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Rational Design of High-Performance Phosphine Sulfonate Nickel Catalysts for Ethylene Polymerization and Copolymerization with Polar Monomers Min Chen, and Changle Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03394 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017
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ACS Catalysis
Rational Design of High-Performance Phosphine Sulfonate Nickel Catalysts for Ethylene Polymerization and Copolymerization with Polar Monomers Min Chen, 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 ABSTRACT: Use of palladium catalysts in olefin polymerization and copolymerization has evolved rapidly. In contrast, Earth-abundant and low-cost nickel catalysts generally suffer from drawbacks that include low thermal stability and generation of low-molecular-weight polymers in the presence of polar monomers. By taking advantage of several design strategies, high-performance phosphine sulfonate-based nickel catalysts were developed. These nickel catalysts demonstrated high activities and thermal stabilities to afford high-molecular-weight polyethylene. Most importantly, high-molecular-weight copolymers could be generated through the copolymerization of ethylene with a variety of polar monomers. Keywords: nickel catalysts, ligand design, olefin polymerization, polar monomer, copolymerization Polyolefins are the most common synthetic polymers, representing almost half of the 300 million tons of plastics 1 produced worldwide. Despite this huge annual production and the wide range of applications of polyolefins, their nonpolar nature remains a significant limitation. Incorporation of polar functional groups can greatly improve the properties of these polymers. The most direct and economical synthetic strategy involving coordination copolymerization of olefins with polar monomers is highly 2 challenging. As major players in the polyolefin industry, early transition metal catalysts are prone to poisoning from polar groups, making them incompetent for the required task. In contrast, because of their low oxophilicity, late transition metal catalysts provide great potential for addressing this issue. A milestone discovery was made in the 1990s by Brookhart et al., who reported the copolymerization of olefins with acrylate monomers using α3 diimine palladium catalysts. Subsequently, great effort has been directed toward the preparation of a variety of polar 4 functionalized copolymers. Significant progress has been made in palladiumcatalyzed olefin/polar monomer copolymerization chemistry since then. However, utilization of nickel metal has proven to be very difficult because of its intrinsically high oxophilicity. This translates into a much higher suppression of polymerization in the presence of polar monomers than observed for the analogous palladium chemistry. For example, Mecking et al. showed that the challenging nature of nickel-catalyzed copolymerization arises from the retarded reactivity of the functionalized olefin insertion 4r products, as well as some deactivation reactions. In addition, many nickel catalysts show greatly reduced o activities at temperatures above 80 C, which is a serious
Chart 1. Strategies to improve the performance of phosphine-sulfonate nickel catalysts. 5
limitation for industrial applications. As a result, relatively few high-performance Ni(II) catalyst systems and narrow polar monomer scope have been reported. Some of the most notable examples include the neutral salicylaldimine Ni(II) catalysts reported by Grubbs et al., which are capable of copolymerizing ethylene with polarsubstituted norbornene and a few other special polar 6 monomers. The groups of Agapi and Takeuchi reported the copolymerization of ethylene with polar monomers using 7a, 7b dinuclear salicylaldimine Ni(II) catalysts. Coates et al. showed that the α-diimine Ni(II)/MAO system can 7c,7d copolymerize ethylene with methyl 10-undecenoate. Nozaki et al. described the copolymerization of ethylene with allyl acetate and allyl ether, using carbene-based nickel 7e catalysts at low activities (~1 kg/mol·h). SHOP (Shell Higher Olefin Process)-type nickel catalysts and a few other nickel-based catalysts have been demonstrated to mediate 8 ethylene copolymerization with polar monomers.
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Recently, some palladium complexes based on phosphine sulfonate ligands and their derivatives have emerged as superior catalysts for the copolymerization of ethylene with 9 a variety of polar monomers. In contrast, the corresponding nickel catalysts usually afford polyethylene of low molecular 10 weight (Mn around 1000). Nozaki et al. demonstrated the phosphine sulfonate nickel catalyzed copolymerization of ethylene with allyl acetate and allyl silyl ether, with low activity (~1 kg/mol·h), low comonomer incorporation (< 11 0.24%), and low copolymer molecular weight (Mn < 1000). Several strategies have been reported for improving the properties of late transition metal catalysts in literature. (1) It is well known that steric bulk at the axial positions in αdiimine Pd(II) and Ni(II) catalysts can decrease the rate of chain transfer to monomer, leading to increased polymer molecular weight.4 This strategy was successfully applied by Mecking et al. and Scott et al. to phosphine sulfonate Pd(II) and Ni(II) catalysts through the incorporation of biaryl substituents (Chart 1, I).12 (2) Electronic perturbation is important for modulating the properties of metal catalysts, and electron-donating substituents may lower the oxophilicity of a nickel center. Recently, Mecking et al. showed that electron-donating substituents (R) on phosphine sulfonate Pd(II) catalysts (Chart 1, II) can increase the polyethylene molecular weight.13 (3) In salicylaldimine Ni(II) and phosphine sulfonate Pd(II) systems, the steric bulk of the ortho R' substituent (Chart 1, II and III) is crucial to obtaining high-molecular-weight polymers.6,9m (4) The presence of strongly coordinating bases (such as PPh3 in catalyst III) may require a scavenger to activate the catalyst precursor. Moreover, the coordinating base may competitively bind to the metal center as opposed to the polar monomer.12a, 14 In this contribution, we demonstrate that the abovementioned drawbacks for nickel catalysts can be addressed through rational catalyst design. The phosphine sulfonate ligands were prepared by the reaction of benzenesulfonic acid with two equiv. of nBuLi, followed by the addition of one equiv. of R1ArPCl (Scheme 1; Ar = 2-[2,6-(MeO)2C6H3]C6H4; R1 = Ph (L1), Cy (L2), tBu (L3)). The reaction of ligand L1 with two equiv. of nBuLi, followed by the addition of C6F6 or Me3SiCl, afforded ligands L4 (R1 = Ph, R2 = C6F5) and L5 (R1 = Ph, R2 = SiMe3), respectively, in 57-64% yields. Our initial target was the nickel 2,6-lutidine complex, because the labile nature of the 2,6-lutidine base may likely lead to the formation of a single component nickel catalyst. Complexes 1, 2 and 4 were generated in 4662% yields using the strategy developed by Nozaki et al.11 Similar procedures utilizing ligands L3 and L5 generated complex mixtures from which the desired products could not be isolated. Therefore, the PPh3 adducts (3 and 5) were prepare instead. These metal complexes were characterized by 1H, 13C, and 31P NMR spectroscopy, and elemental analysis. The molecular structures of 2 and 5 were determined by x-ray diffraction (Figure 1). The geometry at the nickel center is square planar, and the R (Me or Ph)
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R1
SO 3H
n
BuLi
R1
Ar
2 n BuLi
PH +
Ar
PH+
C6 F6 or Me3 SiCl S OS OR2 O O O R1 = Ph; R1 = Ph (L1), Cy (L2), tBu (L3) R2 = C 6F5 (L4), SiMe3 (L5) ArR1 PCl
O
R1
Ar +
PH R2
O
S OO
Ar= MeO
OMe
1) Ni(COD) 2 2,6-lutidine 2) allyl chloride 3) AlMe3
R1
Ar
P Ni R2
O
2) (PPh3)2 NiPhCl R 2
S O O R1 Ar P
1) NaH
Ni O
Me 1: R 1 =Ph, R 2=H; 2: R 1 =Cy, R 2=H; Lut 4: R 1 =Ph, R 2=C6 F5
S O O
Ph 3: R =tBu, R =H; 1 2 5: R 1=Ph, R 2 =SiMe 3 PPh 3
Scheme 1. Synthesis of the phosphine sulfonate ligands and the corresponding nickel complexes.
Figure 1. Molecular structures of catalysts 2 and 5. Hydrogen atoms and the Ph3P phenyl groups in 5 were omitted for clarity. Selected bond lengths (Å) and angles (deg) for 2: Ni2-P2 = 2.1397(7), Ni2-C41 = 1.939(3), Ni2-N2 = 1.979(2), Ni2-O6 = 1.9775(18), C41-Ni2-N2 = 88.86(11), O6-Ni2-P2 = 94.11(6); for 5: Ni2-P3 = 2.2314(11), Ni2-C83 = 1.894(4), Ni2-P4 = 2.2615(11), Ni2-O6 = 1.984(3); C83-Ni2-P4 = 88.03(11), O6Ni2-P3 = 90.65(9). group is positioned cis to the phosphine. The biaryl substituent does not lie on the axial position of the nickel metal, however, during polymerization, and after the dissociation of the coordinating base, chelate ring inversion and/or pseudoaxial aryl group rotation could bring the 15 biaryl substituent to the axial positions. First of all, these nickel complexes (1-5) exhibit good o thermal stability, maintaining high activities at 80 or 100 C 6 -1 -1 (Table 1). With activities well above 10 g·mol ·h , they are among the most active neutral nickel catalysts for ethylene 6a,10e,14,16 polymerization. Complexes 2 and 3, bearing electrondonating R1 groups, are less active than complex 1, affording polyethylene of lower molecular weight. This is the most important reason that we decided to prepare ligands L4 and L5, bearing phenyl groups at the R1 position. With the extra ortho substituent, catalyst 4 demonstrated much higher activities, as well as higher polymer molecular weights. Interestingly, the polymer molecular weights produced from the various catalysts followed the order: 4 > 1 > 5. This is
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Table 1. Ethylene homopolymerization at 80 oC and 100 o a C. o
Ent Cat. T( C)
Yield
b
[b]
6 b
Act.(10 )
c
c
o
Mn
PDI Tm ( C)
1
1
80
3.7
3.7
3200
3.02
122.6
2
2
80
2.3
2.3
2900 3.17
121.8
3
3
80
3.0
3.0
1700
2.74
120.4
4
4
80
4.5
4.5
4700 2.70
5
5
80
3.2
3.2
6
1
100
3.2
7
2
100
2.0
8
3
100
9
4
10
5
Table 2. Ethylene-polar monomer copolymerization at o a 80 C.
d
Ent. Cat. Comonom [M] Yield Act. XM er mol/L (g)b (104)b (%)c
d
Mn
PDI
d
Tm (oC)e
1
1
2
1.2
6.0
3.4
12500 1.49 124.1
126.1
2
1
1
1.2
6.0
2.1
12200 2.13 119.5
2900 2.64
121.9
3
1
1
0.2
1.0
1.6 19600 1.78 128.8
3.2
2300
3.56
121.1
4
1
1
0.2
1.0
0.7
4300 2.52 116.3
2.0
2000 3.33
121.0
f
1
1
0.1
0.5
0.8
2000 1.58 122.6
2.2
2.2
1500
3.12
120.0
100
4.0
4.0
3800 3.49
124.6
6
1
2
1.0
5.0
3.1
10400 2.61 126.4
100
2.9
2.9
2100
120.8
7
1
2
2.3
11.5
1.6
10400 2.50 125.9
Polymerization conditions: catalyst = 2 μmol; toluene = 48 8 b mL, CH2Cl2 = 2 mL, ethylene = 8 atm, time = 0.5 h. The 9 yields and activities are average of at least two runs. Activity 6 -1 -1 c 10 is in unit of 10 g·mol ·h . Determined by GPC in o d trichlorobenzene at 150 C. Determined by differential 11 scanning calorimetry (DSC). 12
1
2
2.3
11.5
2.4
3700 2.14 114.7
1
2
0.9
4.5
1.6
7900 2.76 119.1
1
1
1.1
5.5
1.8
4800 2.50 122.5
1
1
1.5
7.5
2
4500 2.76 118.7
1
3
0.8
4.0
7.6
3000 2.60 94.5
2
1
3.1
15.5
3
2800 2.87 118.0
3
1
2.7
13.5
1.9
1600 3.53 120.0
4
1
2.1
10.5
4.4
9800 3.54 118.7
16
5
1
1.5
7.5
3
4000 3.52 118.9
17
2
2
1.2
6.0
4.4
4700 2.38 117.4
18
3
2
1.5
7.5
2.5
3600 2.33 120.4
19
4
2
1.2
6.0
6
12000 2.68 115.5
20
5
2
0.8
4.0
3.4
4500 2.53 118.0
21
4
1
0.2
1.0
1.2
5400 2.06 118.5
22
4 CH3COOEt
1
1.6
8.0
-
10700 3.70 126.9
23
4
1
4.9
24.5
-
48000 1.80 134.9
3.28
5
a
consistent with comparisons made between catalysts 2/3 13 o and 1. Based on time-dependence studies at 80 C (Figure 14 S1), catalyst 1 became deactivated within 20 min, while 15 catalyst 4 maintained high activity for up to 1 h. Secondly, this is one of the very few nickel catalyst systems that can copolymerize ethylene with a variety of polar monomers (Table 2). Without the use of large amounts of cocatalyst or protecting reagents, very few single-component nickel catalysts are capable of initiating the efficient copolymerization of ethylene with polar monomers. In this system, methyl acrylate completely shuts down polymerization.4r Moderate polymerization activity was observed with butyl vinyl ether, and no comonomer incorporation was observed. High activity, good comonomer incorporation (0.7-7.6%), and high copolymer molecular weights (Mn up to 19600) were achieved during 1-catalyzed ethylene copolymerizations with silicon containing vinyl/allyl comonomers, allyl ether, allyl cyanide, polar functionalized norbornenes, and some OH/COOMe/Br/Cl containing, long chain, comonomers. Among these nickel catalysts, 4 gave the highest comonomer incorporation, and the highest copolymer molecular weight. Interestingly, catalysts 2 and 3, bearing electron donating substituents, showed higher activities than 1. This may originate from the lower electrophilicity of the nickel center in catalysts 2 and 3, compared to catalyst 1. Nozaki et al. previously showed that allyl chloride completely deactivated phosphine sulfonate nickel catalysts.11 In our system, small amounts of product were generated during 1-catalyzed ethylene copolymerization with allyl chloride. We previously showed that the addition of B(C6F5)3 to phosphine sulfonate nickel catalysts greatly increases their ethylene polymerization activity.10c Here, 1/B(C6F5)3 was shown to initiate efficient ethylene/allyl chloride copolymerization, with moderate activity, comonomer incorporation, and molecular weight. The ability of this catalyst system to incorporate the allyl cyanide comonomer (Table 2, entries 4 and 21) represents a big advantage, since SHOP-type catalysts are unable to
Et2O
a
Polymerization conditions: total volume of solvent and polar monomer = 20 mL, catalyst = 20 μmol, ethylene = 8 atm, 80 oC, time = 1 h. bThe yields and activities are average 4 -1 -1 of at least two runs. Activity is in unit of 10 g·mol ·h . c 1 o d Determined by H NMR in C2D2Cl4 at 120 C. Determined o e by GPC in trichlorobenzene at 150 C. Determined by DSC. f 100 μmol B(C6F5)3 was added. incorporate these types of comonomer.8 Interestingly, acetonitrile was shown to greatly inhibit polymerization in the phosphine sulfonate palladium system, even at very low acetonitrile loadings (0.05 M).14a In our system, this inhibition effect is less dramatic; 1.2 g of polyethylene was generated by catalyst 4 in the presence of 1 M of acetonitrile under the conditions shown in Table 2. Thirdly, very high molecular weights are able to be achieved in both ethylene polymerization and copolymerization reactions (Table 3). At 25 oC, semicrystalline polyethylenes, with high molecular weights (Mn up to 446,000), high melting temperatures (Tm up to 138.5 oC), and low branching densities (5/1000 carbon atoms for 3 and 2/1000 carbon atoms for 4) were generated. Similar to observations made under high temperatures, catalyst 4
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Table 3. Ethylene (co)polymerization at 25 oC.a Ent. Cat.
Mn Tm d 4 d PDI o e (10 ) ( C)
1
1
-
2.7
13.5
-
36.2
2
2
-
trace
--
-
--
3
3
-
0.7
3.5
-
6.2
2.42 134.5
4
4
-
2.6
13.0
-
44.6
1.96 138.5
5
5
-
4.3
21.5
-
26.4
2.42 137.5
6
1
0.4
2.0
0.35
8.9
2.14
7
1
1.3
6.5
0.35
15.0
1.85 130.0
8
1
1.0
5.0
0.46
14.6
1.85 126.4
9
3
1.2
6.0
0.30
5.9
2.02 132.9
10
4
2.0
10.0
0.6
17.9
2.12 128.7
11
5
3.3
16.5
0.7
12.4
2.39 130.6
f
3
-
0.9
4.5
-
7.5
1.96 136.2
f
5
-
4.4
22.0
-
29.0
1.90 136.5
f
3
1.1
5.5
0.5
5.5
2.32
133.1
f
5
3.8
19.0
0.9
13.7
2.27
131.2
12 13
14 15 a
Comonom Yield Act. XM b 4 b c (g) (10 ) (%) er
2.21
137.2
--
--
131.3
Polymerization conditions: total volume of solvent and polar monomer = 20 mL, polar monomer = 1 M, ethylene = 8 o atm, 25 C, catalyst = 20 μmol, time = 1 h (10 μmol and 2 h b for ethylene homopolymerization). The yields and activities 4 are average of at least two runs. Activity is in unit of 10 -1 -1 c 1 o g·mol ·h . Determined by H NMR in C2D2Cl4 at 120 C. d o Determined by GPC in trichlorobenzene at 150 C. e f Determined by DSC. 2 eq. Ni(COD)2 was added.
ethylene homopolymerization and copolymerization reactions (Table 3, entries 3, 5 vs 12, 13, entries 9, 11 vs 14, 15, o similar behavior was observed at 80 C, see Table S1). Probably, the strong trans influence of the phosphine moiety increases the lability of the PPh3 coordination, such that it no longer affects ethylene coordination or insertion under polymerization conditions. This is an important advantage of this system, since the PPh3 complex is more stable and much easier to synthesize, compared to the 2,6lutidine complex. Copolymerization always resulted in lower molecular weights because insertion at a tertiary carbon (resulting from the 2,1 insertion of the monomer into the metal-carbon bond) is always slower than insertion into a secondary carbon (product of ethylene insertion). In addition, chain transfer is more likely to occur when the insertion process is slower; the polymerization and copolymerization studies at o 25 C agree with this trend. It is quite surprising and unexpected that the opposite trend was observed for studies o at 80 C (Table 1 vs. Table 2). The activities of these nickel o catalysts are so high at 80 C that polymerization becomes o highly exothermic. Although the temperature of the 80 C oil bath remained unchanged, it is highly possible that the temperature of the polymerization solution itself may have 17 increased. The presence of polar monomers greatly reduced these catalytic activities by one to two orders of magnitude. This temperature difference may lead to different polymer molecular weights, considering the high sensitivity of molecular weight toward temperature observed in our system. The presence of 1 M of CH3COOEt or Et2O could lower the catalytic activities and increase the polymer molecular weights (Table 2, entries 22 and 23). To conclude, high-performance phosphine sulfonate nickel catalysts were designed and prepared by the installation of sterically bulky substituents at axial positions on the metal, ortho positions on the ligands as well as the installation of substituents with different electronic properties and labile coordinating bases. These nickel catalysts can initiate ethylene homopolymerization in the o 25-100 C temperature range, without the need for any 6 -1 -1 cocatalyst, and with activities of up to 4.5 × 10 g·mol ·h , providing Mn values of up to 446,000. A large number of polar monomers were copolymerized with ethylene with 5 -1 -1 high activities (up to 1.5 × 10 g·mol ·h ), good comonomer incorporations (up to 7.6%) and high molecular weights (Mn up to 179,000). This is a very rare example of a nickel-based catalyst system that possesses the combined properties of high activity and high thermal stability to produce polymers of high molecular weight, while displaying wide scope for polar monomer substrates. This also represents an excellent demonstration of the capacity of rational ligand design for the improvement of the performance of olefin polymerization catalysts.
afforded the highest-molecular-weight polymers. This can be explained by the electron-withdrawing feature of the C6F5 substituent and is consistent with the fact that catalysts 2 and 3, bearing electron-donating groups, gave lowermolecular-weight polyethylene than catalyst 1. In addition, the control over the orientation of the -SO2- moiety in relation to the ortho substituent may also contribute to this 9d,9m phenomenon. At low temperature, the comonomer incorporation ratio was reduced by factors of 4-7. Probably, the ethylene concentration is significantly higher at lower temperatures, resulting in a lower fraction of comonomer in solution and thus copolymer. However, the copolymer molecular weights were improved by up to 37 times. In all cases, semicrystalline copolymers, with high molecular weights (Mn up to 179,000) and high melting temperatures o (Tm up to 132.9 C), were generated. In some nickel catalysts containing coordinating phosphine bases (such as triphenylphosphine in catalyst III), a phosphine scavenger, such as Ni(COD)2, is usually required because of the difficulties in activating the catalyst 6 precursor. In this system, the presence or absence of AUTHOR INFORMATION Ni(COD)2 had minimal effect on the catalytic activities of 3 Corresponding Author or 5, or the polymer molecular weights obtained, in both
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[email protected] Notes The authors declare no competing financial interests.
ASSOCIATED CONTENT Supporting Information Synthesis and characterization of the ligands and metal complexes, X-ray diffraction information, polymerization data and polymer characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
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