Pentiptycenyl Substituents in Insertion Polymerization with α-Diimine

Apr 23, 2019 - Motivated by the need for a new generation of α-diimine Ni(II) and Pd(II) catalysts for tuning the catalytic activity, polymer molecul...
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Pentiptycenyl Substituents in Insertion Polymerization with α‑Diimine Nickel and Palladium Species Yudan Liao,†,‡ Yixin Zhang,† Lei Cui,† Hongliang Mu,† and Zhongbao Jian*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China

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S Supporting Information *

ABSTRACT: Motivated by the need for a new generation of α-diimine Ni(II) and Pd(II) catalysts for tuning the catalytic activity, polymer molecular weight, comonomer incorporation, and branching density in ethylene polymerization and copolymerization with polar monomers, a family of α-diimine Ni(II) and Pd(II) catalysts Ipty-Ni1−4 and Ipty-Pd1−4 derived from sterically demanding and rotationally restricted pentiptycenyl N-aryl substituents were synthesized and fully characterized by NMR, IR, MALDI-TOF, elemental analysis, and X-ray diffraction. Pentiptycenyl-substituted Ni(II) and Pd(II) catalysts were further probed in ethylene (co)polymerization as a comparison with the rotationally free dibenzhydryl substituent reported previously. In the Ni-catalyzed ethylene polymerization (20−80 °C), catalytic activities ((0.64−3.74) × 106 g mol−1 h−1), polymer molecular weights ((1.1− 37.7) × 104 g mol−1), branching densities (6−55/1000C), and melting points (94−135 °C) could be tuned over a broad range. In the Pd-catalyzed ethylene polymerization, these catalysts gave varied catalytic activities ((1.4−54.7) × 104 g mol−1 h−1) and polymer molecular weights ((0.8−39.6) × 104 g mol−1), but similar branching densities (62−72/1000C). Furthermore, these palladium catalysts exhibited a high MA incorporation of 1.0−4.1 mol % in the copolymerization of ethylene and methyl acrylate (MA). On the basis of these results, comparisons of the pentiptycenyl-derived and the dibenzhydryl-derived α-diimine Ni(II) and Pd(II) catalysts on ethylene (co)polymerization were made in detail.

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INTRODUCTION Since the seminal discovery by Ziegler and Natta opened the door for accessing versatile polyolefins, transition-metalcatalyzed coordination−insertion olefin (co)polymerization has attracted much attention and shown particularly great success in both the academic and industrial communities over the past decades.1 In comparison with the early transitionmetal-based milestone catalysts such as Ziegler/Natta catalysts, metallocene catalysts, and CGC catalysts (Constrained Geometry Catalysts), one of the most pivotal breakthroughs in the late-transition-metal-based catalysts was the Brookhart α-diimine nickel (Ni) and palladium (Pd) catalysts (Chart 1, I).2 However, the conventional α-diimine Ni(II) and Pd(II) catalysts suffer from poor thermal stability at elevated temperatures,3−5 which greatly limits potential industrial application. To address these issues and develop catalysts with enhanced thermal stability and others, advanced strategies on modification of sterically bulky N-o-aryl substituent and ligand backbone have been extensively employed.6−14 Among them, these α-diimine Ni(II) and Pd(II) catalysts bearing a sterically demanding 2,6-dibenzhydryl substituent on the N-aryl group have drawn much attention. Sun et al. designed a series of αdiimine Ni(II) catalysts ligated by asymmetric N-aryl groups © XXXX American Chemical Society

with a dibenzhydryl moiety and a less bulky moiety, which show a high activity of 106 g mol−1 h−1 for ethylene polymerization at 60 °C to afford a high molecular weight polymer of 105 g mol−1 (Chart 1, II).15 In particular, Long et al. reported symmetrically dibenzhydryl-derived α-diimine Ni(II) catalysts that are highly active for ethylene polymerization even at 100 °C.16 Chen et al. comprehensively studied and developed the related Ni(II) and Pd(II) catalysts derived from various dibenzhydryl moieties with electron-donating and -withdrawing substituents and their derivatives such as naphthalene and benzothiophene (Chart 1, III).17 These fantastic catalysts not only were capable of generating welldefined polyethylenes at 20−100 °C with tunable branching densities and particularly high molecular weights but also enabled the copolymerization of ethylene with a broad scope of polar monomers including challenging polar allyl monomers. Iptycenes are a class of aromatic compounds with arene units fused to a bicyclo[2.2.2]octatriene bridgehead system. Over the past years, iptycenes, especially triptycenes and pentiptycenes, have attracted much attention and found Received: February 15, 2019

A

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Organometallics Chart 1. α-Diimine Ni(II) and Pd(II) Catalysts Bearing Various Bulky N-Aryl Substituents and Backbones

Scheme 1. Synthesis of α-Diimine Ligands and Corresponding Ni(II) Complexes (Ipty-Ni1−4) and Pd(II) Complexes (IptyPd1−4)

numerous applications in material science, conjugated polymers, microporous polymers, supramolecular chemistry, coordination chemistry, host−guest chemistry, molecular machines, sensor applications, and other areas.18 However, as a large steric blocking substituent, three-dimensional geometry iptycenes, especially pentiptycenes, are relatively underutilized

in transition-metal complexes. Limited examples are the applications of triptycenyl or pentiptyceyl functionality in bulky N-heterocyclic carbene (NHC) ligated metal complexes for catalytically organic synthesis and ring-opening metathesis polymerization.19,20 Despite these promises, however, the use of iptycenes, like pentiptycene, as the potential substituent in B

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Figure 1. Molecular structures of complexes Ipty-Ni1 (CCDC 1893875) (a), Ipty-Pd1 (CCDC 1893877) (b), and Ipty-Pd2 (CCDC 1893876) (c) drawn with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity except the OH group. Selected bond lengths (Å): (a) Ni1−N1 2.008(7), Ni1−Br1 2.338(3), Ni1−Br2 2.300(3); (b) Pd1−N1 2.090(11), Pd1−C42A 1.670(3), Pd1−Cl1 2.223(8); (c) Pd1−N1 2.098(5), Pd1− N2 2.193(5), Pd1−C83 2.105(19), Pd1−Cl1 2.253(3). Selected angle (deg): (a) N1−Ni1−N1A 83.2(4), Br1−Ni1−Br2 128.5(1); (b) N1−Pd1− N1A 79.4(5), Cl1−Pd1−C42A 92.589(5); (c) N1−Pd1−N2 78.4(2), Cl1−Pd1−C83 85.2(5). Molecular structure of complex Ph-Pd4 from literature17 as citation (d).

Table 1. Effect of Nickel Catalyst and Temperature on Ethylene Polymerizationa entry

cat.

T (°C)

yield (g)

act. (106)b

Mn (104)c

Mw/Mnc

brsd

Tme (°C)

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

Ipty-Ni1 Ipty-Ni1 Ipty-Ni1 Ipty-Ni1 Ipty-Ni2 Ipty-Ni2 Ipty-Ni2 Ipty-Ni2 Ipty-Ni3 Ipty-Ni3 Ipty-Ni3 Ipty-Ni3 Ipty-Ni4 Ipty-Ni4 Ipty-Ni4 Ipty-Ni4 Ph-Ni4 Ph-Ni4 Ph-Ni4 Ph-Ni4

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

0.84 2.63 3.39 2.74 0.64 2.93 3.60 2.55 2.46 2.71 3.74 3.13 2.13 2.53 3.13 2.67 0.57 0.83 1.54 1.83

0.84 2.63 3.39 2.74 0.64 2.93 3.60 2.55 2.46 2.71 3.74 3.13 2.13 2.53 3.13 2.67 0.57 0.83 1.54 1.83

4.0 2.2 2.0 1.1 5.3 3.8 2.9 1.8 4.8 4.1 2.6 2.1 37.7 33.1 10.5 8.6 71.6 97.7 104.6 103.3

5.2 5.0 4.5 3.8 3.2 3.2 2.7 2.5 5.0 4.8 4.6 4.0 2.2 2.1 2.4 2.3 1.8 2.0 2.1 2.3

6 12 15 26 6 12 21 37 14 22 38 46 9 13 36 55 42 48 53 56

135 128 125 121 133 127 115 103 128 122 118 94 128 117 102 99 73 64 56 54

a Reaction conditions: Ni catalyst (2 μmol), MAO (500 equiv), toluene/CH2Cl2 (19 mL/1 mL), ethylene (6 bar), polymerization time (30 min); all entries are based on at least two runs, unless noted otherwise. bActivity is in unit of 106 g mol−1 h−1. cDetermined by GPC in 1,2,4trichlorobenzene at 150 °C vs polystyrene standards. dbrs = Number of branches per 1000C, as determined by 1H NMR spectroscopy. e Determined by DSC (second heating).

transition-metal catalysts for insertion polymerization of olefins remains less explored.21 There are two key factors that prompt us to investigate the pentiptycenyl substituents in the insertion polymerization. First, Coates and Gao et al. described the α-diimine Ni(II) and Pd(II) catalysts with a bulky dibenzobarrelene ligand backbone that show enhanced thermal stability over 80 °C for ethylene polymerization achieving even living fashion (Chart 1, IV).22 The steric bulk of the dibenzobarrelene backbone inhibits the N-aryl rotations by repulsive interaction and forces the aryl groups closer to the active metal center. This story teaches us that the iptycenyl substituent could be also anticipated to be a sterically demanding moiety for retarding the N-aryl rotations because of the extremely similar framework between dibenzobarrelene and triptycene. Second, as aforementioned, the bulky dibenzhydryl substituent of N-aryl group in the αdiimine ligand remarkably favors ethylene (co)polymerization. Note that two phenyl groups in the dibenzhydryl framework rotate freely around the central carbon atom (C*, Chart 1,

III). In comparison, generating a more rigid steric hindrance is believed to result in different polymerization properties. Intrigued by these exciting results based on both the dibenzobarrelene backbone and dibenzhydryl substituent of the N-aryl group, in this contribution we probe the role of pentiptycenyl as the N-aryl substituent in the α-diimine Ni(II) and Pd(II) catalysts. These catalysts show different behaviors for ethylene polymerization and copolymerization with methyl acrylate from those catalysts based on dibenzhydryl substituents.

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RESULTS AND DISCUSSION Synthesis of α-Diimine Ligands and Corresponding Ni(II) and Pd(II) Complexes. Starting from readily available 1,4-benzoquinone and anthracene chemicals, the desired pentiptycene aminophenol was prepared over three steps using the literature procedure in high yields (Scheme 1).19c,23 There are a number of methods for the synthesis of an αdiimine ligand from the reaction of α-dione and aniline. In this C

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Figure 2. Plot of yield, molecular weight, branching density, and melting point of polyethylene generated vs polymerization temperature with Ni(II) catalysts Ipty-Ni1−4.

case, the targeted α-diimine ligands Ipty−OH1 and Ipty− OH3 containing −OH moieties were synthesized from the reaction of acenaphthene-1,2-dione with pentiptycene aminophenol in CH3CN in the presence of AcOH at 90 °C and from the reaction of 2,3-butanedione with the corresponding aniline in iPrOH in the presence of HCOOH at 80 °C, respectively. Moreover, methylation of the phenolic −OH group with MeI gave another two α-diimine ligands Ipty-OMe2 and IptyOMe4 containing −OMe moieties as a comparison.19c All four α-diimine ligands further respectively reacted with 1 equiv of NiBr2(DME) overnight to generate the corresponding nickel (Ni(II)) complexes Ipty-Ni1−4 in excellent yields of 80−85%. Due to the large steric hindrance, the palladium (Pd(II)) complexes Ipty-Pd1−4 were prepared from the reaction of the corresponding α-diimine ligands with PdMeCl(COD) for at least 3 days in 48−94% yields. All these Ni(II) and Pd(II) complexes, especially containing the −OH group, were of relatively low solubility even in dichloromethane or chloroform. Ni(II) complexes were characterized by elemental analysis, mass spectrometry (MALDI-TOF), IR, and NMR spectroscopy (due to the paramagnetic nature, only characteristic signals are identified in the 1H NMR spectra; see Supporting Information). Likewise, the diamagnetic Pd(II) complexes were comprehensively identified by NMR spectroscopy, elemental analysis, and MALDI-TOF. The newly generated Pd−Me group gives a singlet at δ = 0.98 (Ipty-Pd1), 0.76 (Ipty-Pd2), 0.97 (IptyPd3), and 0.96 (Ipty-Pd4) ppm in the 1H NMR spectra. Fortunately, suitable X-ray single crystals of some of these Ni(II) and Pd(II) complexes were grown after significant effort by layering a saturated dichloromethane solution with hexane at ambient temperature. As shown in Figure 1, the Ni(II) complex Ipty-Ni1 shows a slightly distorted tetrahedral geometry with typical bond lengths and angles, while the Pd(II) complexes Ipty-Pd1 and Ipty-Pd2 adopt a slightly distorted square-planar geometry. Compared with the Ph-Pd4 catalyst structure (Figure 1d), less steric blockage (by two bowl-shaped pentiptycenyl units) at the axial positions of the metal center could be observed in the solid-state structures. Ethylene Polymerization by Ni(II) Catalysts. In situ activated with MAO, dibromo Ni(II) precatalysts Ipty-Ni1−4 were highly active in ethylene polymerization (Table 1). The catalytic activities are on the high level of 106 g mol−1 h−1 at almost all polymerization temperatures from 20 to 80 °C. With the increase of temperature from 20 to 60 °C, the activity increases correspondingly; but at 80 °C, the activity drops although it is still maintained at a high level (Figure 2). This indicates that these Ni(II) catalysts Ipty-Ni1−4 are highly thermally stable due to the steric blockage of pentiptycenyl substituents that greatly inhibits N-aryl rotation preventing

premature catalyst decomposition and are suitable for industrially used gas-phase ethylene polymerization (70−110 °C).24 Moreover, under otherwise identical conditions, the molecular weight (Mn) of polyethylene obtained decreases with increasing polymerization temperature. It is attributed to the increased rate of chain transfer (via β-H elimination) relative to chain propagation at elevated temperature.4 Likewise, it should be noted that the branching densities of the generated polyethylenes are also dependent on the polymerization temperature. Decreasing temperature results in the drop of branching density, indicating the branching density can be tuned only by readily varying the temperature. As anticipated, the melting point (Tm) of polymer raises with decreasing branching density. This suggests that the chain walking rate decreases with decreasing temperature. At 20 °C, it is remarkable that these obtained polyethylenes have an extremely low degree of branching (6−14/1000C) and thus possess particularly high Tm (128−135 °C). To the best of our knowledge, this is one of the lowest levels of branching density produced by α-diimine Ni(II) species, and is significantly different from that (>55/1000C) generated by the classic (iPr2) Brookhart Ni(II) catalysts.2,13b,17f The effect of the backbone and para-substituent of Ni(II) catalysts Ipty-Ni1−4 on the ethylene polymerization was also studied. For the Ni(II) catalysts Ipty-Ni1 and Ipty-Ni3 containing the −OH group at the para-position, the dimethyl backbone gives polyethylenes with slightly higher activity, a similar molecular weight, and higher branching density than those with the acenaphthyl backbone. Likewise, for the Ni(II) catalysts Ipty-Ni2 and Ipty-Ni4 containing the −OMe group at the para-position, the dimethyl backbone displays comparable activity, higher branching density, but a significantly increased molecular weight (4−9 times) than those with the acenaphthyl backbone. These results reveal that dimethyl is superior to acenaphthyl as a backbone in the α-diimine Ni(II) system, which is in line with observed results previously.4 The polyethylenes obtained by the Ni(II) catalysts Ipty-Ni1 and Ipty-Ni3 containing the −OH group at the para-position have broad polydispersities (PDI = 3.8−5.2). The PDI is greater than 2.0 in entries 1−4 and 9−12, revealing probable multiple active species. In situ activated with MAO, it is possible that deprotonation of the hydroxyl group by MAO could lead to formation of metal clusters/oligomers due to metal bridging of the phenolate with Al. In addition, in the dibenzhydryl-substituted α-diimine Ni(II) and Pd(II) systems, Chen et al. investigated the effect of a series of electron-donating and -withdrawing substituents (MeO, Me, Cl, CF3) at the para-position of the N-aryl group on ethylene polymerization.17 Here, the rare and underutilized −OH moiety was studied for comparison with D

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Figure 3. Comparisons on yield, molecular weight, branching density, and melting point of polyethylene generated with Ipty-Ni4 and Ph-Ni4.

the −OMe group. According to the Hammett substituent constant,25 the σP value of the OH group is −0.37, and the σP value of the OMe group is −0.27. In the presence of a large excess of MAO, the deprotonation of the OH group in the Ni(II) complexes takes place to generate aluminum aryloxide with an ionic O−Al bond.26 The O−Al bond is partially polarized that is closer to the O− group (σP = −0.81). This indicates the formation of the O−Al group with a value of −0.37 < σP < −0.81. As a result, the order of electron-donating ability is O−Al > OH > OMe. By employing acenaphthylderived Ipty-Ni1 (O−Al) and Ipty-Ni2 (OMe) in ethylene polymerization, results are comparable with regard to the catalytic activity, molecular weight, and branching density (corresponding Tm) at various temperatures. In contrast, although the catalytic activity and the Tm are respectively similar in the dimethyl-derived Ipty-Ni3 (O−Al) and Ipty-Ni4 (OMe) systems, Ipty-Ni4 produces a remarkably higher molecular weight (4−9 times) than Ipty-Ni3. As shown in Chart 1, the dibenzhydryl-substituted α-diimine Ni(II) catalyst Ph-Ni4 possesses a rotationally free dibenzhydryl blockage, but the pentiptycenyl-derived α-diimine Ni(II) catalyst Ipty-Ni4 is of a fixed dibenzhydryl-like blockage; thus, a comparative study on ethylene polymerization using both of them is significantly helpful to probe the role of the pentiptycenyl substituent on the polymerization. Under otherwise identical conditions, compared to Ph-Ni4, Ipty-Ni4 shows higher activity at all polymerization temperatures (especially at lower temperature), but the molecular weight of polyethylenes obtained (up to 377 kg mol−1) is obviously lower (especially at higher temperature) (Figure 3). To gain further insight into the catalyst stability at different temperatures, polymerizations with different reaction times were performed at 20, 40, 60, and 80 °C (Table S1, Figure S1 in the Supporting Information). At lower temperatures (20 °C, 40 °C), the catalysts Ph-Ni4 and Ipty-Ni4 almost showed constant activity over 30 min. At higher temperatures (60 °C, 80 °C), the catalyst Ph-Ni4 almost maintained the same activity, while the catalyst Ipty-Ni4 showed a slight decrease of activity after 20 min. These results indicated that the catalyst Ipty-Ni4 deactivation happened after 20 min at higher temperatures, which showed the catalyst Ph-Ni4 was more stable at higher temperatures than the catalyst Ipty-Ni4. This corresponds to the larger steric blockage of dibenzhydryl substituents. Notably, the branching densities of polyethylenes produced by Ipty-Ni4 are lower than those produced by PhNi4, and it is drastically different at low temperatures (9/ 1000C vs 42/1000C at 20 °C; 13/1000C vs 48/1000C at 40 °C). Correspondingly, the Tm of polymers generated by IptyNi4 is clearly higher (99−128 °C vs 55−73 °C). In addition, Ph-Ni4 generates a significantly higher molecular weight, indicating a large steric blockage around the metal center results in a higher ratio of chain propagation rate to chain transfer rate. The single crystal structure diagrams

from Figure 1 directly show that the steric blockage of the pentiptycenyl substituent around the metal center is less than that of the dibenzhydryl substituent. At the same time, IptyNi4 displays a remarkably lower branching density and higher Tm, revealing a slower chain walking rate (the greater ratio of insertion rate versus chain running rate). As a result, Ipty-Ni4 readily produces a more linear polyethylene topology (high density polyethylene, HDPE), but Ph-Ni4 tends to generate a linear low density polyethylene (LLDPE). Ethylene Polymerization by Pd(II) Catalysts. A difference between the Ni(II) catalysts derived from the pentiptycenyl substituted and the dibenzhydryl substituted αdiimine ligand prompted us to investigate the corresponding Pd(II) catalysts for the ethylene polymerization. With the activation of tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBArF), Pd(II) catalysts Ipty-Pd1−4 showed moderate activities for ethylene polymerization at given temperatures (Table 2). Considering the effect of the backbone and paraTable 2. Ethylene Polymerization with Palladium Catalystsa entry

cat.

T (°C)

yield (g)

act.b

Mnc

Mw/Mnc

brsd

1 2 3 4 5e 6e 7

Ipty-Pd1 Ipty-Pd2 Ipty-Pd3 Ipty-Pd4 Ipty-Pd4 Ipty-Pd4 Ph-Pd4

30 30 30 30 30 50 30

1.6 3.6 4.3 11.8 3.1 10.9 22.5

1.36 3.00 3.56 9.84 15.6 54.7 18.7

1.1 0.8 39.6 26.2 18.2 9.9 48.4

1.5 1.6 1.4 1.7 1.5 2.0 2.4

62 65 71 71 72 72 25

a Reaction conditions: Pd catalyst (10 μmol), NaBArF (1.5 equiv), CH2Cl2 (40 mL), ethylene (4 bar), polymerization time (12 h), unless noted otherwise. bActivity is in unit of 104 g mol−1 h−1. cMn is in unit of 104 g mol−1. Determined by GPC in THF at 40 °C or in 1,2,4-trichlorobenzene at 150 °C vs polystyrene standards. dbrs = Number of branches per 1000C, as determined by 1H NMR spectroscopy. ePolymerization time (2 h).

substituent on the polymerization property, the acenaphthylderived Pd(II) catalysts Ipty-Pd1−2 were 3 times lower in activity than the dimethyl-derived Pd(II) catalysts Ipty-Pd3− 4; meanwhile, the molecular weight of polyethylene obtained was significantly lower (11 vs 396 kg mol−1, 8 vs 262 kg mol−1), but the branching density was quite similar (62−71/ 1000C) (Table 2, entries 1−4). Not surprisingly, OHsubstituted Ipty-Pd1 and Ipty-Pd3 exhibited poorer activity than MeO-substituted Ipty-Pd2 and Ipty-Pd4, which may be due to the reactivity of the acidic OH group. As a representative example, a time-dependence study on the activity of ethylene polymerization with Ipty-Pd4 was performed. A prolonged reaction time resulted in a further increase in polymer yield, demonstrating that catalyst activity is retained for a long time (more than 12 h) under the polymerization conditions, albeit with a slight decrease E

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ethylene/MA copolymerization investigation with Ipty-Pd1−4 (Table 3), similar trends were observed to those for ethylene homopolymerization, considering activity, polymer molecular weight, and branching density. At a given MA concentration of 1.0 M, all Pd(II) catalysts Ipty-Pd1−4 gave a high MA incorporation of 1.0−1.8 mol %, with Ipty-Pd3 possessing the highest molecular weight of 90.1 kg mol−1 and Ipty-Pd4 exhibiting the highest activity of 19.1 kg mol−1 h−1. As expected, at a given ethylene pressure, the incorporation of MA in the copolymers increased with increasing concentration of MA in the reaction mixture to reach 1.6−3.6 mol % at an initial concentration of 2.0 M. Likewise, under the same MA concentration, a lower ethylene pressure generated a higher MA incorporation of up to 2.2−4.1 mol %. It should be noted that the branching density of copolymers obtained was almost independent of comonomer concentration and ethylene pressure. The copolymerization of ethylene and MA was further utilized to compare Ipty-Pd4 with Ph-Pd4 regarding copolymerization behavior (Table 3, entries 10, 11, 13, and 14). As shown in Figure 5, compared to Ph-Pd4, Ipty-Pd4

(Table 2, entries 4 and 5). Under otherwise identical conditions, an elevated temperature resulted in a remarkably higher activity but a decreased molecular weight, as expected (Table 2, entries 5 and 6). Comparison of the pentiptycenyl substituted Ipty-Pd4 and the dibenzhydryl substituted Ph-Pd4 on the ethylene polymerization was more meaningful to understand the principle of catalyst design. Under the same conditions, Ph-Pd4 displayed a ca. twice higher activity and a ca. twice higher molecular weight than that of Ipty-Pd4 (Figure 4, Table 2, entries 4 and

Figure 4. Comparisons on yield, molecular weight, and branching density of polyethylene generated with Ipty-Pd4 and Ph-Pd4.

7). However, the branching density was totally different, where Ipty-Pd generated a branching density of 72/1000C that is higher than that (25/1000C) of Ph-Pd4, but is lower than that (>100/1000C) of the classic Brookhart catalyst (Chat 1, I). In the α-diimine Pd(II) catalysts systems, it has been found regularly that the more sterically encumbering the substituents, the higher the catalytic activities, the higher the polymer molecular weights, and the lower the branching densities.17 This comparison again demonstrated that the rotationally restricted pentiptycenyl substituent has less steric blockage at the axial positions of the metal center than the rotationally free dibenzhydryl substituent. We can also understand it directly from the comparison of single crystal structure diagrams from Figure 1. Ethylene/Methyl Acrylate Copolymerization by Pd(II) Catalysts. The copolymerization of ethylene and methyl acrylate (MA) is the most extensively used reaction to evaluate the copolymerization property of a new catalyst. In the

Figure 5. Comparisons on MA incorporation, molecular weight, and branching density of ethylene/MA copolymers generated with IptyPd4 and Ph-Pd4.

produced ethylene/MA copolymers with a significantly 6-fold higher MA incorporation (1.0 vs 0.16 mol %, 1.6 vs 0.33 mol %) and with an expectedly increased branch density; however,

Table 3. Copolymerization of Ethylene and Methyl Acrylate with Palladium Catalystsa entry

cat.

cMA (mol L−1)

yield (g)

act. (103)b

XMA (mol %)c

Mn (103)d

Mw/Mnd

brsc

1 2 3e 4 5 6e 7 8 9e 10 11 12e 13 14

Ipty-Pd1 Ipty-Pd1 Ipty-Pd1 Ipty-Pd2 Ipty-Pd2 Ipty-Pd2 Ipty-Pd3 Ipty-Pd3 Ipty-Pd3 Ipty-Pd4 Ipty-Pd4 Ipty-Pd4 Ph-Pd4 Ph-Pd4

1.0 2.0 1.0 1.0 2.0 1.0 1.0 2.0 1.0 1.0 2.0 1.0 1.0 2.0

0.18 0.10 0.08 0.25 0.14 0.11 0.83 0.36 0.37 2.29 0.86 0.83 4.83 2.59

1.50 0.83 0.67 2.10 1.20 0.92 6.90 3.00 3.10 19.1 7.20 6.90 40.3 21.6

1.8 3.6 4.1 1.8 3.6 3.0 1.2 2.2 2.4 1.0 1.6 2.2 0.16 0.33

7.8 5.4 6.8 8.8 5.2 5.1 90.1 29.0 20.5 50.7 15.4 18.6 124.1 54.8

2.3 2.4 2.4 2.4 2.2 2.2 2.2 2.4 2.2 2.4 2.2 2.6 2.2 1.9

65 58 62 61 60 67 71 70 75 71 70 76 25 22

a Reaction conditions: Pd catalyst (10 μmol), NaBArF (1.5 equiv), CH2Cl2 + MA (20 mL), ethylene (4 bar), polymerization temperature (30 °C), polymerization time (12 h), unless noted otherwise. bActivity is in unit of 103 g mol−1 h−1. cXMA = Incorporation of MA, brs = Number of branches per 1000C, as determined by 1H NMR spectroscopy. dDetermined by GPC in THF at 40 °C or in 1,2,4-trichlorobenzene at 150 °C vs polystyrene standards. eEthylene (2 bar).

F

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the activity and the copolymer molecular weight were again lower. The low incorporation of MA probably originates from the steric bulk of the dibenzhydryl substituted α-diimine ligand, which makes the monomer binding more unfavorable for MA. This also reveals that steric blockage of the pentiptycenyl substituent at the axial positions of the metal center is less than that of the dibenzhydryl substituent.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support from the National Natural Science Foundation of China (No. 21871250) and the Jilin Provincial Science and Technology Department Program (No. 20190201009JC).

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CONCLUSIONS In summary, we have demonstrated a new sterically demanding pentiptycenyl N-aryl substituent that combines the excellent features of both a dibenzhydryl substituent and a dibenzobarrelene backbone applied to α-diimine Ni(II) and Pd(II) catalysts for ethylene (co)polymerization. Compared to the dibenzhydryl-derived Ni(II) catalyst, the pentiptycenyl-derived Ni(II) catalyst shows higher catalytic activity, a remarkably lower branching density, and a higher Tm, but a lower molecular weight (especially at elevated temperatures). Moreover, compared to the dibenzhydryl-derived Pd(II) catalyst, the pentiptycenyl-derived Pd(II) catalyst exhibits reduced catalytic activity, a reduced molecular weight, but an increased branching density for ethylene polymerization, meanwhile providing much higher comonomer incorporation for ethylene/MA copolymerization. In comparison with the rotationally free dibenzhydryl substituent, the rotationally restricted pentiptycenyl substituent offers superior activity and slower chain walking for Ni(II) species and enhanced comonomer incorporation for Pd(II) species. Nevertheless, less steric blockage of the rotationally restricted pentiptycenyl substituent at the axial positions of the metal center leads to the decrease of polymer molecular weight. Thus, we are currently installing suitable substituents into the pentiptycenyl substituent to modify the steric bulk, while also applying this pentiptycenyl substituent to other catalytic systems for olefin polymerization and copolymerization with polar monomers. This work again reveals the power of sterically bulky substituents in α-diimine Ni(II) and Pd(II) catalysts to address issues in the insertion (co)polymerization of olefin and polar monomers.

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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.9b00106. Experimental details and characterization and analysis data (NMR, MALDI-TOF, DSC, and GPC) for complexes, polyethylenes, and copolymers (PDF) Accession Codes

CCDC 1893875−1893877 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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REFERENCES

(1) (a) Eagan, J. M.; Xu, J.; Di Girolamo, R. D.; Thurber, C. M.; Macosko, C. W.; LaPointe, A. M.; Bates, F. S.; Coates, G. W. Combining Polyethylene and Polypropylene: Enhanced Performance with PE/iPP Multiblock Polymers. Science 2017, 355, 814−816. (b) Stürzel, M.; Mihan, S.; Mülhaupt, R. From Multisite Polymerization Catalysis to Sustainable Materials and All-Polyolefin Composites. Chem. Rev. 2016, 116, 1398−1433. (c) Mu, H. L.; Pan, L.; Song, D. P.; Li, Y. S. Neutral Nickel Catalysts for Olefin Homo- and Copolymerization: Relationships between Catalyst Structures and Catalytic Properties. Chem. Rev. 2015, 115, 12091− 12137. (d) Delferro, M.; Marks, T. J. Multinuclear Olefin Polymerization Catalysts. Chem. Rev. 2011, 111, 2450−2485. (e) Hustad, P. D. Frontiers in Olefin Polymerization: Reinventing the World’s Most Common Synthetic Polymers. Science 2009, 325, 704−707. (f) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Catalytic Production of Olefin Block Copolymers via Chain Shuttling Polymerization. Science 2006, 312, 714−719. (g) Coates, G. W. Precise Control of Polyolefin Stereochemistry Using Single-Site Metal Catalysts. Chem. Rev. 2000, 100, 1223−1252. (2) (a) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. Mechanistic Studies of the Palladium-Catalyzed Copolymerization of Ethylene and α-Olefins with Methyl Acrylate. J. Am. Chem. Soc. 1998, 120, 888−899. (b) Johnson, L. K.; Mecking, S.; Brookhart, M. Copolymerization of Ethylene and Propylene with Functionalized Vinyl Monomers by Palladium(II) Catalysts. J. Am. Chem. Soc. 1996, 118, 267−268. (c) Johnson, L. K.; Killian, C. M.; Brookhart, M. New Pd(II)- and Ni(II)-Based Catalysts for Polymerization of Ethylene and α-Olefins. J. Am. Chem. Soc. 1995, 117, 6414−6415. (3) (a) Ye, Z. B.; Xu, L. X.; Dong, Z. M.; Xiang, P. Designing Polyethylenes of Complex Chain Architectures via Pd−DiimineCatalyzed ‘‘Living’’ Ethylene Polymerization. Chem. Commun. 2013, 49, 6235−6255. (b) Gao, H. Y.; Hu, H. B.; Zhu, F. M.; Wu, Q. A Thermally Robust Amine−Imine Nickel Catalyst Precursor for Living Polymerization of Ethylene above Room Temperature. Chem. Commun. 2012, 48, 3312−3314. (c) Gottfried, A. C.; Brookhart, M. Living and Block Copolymerization of Ethylene and α-Olefins Using Palladium(II)-α-Diimine Catalysts. Macromolecules 2003, 36, 3085− 3100. (d) Guan, Z. B. Control of Polymer Topology by ChainWalking Catalysts. Chem. - Eur. J. 2002, 8, 3086−3092. (e) Gottfried, A. C.; Brookhart, M. Living Polymerization of Ethylene Using Pd(II) α-Diimine Catalysts. Macromolecules 2001, 34, 1140−1142. (f) Deng, L. Q.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. The Role of Bulky Substituents in Brookhart-Type Ni(II) Diimine Catalyzed Olefin Polymerization: A Combined Density Functional Theory and Molecular Mechanics Study. J. Am. Chem. Soc. 1997, 119, 6177−6186. (4) Gates, D. P.; Svejda, S. A.; Oñate, E.; Killian, C. M.; Johnson, L. K.; White, P. S.; Brookhart, M. Synthesis of Branched Polyethylene Using (α-Diimine)nickel(II) Catalysts: Influence of Temperature, Ethylene Pressure, and Ligand Structure on Polymer Properties. Macromolecules 2000, 33, 2320−2334. (5) (a) Liu, F. S.; Hu, H. B.; Xu, Y.; Guo, L. H.; Zai, S. B.; Song, K. M.; Gao, H. Y.; Zhang, L.; Zhu, F. M.; Wu, Q. Thermostable αDiimine Nickel(II) Catalyst for Ethylene Polymerization: Effects of the Substituted Backbone Structure on Catalytic Properties and Branching Structure of Polyethylene. Macromolecules 2009, 42, 7789− 7796. (b) Berkefeld, A.; Mecking, S. Deactivation Pathways of Neutral Ni(II) Polymerization Catalysts. J. Am. Chem. Soc. 2009, 131, 1565− 1574. (c) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.;

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhongbao Jian: 0000-0002-2627-454X G

DOI: 10.1021/acs.organomet.9b00106 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Brookhart, M. Mechanistic Studies of Pd(II)-α-Diimine-Catalyzed Olefin Polymerizations. J. Am. Chem. Soc. 2000, 122, 6686−6700. (6) (a) Chen, Z.; Brookhart, M. Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts. Acc. Chem. Res. 2018, 51, 1831−1839. (b) Chen, C. L. Designing Catalysts for Olefin Polymerization and Copolymerization: Beyond Electronic and Steric Tuning. Nat. Rev. Chem. 2018, 2, 6−14. (c) Nakamura, A.; Anselment, T. M. J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.; van Leeuwen, P. W. N. M.; Nozaki, K. Ortho-Phosphinobenzenesulfonate: A Superb Ligand for Palladium-Catalyzed Coordination−Insertion Copolymerization of Polar Vinyl Monomers. Acc. Chem. Res. 2013, 46, 1438−1449. (d) Franssen, N. M. G.; Reek, J. N. H.; de Bruin, B. Synthesis of Functional ‘Polyolefins’: State of the Art and Remaining Challenges. Chem. Soc. Rev. 2013, 42, 5809−5832. (e) Nakamura, A.; Ito, S.; Nozaki, K. Coordination-Insertion Copolymerization of Fundamental Polar Monomers. Chem. Rev. 2009, 109, 5215−5244. (f) Chen, E. Y. -X. Coordination Polymerization of Polar Vinyl Monomers by SingleSite Metal Catalysts. Chem. Rev. 2009, 109, 5157−5214. (g) Boffa, L. S.; Novak, B. M. Copolymerization of Polar Monomers with Olefins Using Transition-Metal Complexes. Chem. Rev. 2000, 100, 1479− 1493. (7) (a) Popeney, C. S.; Guan, Z. B. A Mechanistic Investigation on Copolymerization of Ethylene with Polar Monomers Using a Cyclophane-Based Pd(II) α-Diimine Catalyst. J. Am. Chem. Soc. 2009, 131, 12384−12393. (b) Popeney, C. S.; Camacho, D. H.; Guan, Z. B. Efficient Incorporation of Polar Comonomers in Copolymerizations with Ethylene Using a Cyclophane-Based Pd(II) α-Diimine Catalyst. J. Am. Chem. Soc. 2007, 129, 10062−10063. (c) Camacho, D. H.; Guan, Z. B. Living Polymerization of α-Olefins at Elevated Temperatures Catalyzed by a Highly Active and Robust Cyclophane-Based Nickel Catalyst. Macromolecules 2005, 38, 2544− 2546. (d) Camacho, D. H.; Salo, E. V.; Ziller, J. W.; Guan, Z. B. Cyclophane-Based Highly Active Late-Transition-Metal Catalysts for Ethylene Polymerization. Angew. Chem., Int. Ed. 2004, 43, 1821− 1825. (8) (a) Allen, K. E.; Campos, J.; Daugulis, O.; Brookhart, M. Living Polymerization of Ethylene and Copolymerization of Ethylene/ Methyl Acrylate Using “Sandwich” Diimine Palladium Catalysts. ACS Catal. 2015, 5, 456−464. (b) 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. (9) (a) Takano, S.; Takeuchi, D.; Osakada, K.; Akamatsu, N.; Shishido, A. Dipalladium Catalyst for Olefin Polymerization: Introduction of Acrylate Units into the Main Chain of Branched Polyethylene. Angew. Chem., Int. Ed. 2014, 53, 9246−9250. (b) Takeuchi, D.; Chiba, Y.; Takano, S.; Osakada, K. Double-Decker-Type Dinuclear Nickel Catalyst for Olefin Polymerization: Efficient Incorporation of Functional Co-monomers. Angew. Chem., Int. Ed. 2013, 52, 12536−12540. (10) (a) Zhai, F.; Jordan, R. F. (α-Diimine)nickel Complexes That Contain Menthyl Substituents: Synthesis, Conformational Behavior, and Olefin Polymerization Catalysis. Organometallics 2017, 36, 2784− 2799. (b) Zhai, F.; Solomon, J. B.; Jordan, R. F. Copolymerization of Ethylene with Acrylate Monomers by Amide-Functionalized αDiimine Pd Catalysts. Organometallics 2017, 36, 1873−1879. (11) (a) 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. (b) Guo, L. H.; Gao, H. Y.; Guan, Q. R.; Hu, H. B.; Deng, J. A.; Liu, J.; Liu, F. S.; Wu, Q. Substituent Effects of the Backbone in α-Diimine Palladium Catalysts on Homo- and Copolymerization of Ethylene with Methyl Acrylate. Organometallics 2012, 31, 6054−6054. (12) (a) 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. (b) 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. (c) O’Connor, K. S.; Watts, A.; Vaidya, T.; LaPointe, A. M.; Hillmyer, M. A.; Coates, G. W. Controlled Chain Walking for the Synthesis of Thermoplastic Polyolefin Elastomers: Synthesis, Structure, and Properties. Macromolecules 2016, 49, 6743−6751. (d) 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. (13) (a) Li, M.; Wang, X. B.; Luo, Y.; Chen, C. L. A SecondCoordination-Sphere Strategy to Modulate Nickel- and PalladiumCatalyzed Olefin Polymerization and Copolymerization. Angew. Chem., Int. Ed. 2017, 56, 11604−11609. (b) Lian, K.; 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. (c) Na, Y. N.; Wang, X. B.; Lian, K. B.; Zhu, Y.; Li, W. M.; Luo, Y.; Chen, C. L. DinuclearαDiimine NiII and PdII Complexes that Catalyze Ethylene Polymerization and Copolymerization. ChemCatChem 2017, 9, 1062−1066. (d) Wang, R. K.; Sui, X. L.; Pang, W. M.; Chen, C. L. Ethylene Polymerization by Xanthene-Bridged Dinuclear α-Diimine NiII Complexes. ChemCatChem 2016, 8, 434−440. (14) Ionkin, A. S.; Marshall, W. J. ortho-5-Methylfuran- and Benzofuran-Substituted η3-Allyl(α-diimine)nickel(II) Complexes: Syntheses, Structural Characterization, and the First Polymerization Results. Organometallics 2004, 23, 3276−3283. (15) (a) Suo, H. Y.; Oleynik, I. V.; Huang, C. B.; Oleynik, I. I.; Solan, G. A.; Ma, Y. P.; Liang, T. L.; Sun, W. H. ortho-Cycloalkyl Substituted N,N′-Diaryliminoacenaphthene-Ni(II) Catalysts for Polyethylene Elastomers; Exploring Ring Size and Temperature Effects. Dalton Trans. 2017, 46, 15684−15697. (b) Mahmood, Q.; Zeng, Y. N.; Yue, E. L.; Solan, G. A.; Liang, T. L.; Sun, W. H. Ultra-High Molecular Weight Elastomeric Polyethylene Using an Electronically and Sterically Enhanced Nickel Catalyst. Polym. Chem. 2017, 8, 6416−6430. (c) 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-DiiminoNickel Catalyst. Polym. Chem. 2017, 8, 2785−2795. (d) Wang, Z.; Liu, Q. B.; 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. (e) Fan, L. L.; Du, S. Z.; Guo, C. Y.; Hao, X.; Sun, W. H. 1-(2,6-Dibenzhydryl-4-fluorophenylimino)-2aryliminoacenaphthylylnickel Halides Highly Polymerizing Ethylene for the Polyethylenes with High Branches and Molecular Weights. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1369−1378. (f) Du, S. Z.; Kong, S. L.; Shi, Q. S.; Mao, J.; Guo, C. Y.; Yi, J. J.; Liang, T. L.; Sun, W. H. Enhancing the Activity and Thermal Stability of Nickel Complex Precatalysts Using 1-[2,6-Bis(bis(4-fluorophenyl)methyl)-4methylphenylimino]-2-aryliminoacenaphthylene Derivatives. Organometallics 2015, 34, 582−590. (g) Kong, S. L.; Guo, C. Y.; Yang, W. H.; Wang, L.; Sun, W. H.; Glaser, R. 2,6-Dibenzhydryl-N-(2phenyliminoacenaphthylenylidene)-4-chloroaniline Nickel Dihalides: Synthesis, Characterization and Ethylene Polymerization for Polyethylenes with Hhigh Molecular Weights. J. Organomet. Chem. 2013, 725, 37−45. (h) Liu, H.; Zhao, W. Z.; Hao, Xi.; Redshaw, C.; Huang, W.; Sun, W. H. 2,6-Dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-4-methylbenzenamine Nickel Dibromides: Synthesis, Characterization, and Ethylene Polymerization. Organometallics 2011, 30, 2418−2424. (16) (a) Brown, L. A.; Anderson, W. C., Jr.; 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. (b) Rhinehart, J.; Mitchell, N. E.; H

DOI: 10.1021/acs.organomet.9b00106 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Long, B. K. Enhancing α-Diimine Catalysts for High-Temperature Ethylene Polymerization. ACS Catal. 2014, 4, 2501−2504. (c) Rhinehart, J. L.; Brown, L. A.; Long, B. K. A Robust Ni(II) α-Diimine Catalyst for High Temperature Ethylene Polymerization. J. Am. Chem. Soc. 2013, 135, 16316−16319. (17) (a) Dai, S. Y.; Chen, C. L. Palladium-Catalyzed Direct Synthesis of Various Branched, Carboxylic Acid-Functionalized Polyolefins: Characterization, Derivatization, and Properties. Macromolecules 2018, 51, 6818−6824. (b) 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. (c) Guo, L. H.; Lian, K. B.; Kong, W. Y.; Xu, S.; Jiang, G. R.; Dai, S. Y. Synthesis of Various Branched UltraHigh-Molecular-Weight Polyethylenes Using Sterically Hindered Acenaphthene-Based α-Diimine Ni(II) Catalysts. Organometallics 2018, 37, 2442−2449. (d) Guo, L. H.; Kong, W. Y.; Xu, Y. J.; Yang, Y. L.; Ma, R.; Cong, L.; Dai, S. Y.; Liu, Z. Large-Scale Synthesis of Novel Sterically Hindered Acenaphthene-Based α-Diimine Ligands and Their Application in Coordination Chemistry. J. Organomet. Chem. 2018, 859, 58−67. (e) Guo, L. H.; Zou, C.; Dai, S. Y.; Chen, C. L. Direct Synthesis of Branched Carboxylic Acid Functionalized Poly(1-octene) by α-Diimine Palladium Catalysts. Polymers 2017, 9, 122. (f) Dai, S. Y.; Chen, C. L. Direct Synthesis of Functionalized High-Molecular-Weight Polyethylene by Copolymerization of Ethylene with Polar Monomers. Angew. Chem., Int. Ed. 2016, 55, 13281− 13285. (g) Dai, S. Y.; Zhou, S. X.; Zhang, W.; Chen, C. L. Systematic Investigations of Ligand Steric Effects on α-Diimine Palladium Catalyzed Olefin Polymerization and Copolymerization. Macromolecules 2016, 49, 8855−8862. (h) Guo, L. H.; Dai, S. Y.; Sui, X. L.; Chen, C. L. Palladium and Nickel Catalyzed Chain Walking Olefin Polymerization and Copolymerization. ACS Catal. 2016, 6, 428−441. (i) Guo, L. H.; Dai, S. Y.; Chen, C. L. Investigations of the Ligand Electronic Effects on α-Diimine Nickel(II) Catalyzed Ethylene Polymerization. Polymers 2016, 8, 37. (j) Dai, S. Y.; Sui, X. L.; Chen, C. L. Highly Robust Palladium(II) α-Diimine Catalysts for Slow-Chain-Walking Polymerization of Ethylene and Copolymerization with Methyl Acrylate. Angew. Chem., Int. Ed. 2015, 54, 9948− 9953. (18) Book: (a) Chen, C. F.; Ma, Y. X. Iptycenes Chemistry: From Synthesis to Applications; Springer Verlag: Berlin, 2012. (b) Jiang, Y.; Chen, C. F. Recent Developments in Synthesis and Applications of Triptycene and Pentiptycene Derivatives. Eur. J. Org. Chem. 2011, 2011, 6377−6403. (c) Swager, T. M. Iptycenes in the Design of High Performance Polymers. Acc. Chem. Res. 2008, 41, 1181−1189. (d) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Conjugated Polymer-Based Chemical Sensors. Chem. Rev. 2000, 100, 2537−2574. (19) (a) Heidrich, M.; Bergmann, M.; Müller-Borges, D.; Plenio, H. Bispentiptycenyl-N-Heterocyclic Carbene (NHC) Gold Complexes: Highly Active Catalysts for the Room Temperature Hydration of Alkynes. Adv. Synth. Catal. 2018, 360, 3572−3578. (b) Bergmann, M.; Savka, R.; Foro, S.; Plenio, H. Synthesis of an ortho-Methyl-N,N′bis(triptycenyl) N-Heterocyclic Carbene Ligand and Its Metal Complexes. Eur. J. Inorg. Chem. 2017, 2017, 3779−3786. (c) Savka, R.; Foro, S.; Plenio, H. Pentiptycene-Based Concave NHC−Metal Complexes. Dalton Trans. 2016, 45, 11015−11024. (d) Savka, R.; Bergmann, M.; Kanai, Y.; Foro, S.; Plenio, H. Triptycene-Based Chiral and meso-N-Heterocyclic Carbene Ligands and Metal Complexes. Chem. - Eur. J. 2016, 22, 9667−9675. (20) Vasiuta, R.; Stockert, A.; Plenio, H. Alternating Ring-Opening Metathesis Polymerization by Grubbs-Type Catalysts with NPentiptycenyl, N-Alkyl-NHC Ligands. Chem. Commun. 2018, 54, 1706−1709. (21) During the preparation of this manuscript, Plenio et al. reported on January 11, 2019 similar bispentiptycenyl-diimine Ni(II) catalysts (R1 = OBu, R2 = acenaphthene; R1 = OBu, R2 = Me in Scheme 1) for ethylene polymerization and copolymerization with long-chain polar monomers. See: Kanai, Y.; Foro, S.; Plenio, H. Bispentiptycenyl− Diimine−Nickel Complexes for Ethene Polymerization and Copoly-

merization with Polar Monomers. Organometallics 2019, 38, 544− 551. (22) (a) Zhong, S. H.; Tan, Y. X.; Zhong, L.; Gao, J.; Liao, H.; Jiang, L.; Gao, H. Y.; Wu, Q. Precision Synthesis of Ethylene and Polar Monomer Copolymers by Palladium-Catalyzed Living Coordination Copolymerization. Macromolecules 2017, 50, 5661−5669. (b) Zhong, L.; Li, G. L.; Liang, G. D.; Gao, H. Y.; Wu, Q. Enhancing Thermal Stability and Living Fashion in α-Diimine−Nickel-Catalyzed (Co)polymerization of Ethylene and Polar Monomer by Increasing the Steric Bulk of Ligand Backbone. Macromolecules 2017, 50, 2675− 2682. (c) Long, B. K.; Eagan, J. M.; Mulzer, M.; Coates, G. W. SemiCrystalline Polar Polyethylene: Ester-Functionalized Linear Polyolefins Enabled by a Functional-Group-Tolerant, Cationic Nickel Catalyst. Angew. Chem., Int. Ed. 2016, 55, 7106−7110. (d) Huo, P.; Liu, W. Y.; He, X. H.; Wei, Z. H.; Chen, Y. W. Substituent Effects and Activation Mechanism of Norbornene Polymerization Catalyzed by Three-Dimensional Geometry α-Diimine Palladium Complexes. Polym. Chem. 2014, 5, 1210−1218. (e) Huo, P.; Liu, W. Y.; He, X. H.; Wang, H. M.; Chen, Y. W. Nickel(II) Complexes with ThreeDimensional Geometry α-Diimine Ligands: Synthesis and Catalytic Activity toward Copolymerization of Norbornene. Organometallics 2013, 32, 2291−2299. (23) (a) Yang, J.-S.; Ko, C. W. Pentiptycene Chemistry: New Pentiptycene Building Blocks Derived from Pentiptycene Quinones. J. Org. Chem. 2006, 71, 844−847. (b) Zhu, X. Z.; Chen, C. F. Iptycene Quinones: Synthesis and Structure. J. Org. Chem. 2005, 70, 917−924. (24) Xie, T. Y.; McAuley, K. B.; Hsu, J. C. C.; Bacon, D. W. Gas Phase Ethylene Polymerization: Production Processes, Polymer Properties, and Reactor Modeling. Ind. Eng. Chem. Res. 1994, 33, 449−479. (25) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91, 165−195. (26) Control experiment shows that the reaction of MAO with phenol in toluene produces a lot of bubbles (methane).

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DOI: 10.1021/acs.organomet.9b00106 Organometallics XXXX, XXX, XXX−XXX