Ligand-Unsymmetrical Phenoxyiminato Dinickel Catalyst for High

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Ligand-Unsymmetrical Phenoxyiminato Dinickel Catalyst for High Molecular Weight Long-Chain Branched Polyethylenes Dongxu Shu,† Aidan R. Mouat,† Casey J. Stephenson, Anna M. Invergo, Massimiliano Delferro,* and Tobin J. Marks* Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: An unsymmetrical bimetallic catalyst, (CF3/ SO2)FI2-Ni2, having a CF3-functionalized phenoxyiminato Ni(II) center, which produces linear high-Mw polyolefin (CF3/Ni) joined to an adjacent SO2-functionalized phenoxyiminato Ni(II) center, which produces highly branched lowMw polyolefin (SO2/Ni), was synthesized and fully characterized. In ethylene homopolymerizations, (CF3/SO2)FI2-Ni2 affords monomodal (Đ = 1.7), highly, long-chain branched (69 branches/1000 C) polyethylenes with Mw = 24 kg/mol. In contrast, bimetallic (CF3)FI2-Ni2 and (SO2)FI2-Ni2 produce high-Mw (25 kg/mol) exclusively methyl-branched (40 branches/1000 C) and low-Mw (4.5 kg/mol) highly branched (105 branches/1000 C) polyethylenes, respectively, while tandem monometallic (CF3)FI-Ni + (SO2)FI-Ni catalyst mixtures yield a bimodal polyolefin mixture (Đ = 6.4). fforts to simulate multimetallic enzyme function, where proximate metal centers play essential roles in achieving superior reactivity and selectivity,1 have led to a blossoming of abiotic multimetallic cooperative catalysis in areas such as epoxidation,2 olefin hydrogenation and hydroformylation,3 cycloaddition,4 and coordinative polymerization.5 In homobimetallic-mediated olefin polymerization, cooperative effects between adjacent catalytic centers are known to significantly enhance product molecular weight, chain branch density, and comonomer enchainment selectivity compared to analogous monometallic catalysts.6,7 Heterobimetallic polymerization catalysts are far less explored and offer new possibilities for accessing novel polymer microstructures, branch types, and branch densities (Chart 1).8 For example, in a series of bimetallic [Ti−Cr] complexes with varying length linkers, n, those having close Ti···Cr proximity exhibit marked and unusual enchainment cooperativity between the catalytic centers in ethylene polymerizations (Chart 1).9 This includes high selectivity for n-butyl branches in conversion-insensitive densities, implying that ethylene trimerization to 1-hexene at the Cr center is closely coupled to copolymerization with ethylene at the Ti center. In another example, CGC2-TiZr-catalyzed ethylene homopolymerization (Chart 1)10 yields high-Mw, monomodal (Đ), long-chain branched (~2.1 LCB/1000 C)11 polyethylenes, with vinyleneterminated oligomers generated at Zr co-enchained with ethylene at Ti. In contrast, a tandem mixture of the corresponding monometallic Ti and Zr catalysts affords a bimodal mixture of high-Mw linear polyethylenes (from Ti) and low-Mw branched polyethylenes (from Zr). While the above

E

© XXXX American Chemical Society

Chart 1. Examples of Heterobimetallic Polymerization Catalysts

heterobimetallic catalysts show good efficiencies in producing linear low density polyethylenes and long-chain branched polyethylenes, respectively, versus the analogous tandem monometallic mixtures, we envisioned an alternative approach. What would be the properties of a homobimetallic catalyst having very different ligand-originated polymerization properties at the two metal centers? Received: November 2, 2015 Accepted: November 4, 2015

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DOI: 10.1021/acsmacrolett.5b00781 ACS Macro Lett. 2015, 4, 1297−1301

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ACS Macro Letters

NaH followed by metalation with trans-NiClMe(PMe3)2 (Scheme 1). Diffraction-quality crystals were grown by slowly

The selective introduction of long-chain branches into polyolefins is of both fundamental and technological research interest since these branches can greatly enhance polymer properties.12 In single-site coordinative polymerizations, longchain branches are typically created via capture and reenchainment of vinyl-terminated polyethylenes that are produced in the polymerization process.13 Hyperbranched polyethylenes (HBPEs) feature high densities of branch types and, in particular, have branch-on-branch formation, which results in many interesting polymer properties. In addition, HBPEs14 can be synthesized by Ni- or Pd-diamine catalysts via chain walking polymerization,15 having product Mws in the 1− 150 kg/mol range and branch densities higher than 110/ 1000 C. Here we report the synthesis and unprecedented cooperative polymerization properties of a ligand-unsymmetrical homobimetallic catalyst (CF3/SO2)FI2-Ni2, which joins in close proximity a Ni olefin polymerization center producing linear high-Mw product,16 and a Ni polymerization center producing highly branched low-Mw product (Chart 2).17,18 It will be seen

Scheme 1. Synthesis and Molecular Structures of Unsymmetrical and Symmetrical Homobimetallic Polymerization Catalysts (CF3/SO2)FI2-Ni2 and (SO2)2FI2Ni2, Respectively

Chart 2. Polymerization Characteristics of Monometallic, Symmetrical Homobimetallic, and Unsymmetrical Homobimetallic Ni Polymerization Catalysts

cooling saturated n-hexane solution. The two Ni centers exhibit significant differences in displacement from the aromatic skeleton mean plane, with the SO2-Ni significantly more displaced than CF3-Ni. In this configuration, the two Ni centers are displaced in opposite directions from the mean plane of the ligand backbone and feature cis-Me groups and trans-PMe3 ligands relative to the phenoxyiminato N atom, as in previous FI2-Ni2 bimetallics.6b In contrast to monometallic (SO2)FINi,18a no π-interaction is observed between the Ni center and the sulfone ligand phenyl moiety, likely due to the sterically encumbered para-t-butyl group. Catalyst (CF3/SO2)FI2-Ni2 was fully characterized by standard spectroscopic and analytical methods. Two geometrically distinct Ni-Me and Ni-PMe3 groups are evidenced by two sets of 1H, 13C, and 31P NMR signals, whereas homobimetallic (CF3)2FI2-Ni2 shows only single peaks for equivalent groups. For comparisons, (SO2)2FI2-Ni2 was also synthesized, following the same sequence and was fully characterized by spectroscopic and analytical methods, and by single-crystal XRD (Scheme 1). Ethylene polymerizations were carried out in toluene using Ni(COD)2 or B(C6F5)3 as phosphine scavengers at constant 8 atm ethylene, with care to minimize exotherm and masstransfer effects. Since Ni(COD)2 is a known “non-innocent” cocatalyst and produces linear high-Mw polyethylenes in the presence of free HFI ligand liberated during polymerization,6b B(C6F5)3 was first investigated as the cocatalyst. Negligible polymer is observed with (SO2)FI-Ni, while (CF3)FI-Ni produces near-linear polyethylenes with Mw = 138 kg/mol at 130 kg PE mol [Ni]−1 atm−1 h−1 (Table 1, entries 1, 2). Note that reaction of (SO2)FI-Ni with 1 or 2 equiv B(C6F5)3 yields unidentified paramagnetic species in which the borane likely coordinates the sulfonyl group (Figure S20).20 This intermediate rapidly decomposes via reductive elimination21 to Ni(0), possibly reflecting electron density transfer to Ni via S O···B(C6F5)3 binding.22 Thus, previously optimized conditions (5 min, constant 8 atm ethylene, 25 °C) for (SO2)FI-Ni and (CF3)FI-Ni with Ni(COD)2 as the cocatalyst were used. Here,

that (CF3/SO2)FI2-Ni2 affords monomodal, highly branched, high-Mw polyethylenes with significant long-chain branching using ethylene as the only feed, whereas the related tandem monometallic (CF3)FI-Ni16b + (SO2)FI-Ni18a catalyst mixture produces a bimodal product under the same polymerization conditions. Furthermore, analogous monometallic (CF3)FI-Ni and bimetallic (CF3)FI2-Ni26b produce more sparsely branched linear polyethylenes having only methyl branches. The syntheses of ligands H2(CF3/SO2)FI2 and H2(SO2)2FI2 are depicted in Scheme S1 of the SI. The reagent 2,7-diformyl1,8-dihydroxynaphthalene is first condensed with 2,6-(3,5ditrifluoromethylphenyl)aniline, followed by reaction with 2((4-(tbutyl)phenyl-sulfonyl)aniline to yield ligand H2(CF3/ SO2)FI2. Note that the imine condensation sequence is crucial for successful synthesis. H2(CF3/SO2)FI2 was fully characterized by spectroscopic and analytical methods, as well as by single-crystal XRD (see SI). Similar to previous phenoxyiminato bimetallic ligands,6b−d H2(CF3/SO2)FI2 undergoes ketoamine/enol-imine tautomerization.19 (CF3/SO2)FI2-Ni2 is obtained in high yield by deprotonation of the ligand with 1298

DOI: 10.1021/acsmacrolett.5b00781 ACS Macro Lett. 2015, 4, 1297−1301

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Table 1. Ethylene Polymerization Data for Catalysts (CF3/SO2)FI2-Ni2, (CF3)FI-Ni, (SO2)FI-Ni, (CF3)FI-Ni + (SO2)FI-Ni, (CF3)2FI2-Ni2, and (SO2)2FI2-Ni2α % branch typee entry

catalyst

cocat.

time (min)

PE (g)

act.b

TOFc (h−1)

ρbrd

Me

Hex+

other

Mwf (kg/mol)

Đf

1 2 3 4 5 6 7

(SO2)FI-Ni (CF3)FI-Ni (SO2)FI-Ni (CF3)FI-Ni (SO2)2FI2-Ni2 (CF3)2FI2-Ni2 (CF3)FI-Ni + (SO2)FI-Ni

B(C6F5)3 B(C6F5)3 Ni(COD)2 Ni(COD)2 Ni(COD)2 Ni(COD)2 Ni(COD)2

40 40 5 5 5 5 5

6.90 0.40 2.23 0.07 0.32 1.83

130 60 335 5 24 273

62727 1488 81888 110 2000 9607

9 140 7 105 40 39

100 45 100 38 100 79

0 19 0 27 0 8

0 36 0 35 0 13

138 5 92 4.5 25 56

2.3 1.6 3.0 1.7 2.4 6.4

8 9 10

(CF3/SO2)FI2-Ni2 (CF3/SO2)FI2-Ni2 (CF3/SO2)FI2-Ni2

Ni(COD)2 Ni(COD)2 Ni(COD)2

5 10 40

0.27 0.50 0.89

20 19 8

2289 2256 1672

69 57 41

58 62 60

16 14 16

26 24 24

24 30 117

1.7 2.0 4.7

Tmg 122.2 119.0 115.8 103.4, 114.1 121.6 124.4 128.3

α Polymerization in 25 mL toluene with 10 μmol catalyst at constant 8.0 atm ethylene pressure using 2 equiv Ni(COD)2/Ni or 1 equiv of B(C6F5)3/ Ni as phosphine scavenger/cocatalyst at 25 °C. All entries performed in duplicate. bUnits: kg PE [Ni]−1 atm−1 h−1. cEthylene turnover frequency (TOF) calculated from the polymer mass. dBranch density (branches/1000 C atoms) determined by 1H NMR. eAnalysis by 13C NMR.28 fGPC vs polystyrene standards. gMelting temperature determined by DSC.

Mw = 25 kg/mol and activity = 24 kg PE mol [Ni]−1 atm−1 h−1. With the aforementioned monometallic and symmetrical bimetallic catalyst data in hand, unsymmetrical (CF3/SO2) FI2-Ni2 was next investigated. Under the above conditions, this catalyst produces monomodal (Đ = 1.7), highly branched polyethylenes with activity = 20 kg PE mol [Ni]−1 atm−1 h−1 and Mw = 24 kg/mol (Table 1, entry 8), similar to polyethylenes obtained by some Pd catalysts.26 Furthermore, only traces of “free” low-Mw oligomers/highly branched polyethylene is found in solution, indicating efficient intramolecular polymeryl transfer (vide infra).9 In general, bimetallic catalysts exhibit lower activities/TOFs than their mononuclear counterparts, likely reflecting steric and electronic constraints.27 1 H NMR and 13C NMR spectra of the (CF3/SO2)FI2-Ni2derived polyethylenes reveal 69 branches/1000 C, including methyl, ethyl, n-propyl, n-butyl, sec-butyl, and C≥6 (16% by 13C NMR analysis) branches (Figure 2).28 The formation of secbutyl groups indicates branch-on-branch formation (“chainwalking”),29while the significant percentage of methyl branches

mononuclear (SO2)FI-Ni produces hyperbranched polyethylenes (140 branches/1000 C; Table 1, entry 3) with Mw = 5 kg/ mol, Đ consistent with single-site behavior, and with activity = 60 kg PE mol [Ni]−1 atm−1 h−1.18a Control experiments and DFT analysis indicate that the proximate coordinating SO2 group is responsible for the hyperbranched polyethylenes.18a In contrast, monometallic (CF3)FI-Ni yields sparsely branched monomodal polyethylenes (7 branches/1000 C, Đ = 3, Table 1, entry 4) with Mw = 92 kg/mol and activity = 335 kg PE mol [Ni]−1 atm−1 h−1. The low branch density is likely due to weak (ligand)C−F···H−C(polymer)23 interactions, which slow chain transfer via β-H elimination.16b Furthermore, a (CF3)FI-Ni + (SO2)FI-Ni tandem catalyst yields a bimodal mixture (Đ = 6.4, Figure 1) of polyethylenes with overall activity = 273 kg PE mol

Figure 1. GPC traces (viscometric detection) of polyethylenes derived from (CF3/SO2)FI2-Ni2 (red line) and a (CF3)FI-Ni + (SO2)FI-Ni tandem catalyst (blue dot line).

[Ni]−1 atm−1 h−1 and Mw = 56 kg/mol (Table 1, entry 7),24 indicating that the two metal centers work independently, without significant cooperativity.25 Regarding bimetallic catalysts, under the above polymerization conditions, (SO2)2FI2-Ni2 produces highly branched polyethylenes (105 branches per 1000 C; Table 1, entry 5) with Mw = 4.5 kg/mol and activity = 5.3 kg PE mol [Ni]−1 atm−1 h−1, while (CF3)2FI2-Ni affords branched monomodal polyethylenes (40 branches per 1000 carbon, Table 1, entry 6) with

Figure 2. 13C NMR Spectra (125 MHz, C2D2Cl4, 120 °C) of PEs produced by (SO2)FI−Ni, (CF3)FI-Ni, and (CF3/SO2)FI2-Ni2 (Table 1, entries 3, 4, and 8, respectively), scaled to the PE CH2 backbone resonance at δ 30 ppm. 1299

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Scheme 2. Proposed Mechanism for Bimetallic Cooperation between Dissimilar Ni Centers To Produce Polyethylenes with Long-Chain Branches

(59%, by 13C NMR analysis) suggests contributions from both the SO2-Ni and CF3-Ni catalytic centers (45% from (SO2)FINi, by 13C NMR analysis). These results argue that vinylic methyl-branched macromonomer formation at the CF3-Ni site occurs in close concert with capture and coenchainment + ethylene homopolymerization at the SO2-Ni site (vide infra). End group analysis reveals that the unsaturation is consistent with a β-H elimination chain transfer process30 and double bond migration (Figure S21).31 To assess the “tightness” of methyl-branched polyethylene transfer from the CF3-Ni center to the SO2-Ni center in (CF3/ SO2)FI2-Ni2, competing 0.10 M 1-pentene was added to the polymerization medium (see SI, Table S1). It is found that mononuclear (SO2)FI-Ni and binuclear (SO2)2FI2-Ni2 incorporate exogenous 1-pentene into the hyperbranched polyethylene backbone at levels of 2.8/ and 1.6/1000 C, respectively, whereas negligible amounts of C5 α-olefin are enchained by mononuclear (CF3)FI-Ni, binuclear (CF3)2FI2Ni2, or tandem (CF3)FI−Ni + (SO2)FI-Ni. In marked contrast, under identical reaction conditions (CF3/SO2)FI2-Ni2 produces monomodal PE with 1.5 n-propyl branches/1000 C, consistent with 1-pentene incorporation into the hyperbranched polyethylene produced at the Ni-SO2 center, followed by intramolecular coenchainment with ethylene and the methyl-branched macromonomer produced by the CF3-Ni site. Note that, in ethylene homopolymerization, (CF3/ SO2)FI2-Ni2 introduces only 0.7 n-propyl branches/1000 C, confirming 1-pentene incorporation in the above experiments. Increasing the (CF3/SO2)FI2-Ni2 ethylene homopolymerization time from 5 to 40 min (Table 1, entries 8−10) evidence the gradual production of bimodal polyethylenes having a small high-Mw content (Figure S1), consistent with the previous findings6b that Ni(COD)2 + free ligand ultimately generates a new catalyst responsible for the high-Mw polyethylene fraction. Ignoring this side reaction, a tentative scenario to accommodate the present observations is shown in Scheme 2. The (CF3)FI ligand fragment rapidly produces methyl-branched macromonomers via β-H elimination/reinsertion. The macromonomers having vinyl end-groups are then efficiently, intra-

molecularly enchained at the Ni coordinated by the (SO2)FI ligand fragment,32 which produces highlybranched polymers, due to non-negligible SO2···Ni interaction.18a This process accounts for the observation of monomodal polyethylenes with long-chain branches. The cooperative effects (elimination of macromolecules/intramolecular reinsertion) observed in homobimetallic (CF3/SO2)FI2-Ni2 resemble those found in [Ti− Cr] heterobimetallic catalyst,9 even if the two systems are completely different and give rise to dissimilar polymers. In conclusion, the synthesis, characterization, and polymerization properties of the ligand-unsymmetrical homobimetallic (CF3/SO2)FI2-Ni2 catalyst are reported. The catalyst produces polyethylenes with long-chain branches, high-Mw, and narrow polydispersity via cooperation between mechanistically very dissimilar Ni centers using ethylene as the only feed. In comparison, the tandem monometallic (CF3)FI-Ni + (SO2)FINi catalyst system produces bimodal mixtures, while the analogous monometallic (CF3)FI-Ni and bimetallic (CF3)2FI2Ni2 yield polyethylenes with only methyl branches. Furthermore, (SO2)2FI2-Ni2 affords highlybranched (105 branches/ 1000 C) low-Mw (4.5 kg/mol) polyethylenes. These results support a cooperative mechanism involving the two proximate Ni centers. The effectiveness of this ligand-asymmetric homobimetallic strategy suggests new opportunities for creating novel catalysts and polymers with unusual microstructures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00781. Experimental details and additional data (PDF). CIF data (CIF). CIF data (CIF). CIF data (CIF). CIF data (CIF). CIF data (CIF). 1300

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

These authors contributed equally (D.S. and A.R.M.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by DOE-BES Grant 86ER13511 is gratefully acknowledged. Use of NMR facilities at the IMSERC center of Northwestern University was supported by NSF under Grant CHE-1048773. We also thank Boulder Scientific Company for a generous gift of B(C6F5)3.

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DOI: 10.1021/acsmacrolett.5b00781 ACS Macro Lett. 2015, 4, 1297−1301