Palladium-Catalyzed Direct Synthesis of Various Branched

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Palladium-Catalyzed Direct Synthesis of Various Branched, Carboxylic Acid-Functionalized Polyolefins: Characterization, Derivatization, and Properties Shengyu Dai†,‡ and Changle Chen*,‡ †

Institute of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, China Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China

‡

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S Supporting Information *

ABSTRACT: Ethylene-co-acrylic acid (E−AA) copolymers are typically produced via high-pressure free radical copolymerization and have great industrial importance because of their many applications. The radical polymerization mechanism usually leads to highly branched products with poor mechanical properties. Transition-metal-catalyzed E−AA copolymerization represents a direct and economical route to access these copolymers with potentially better control over their microstructures and material properties. However, this is highly challenging due to catalyst poisoning from both the oxygen and carboxylic acid moieties in the monomers. In this contribution, we demonstrate that a series of α-diimine-based palladium catalysts can mediate efficient copolymerizations of ethylene with AA, allylacetic acid, and 10-undecenoic acid, leading to the formation of various branched, carboxylic acid-functionalized polyolefin materials. These comonomers exist as carboxylic acid-based dimeric species at ambient temperatures, which is proposed as the key reason for the successful copolymerizations. These polar, functionalized polyolefins demonstrate greatly improved surface properties based on water contact angle measurements and dyeing experiments. Furthermore, these copolymers can be converted to sodium-, zinc-, and iron-based ionomers. The metal ions can act as physical cross-links and dramatically improve the mechanical properties of these copolymers.

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INTRODUCTION Ethylene-co-acrylic acid (E−AA) copolymers are industrially important materials with many applications.1 They are produced via a typical high-pressure, free-radical copolymerization technique and are used in both the acidic form and as the partially neutralized derivatives (ionomers). The hydrogen bonds among the carboxyl groups and the metals ions in the ionomers may act as physical cross-links and dramatically alter the materials’ properties including their flexibility, toughness, crack resistance, impact strength, and gas impermeability.2−4 Ethylene can be renewably produced via the dehydration of bioethanol,5,6 and several renewable routes have emerged for the preparation of AA using bio-based raw materials,7−9 making this type of copolymer even more attractive. Recently, Wagener et al. described the synthesis of some E−AA copolymers with well-defined microstructures by acyclic diene metathesis or by the ring-opening metathesis polymerization of substituted cyclooctene followed by catalytic hydrogenation.10,11 In addition to this two-step synthetic strategy, the monomers used required a multistep synthesis, and the carboxylic acid groups needed to be protected during the polymerization reactions. Bielawski et al. reported the synthesis of high molecular weight linear E−AA copolymers via the tandem hydrocarboxylation/hydrogenation of cispolybutadiene.12 As the most direct and economical procedure, © XXXX American Chemical Society

the synthesis of these copolymers from the transition-metalcatalyzed copolymerization of basic industrial monomers (E and AA) is highly fascinating. Despite decades of research and some recent developments, the transition-metal-catalyzed copolymerization of ethylene with polar monomers remains a great challenge in this field.13 Brookhart-type α-diimine palladium catalysts represent a great breakthrough and have been applied for the copolymerizations of ethylene with methyl acrylate (MA) and a few other polar monomers.14−30 However, these catalysts have not been investigated for E−AA copolymerization until very recently.31,32 Drent-type phosphine sulfonate palladium catalysts represent another major advance in this field, as they enable copolymerizations of ethylene with a surprisingly wide range of polar monomers.33−40 Most importantly, the Mecking group, the Claverie group, and our group have demonstrated successful examples of transition-metal-catalyzed E−AA copolymerization using phosphine sulfonate palladium catalysts.41−43 In this direct copolymerization, good AA incorporation (3.0−9.6%) was achieved, but with a relatively low copolymer molecular weight (Mn = 6100−8000). Received: June 12, 2018 Revised: August 9, 2018

A

DOI: 10.1021/acs.macromol.8b01261 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Diaryl-Based α-Diimine Pd(II) Catalysts 1−8 with Varied Ligand Sterics

Table 1. E−AA Copolymerization Catalyzed by 1−8 with 1 atm of Ethylene Pressurea entry

cat.

[AA] (M)

yield (g)

activityb

XAAc (%)

Mnd (×10−3)

PDI

Be

Tmf (°C)

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 5 6

1 1 1 1 1 1 1 1 2 2

0.08 0.12 0.18 0.14 0.19 0.17 0.24 0.27 0.05 0.12

2.7 4.0 6.0 4.7 6.3 5.7 8.0 9.0 1.7 4.0

7.3 5.6 4.2 3.2 2.7 2.2 0.5 0.7 5.6 4.2

1.2 5.2 6.1 10.6 21.8 36.3 54.0 29.6 10.2 13.8

2.3 2.0 2.2 2.3 1.8 1.7 2.6 1.3 1.8 1.6

84/119 83/113 66/86 44/60 41/54 36/48 12/15 10/13 33/61 29/49

g g g g 37 55 121 122 43 54

Conditions: 0.010 mmol of precatalyst, 1.2 equiv of NaBAF, total volume of CH2Cl2 and AA: 25 mL, 1 atm, 3 h, 40 °C. bActivity = 103 g/(mol Pd·h). cXAA= AA incorp. (mol %). dMolecular weight was determined by GPC. eB = branches per 1000 carbons. Branching numbers were determined using 1H NMR spectroscopy; the first numbers represent the branches ending without functional groups, and the second numbers include the branches ending with functional groups. fMelting temperature determined by DSC. gAmorphous polymers. a

Table 2. Ethylene−AA Copolymerization under Various Conditions Using 6a entry

T (°C)

t (h)

P (atm)

mon.

[AA] (M)

yield (g)

activityb

XAAc (%)

Mnd (×10−4)

PDId

Be

Tmf (°C)

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

20 20 20 20 20 40 20 20 20 20 20 20 40

1 1 1 10 3 3 3 1 1 10 10 3 3

4 2 2 2 1 1 1 8 8 2 2 1 1

AA AA AA AA AA AA AA AAA UA AAA UA MA MA

1 1 2 2 1 1 1 1 1 1 1 1 1

0.41 0.35 0.22 2.14 0.21 0.17 0.24 2.19 1.06 1.37 0.65 0.19 0.12

41.0 35.0 22.0 21.4 7.0 5.7 8.0 219.0 106.0 13.7 6.5 6.3 4.0

0.1 0.3 0.6 0.7 1.0 2.2 0.8 0.7 2.7 3.0 7.0 0.7 1.6

14.5 8.0 6.3 56.2 7.9 3.6 5.2 63.4 28.1 34.0 8.7 4.7 2.7

1.8 1.9 1.7 2.2 1.7 1.7 1.5 1.5 1.8 2.4 2.1 1.8 1.9

25/25 28/29 26/29 26/29 26/31 36/48 37/41 21/25 29/43 26/41 35/70 35/39 45/53

103 101 101 91 103 55 99 97 97 88 63 55 52

a

Conditions: 0.010 mmol of precatalyst 6, 1.2 equiv of NaBAF, total volume of CH2Cl2 and AA: 25 mL. bActivity = 103 g/(mol Pd·h). cXAA= AA incorp. (mol %). dMolecular weight was determined by GPC. eB = branches per 1000 carbons. Branching numbers were determined using 1H NMR spectroscopy; the first numbers represent the branches ending without functional groups, and the second numbers include the branches ending with functional groups. fMelting temperature determined by DSC. gUsing toluene as solvent.

Generally, α-diimine palladium catalysts are less tolerant toward polar functional groups than phosphine sulfonate palladium catalysts. Thus, it is less likely that α-diimine-type palladium catalysts could initiate E−AA copolymerization with better results than phosphine sulfonate type palladium catalysts. However, our group recently reported that some sterically bulky α-diimine palladium catalysts (Scheme 1, 1−8) bearing diaryl−hydryl moieties can greatly expand the polar monomer substrate scope and generate semicrystalline

copolymers.44−46 This result inspired us to investigate the reactivities of these α-diimine palladium catalysts toward an AA comonomer. Our initial screening showed that no AA was incorporated using sterically very bulky catalysts 6−8 under 8 atm of ethylene pressure.31 However, moderate AA incorporation was observed in 1-octene polymerization. In this contribution, we wish to demonstrate that efficient E−AA copolymerization can be achieved with α-diimine palladium catalysts by manipulating the polymerization conditions B

DOI: 10.1021/acs.macromol.8b01261 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (especially ethylene pressure). Surprisingly, these α-diimine palladium catalysts possess better tolerance toward the carboxyl functionalities than phosphine sulfonate palladium catalysts, and the origin of this unexpected observation was investigated. Furthermore, the physical properties of the resulting copolymers, as well as the derived ionomers, were studied in detail.

slightly affected by changing the polymerization solvent from CH2Cl2 to toluene (Table 2, entry 7 vs 5). Furthermore, two comonomersallylacetic acid (AAA) and 10-undecenoic acid (UA)with CH2 spacers between the alkene and the carboxylic acid were studied. In these two cases, good comonomer incorporation ratios (0.7% and 2.7%) were observed, even under 8 atm of ethylene (Table 2, entries 8 and 9). As mentioned in the Introduction, no comonomer was incorporated in E−AA copolymerization catalyzed with 6 at 8 atm of ethylene. These results suggested that these two special comonomers bind to the palladium center better than the AA comonomer or that their insertion barriers are lower than that of the AA comonomer. At 2 atm of ethylene, very high comonomer incorporation ratios (3.0−7.0%) along with high copolymer molecular weights (Mn: 8.7 × 104−34.0 × 104) were achieved (Table 2, entries 10 and 11). This provides an alternative strategy for the synthesis of carboxylic acidfunctionalized polyethylenes with both high molecular weights and high contents of polar groups. Most surprisingly, all of the properties (activity, comonomer incorporation, molecular weight, and melting point) achieved for E-AA copolymerization are better than those of E-MA (methyl acrylate) copolymerization under the same conditions (Table 2, entry 12 vs 5). Specifically, the comonomer incorporation ratio was enhanced (1.0% vs 0.7%) when AA was used, along with a greatly enhanced copolymer molecular weight (7.9 × 104 vs 4.7 × 104). This is counterintuitive considering that the AA comonomer possesses acidic carboxylic acid moieties in addition to the ester functionalities, which should lead to greater catalyst poisoning for the AA comonomer. Similar property trends were also observed at higher copolymerization temperatures (40 °C, Table 2, entry 13 vs 6). Another surprising observation is that the α-diimine palladium system possesses comparable or even better properties than the extensively studied phosphine sulfonate palladium system. This is also counterintuitive considering that the phosphine sulfonate palladium system can tolerate a much wider scope of polar functionalities than the α-diimine palladium system.13−18,31−39 Some control experiments were performed to understand these unexpected results. First, the effect of propionic acid on ethylene polymerization for these two systems was investigated (Figure 1). The TOF values were reduced by ca. 86% and 89% in the presence of 0.1 and 0.3 mol/L of propionic acid with the phosphine sulfonate palladium system. In contrast, the TOF values were only reduced by 7% and 28% in the presence of 1.0 and 2.0 mol/L of propionic acid with the α-diimine palladium system. The poisoning effect is much greater at 100 °C, at which the TOF value was reduced by 47% in the presence of 1.0 mol/L of propionic acid for the α-diimine palladium system. The TOF values of catalyst 6 at 20 °C remained constant over a period of 60 min in both the presence or absence of 1 mol/L of propionic acid (Table S1 and Figure S1). The propionic acid probably inhibits the polymerization through the reversible coordination/dissociation of the oxygen functionality to the metal center, but no catalyst decomposition is induced in this process. In the presence of 1.0 mol/ L of ethyl propionate, the TOF value of catalyst 6 was reduced by 47%, in contrast to the 7% observed with propionic acid. Clearly, a much greater poisoning effect was induced by ethyl propionate versus propionic acid, which is consistent with the results observed in the copolymerizations using AA versus MA.

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RESULTS AND DISCUSSION When activated using 1.2 equiv of sodium tetrakis(3,5bis(trifluoromethyl)phenyl)borate (NaBAF), these palladium catalysts can mediate efficient E−AA copolymerization using an ethylene pressure of 1 atm (Table 1, entries 1−8). Copolymers with a wide range of molecular weights (Mn: 1.2 × 103−54.0 × 103) and a wide range of AA incorporation ratios (0.5−7.3%) were generated. Drent-type phosphine sulfonate palladium catalysts afforded highly linear E−AA copolymers with branching numbers of ca. 3/1000 carbon atoms and melting temperatures of ca. 100 °C.39 In this case, the branching of the copolymer products (13−119/1000 carbon atoms) varied, and amorphous to semicrystalline materials (Tm between 37 and 122 °C) were generated. At higher AA concentrations, 5 and 6 gave higher AA incorporation ratios, but at the expense of the copolymer molecular weight (Table 1, entries 9 and 10). The COOH group was located at the chain-end position for the copolymers obtained by these αdiimine palladium catalysts. The characteristic peak for the incorporated AA unit was observed at ca. δ 2.35 ppm (t) corresponding to −CH2CH2COOH (the integration ratio versus −CH2CH2COOH at ca. 1.62 (m) ppm is close to 1/1, Figures S5−S7). In contrast, phosphine sulfonate palladium catalysts led to in-chain incorporated AA unit.41−43 For that case, the characteristic AA peak was observed at ca. δ 2.45 ppm corresponding to −CH2CH(COOH)CH2− (the integration ratio versus −CH2CH(COOH)CH2− at ca. 1.66 (m) ppm is close to 1/2, Figure S8). This was further confirmed through 13 C NMR analysis of the AA copolymer (sample from Table 1, entry 4, Figure S9). Subsequently, the properties of 6 in E−AA copolymerizations under various conditions were investigated. At higher ethylene pressures (Table 2, entries 1 and 2, vs Table 2, entry 6), the catalytic activities, copolymer molecular weights, and copolymer melting temperatures were greatly enhanced; however, the AA incorporation ratios were reduced. Similar to the scenario at 1 atm of ethylene, increasing the AA concentration can greatly increase the AA incorporation ratio (Table 2, entry 3 vs 2). Importantly, the activity and AA incorporation remained constant for over 10 h under these conditions (Table 2, entry 4 vs 3). In addition, the molecular weight of the copolymer increased dramatically (almost 10 times). These results suggested the great stability of this catalyst under these conditions. Furthermore, time dependence studies showed that both the polymer yield and the polymer molecular weight (Mn) increased roughly linearly as a function of time over 10 h (Table S2 and Figure S2). This suggests “quasi-living” behavior for the copolymerization process under these conditions.47,48 By increasing the polymerization temperature from 20 to 40 °C, both the catalytic activity and the copolymer molecular weight decreased (Table 2, entry 6 vs 5); however, the AA incorporation ratio doubled. This is likely due to the decrease in the ethylene concentration at higher temperatures. The copolymerization parameters were only C

DOI: 10.1021/acs.macromol.8b01261 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

temperature, and the corresponding bulky structure can efficiently alleviate the poisoning effect from both the oxygen and carboxylic acid moieties. At elevated temperatures, the equilibrium is shifted to the monomer side, leading to a greater poisoning effect. The polymerization studies with phosphine sulfonate palladium were all performed at high temperatures, which led to the less desirable results compared to those of the α-diimine palladium-catalyzed polymerizations at low temperatures. For α-diimine palladium-catalyzed E−MA copolymerization, the catalyst resting state is the six-membered chelate species.14,15 For E−AA copolymerization, the dimerization equilibrium probably also exists for the six-membered chelate species (Scheme 2). Correspondingly, it may become easier for ethylene to open the chelate and resume the copolymerization process. In the FT-IR spectrum of the E−AA copolymer, the characteristic peak at 1704 cm−1 indicates the presence of hydrogen-bridged cross-links.50 No characteristic bands at 1750 and 3500 cm−1 for the free carboxyl group were observed, confirming the dominance of the dimeric carboxyl groups in the copolymers. The reaction with NaOtBu can convert the E−AA copolymer to sodium ionomers (Scheme 3a).51 This finding was confirmed by the disappearance of the peak at 1704 cm−1, the appearance of the symmetrical vibration ca. 1365 cm−1, and the asymmetrical vibration ca. 1567 cm−1 of the carboxylate anion in the IR spectrum (Scheme 3b).52 These observations further supported the above-mentioned hypothesis regarding the important role of the carboxyl-based dimeric species in the polymerization reactions. It has been demonstrated in the literature that the incorporation of polar groups into polyolefins can enhance important material properties such as toughness, adhesion, compatibility, barrier properties, and surface properties. Here, the surface properties of the E−AA copolymers were studied by measuring their water contact angles (WCA). Thin films were prepared on a glass substrate by evaporating a toluene solution of the polymer (see the Supporting Information for details). The WCA value was 104° for pure PE prepared by catalyst 6 (Scheme 3c and Figure S4). The incorporation of 2.2% AA comonomer can induce a ca. 20° WCA reduction to

Figure 1. Ethylene polymerization in the presence of propionic acid and ethyl propionate using a phosphine sulfonate palladium catalyst (data taken from ref 41, 3.5 μmol of catalyst, 5 atm of ethylene, 30 min) and α-diimine catalyst 6 (10 μmol of 6, 1.2 equiv of NaBAF, 8 atm of ethylene, 60 min). [M] represents the concentration of propionic acid or ethyl propionate.

This is probably due to the equilibrium between the monomeric and dimeric species of propionic acid and AA (Scheme 2).49 The dimeric species dominates at room Scheme 2. Equilibrium between the Monomeric and Dimeric Species of AA, Propionic Acid, and the SixMembered Chelate Species

Scheme 3. (a) Conversion of Carboxylic Acid Moiety to Sodium, Zinc, and Iron Salts; (b) FT-IR Spectra of Polyethylene and Copolymer Samples; (c) Gradual Change in Water Contact Angles through Increased AA Incorporation Ratios; (d) Dyeing Experiments for Polyethylene and Copolymer Samples; (e) Tensile-Test Specimens of the Polyethylene and Copolymer Samples; and (f) Stress vs Strain Curves for the Copolymer Samplesa

a

The results on multiple specimens were shown to represent the reproducibility (Figures S48−S51). D

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COOFe copolymer (C, 77.85%, H, 12.46%, N, 0.01%). This suggests the high efficiency of these metalation reactions.

85°. The WCA values gradually decreased with an increased incorporation ratio of the AA comonomer. Eventually, a 7.3% AA incorporation led to a WCA value of 58°, which is close to half of that of the pure PE sample. The influence of the AA incorporation on the surface properties was further studied using a dyeing experiment (Scheme 3d). The E−AA copolymer and a certain amount of an azo-type dye powder ((E)-4-((4-(diethylamino)phenyl)diazenyl)benzoyl fluoride)53 were stirred at 60 °C in THF for 60 min. Evaporation of the THF solution on top of a polyethylene sample gives a thin film of the copolymer. The process was repeated three times to make the film thicker. Subsequently, the copolymer film was washed with acetone three times and dried in a vacuum oven to a constant weight. The color of the samples clearly depends on the incorporation of the AA comonomer (Scheme 3d). Furthermore, the Kubelka−Munk equation was used to determine the color strength of the extracted copolymer.54 Bigger K/S values (where K and S represent the spectral absorption and scattering coefficients, respectively) indicate higher color strength. The K/S value of pure PE is very small due to the nonpolar nature of polyethylene and the correspondingly low compatibility with the highly polar dye molecules. With the incorporation of 2.2% AA comonomer (sample from Table 1, entry 6), the copolymer sample demonstrated a K/S value of 0.39. This value can be further increased to 2.08 with an increase in the AA incorporation ratio (4.2%, sample from Table 1, entry 3). Clearly, the carboxylic acid functional group can efficiently increase the affinity of the polymeric material to the dye molecules, even at very low percentage of carboxylic acid incorporation. The mechanical properties of selected copolymer samples were also studied (Scheme 3e,f). Dog-bone-shaped tensile-test specimens were obtained by melt-pressing the polymeric products at certain temperatures. The E−AA copolymers obtained in this system range from amorphous to semicrystalline with high melting temperatures. However, they are all very sticky materials that stick to the equipment, preventing the preparation of test specimens. In contrast, the E−UA copolymer (Table 2, entry 11) can be melt-processed to give test specimens. This copolymer sample exhibited typical thermoplastic behavior, with stress-at-break and strain-atbreak values of 9.1 MPa and 467%, respectively. The reaction of the copolymer with NaOtBu (1.2 equiv with respect to acrylic acid moieties) can convert the copolymer to a sodiumbased ionomer. The sodium-based ionomer possesses similar tensile properties as the original COOH-based copolymer. Furthermore, the sodium-based ionomer can be converted to a zinc-based ionomer by reacting with ZnCl2 (0.5 equiv with respect to acrylic acid moieties). The zinc ions can act as physical cross-links, leading to a greatly increased stress-atbreak value (28.6 MPa). The reaction of the sodium-based ionomer with 0.33 equiv of Fe(NO3)3 led to very stiff materials, which could not be pressed at all. When the amount of Fe(NO3)3 was reduced to 0.05 equiv, the resulting ionomer can be melt-processed to give test specimens. Although only a small amount of the sodium ions were replaced with the iron ions, the resulting sample showed greatly increased strain-atbreak values (788%). Elemental analyses were performed for the copolymers. Only trace amount of chlorine element was observed in the P-COOZn copolymer (Cl, 0.03%), and only a trace amount of nitrogen element was observed in the P-

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CONCLUSIONS Efficient E−AA copolymerizations were achieved using αdiimine-based palladium catalysts. In this system, moderate activities (up to 4.1 × 104 g/(mol Pd·h)), appreciable AA incorporation ratios (0.1−7.3%), and high copolymer molecular weights (Mn up to 5.6 × 105) were observed. The use of AAA and UA comonomers with CH2 spacers between the alkenes and the carboxylic acid groups led to increased activities along with increased copolymer molecular weights. Control experiments indicated the critical role of the equilibrium between the monomeric and dimeric forms of the carboxylic moiety in these copolymerization reactions. The incorporation of the carboxylic functional group into polyolefins can dramatically improve the surface properties of these materials, as evidenced by the greatly reduced water contact angles and greatly enhanced affinity for an azo-type dye compound. Furthermore, the conversion of the carboxylic acidfunctionalized copolymers to sodium/zinc/iron-based ionomers can significantly alter the mechanical properties of these materials.

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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.macromol.8b01261. Experimental procedures, NMR spectra for the copoly-

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mers (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.C.). ORCID

Shengyu Dai: 0000-0003-4110-7691 Changle Chen: 0000-0002-4497-4398 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC, 21690071, 51703215, and 51522306) and the China Postdoctoral Science Foundation (2017M612077).

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DOI: 10.1021/acs.macromol.8b01261 Macromolecules XXXX, XXX, XXX−XXX