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Article

Copolymerization of Ethylene and Vinyl Fluoride by Self-Assembled Multinuclear Palladium Catalysts

by
Qian Liu
and
Richard F. Jordan
*
Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA
*
Author to whom correspondence should be addressed.
Polymers 2020, 12(7), 1609; https://doi.org/10.3390/polym12071609
Submission received: 10 June 2020 / Revised: 8 July 2020 / Accepted: 14 July 2020 / Published: 19 July 2020
(This article belongs to the Special Issue Coordination Catalysis in Additive Polymerization)
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Abstract

:
The self-assembled multinuclear PdII complexes {(Li-OPOOMe2)PdMe(4-5-nonyl-pyridine)}4Li2Cl2 (C, Li-OPOOMe2 = PPh(2-SO3Li-4,5-(OMe)2-Ph)(2-SO3−-4,5-(OMe)2-Me-Ph)), {(Zn-OP-P-SO)PdMe(L)}4 (D, L = pyridine or 4-tBu-pyridine, [OP-P-SO]3− = P(4-tBu-Ph)(2-PO32−-5-Me-Ph)(2-SO3−-5-Me-Ph)), and {(Zn-OP-P-SO)PdMe(pyridine)}3 (E) copolymerize ethylene and vinyl fluoride (VF) to linear copolymers. VF is incorporated at levels of 0.1–2.5 mol% primarily as in-chain -CH2CHFCH2- units. The molecular weight distributions of the copolymers produced by D and E are generally narrower than for catalyst C, which suggests that the Zn-phosphonate cores of D and E are more stable than the Li-sulfonate-chloride core of C under copolymerization conditions. The ethylene/VF copolymerization activities of C–E are over 100 times lower and the copolymer molecular weights (MWs) are reduced compared to the results for ethylene homopolymerization by these catalysts.

Graphical Abstract

1. Introduction

The coordination–insertion copolymerization of ethylene with polar vinyl monomers by PdII catalysts has been extensively studied [1,2,3,4,5,6,7,8]. Vinyl halides are particularly challenging polar comonomers due to (i) their poor competition with ethylene for binding to PdII catalysts [9,10], (ii) the formation of inactive LnPd-X complexes by β-X elimination of LnPdCH2CXR species (generated by 1,2 CH2=CHX insertion or 2,1- CH2=CHX insertion followed by chain walking) [11,12,13,14,15,16,17,18,19,20], and (iii) the low insertion reactivity of LnPdCHXCH2R species formed by 2,1 CH2=CHX insertion [16,21]. We previously reported that (PO)PdMe(L) catalysts (A, Figure 1) that contain phosphine-arenesulfonate ligands (PO−) copolymerize ethylene and vinyl fluoride (VF) to linear copolymers with up to 0.55 mol% VF incorporation [22,23,24,25]. The catalyst activities are significantly reduced and the polymer molecular weights (MWs) are also reduced in ethylene/VF copolymerization compared to ethylene homopolymerization under the same conditions. For example, {P(2-Et-Ph)2(2-SO3-Ph)}PdMe(py) exhibits an activity of 296 kg∙mol−1∙h−1 in ethylene homopolymerization and produces polyethylene (PE) with Mn = 16,560 Da (300 psi ethylene, toluene, 80 °C), while in ethylene/VF copolymerization the activity and polymer Mn are reduced to 5 kg∙mol−1∙h−1 and 6800 Da, respectively (300 psi total pressure, VF/ethylene = 4/1, toluene, 80 °C) [22]. The ethylene/VF copolymers produced by (PO)PdMe(L) catalysts contain internal -CH2CHFCH2- units formed by 1,2 and/or 2,1 VF insertion into growing (PO)PdR species. The copolymers also contain -CH2CHFCH3, -CH2CHF2, and -CH2CH2F chain ends. The -CH2CHFCH3 units are formed by 2,1 VF insertion into (PO)PdH species followed by chain growth. It was proposed that the -CH2CHF2 and -CH2CH2F groups are generated by VF or ethylene insertion of (PO)PdF species (formed by β-F elimination of (PO)Pd(CH2CHFR) species), followed by chain growth. Strong support for this proposal was provided by the demonstration that (POBpOMe)PdF(2,6-lutidine) ([POBp,OMe]− = [P(2′,6′-(OMe)2-2-biphenyl)(2-OMe-Ph)(2-SO3-5-Me-Ph)]-) reacts with VF to yield the 1,2-insertion product (POBp,OMe)Pd(CH2CHF2)(2,6-lutidine) and with ethylene to yield PE with -CH2CH2F end groups [23]. It was also demonstrated that (POBp,OMe)Pd(CH2CHF2)(2,6-lutidine) reacts with ethylene to produce PE with -CH2CHF2 chain ends. The ability of (PO)PdF species to undergo olefin insertions is key to the ability of (PO)PdRL species to catalyze ethylene/VF copolymerization. The copolymerization of ethylene with other CH2=CHX monomers by (PO)PdRL catalysts has been extensively studied [26,27,28,29,30,31,32,33,34].
The related (Li-OPO)PdMe(py’) complex (py’ = 4-5-nonyl-pyridine) based on the phosphine-bis(arensulfonate) ligand PPh(2-SO3Li-5-Me-Ph)(2-SO3−-5-Me-Ph) (Li[OPO]−) self-assembles into the tetrameric {(Li-OPO)PdMe(py’)}4 species B, which is held together by a Li4S4O12 double 4-ring (D4R) cage (Figure 1) [35,36,37]. “Pd4 cage” catalyst B produces linear PE with a high MW and linear ethylene/VF copolymers that contain up to 3.6 mol% VF (Table 1, entries 8,9). The ethylene/VF copolymers produced by B contain internal -CH2CHFCH2- units as well as -CH2CHFCH3 and -CH=CHF end groups, but no NMR-detectable -CH2CF2H or -CH2CH2F chain ends. The -CH=CHF chain ends are most likely formed by 2,1 VF insertion into growing (Li-OPO)Pd-R species, followed by β-H elimination. However, B undergoes partial disassembly to the monomeric (Li-OPO)PdMe(py’) “Pd1” species under polymerization conditions, which strongly influences the MWs and molecular weight distributions (MWDs) of the polyethylene and ethylene/VF copolymers it produces. For example, B produces a high-MW ethylene/VF copolymer with a broad bimodal MWD (Mw = 494 kDa, PDI = 310; Table 1, entry 9) in hexanes suspension, due to competing copolymerization by intact B (which generates the high-MW fraction) and monomeric (Li-OPO)PdR species (which generates the low-MW fraction). In contrast, B produces a low-MW copolymer with a narrow MWD (Mw = 4200, PDI = 2.4; entry 8) in toluene solution, indicative of nearly complete dissociation to monomeric species.
We recently reported several new multinuclear Pd “cage” catalysts (C‒E) [39,40]. The sterically-expanded phosphine-bis(arensulfonate) ligand PPh(2-SO3Li-4,5-(OMe)2-Ph)(2-SO3−-4,5-(OMe)2-Me-Ph) ([Li-OPOOMe2]−) directs the self-assembly of the tetranuclear complex (Li-OPOOMe2)PdMe(py’)}4Li2Cl2 (C, Figure 1), in which four (Li-OPOOMe2)PdMe(py’) units are arranged around the periphery of a Li4S4O12•Li2Cl2 cage [39]. Compound C is much less susceptible to cage dissociation than B. For example, C undergoes only 6.5% dissociation to monomeric species in CDCl2CDCl2 solution at 80 °C ([Pd4]initial = 4.7 mM), whereas B undergoes 38% dissociation under these conditions. In toluene, C is only partially dissociated into monomeric species and therefore produces PE with broad MWD as expected for a multi-site catalyst. In contrast, in hexanes suspension at 80 °C, C is resistant to disassembly and exhibits nearly ideal single-site behavior in ethylene homopolymerization and produces high-MW PE (Mw = 1473 kDa, PDI = 2.3, Figure 2). The solvent effects on the MWDs of the PEs produced by B and C may appear to be opposite but simply reflect the relative stabilities of B and C toward disassembly to monomeric species.
The phosphine-arenephosphonate-arenesulfonate ligand P(4-tBu-Ph)(2-PO32−-5-Me-Ph)(2-SO3−-5-Me-Ph) ([OP-P-SO]3−) directs the self-assembly of the tetrameric complex {(Zn-OP-P-SO)PdMe(L)}4 (D, Figure 1, L = py or 4-tBu-py), which is a direct structural analogue of B [40]. The Zn-phosphonate interactions in the central D4R Zn4P4O12 cage of D are stronger than the Li-sulfonate and Li-chloride interactions in the cores of B and C and, as a result, D is more thermally stable, exhibiting no NMR-detectable dissociation to monomeric species at 80 °C in CDCl2CDCl2 solution. However, D (L = 4-tBu-py) is isolated as a mixture of the major diastereomer shown in Figure 1 and ca. 20% of minor diastereomers that differ in the relative stereochemical configurations at the phosphorus centers. In the presence of 1 equiv B(C6F5)3 per 4-tBu-py, D produces PE with a high MW but a broad MWD (Mw = 436 kDa, PDI = 15; toluene/chlorobenzene solvent, 80 °C, 410 psi ethylene), consistent with the expected multi-site catalysis.
D (L = py) is converted to trimeric complex {(Zn-OP-P-SO)PdMe(py)}3 (E, Figure 1) in the presence of methanol. E adopts a cage structure composed of Zn3P3O6 and Pd3O3 rings linked through bridging (aryl)PO32− groups and is thermally stable at 80 °C in CDCl2CDCl2 solution. In the presence of 1 equiv of B(C6F5)3 per pyridine, E produces high-MW linear PE with a narrow MWD indicative of single-site catalysis (Mw = 691 kDa, PDI = 2.7; toluene/chlorobenzene solvent, 80 °C, 410 psi ethylene).
The availability of these new, more stable multinuclear Pd assemblies provides an opportunity to probe the reactivity of Pd cage catalysts while reducing the complications from thermal cage disassembly. In this paper we report the ethylene/VF copolymerization behavior of C–E.

2. Results and Discussion

Ethylene/VF copolymerizations were carried out using C as the catalyst at 80 °C in toluene or hexanes (Scheme 1), and the results are shown in Table 1. Experimental details and copolymer characterization data are provided in the Supplementary Materials. In toluene, C produces low-MW copolymers with ≤0.25 mol% VF incorporation (entries 1,2). In contrast, in hexanes slurry, C generates a high-MW copolymer with 0.87 mol% VF incorporation at a VF/ethylene feed ratio of 0.36 (entry 3), which is increased to 2.5 mol% at a VF/ethylene feed ratio of 0.92 (entry 4). The microstructure of the copolymer produced by C is similar to that from B (Table 2, Figure 3), with major in-chain -CH2CHFCH2- units and minor VF-derived -CH2CFHCH3 and cis-CH=CHF end groups [41,42,43,44,45,46,47].
The ethylene/VF copolymers produced by C in hexanes exhibit broad MWDs (Table 1, entries 3,4; Figure 2). Given the nearly ideal single-site behavior observed in ethylene homopolymerization by C in hexanes, it is unlikely that the broadening of the MWD of the ethylene/VF copolymers formed in this solvent is due to thermal disassembly of the cage structure (Figure 2). One possibility is that nucleophilic Pd-F species generated by 1,2-VF insertion and β-F elimination react with the Li+ ions in the central cage, leading to the formation of new active Pd species [23,25,48,49,50,51,52]. Consistent with this proposal, -CH2CH2F and –CH2CHF2 end groups, which are formed by ethylene and VF insertion of Pd-F species in copolymerization by mononuclear catalysts A, are not observed in the copolymer produced by C (Figure 3) [23]. This process provides a potential catalyst deactivation pathway.
Ethylene/VF copolymerization by D (L = 4-tBu-py) was investigated in a mixed toluene/chlorobenzene (49/1) solvent at a VF/ethylene feed ratio of 0.92 (Table 1, entry 5). D decomposes in the presence of free 4-tBu-py (and other Lewis bases). Therefore, 1 equiv B(C6F5)3 per 4-tBu-py was added to a solution of D in chlorobenzene prior to dilution with toluene to sequester the 4-tBu-py that will be displaced by monomer, in order to minimize catalyst deactivation. Under these conditions, D produces a linear copolymer with 0.96 mol% VF incorporation. The MWD of the copolymer formed by D is unimodal but somewhat broadened (PDI = 4.1), which can be attributed to the presence of several diastereomeric forms of the catalyst. The copolymer contains in-chain -CH2CFHCH2- and a small amount of VF-derived -CH2CFHCH3 chain ends. Catalyst E behaves similarly to D, incorporating 1.1 mol% VF in the form of both -CH2CFHCH2- (major) and -CH2CFHCH3 (minor) units (Table 1, entry 7). The MWDs of the copolymers formed by E are narrow (PDI ≤ 2.4), indicative of nearly ideal single-site catalysis. These results suggest that the multinuclear structures of D and E are substantially retained during ethylene/VF copolymerization.
The ethylene/VF copolymerization activities of C–E are over 100 times lower than the ethylene homopolymerization activities, and the copolymer MWs are reduced compared to the results for ethylene homopolymerization, as observed previously for A and B and mononuclear (PO)PdRL catalysts.

3. Conclusions

“Pd cage” catalysts C‒E copolymerize ethylene with VF to linear copolymers with 0.1 to 2.5 mol% VF incorporation depending on the catalyst and reaction conditions. VF is incorporated predominantly as in-chain -CH2CHFCH2- units, with minor -CH2CFHCH3 and, for C, cis-CH=CHF end groups. C produces an ethylene/VF copolymer with a broad MWD in hexanes, which contrasts results for ethylene homopolymerization where nearly ideal single-site behavior is observed. These results suggest that the presence of VF promotes structural disruption of the catalyst, possibly through reaction of Pd-F species with the Li+ ions in the cage. The ethylene/VF copolymerization behavior of C is very similar to that of B, except the extent of VF incorporation by C is somewhat lower than by B. D and E produce ethylene/VF copolymers with narrow MWDs, suggesting that the cage assemblies remain substantially intact during copolymerization. The activities of C–E in ethylene/VF copolymerization are >100 times lower than in ethylene homopolymerization. This inhibition effect is a major roadblock to the development of more practical ethylene/VF copolymerization catalysts.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4360/12/7/1609/s1.

Author Contributions

Conceptualization: R.F.J.; funding acquisition: R.F.J.; investigation: Q.L.; supervision: R.F.J.; writing—original draft: Q.L.; writing—review and editing: R.F.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the US National Science Foundation under Grant CHE-1709159.

Conflicts of Interest

The authors declare no conflict of interest.

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Polymers 12 01609 g001 550
Figure 1. (PO)PdR Complexes. Py’ = 4-(5-nonyl)pyridine. The lower (Li-OPO)PdMe(py’) or (OP-P-SO)PdMeL units in the schematic structure of B, C, and D are denoted by “Pd”.
Figure 1. (PO)PdR Complexes. Py’ = 4-(5-nonyl)pyridine. The lower (Li-OPO)PdMe(py’) or (OP-P-SO)PdMeL units in the schematic structure of B, C, and D are denoted by “Pd”.
Polymers 12 01609 g001
Polymers 12 01609 g002 550
Figure 2. Molecular weight distributions of polyethylene (PE) [39] and ethylene/VF copolymer (Table 1, entry 4) generated by C determined by high temperature GPC. Polymerization conditions: hexanes solvent, 80 °C.
Figure 2. Molecular weight distributions of polyethylene (PE) [39] and ethylene/VF copolymer (Table 1, entry 4) generated by C determined by high temperature GPC. Polymerization conditions: hexanes solvent, 80 °C.
Polymers 12 01609 g002
Polymers 12 01609 sch001 550
Scheme 1. Ethylene/VF Copolymerization by Catalyst B.
Scheme 1. Ethylene/VF Copolymerization by Catalyst B.
Polymers 12 01609 sch001
Polymers 12 01609 g003 550
Figure 3. 19F{1H} NMR spectrum of ethylene/VF copolymer (o-dichlorobenzene-d4, 120 °C) produced by C (Table 1, entry 4).
Figure 3. 19F{1H} NMR spectrum of ethylene/VF copolymer (o-dichlorobenzene-d4, 120 °C) produced by C (Table 1, entry 4).
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Table
Table 1. Ethylene/vinyl-fluoride copolymerization by catalysts C, D, and E [38].
Table 1. Ethylene/vinyl-fluoride copolymerization by catalysts C, D, and E [38].
EntryCat.SolventPC2H4
(psi)		PVF
(psi)		Acitivity d
(kg∙mol−1∙h−1)		Mw e
(103)		PDI eVF Incorp f
(mol%)		Tm g
(°C)
1 aCtoluene220801.420.38.00.10128.8
2 aCtoluene1301200.441.921.40.25ND h
3 aChexanes2208012.049818.20.87134.4
4 aChexanes1301203.441926.22.5132.8
5 a,bDtoluene + PhCl1301202.042.24.10.96131.4
6 a,bEtoluene + PhCl220804.950.32.40.43134.7
7 a,bEtoluene + PhCl1301201.023.02.31.1131.6
8 a,cBtoluene1301201.94.22.42.4127.8
9 a,cBhexanes1301201.44943103.6127.8
a Conditions: [Pd] = 10 μmol, temperature = 80 °C, time = 2 h, 50 mL solvent. b 1 equiv B(C6F5)3 per L, 49/1 toluene/chlorobenzene solvent. c cited from reference 36. d Activity is reported per mol Pd. e Determined by Gel Permeation Chromatography (GPC) [38]. f VF incorporation in copolymer determined by 1H NMR. g DSC. h Not determined due to the limited quantity of copolymer.
Table
Table 2. Microstructures of E/VF copolymers produced by catalysts C, D, and E [41,42,43,44,45,46,47].
Table 2. Microstructures of E/VF copolymers produced by catalysts C, D, and E [41,42,43,44,45,46,47].
CatalystEntry in Table 1Distribution of VF Incorporation Modes (%) a
Polymers 12 01609 i001 Polymers 12 01609 i002 Polymers 12 01609 i003
C1100Not observedNot observed
C285.15.09.9
C399.10.9Not observed
C499.20.40.4
D5>99Trace bNot observed
E699.20.8Not observed
E798.81.2Not observed
a Determined by 19F NMR spectroscopy; -CH2CH2F and -CH2CHF2 end groups were not observed. b Resonance observed but intensity too low for accurate integration.

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Liu, Q.; Jordan, R.F. Copolymerization of Ethylene and Vinyl Fluoride by Self-Assembled Multinuclear Palladium Catalysts. Polymers 2020, 12, 1609. https://doi.org/10.3390/polym12071609

AMA Style

Liu Q, Jordan RF. Copolymerization of Ethylene and Vinyl Fluoride by Self-Assembled Multinuclear Palladium Catalysts. Polymers. 2020; 12(7):1609. https://doi.org/10.3390/polym12071609

Chicago/Turabian Style

Liu, Qian, and Richard F. Jordan. 2020. "Copolymerization of Ethylene and Vinyl Fluoride by Self-Assembled Multinuclear Palladium Catalysts" Polymers 12, no. 7: 1609. https://doi.org/10.3390/polym12071609

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