Letter Cite This: ACS Macro Lett. 2018, 7, 213−217
pubs.acs.org/macroletters
Nickel-Catalyzed Propylene/Polar Monomer Copolymerization Yohei Konishi,†,# Wen-jie Tao,‡,# Hina Yasuda,‡ Shingo Ito,‡ Yasuo Oishi,† Hisashi Ohtaki,† Akio Tanna,§ Takao Tayano,*,§ and Kyoko Nozaki*,‡ †
Japan Polychem Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama, Kanagawa 227-8502, Japan Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Japan Polychem Corporation, 1 Toho-cho, Yokkaichi, Mie 510-0848, Japan ‡
S Supporting Information *
ABSTRACT: Nickel complexes bearing bidentate alkylphophine− phenolate ligands were synthesized and applied to the polymerization of propylene and the copolymerization of propylene and polar monomers. Therein, the use of bulky alkyl substituents on the phosphorus atom was crucial for the formation of highly regioregular polypropylenes. Theoretical calculations suggested that the 1,2insertion of propylene is favored over the 2,1-insertion in these nickel-catalyzed (co)polymerization reactions. The substituent at the ortho position relative to the oxido group greatly influences the polymerization activity, the molecular weight, and stereoregularity of the polypropylenes. This method delivers moderately isotactic ([mm] ≤ 68%) crystalline polypropylenes. The present system represents the first example for a nickel-catalyzed regiocontrolled copolymerization of propylene and polar monomers such as but-3-en-1-ol, but-3-en-1-yl acetate, and tert-butyl allylcarbamate.
P
subsequent 1,2-reinsertion. In 2015, we have developed palladium/imidazo[1,5-a]quinolin-9-olate-1-ylidene catalysts8 that have promoted the first synthesis of regioregular polar polypropylenes, albeit that their stereoregularity was atactic. Subsequently, moderately isotactic polar polypropylenes were synthesized using palladium/phosphine−sulfonate catalysts.9 Despite the aforementioned progress,8,9 the catalytic systems currently available are mainly restricted to palladium,8,9 even though nickel would be a more desirable candidate owing to its abundance and less costly nature, which are essential for industrial applications. Thus far, the nickel-catalyzed regiocontrolled copolymerization of propylene remains challenging, given that (i) the polymerization reaction needs to be carried out below room temperature in order to obtain highly regioregular polypropylenes, even when using state-of-the-art nickel catalysts,10−13 and that (ii) the polymerization activity is severely reduced in the presence of large amounts of polar monomers, due to the higher oxophilic nature of nickel compared to palladium.14 The coordination−insertion copolymerization of propylene and polar monomers using nickelbased catalysts thus remains as one of the major challenges in olefin polymerization. Here we report the first nickel-catalyzed regio- and stereocontrolled propylene polymerization and propylene/polar monomer copolymerization reactions (Scheme 1). A series of novel nickel catalysts bearing bidentate alkylphosphine−
olypropylene is one of the most important polymers used to manufacture a wide range of commodity plastic products.1 Despite the versatile usage, the low hydrophilicity of polypropylene limits its range of applications. Thus, intensive efforts have been devoted to the introduction of polar functional groups into hydrophobic polypropylenes to improve their surface and material properties.2 The currently most common industrial approach to such “functional polypropylenes” is based on postpolymerization functionalization techniques. However, these processes require relatively harsh conditions that entail certain limitations, such as chain shortening and a comparatively narrow scope of accessible functional groups.3 Meanwhile, the coordination−insertion copolymerization of propylene and polar monomers using transition-metal complexes would be a straightforward method for the controlled generation of functionalized polypropylenes. Although some early transition metal catalysts were reported to promote the copolymerization of propylene with polar monomers,4,5 the attention of polymer chemists have been shifted toward late-transition metal catalysts because they exhibit high tolerance toward polar functional groups.6 In late-transition-metal-catalyzed propylene polymerization, the regio- and stereocontrol of propylene insertion is indispensable. The first example of a copolymerization of propylene and a polar monomer (methyl acrylate) using palladium/α-diimine catalysts was reported by Brookhart in 1996.7 However, the obtained copolymers contained substantial amounts of chain-straightening regioerror units resulted from a formal 3,1-insertion of propylene, that is, a 2,1-insertion of propylene followed by a β-hydrogen elimination and a © XXXX American Chemical Society
Received: November 16, 2017 Accepted: January 19, 2018
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DOI: 10.1021/acsmacrolett.7b00904 ACS Macro Lett. 2018, 7, 213−217
Letter
ACS Macro Letters Scheme 1. Copolymerization of Propylene and Polar Monomers Using Nickel/Alkylphosphine−Phenolate Catalysts
Figure 1. Molecular structures of (a) 1c and (b) 1e with thermal ellipsoids set at 50% probability. Hydrogen atoms are omitted for clarity.
phenolate ligands were synthesized inspired by the nickel catalyst we have recently reported for ethylene polymerization.15 The introduction of bulky alkyl substituents on the phosphorus atom9 and a substituent at the ortho position relative to the oxido group9,16,17 is the key for our success to generate highly regioregular polypropylenes with high molecular weight even at high temperature (50 °C). The observed isotacticity was the highest among the group 10 metal-catalyzed propylene polymerization at such high temperature. Moreover, the copolymerization of propylene and polar monomers was accomplished to furnish polar polypropylenes. The synthetic routes to nickel/alkylphosphine−phenolate complexes 1a−1f are shown in Scheme 2. The ligand
P1 bond length in 1b (2.2301(13)Å), which is noticeably longer than that in 1d (2.1829(7) Å) and 1e (2.1825(15) Å). This is presumably due to the stronger trans influence of triethylphosphine compared to that of pyridine. Initially, we investigated the homopolymerization of propylene using nickel catalysts 1a−1f (Table 1). Catalyst 1a, a conventional nickel/diphenylphosphine−phenolate complex that is used for the polymerization of ethylene,19 exhibited a relatively low catalytic activity and afforded only trace amounts of propylene oligomers (entry 1). The yield and molecular weight of these oligomers were in fact too low for a quantitative NMR analysis of their regiodefects and stereoregularity. Changing the substituents on the phosphorus atom from phenyl to tert-butyl or menthyl groups9,20 led to an increase of the molecular weight of the polymeric products, affording polypropylenes with weight-averaged molecular weights (Mw) of 7.0 × 103 (entry 2) and 4.6 × 103 (entry 3), respectively. As expected from our previous paper,15 the introduction of either a 2,3,4,5,6-pentafluorophenyl (entry 4, catalyst 1d) or tert-butyl group (entry 5, catalyst 1e) at the ortho position relative to the oxido group of 1b led to a drastic increase of the polymerization activity (62 and 110 g/mmol·h, respectively).21 Catalyst 1e also afforded an improved Mw of 1.7 × 104. The combination of a di-tert-butylphosphino group and a tert-butyl substituent ortho to the oxido group emerged as the most effective in terms of catalytic activity and polymer molecular weight. Similar enhancement of the catalytic activity and polymer molecular weight by introducing an ortho substituent was observed in the case of ligands bearing a dimenthylphosphino group (entry 6). The regiodefects in the obtained polypropylenes were determined by NMR analysis. Quantitative 13C NMR analyses revealed that highly regioregular polypropylenes were obtained from 1b−1f (entries 2−6). In particular, the almost perfect regio-controlled polymerization of propylene was realized by 1e at 50 °C (entry 5). It should be noted that the polypropylenes obtained from 1b and 1d−1f contained no sections due to 1,3enchainment, whereas those produced by previously reported nickel catalysts, such as Ni/α-diimine10 or Ni/α-keto-βdiimine,11 contained significant amounts of units that stem from a 1,3-enchainment (56 mol % and 25 mol %, respectively) even when the polymerization was performed at 22−25 °C. In order to elucidate the origin of the observed high regioregularity, we performed DFT calculations using the B3LYP functional in combination with lanl2dz (Ni) or 631G(d) (all other atoms) basis sets. As shown in Figure 2, the transition states for all possible combinations of 1,2-/2,1insertion of propylene after another 1,2-/2,1-insertion of
Scheme 2. Synthesis of Nickel/Phosphine−Phenolate Complexes 1a−1f
precursors La−Lc, bearing a hydro group ortho to the hydroxy group, were deprotonated and treated with the nickel precursors to form the corresponding arylnickel(phosphine) complexes (1a−1c). In these cases, the labile phosphine ligands as L are important to suppress the undesired formation of bisligated complexes Ni(R12PC6H4O)2.18 As for ligands Ld−Lf with a bulky ortho substituent (R2), the corresponding complexes 1d−1f bearing a weaker labile ligand, such as pyridine or its derivatives, were isolated, while no undesired bisligated complexes were given. The structures of 1b−1e were also confirmed by X-ray crystallographic analysis (Figures 1 and S55−S58). Complexes 1b−1d exhibit similar bond lengths and angles around the nickel center, with the exception of the Ni1− 214
DOI: 10.1021/acsmacrolett.7b00904 ACS Macro Lett. 2018, 7, 213−217
Letter
ACS Macro Letters Table 1. Propylene Polymerization by Nickel/Alkylphosphine−Phenolate Catalystsa activity entry f
1 2f 3f 4 5 6
catalyst
g/mmol·h
TON (103)
Mwb (103)
Mw/Mnb
[mm]c (%)
regiodefectsd (%)
1a 1b 1c 1d 1e 1f
trace 1.0 1.4 62 110 8.3
0.07 0.10 4.4 7.9 0.59
nd 7.0 4.6 6.1 17 16
nd 1.4 1.3 1.6 1.8 1.8
nd 46 68 25 19 60
nd 0.62 3.2 2.0 0.61 7.7
Tme (°C)
87, 43
57, 42
A mixture of the catalyst (30 μmol) and propylene (ca. 250 g) was stirred for 3 h at 50 °C in a 2.0 L autoclave. bMolecular weights determined by size-exclusion chromatography (SEC) analysis using polystyrene standards. cTriad ratios determined by quantitative 13C NMR analysis. d Regiodefects determined by quantitative 13C NMR analysis. eMelting temperatures determined by differential scanning calorimetry (DSC) analysis. f The reaction was performed in the presence of Ni(cod)2 (60 μmol). a
Figure 3. Quantitative 13C NMR spectra (18−23 ppm; −(CH(CH3)CH2)n−) of the polypropylenes obtained from entries (a) 2, (b) 3, (c) 5, and (d) 6 in Table 1.
Figure 2. Relative energy difference (ΔG, kcal/mol) of the transition states during the insertion of propylene relative to corresponding alkylnickel(propylene) complexes: (a) 1,2-insertion into [P−O]NiCH2CH(CH3)2, (b) 2,1-insertion into [P−O]NiCH2CH(CH3)2, (c) 1,2-insertion into [P−O]NiCH(CH3)CH2CH3, and (d) 2,1insertion into [P−O]NiCH(CH3)CH2CH3.
phosphorus atom. Notably, the differential scanning calorimetry (DSC) analysis of the polypropylene obtained from 1c (entry 3) revealed an endothermic peak at Tm = 87 °C (ΔH = 43 J/g), which is significantly higher than those previously reported for polymers obtained from menthyl-substituted palladium/phosphine−sulfonate catalysts (up to 18 J/g).9 In terms of isotacticity, the catalysts bearing a hydro group at the ortho position relative to the oxido moiety were found to be more suitable than catalysts bearing a bulky substituent (entry 2 vs 5 and entry 3 vs 6). Furthermore, the nickel/menthyl-substituted phosphine−phenolate catalysts displayed a tendency toward higher triad ratios than the nickel/tert-butyl-substituted phosphine−phenolate catalysts (entry 3 vs 2 and entry 6 vs 5). The copolymerization of propylene with polar monomers was investigated using catalyst 1e, which exhibited the highest catalytic activity and polymer molecular weight for the homopolymerization of propylene among all the studied catalysts (Table 2). Hydroxy, carboxy, and amino groups were successfully incorporated into polypropylene by using the polar monomers but-3-en-1-ol, but-3-en-1-yl acetate, and Bocprotected allylamine (tert-butyl allylcarbamate), respectively. Unprotected 3-buten-1-ol was incorporated (up to 1.4 mol %) into polypropylene (entries 1 and 2), although the copolymerization activity was much lower than that for the homopolymerization of propylene (entry 5, Table 1). NMR analyses and comparison with literature data5b revealed that the majority of 3-buten-1-ol was incorporated at the chain end.23 As for the copolymerization of propylene and but-3-en-1-yl acetate, (entries 3 and 4), up to 2.4 mol % of but-3-en-1-yl acetate were incorporated in the main chain of polypropylene. In entry 4, poropylene/but-3-en-1-yl acetate copolymer was
propylene were calculated on model complexes containing isobutyl and sec-butyl groups.22 The energy differences are expressed relative to the corresponding arylnickel(phosphine) complexes. For the system bearing ligand La, the 1,2-insertion (10.9 kcal/mol) was found to be favored over the 2,1-insertion (11.5 kcal/mol) for the nickel−isobutyl bond, whereas the 2,1insertion (14.0 kcal/mol) of propylene was preferred over the 1,2-insertion (15.8 kcal/mol) for the nickel−sec-butyl bond. In comparison, the system bearing ligand Le showed that the 1,2insertion of propylene was predominant for both nickel−alkyl bonds (7.1 vs 12.1 kcal/mol for the nickel−isobutyl bond and 10.0 vs 11.6 kcal/mol for the nickel−sec-butyl bond), suggesting that the high regioregularity of the polypropylene afforded by catalyst 1e arises from the selective 1,2-insertion of propylene during the propagation. The stereoregularity of the resulting polypropylenes should thus be greatly influenced by the substituents on the phosphorus atom and by those at the ortho position relative to the oxido group (Figure 3). The polymers produced by 1d and 1e were almost atactic; in contrast, the polypropylene obtained from 1b was moderately isotactic with an [mm] value of 46% (entry 2). Furthermore, menthyl-substituted 1c and 1f improved the [mm] values to 68% (entry 3) and 60% (entry 6), respectively. The increase of [mm] values in the case of 1c and 1f could be attributed to the enhancement of enatiomorphic site control by introducing bulky chiral menthyl groups on the 215
DOI: 10.1021/acsmacrolett.7b00904 ACS Macro Lett. 2018, 7, 213−217
Letter
ACS Macro Letters Table 2. Propylene/Polar Monomer Copolymerization by Nickel/Alkylphosphine−Phenolate Catalystsa activity entry
catalyst (μmol)
1 2 3
1e (20) 1e (20) 1e (10)
4
1e (10)
5
1f (20)
6
1e (20)
7
1e (10)
polar monomer but-3-en-1-ol (0.60 mL) but-3-en-1-ol (0.30 mL) but-3-en-1-yl acetate (1.0 mL) but-3-en-1-yl acetate (0.50 mL) but-3-en-1-yl acetate (0.10 mL) tert-butyl allylcarbamate (0.25 g) tert-butyl allylcarbamate (0.12 g)
time (h)
g/mmol·h
TON (103)
Mwb (103)
Mw/Mnb
incorp.c (mol %)
[mm]d (%)
regiodefectse (%)
12 12 3.0
0.25 0.47 6.8
0.07 0.13 0.48
5.2 6.8 6.4
1.9 1.3 2.2
1.4 0.63 2.4
19 18 18
0.36 0.17 0.45
3.0
44
3.1
12
1.7
1.5
19
0.31
12
1.8
0.51
9.1
1.5
0.49
61
9.2
12
0.23
0.07
7.1
1.3
0.91
19
0.80
12
0.54
0.15
11
1.9
0.39
19
0.39
Tmf (°C)
55, 43
a
A mixture of catalyst (indicated amount), propylene (10 g), and a polar monomer (indicated amount) in toluene (10 mL) was stirred for the indicated time at 50 °C in a 50 mL autoclave. bMolecular weights determined by size-exclusion chromatography (SEC) analysis using polystyrene standards. cIncorporation ratios of the polar monomers determined by 1H NMR analysis. dTriad ratios determined by quantitative 13C NMR analysis. eRegiodefects determined by quantitative 13C NMR analysis. fMelting temperatures determined by differential scanning calorimetry (DSC) analysis.
ORCID
afforded with moderate copolymerization activity (44 g/mmol· h) and high molecular weight (Mw = 12 × 103). The copolymerization using 1f afforded a moderately isotactic crystalline polar polypropylene (Tm = 55 and 43 °C) with a comonomer content of 0.49 mol % and an [mm] value of 61% (entry 5). Moreover, tert-butyl allylcarbamate, a polar allyl monomer, was copolymerized by using catalyst 1e in both of the main chain and the chain end with total incorporation ratios of up to 0.91 mol % (entries 6 and 7). In summary, we have achieved the regio-controlled homopolymerization of propylene and the copolymerization of propylene and polar monomers using nickel/alkylphophine− phenolate catalysts. The concomitant introduction of bulky alkyl groups such as tert-butyl and menthyl groups on the phosphorus atom and of a tert-butyl group at the ortho position relative to the oxido group was found to be essential to achieve high regioregularity, catalytic activity, and molecular weights of the polymers in these (co)polymerization reactions of propylene. Moderately isotactic ([mm] ≤ 68%) crystalline polypropylenes (Tm ≤ 87 °C) and propylene/polar monomer copolymers (Tm ≤ 55 °C) were obtained. In addition, but-3-en1-ol, but-3-en-1-yl acetate, and tert-butyl allylcarbamate were introduced in polypropylene (incorporation ratio: 0.39−2.4 mol %) using highly active nickel complex 1e.
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Yohei Konishi: 0000-0002-7752-5193 Hina Yasuda: 0000-0002-8770-0039 Shingo Ito: 0000-0003-1776-4608 Kyoko Nozaki: 0000-0002-0321-5299 Author Contributions #
Y.K. and W.T. contributed equally to this paper.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JST CREST Grant No. JPMJCR1323, Japan. A part of this work was conducted in Research Hub for Advanced Nano Characterization, The University of Tokyo, supported by MEXT, Japan.
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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/acsmacrolett.7b00904. Experimental procedures, characterization details for ligands, nickel complexes, and polymers (PDF). Crystallographic data for nickel complex 1b (CIF). Crystallographic data for nickel complex 1c (CIF). Crystallographic data for nickel complex 1d (CIF). Crystallographic data for nickel complex 1e (CIF).
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. 216
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