were determined by temperature-sweep experiments, at a scan rate of 1 °C/min and a frequency of 1.0 Hz. The complex viscosities of these samples were observed to decrease with increasing temperature. The samples prepared at low temperature and low MAO loading possess higher percentages of the highmolecular-weight fraction. Clearly, increasing the proportion of the low-molecular-weight fraction dramatically decreases the complex viscosity of the polyethylene sample, thereby improving its processability. Palladium-based catalysts have dominated the field of olefin/ polar-monomer copolymerization. In contrast, the more cost-
Figure 2. Molecular-weight distributions as functions of temperature for the various catalysts: (a) Ni-Ph and (b) Ni-Np. Molecular-weight distributions as functions of MAO content for various catalysts: (c) NiPh and (d) Ni-Np.
shown in Scheme 2 shifts toward the right at high temperatures, leading to higher proportions of the mono-ligated complex and correspondingly higher percentages of the low-molecularweight fraction. Based on the GPC analysis, the mono-ligated complex is more active than the bis-ligated complex at low polymerization temperatures. It is possible that the sterically D
DOI: 10.1021/acs.macromol.7b02156 Macromolecules XXXX, XXX, XXX−XXX
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entry 11), which is consistent with the ethylene-homopolymerization scenario and provides supporting evidence for our above-mentioned hypothesis concerning the effect of MAO loading. Fractionation of this bimodal copolymer (Table S2 and Figure S5) showed that the low-molecular-weight fraction possesses much higher methyl 10-undecenoate incorporation (0.9%) than the high-molecular-weight fraction (0.1%). This is consistent with our proposed mechanism that the sterically open monoligated nickel complex affords low molecular weight fraction, along with high comonomer incorporation. However, it should be noted that this is probably true for most catalytic system, since the incorporation of more polar comonomers will lead to lower molecular weight due to the poisoning effect of the polar groups. In this system, the polyethylene polydispersity (GPC curve) could be tuned from monomodal to bimodal, with the ratios of the two components also adjustable. This was easily achieved using different catalysts, different polymerization temperatures, different amounts of the MAO cocatalyst, and the addition of extra ligands. This represents an important advantage since polydispersity is a critical parameter that determines many polymer properties.50,51 Specifically for polyolefins, the ability to modulate the molecular-weight distribution is particularly important. Low-molecular-weight polyethylenes are highly stiff, crystallize fast, and possess low melt viscosities. In contrast, high-molecular-weight polyethylenes are highly tough, crystallize slowly, and possess high melt viscosities. Most importantly, polyethylenes with broad molecular-weight distributions are capable of limiting the shear forces involved during extrusion, which is highly desired in industry.52,53
Figure 4. Complex viscosities as functions of temperature for the polyethylene samples prepared using Ni-Ph (Table 1, entries 1, 2, 3, and 6).
effective and earth-abundant nickel counterpart is much less developed. Limited successes using nickel catalysts have been reported in this field, including the use of salicylaldimine nickel,5 α-diimine nickel,37−40 phosphine−sulfonate nickel,41,42 phosphinophenolate nickel,43 SHOP (Shell Higher Olefin Process)-type nickel, and a few other nickel systems.44−49 The Ni-Ph and Ni-An catalysts were tested for their abilities to catalyze the copolymerization of ethylene with methyl 10undecenoate in the presence of 1000 equiv of MAO (Table 2). Ni-An is 2−7 times less active than Ni-Ph for ethylene polymerization; however, it is only slightly less active than NiPh in these copolymerization reactions (Table 2, entries 1−9). In addition, the molecular weight of the copolymer generated by Ni-An is 20−33 times higher than that produced by Ni-Ph, which demonstrates that sterically bulkier catalysts are superior for these ethylene/polar-monomer copolymerization reactions. For both catalysts, an increase in comonomer concentration led to decreases in both activity and copolymer molecular weight and an increase in the comonomer incorporation ratio. Semicrystalline copolymers with molecular weights of between 3.5 × 103 and 2.4 × 105, with comonomer incorporation ratios 0.1−1.5%, and melting points in the 46.5−119.1 °C range were generated. When the proportion of MAO was reduced to 500 equiv, the catalytic activity of Ni-Ph and the comonomer incorporation ratio were also observed to decrease (Table 2, entry 10). However, the copolymer molecular weight was unaffected, suggesting that the catalytically active species did not change. With 200 equiv of MAO, bimodal GPC distributions were observed for both copolymers (Table 2,
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CONCLUSIONS
Some 2-iminopyridine-N-oxide nickel complexes were synthesized and investigated for their abilities to catalyze ethylene polymerization and copolymerization with methyl 10-undecenoate. In addition to high catalytic activities, very high polyethylene molecular weights could be achieved. This nickel system possesses several key advantages: the requirement of a very low cocatalyst loading and the ability to easily tune the polymer molecular weight and molecular-weight distribution as well as the polymer mechanical properties and complex viscosities.
Table 2. Copolymerization of Ethylene and Methyl 10-Undecenoate with Nickel Complexesa entry
Cat.
comonomer [M]
yield (g)
Act.b
Tmc (°C)
1 2 3 4 5 6 7 8 9 10f 11g
Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-Ph Ni-An Ni-An Ni-An Ni-An Ni-Ph Ni-Ph
0.12 0.10 0.08 0.05 0.02 0.10 0.08 0.05 0.02 0.05 0.05
0.09 0.25 0.94 2.75 3.24 0.11 0.62 1.94 2.18 0.16 0.09
0.9 2.5 9.4 27.5 32.4 1.1 6.2 19.4 21.8 1.6 0.9
53.6 58.3 79.9 84.5 91.5 46.5 49.9 51.3 85.0 79.8/119.1
comonomer incorp (mol %)d
Mwe
Mw/Mne
1.5 1.2 0.8 0.7 0.1 1.5 1.1 1.0 0.3 0.4 0.3
3.8 3.5 7.7 7.2 9.2 102.4 176.3 184.0 242.2 8.4 184.7
2.6 2.4 2.2 1.6 1.8 2.1 2.2 2.1 2.0 1.7 36.1
a General conditions: Ni = 10 μmol, MAO:Ni = 1000, CH2Cl2 = 2 mL, toluene = 18 mL, ethylene = 2 atm, time = 1 h, T = 20 °C. bActivity (Act.) = 104 gPE molNi−1 h−1. cMelting temperature determined by DSC. dDetermined by 1H NMR spectroscopy. eMw: 103 g mol−1, Mw and Mw/Mn determined by GPC. fMAO:Ni = 500. gMAO:Ni = 200.
E
DOI: 10.1021/acs.macromol.7b02156 Macromolecules XXXX, XXX, XXX−XXX
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EXPERIMENTAL SECTION
Article
ASSOCIATED CONTENT
S Supporting Information *
General Considerations. All experiments were carried out in a dry nitrogen atmosphere using standard Schlenk techniques or in a glovebox. Deuterated solvents used for NMR spectroscopy were dried and distilled prior to use. 1H and 13C NMR spectra were recorded on a Bruker Ascend Tm 400 spectrometer at ambient temperature unless otherwise stated. 1H and 13C NMR chemical shifts were referenced to the residual solvent, and coupling constants are in hertz. Elemental analyses were performed by the Analytical Center of the University of Science and Technology of China. X-ray diffraction data were collected at 298(2) K with a Bruker Smart CCD area detector and graphitemonochromated Mo Kα radiation (λ = 0.710 73 Å). The molecular weights and molecular-weight distributions of the polymers were determined by gel permeation chromatography (GPC) using an Agilent PL-220 instrument equipped with two Agilent PLgel Olexis columns operating at 150 °C with trichlorobenzene as the solvent. GPC was calibrated using a polystyrene standard and are corrected for linear polyethylene by universal calibration using the Mark−Houwink parameters of Rudin: K = 1.75 × 10−2 cm3/g and R = 0.67 for polystyrene and K = 5.90 × 10−2 cm3/g and R = 0.69 for polyethylene. Stress/strain experiments were performed at room temperature at 10 mm/min using a UTM2502 universal tester. At least three specimens of each polymer were tested. Each polymer was meltpressed at 30−35 °C above its melting point to obtain the test specimen. The test specimens had the following dimensions: gauge length, 28 mm; width, 3 mm; and thickness, 1 mm. Complex viscosity was determined by temperature-sweep experiments using an Anton Paar MCR302 rheometer (plate: 25 mm diameter). The temperature scan rate was set to 1 °C/min, and the frequency was set to 1.0 Hz. Fractionation of Bimodal Polymer. The polymer sample was stirred in 200 mL of toluene at 120 °C for 3 h. The insoluble fraction at 120 °C is referred to as “fraction 1”. The transparent 120 °C toluene solution was then was cooled to 80 °C and maintained statically at this temperature for 12 h to complete the isothermal crystallization process. The insoluble fraction at 80 °C is referred to as “fraction 2”. Finally, the soluble fraction at 80 °C (“fraction 3”) was precipitated with excess ethanol. All fractions were dried at 40 °C under vacuum for 48 h before weighing and testing. Procedure for Ethylene Polymerization. In a typical experiment, a 350 mL glass thick-walled pressure vessel was charged with the required amount of the cocatalyst and 18 mL of toluene, in a glovebox, and a magnetic stirrer bar added. The pressure vessel was connected to a high-pressure polymerization line, and the solution was degassed. The vessel was warmed to the desired temperature using an oil bath and allowed to equilibrate for 5 min. 1.0 μmol of nickel complex in 2 mL of CH2Cl2 was injected into the vessel via a syringe at 1 atm of ethylene pressure. With rapid stirring, the reactor was pressurized with ethylene to a pressure of 8.0 atm. After the required amount of polymerization time, the vessel was vented; the polymer precipitated with acidified methanol (methanol/HCl = 50/1) and dried at 45 °C for 36 h under vacuum. Polymer branching was analyzed by 1H NMR spectroscopy: BD = 1000 × 2(ICH3)/3(ICH2+CH+ICH3). 0.77−0.95 (alkyl methyl, alk-CH3, m); ∼1.0−1.45 (alk-CH and alk-CH2, m). Copolymerization of Ethylene and Methyl 10-Undecenoate. In a typical experiment, a 350 mL glass thick-walled pressure vessel was charged with cocatalyst, toluene, and methyl 10-undecenoate to total volume of 18 mL, in a glovebox, and a magnetic stirrer bar added. The pressure vessel was connected to a high-pressure line, and the solution was degassed. The vessel was warmed to 20 °C (water bath) and allowed to equilibrate for 15 min. 10 μmol of nickel complex in 2 mL of CH2Cl2 was injected into the polymerization system via a syringe at 1 atm of ethylene pressure. With rapid stirring, the reactor was pressurized with ethylene to a pressure of 2.0 atm. After 1 h, the pressure vessel was vented; the polymer was precipitated with acidified methanol (methanol/HCl = 50/1) and dried under vacuum at 45 °C for 36 h.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02156. Experimental procedures, NMR spectra of ligands, nickel complexes, polyethylene, and copolymers (PDF) X-ray crystallographic data of Ni-Ph (CIF) X-ray crystallographic data of Ni-Ph-Ph (CIF)
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AUTHOR INFORMATION
Corresponding Author
*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, 21690071) and the Fundamental Research Funds for the Central Universities (WK3450000001).
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Macromolecules
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DOI: 10.1021/acs.macromol.7b02156 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.7b02156 Macromolecules XXXX, XXX, XXX−XXX
