Combination of Ethylene, 1,3-Butadiene, and Carbon Dioxide into

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Combination of Ethylene, 1,3-Butadiene, and Carbon Dioxide into Ester-Functionalized Polyethylenes via Palladium-Catalyzed Coupling and Insertion Polymerization Yixin Zhang,† Jian Xia,†,§,∥ Jiawen Song,‡ Jianfu Zhang,§,∥ Xufeng Ni,*,‡ and Zhongbao Jian*,†

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State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun 130022, China ‡ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China § School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, China ∥ Jilin Provincial Science and Technology Innovation Center of Optical Materials and Chemistry, Changchun 130022, China S Supporting Information *

ABSTRACT: The combination of ethylene (E), 1,3-butadiene (BD), and carbon dioxide (CO2), three extensively utilized feedstocks, into a polymer via the copolymerization pathway is of great interest in both academia and industry. However, copolymerization for even two of them (E/BD, E/ CO2, BD/CO2) is highly challenging; thus, copolymerization for three of them (E/BD/CO2) together is more elusive and remains unexplored. In this contribution, by employing a twostep strategy of palladium-catalyzed coupling and subsequent insertion polymerization, the E/BD/CO2 copolymerization via an allyl acrylate-type intermediate was achieved for the first time. The palladium-catalyzed [Pd(acac)2/PCy3] C−C coupling reaction of BD and CO2 followed by ring cleavage and esterification first generated the desired trifunctional monomer methyl-2-ethylidene-5-hydroxyhept-6-enoate acrylate (II). Subsequent palladium-catalyzed [(P^O)PdMedmso] coordination−insertion copolymerization of E and II afforded ester-functionalized polyethylenes including three components of E/BD/CO2. These resultant copolymers are chemoselective (reactive acrylate and allyl ester, inert 1,2-disubstituted acrylate), stereoselective (different diastereomers), and regioselective (five-membered γ-butyrolactone and six-membered δ-valerolactone) and thus are of highly novel microstructures with incorporation of noncyclic ester units and cyclic ester units into the main chain. They are comprehensively identified by 1H NMR, 13C NMR, DEPT, 1H−1H COSY, 1H−13C HSQC, and 1H−13C HMBC plus ATR-IR, DSC, and GPC.

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INTRODUCTION Ethylene (E), 1,3-butadiene (BD), and carbon dioxide (CO2) as the C2, C4, and C1 chemical feedstocks, respectively, are the most important and the highly attractive commercialized monomers in the polymer industry. Ethylene can be used to produce a rich variety of polyethylenes (PEs), which are the most commercialized synthetic polymer.1−4 1,3-Butadiene can be utilized to generate the regio- and stereoselective polybutadiene (PBD), which widely acts as the synthetic rubber (cis-1,4 PBD).5−9 Carbon dioxide as a comonomer with epoxide is largely converted to polycarbonate (PC),10−15 which is the only commercialized CO2-derived polymer. Thus, chemistry leading to new polymers made from the copolymerization of E, BD, and CO2 is of significant academic interest and is highly desired for industrial utility. The copolymerization of E and BD is highly challenging because of the distinctly different coordination and insertion © XXXX American Chemical Society

mechanism. After the insertion of E into metal−alkyl active sites, a new metal−σ-alkyl sites is generated, but after the insertion of BD into metal−alkyl active sites, a new metal−πη3-allyl intermediate is formed that is a trap for ethylene coordination/insertion.16−18 To date, there are a few early transition-metal catalysts (such as Sc, Ti, Zr, and Nd) that enable the copolymerization of E and BD.19−25 In contrast, late-transition-metal catalysts (such Ni and Pd) are capable of homopolymerizing E26,27 or BD28−30 but are only able to cooligomerize E and BD to afford the product of 2,4-hexadiene.31 Because of both the thermodynamic and the kinetic limitations,32 methods to produce polymers derived from the reaction of CO2 and olefins are not accessible to date. Received: January 28, 2019 Revised: February 25, 2019

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DOI: 10.1021/acs.macromol.9b00195 Macromolecules XXXX, XXX, XXX−XXX

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Reported examples on the copolymerization of CO2 and olefins such as vinyl ethers or acrylonitrile only generated the oligomers of low molecular weight.33−35 Although in 2016 Müller et al. elucidated the feasibility of copolymerization of CO2 and E with a DFT study by developing suitable Pd catalysts,36 successful catalyst is still unexplored so far. The most effective reaction of CO2 and E is still producing acrylic acid by using Ni or other metal catalysts.37,38 Similar to the obstacle on the copolymerization of CO2 and E, the copolymerization of CO2 and BD is also impossible due to the thermodynamic and kinetic barriers. Until 2014, Nozaki et al. unprecedentedly revealed the feasibility of indirect copolymerization of CO2 and BD by use of a metastable lactone intermediate.39 The δ-lactone monomer, 3-ethylidene6-vinyltetrahydro-2H-pyran-2-one (EVL) (see Scheme 1), was

Article

EXPERIMENTAL SECTION

General Procedures and Materials. All syntheses involving airand moisture-sensitive compounds were performed using standard Schlenk-type glassware (or in a glovebox) under an atmosphere of nitrogen. NMR spectra for the complexes and polymers were recorded on a Bruker AV400 (1H: 400 MHz; 13C: 100 MHz; 31P: 162 MHz) or a Bruker AV500 (1H: 500 MHz; 13C: 125 MHz; 31P: 202 MHz). NMR assignments were confirmed by 1H−1H COSY, 1 H−13C HSQC, and 1H−13C HMBC experiments when necessary. The molecular weights (Mn) and molecular weight distributions (Mw/ Mn) of polyethylenes and copolymers were measured by means of gel permeation chromatography (GPC) on a PL-GPC 220-type hightemperature chromatograph equipped with three PL-gel 10 μm Mixed-B LS type columns at 150 °C or on a TOSOH HLC-8220 GPC with THF as eluent at 40 °C. Melting points (Tm) of polyethylenes and copolymers were measured through DSC analyses, which were performed on a Q 100 DSC from TA Instruments under a nitrogen atmosphere at heating and cooling rates of 10 °C/min (temperature range: 20−160 °C). Elemental analysis was performed at the National Analytical Research Centre of Changchun Institute of Applied Chemistry. IR spectra were acquired on a VERTEX 70 Fourier transform infrared spectrometer. All solvents were purified from the MBraun SPS system. Pd catalysts 1−4 were prepared according to the literature procedures.27,55−57 Monomer I was commercially available, which was further dried over CaH2 for 2 days and distilled. All other reagents were commercially available and used as received. Synthesis of Monomer II (Methyl-2-ethylidene-5-hydroxyhept-6-enoate Acrylate). II is synthesized through the ring cleavage of 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVL), a substituent δ-lactone synthesized through telomerization of CO2 with 1,3-butadiene, and subsequent esterification with acryloyl chloride. 20 g (0.13 mol) of EVL was dissolved in 100 mL of methanol with 6.6 g (0.065 mol) of TEA and refluxed under 70 °C for 24 h. Methanol and TEA were removed by rotary evaporation, and the crude alcoholized EVL was obtained with a 80.6% conversion determined by 1H NMR. The product was directly used in the next step without further purification. Ten grams of alcoholized EVL was dissolved in 60 mL of dry THF with 5.5 g of TEA and kept under 0 °C. A solution of acryloyl chloride (4.9 g, 20 mL THF) was added dropwise within 30 min and kept stirred overnight after addition. NaCl and THF were removed by filtration and rotary evaporation, respectively; the concentrated solution was dissolved in dichloromethane and washed with saturated Na2CO3 aqueous solution three times and then dehydrated with anhydrous Na2SO4. Column chromatography was conducted with hexane/EtOAc = 15:1. Pure monomer II was obtained as a colorless oil with Rf = 0.22. Total yield 72%. 1H NMR (400 MHz, CDCl3): δ = 6.85 (q, J = 7.1 Hz, 1H, H10), 6.41 (dd, J = 17.3, 1.4 Hz, 1H, H1), 6.13 (dd, J = 17.3, 10.4 Hz, 1H, H2), 5.90−5.72 (m, 2H, H1 and H5), 5.35−5.12 (m, 3H, H4 and H6), 3.70 (s, 3H, H13), 2.35 (dd, J = 9.1, 6.8 Hz, 2H, H8), 1.79−1.74 (m, 5H, H7 and H11). 13C{1H} NMR (125 MHz, CDCl3): δ = 168.02 (C12), 165.52 (C3), 138.26 (C10), 136.16 (C5), 132.24 (C9), 130.77 (C1), 128.79 (C2), 116.97 (C6), 74.54 (C4), 51.72 (C13), 33.24 (C7), 22.20 (C8), 14.25 (C11). Anal. Calcd for C13H18O4: C, 65.53; H, 7.61. Found: C, 65.32; H, 7.48. Copolymerization Procedure. In a typical experiment, a 150 mL glass thick-walled pressure vessel was charged with 23 mL of toluene and a magnetic stir bar in the glovebox. The pressure vessel was connected to a high-pressure line. The vessel was warmed to 85 °C using an oil bath and allowed to equilibrate for 15 min. The desired polar monomer and the desired amount of catalyst in 2 mL of CH2Cl2 were injected into the polymerization system via syringe subsequently. With a rapid stirring, the reactor was pressurized and maintained at the desired pressure of ethylene. After desired time, the pressure reactor was vented, and the copolymer was precipitated in ethanol, filtered, and dried at 50 °C for at least 24 h under vacuum.

Scheme 1. Synthesis of the Monomer II Derived from BD and CO2 Feedstocks

first prepared by the palladium-catalyzed coupling of CO2 and BD,40−42 which was then polymerized by a free-radical method to produce CO2-derived polymer with versatile cyclic ester units.43 Recently, we were also interested in the catalytic coupling reaction of CO2 and BD and the chemistry of EVL monomer containing a six-membered ring and two types of vinyl groups. We for the first time developed a highly reactive trivinyl methacrylate-type monomer III (see Scheme 1) derived from EVL and then described its chemoselective RAFT polymerizations to obtain well-defined polymers with linear and hyperbranched topologies.44,45 At the same time, our interests also focused on the field of late-transition-metal (especially Ni and Pd) catalyzed copolymerization of olefin and polar monomers to prepare high-valued functionalized polyolefins.46−54 Because the copolymerization reaction for two of E, BD, and CO2 remains highly challenging as abovementioned, the copolymerization for three of E, BD, and CO2 together is more elusive. In this contribution, intrigued by these previous advances, we first used the strategy of synthesizing EVL by the Pd-catalyzed C−C coupling reaction of CO2 and BD, which further facilely reacted with methanol and subsequently with acryloyl chloride to yield the desired monomer II with three different reactive vinyl groups (acrylate, allyl ester, and 1,2-disubstituted acrylate). The chemoselective coordination−insertion copolymerization of E and II was then achieved by functional group tolerant Pd catalysts. Thus, we for the first time report the coordination−insertion copolymerization of E, BD, and CO2 as three feedstocks via a monomer II intermediate for the synthesis of a new class of ester-functionalized polyethylenes. B

DOI: 10.1021/acs.macromol.9b00195 Macromolecules XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Synthesis of the Targeted Monomer II Derived from the Coupling of BD and CO2 Feedstocks. The Pdcatalyzed coupling reaction of BD and CO2 as feedstocks to generate the δ-lactone monomer 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVL) has been extensively described previously.39−43 Because the EVL monomer is of poor polymerizability due to the structural similarities with tiglic acid ester and α-substituted allyl ester that are difficult to be polymerized, we recently developed a new highly reactive trivinyl methacrylate-type monomer III (R = Me, Scheme 1) derived from the reaction of EVL with alcohol and subsequent methacryloyl chloride for chemoselective RAFT polymerization.44,45 However, monomer III containing one 2monosubstituted acrylate, one 1,2-disubstituted acrylate, and one allyl ester unit is again difficult to be copolymerized with ethylene by the metal-catalyzed coordination−insertion method; thus, we proposed the synthesis of an alternative acrylate-type monomer II including one nonsubstituted acrylate unit (R = H, Scheme 1) for the application in ethylene copolymerization. This is inspired by these results revealed by Mecking et al. that the nonsubstituted acrylate monomer (such as MA) is of remarkably higher reactivity in the insertion copolymerization of ethylene than the substituted acrylate (such as MMA).57−60 Thus, monomer II was synthesized by using a similar procedure with monomer III for three steps, and its purity and structure were comprehensively identified by 1D and 2D NMR spectroscopy plus elemental analysis. In the 1H NMR and 13C NMR spectra (Figure 1), three key vinylic resonances are

Chart 1. Monomers I and II and Pd Catalysts 1−4 Applied in Copolymerizations

In addition, since monomer II contains polymerizable acrylate and allyl units and nonpolymerizable 1,2-disubstituted acrylate unit, the commercialized allyl acrylate monomer I was also used for comparison (Chart 1). Under the activation of sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF), exposure of the α-diimine Pd catalyst 1 to the reaction mixture of ethylene resulted in the formation of highly branched polyethylene in a high activity, as anticipated (Table 1, entry 1). It is well-known that the Pd catalyst 1 promotes the ethylene/acrylate copolymerization but disables the copolymerization of ethylene and polar allyl monomer presumably because of fast chain walking, β-X/H elimination processes, and formation of stable chelates.27,61−67 Thus, the copolymerizations of ethylene with monomers I and II were also performed with the Pd catalyst 1. At a given comonomer concentration of 0.1 M, catalyst 1 mediated the E/I or E/II copolymerization reaction with a lower activity, as expected (Table 1, entries 2 and 4). However, 1H NMR spectra of these polymers obtained indicate no observable incorporation of polar group in the NMR scale (40 mg/mL, 64 scans, see the Supporting Information). To afford a more solid evidence, copolymerization reactions were performed at a higher comonomer concentration of 0.5 M. Nevertheless, no detectable incorporation of polar group was found again, and the activity drastically decreased (Table 1, entries 3 and 5). In fact, very recently Takeuchi et al. also found that Brookhart catalyst could mediate the copolymerization of ethylene and acrylic anhydride, but the activity was extremely low (0.17 kg mol−1 h−1) and the comonomer incorporation was just 0.1 mol % at a comonomer concentration of 0.5 M.68 The failure of Brookhart’s α-diimine Pd catalyst 1 applied to the copolymerizations prompted us to study the Drent-type phosphine−sulfonate Pd catalysts that have been known to promote both ethylene/acrylate and ethylene/polar allyl monomer copolymerizations.26,69 In 2012, Claverie et al. for the first time preliminarily probed the regiochemistry on the copolymerization of E and monomer I by using the strongly pyridine-coordinated Pd catalyst 2-pyr.70 To enhance the

Figure 1. 1H NMR and 13C NMR spectra of monomer II.

clearly observed at δ = 6.41, 5.83, and 6.14 ppm (13C: 130.66 and 128.68 ppm), 6.86 ppm (13C: 138.15 and 132.13 ppm), and 5.82 and 5.22 ppm (13C: 136.05 and 116.86 ppm), which can be assigned to the acrylate unit, the 1,2-disubstituted acrylate unit, and the allyl unit, respectively. Synthesis of Ester-Functionalized Polyethylenes Derived from the Copolymerization of E and Monomer II. Because of the less oxophilic and functional group more tolerant nature, late-transition-metal catalysts are powerful tools for the coordination−insertion copolymerization of olefin and polar monomers. Among of them, there are two kinds of complementary benchmark catalysts discovered by Brookhart and Drent and co-workers. Brookhart’s α-diimine Pd catalysts enable the copolymerization reaction to generate highly branched functional polyolefins, and Drent’s phosphinesulfonate Pd catalysts are capable of producing highly linear functional polyolefins (Chart 1).26,27 With these catalysts in hand, monomer II was applied to ethylene copolymerization. C

DOI: 10.1021/acs.macromol.9b00195 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Copolymerization of Ethylene and Monomers I and IIa

entry

cat.

comonomer

c(M) (mol L−1)

yield (g)

act.b (103)

XMc (mol %)

Mnd (103)

Mw/Mnd

Tme (°C)

brsc

1f 2 3 4 5 6 7g 8 9g 10 11g 12 13g 14 15g 16 17g

1 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

I I II II I I II II I I II II I I II II

0.0 0.1 0.5 0.1 0.5 0.1 0.3 0.1 0.3 0.1 0.3 0.1 0.3 0.1 0.3 0.1 0.3

2.89 0.64 0.06 1.28 0 0.10 0.05 0.28 0.26 0.28 0.23 0.30 0.25 0 0 0.32 0.20

36.1 6.4 0.6 12.8

0.0 0.0 0.0 0.0

123.9 105.3 10.2 116.1

1.8 1.6 1.3 1.7

−h −h −h −h

112 112 107 109

2.5 0.4 7.0 2.2 7.0 1.9 7.5 2.1

2.8 6.0 1.3 3.1 1.6 4.1 1.1 2.8

5.3 4.4 8.8 7.4 9.7 7.4 10.7 8.6

2.1 1.9 2.0 2.1 2.1 2.2 2.1 2.1

115 101 120 114 121 108 122 115

2 2 3 2