Precision Synthesis of Ethylene and Polar Monomer Copolymers by

Jul 18, 2017 - The incorporation of polar functionalities into polyolefins by the copolymerization of ethylene and polar monomers is of substantial in...
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Precision Synthesis of Ethylene and Polar Monomer Copolymers by Palladium-Catalyzed Living Coordination Copolymerization Shuhuang Zhong,†,‡ Yingxin Tan,‡ Liu Zhong,† Jie Gao,† Heng Liao,† Long Jiang,‡ Haiyang Gao,*,† and Qing Wu† †

School of Materials Science and Engineering, PCFM Lab, and ‡School of Chemistry, GD HPPC Lab, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Coordination insertion polymerization is unsurpassed as a straightforward method for synthesis of highvalue polyolefins by the copolymerization of ethylene and polar monomers, but poison effects of polar groups on the metal center result in a lack of fine control over the polymer architecture. Herein we reported a thermally stable dibenzobarrelene-derived α-diimine palladium catalyst for the precision synthesis of functionalized polyolefins by living copolymerization of ethylene and a variety of acrylate comonomers. The introduction of the bulky dibenzobarrelene backbone can improve migratory insertion selectivity of methyl acrylate (MA) in a 2,1-mode, thus preventing polar groups from poisoning palladium centers by stable five-membered palladacycle intermediates formed by 1,2-insertion of MA. In this living chain-walking catalyst system, the composition, molecular weight, and branching topology of the copolymer can be facilely tunable by simple variation of the ethylene pressure.

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INTRODUCTION The incorporation of polar functionalities into polyolefins by the copolymerization of ethylene and polar monomers is of substantial interest as a potential route to polyolefin materials with improved properties.1 A prominent example is the industrial ethylene−vinyl acetate (EVA) copolymers with an annual production of ∼3.0 million tons, being produced by the free radical copolymerization of the two monomers.2 Although these ethylene-based copolymers have widespread applications in adhesives, coating, packaging films, foams, and polymer modifiers, the harsh polymerization conditions of 250−3000 bar pressure and 150−375 °C high temperature lead to broad molecular weight distribution and poorly defined structure of copolymers.3 The synthesis of well-defined functionalized polyolefins with tunable compositions and topologies under mild reaction conditions is therefore highly desirable and enhance considerably the versatility of this class of polymers. Two approaches including controlled radical polymerization (CRP)4 and coordination insertion polymerization (CIP)1,5−8 have been applied to the challenge of controlling the copolymerization of ethylene and polar monomers. In CRP, the copolymerization of α-olefins and acrylate comonomers have been successfully achieved via nitroxide-mediated radical polymerization (NMRP),9 atom-transfer radical polymerization (ATRP),10−14 reversible addition−fragmentation chain-transfer polymerization (RAFT),15,16 and organometallic-mediated radical polymerization (OMRP).17,18 These radical techniques usually produce copolymers with low olefin contents because of a huge difference in monomer reactivity ratios, which limits © XXXX American Chemical Society

their potential as polyolefin materials. CIP is unsurpassed as a straightforward method for the preparation of high-value polyolefins by the copolymerization of olefins and polar monomers. However, CIP normally suffers from heavily reduced activity and low molecular weight of the copolymer produced because of poison effects of the polar monomers to the active metal center.19 Early transition metal catalysts such as group 4−6 metals tend to form stable σ-complexes by a κ-X coordination mode (X is polar groups), thus preventing the formation of the πcomplexes need for activation of the CC double bond because of the high oxophilicity of metal centers.18,19 In contrast, late transition metal catalysts (especially nickel and palladium) are prone to π-coordination of polar monomers because of their low oxophilicity, thereby effectively copolymerizing α-olefins and polar comonomers.18,19 A tremendous amounts effort has been devoted to design and synthesis of novel nickel and palladium catalysts for the copolymerization of ethylene and various polar monomers.20−34 Two kinds of noteworthy examples are α-diimine palladium catalysts (Brookhart type) that can usually produce highly branched copolymers containing polar groups at the end of the branches35−47 and phosphine−sulfonato palladium catalysts (Drent type) that are capable of affording linear copolymers with polar groups built into the polyethylene main chain.48−68 Received: May 31, 2017 Revised: July 10, 2017

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DOI: 10.1021/acs.macromol.7b01132 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis Route of α-Diimine Palladium Complexes

The single-crystal X-ray diffraction analysis showed that the chloromethylpalladium complex 1 displayed a distorted squareplanar coordination around the palladium center (Figure 1).

In the copolymerization of ethylene with methyl acrylate (MA) using α-diimine palladium catalysts, MA monomer binds palladium center by π-coordination and selectively inserts into a Pd−Me bond in a 2,1-mode to form a six-membered chelate complex by rapid isomerization. Further chain growth requires the opening of the chelates by coordination and insertion of ethylene, thus yielding high branched polymers with terminal ester functionality.35,36 Despite these contributions, the living coordination copolymerization of ethylene with polar monomers to synthesize well-defined ethylene-based copolymers is highly challenging up to date because of κ-X coordination of polar monomers, deactivation of catalysts, poor insertion selectivity of polar monomers, and rapid β-H and β-X eliminations.26,41 Herein we report the living copolymerization of ethylene and a variety of acrylates by palladium-catalyzed coordination copolymerization. A thermally stable α-diimine palladium catalyst featuring the bulky dibenzobarrelene backbone has been designed and synthesized for living homo- and copolymerizations of ethylene and acrylate monomers via inhibition of the N−aryl rotation of the α-diimine ligand originating from the repulsive interaction.69 We also demonstrate the potential of this chain-walking catalyst as a macromolecular-tailoring tool by the synthesis of ethylenebased copolymers with controllable topologies. In this catalyst system, the composition, molecular weight, and branching topology of the copolymer can be facilely tunable by simple variation of the ethylene pressure.

Figure 1. Crystal structure of α-diimine palladium complex 1. Hydrogen atoms are omitted for clarity.

The observed bond lengths and angles are typical for α-diimine palladium complexes.74 It is noteworthy that the aryl rings of aniline moieties are nearly perpendicular to the five-membered Pd−diimine ring plane (dihedral angle of 83.4° and 79.1°). These twist angles are also greater than those of α-diimine palladium complexes with dimethyl (79.7° and 77.7°) and dihydrogen (78.7° and 69.2°) backbone.74 This is a result from sterically repulsive interactions of bulky dibenzobarrelene backbone with aryl moieties, which is anticipated to suppress C−H activation originating from N−aryl rotations at elevated temperatures.74−79 Additionally, rigid dibenzobarrelene backbone can effectively shield the back space of palladium metal, thus providing the more enclosed space around palladium metal. The single crystal of cationic palladium complex 2 suitable for X-ray diffraction analysis was also obtained by slow evaporation of the palladium complex solution in CH2Cl2, which is one of extremely rare crystal structure of cationic αdiimine palladium complexes.78 The cationic palladium complex 2 featuring the methyl group and the coordinated CH3CN (Figure 2) showed a similar surrounding space around palladium metal center and the same steric effect to palladium complex 1. Ethylene Polymerization. Ethylene polymerizations using cationic palladium catalyst 2 without any cocatalysts were screened from 15 to 100 °C at 3 psig pressure of ethylene (entries 1−6 in Table 1). As we initially envisioned, the introduction of dibenzobarrelene backbone significantly

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RESULTS AND DISCUSSION Synthesis and Crystal Structure of Palladium Complexes. 9,10-Dihydro-9,10-ethanoanthracene-11,12-dione was prepared by Diels−Alder addition of commercially available anthracene with vinylene carbonate, hydrolysis, and Swern oxidation reactions.70,71 The condensation reaction of the αdione compound with the 2,6-diisopropylaniline facilely produced the α-diimine ligand L1 in a high yield.69 The new chloromethylpalladium complex 1 was obtained by complexation of the ligand L1 and Pd(COD)MeCl (COD: 1,5cyclooctadiene) (Scheme 1).72,73 The chloromethylpalladium complex was further treated with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBArF) and acetonitrile to yield the cationic palladium complex 2 [(α-diimine)Pd(CH3CN)Me]+BArF−, which was confirmed by 1H and 13C NMR spectroscopy, elemental analysis, and X-ray crystallography (see Experimental Section). B

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Figure 3. Plots of Mn (▲) and Mw/Mn (PDI) (■) as a function of polymerization time using 2 at 25 °C and GPC traces at different times. Figure 2. Crystal structure of α-diimine palladium complex 2. Hydrogen atoms and the anion BArF− are omitted for clarity.

1000C. The branching density is nearly independent to polymerization temperature (25−100 °C) and ethylene pressure (entries 9 and 10 in Table 1), except that a slightly decreasing branching density was observed at 15 °C. This observation is similar to those reported for polymers produced using the standard α-diimine palladium 3.80−82 Strikingly, the dibenzobarrelene derived α-diimine palladium catalyst 2 shows improved thermal stability, which is attributed to the unique nature of the bulky backbone. The steric demand of the dibenzobarrelene backbone is expected to inhibit the rotation of N−aryl bonds by the repulsive interaction of the bulky backbone with aniline moieties, thereby preventing the palladium catalyst decomposition by the C−H activation.69,75−79 Copolymerization of Ethylene and Acrylate Monomers. High thermal stability and living nature of palladium catalyst 2 for ethylene polymerization motivated us to conduct living copolymerizations of ethylene and MA because of the high regioselectivity of α-diimine palladium catalysts for this most common comonomer.35,36 The MA incorporation in copolymer was clearly identified by 1H NMR spectroscopy of ethylene−methyl acrylate copolymer (EMA) (Figure S6), and no MA homopolymer was present in copolymerization products.38 The influences of reaction parameters on the copolymerization of ethylene and MA using cationic palladium catalyst 2 were investigated in detail. With increased reaction temperature from 15 to 50 °C, copolymerization yield and copolymer molecular weight reached the maximum values at 25 °C while the MA incorporation increased consistently and PDI became broad (entries 1−4 in Table 2). Increasing MA concentration resulted in an increasing incorporation of comonomer up to 4.32 mol %; however, copolymerization yield and copolymer molecular weight decreased and PDI became broad (entries 2, 5, and 6 in Table 2). In comparison to the standard α-diimine palladium catalyst 3 (PDI = 1.6−1.8), the dibenzobarrelene derived α-diimine palladium catalyst 2 produced more narrowly dispersed EMA copolymers.35 Narrowly dispersed ethylene−methyl acrylate copolymers (EMA) (PDI < 1.10) were obtained by catalyst 2, suggesting living fashion. Copolymerization results catalyzed by 2 at 25 °C also showed that Mn of EMA copolymers grew linearly and Mw/Mn (PDI) values were below 1.20 with prolonged polymerization time, and the incorporation of MA remained nearly constant (∼1.85 mol %) (Figure 4). This result suggests that the comonomer MA is equally incorporated into polyethylene chains, and palladium-catalyzed living copolymerization of ethylene and MA by CIP is undoubtedly achieved. In contrast to the previous copolymerization using the sandwich

Table 1. Ethylene Polymerization Results Using Palladium Catalyst 2a entry

T (°C)

press. (psig)

yield (mg)

act.b

Mn (kg/mol)

PDIc

BDd

1 2 3 4 5 6 7 8 9e 10e

15 25 35 45 55 65 80 100 25 25

3 3 3 3 3 3 3 3 75 300

207 284 434 531 592 457 389 134 677 765

10.4 14.2 21.7 26.5 29.6 22.8 19.5 6.7 67.7 76.5

16.8 40.3 31.7 24.6 24.2 12.2 7.0 3.0 50.4 54.2

1.05 1.04 1.45 1.48 1.52 1.55 1.45 1.36 1.17 1.19

87 96 97 98 98 97 99 101 95 94

Polymerization conditions: 10 μmol of Pd, 2 h, 28 mL of toluene, and 2 mL of CH2Cl2. bActivity: kg PE/(mol Pd·h). cMn and PDI were determined by gel permeation chromatography (GPC) in 1,2,4trichlorobenzene at 150 °C using a light scattering detector. d Branching density, branches per 1000 carbon atoms determined by 1 H NMR spectroscopy. eReaction time: 1 h. a

enhanced the thermal stability of α-diimine palladium catalysts.69 Under atmospheric pressure, catalyst 2 showed the highest activity at 55 °C. When the reaction temperature was increased from 55 to 100 °C, the polymerization activity gradually decreased. Importantly, the palladium black generating from the decomposition of the palladium catalyst in the polymerizing mixture was not observed for catalyst 2 below 55 °C, whereas it usually appeared for the standard palladium catalyst 3 with dimethyl backbone (see Scheme 1) above room temperature.79 The plot of turnover frequency (TOF) versus reaction time further showed a nearly unchanged ethylene consumption (Figure S3), suggesting that the palladium catalyst 2 is stable at 55 °C over a 2 h period under the adopted conditions. Living ethylene polymerizations were successfully achieved using catalyst 2 below 25 °C (entries 1 and 2 in Table 1). Figure 3 shows that GPC traces of the polymers obtained at different polymerization times at 25 °C are symmetric and shift to the high molecular weight region with the prolonging polymerization time. Plots of number-average molecular weight (Mn) as a function of polymerization time also illustrate that Mn grows linearly with the polymerization time, and Mw/Mn (PDI) values are below 1.10. The amorphous polyethylenes obtained by catalyst 2 are highly branched and have the branching densities of 87−101/ C

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Macromolecules Table 2. Copolymerization Results of Ethylene and Acrylate Monomers Using Palladium Catalyst 2a entry

comonomer (M)

temp (°C)

conc (M)

PE (psig)

time (h)

yield (mg)

incorpb (mol %)

Mnc (kg/mol)

PDIc

BDd

1 2 3 4 5 6 7e 8e 9 10

MA MA MA MA MA MA MA MA BA DA

15 25 35 50 25 25 25 25 25 25

1.1 1.1 1.1 1.1 0.55 2.2 2.2 4.4 1.1 1.1

3 3 3 3 3 3 75 300 3 3

6 6 6 6 6 6 1 0.5 6 6

373 430 239 145 531 157 483 308 219 280

1.35 1.84 3.14 4.64 1.01 4.32 0.69 0.55 2.03 2.66

33.0 36.3 17.9 12.6 39.2 13.4 32.7 31.1 20.7 17.1

1.07 1.08 1.19 1.30 1.06 1.12 1.20 1.15 1.18 1.20

100 99 93 91 102 93 105 100 109 111

a Polymerization conditions: 20 μmol of Pd, 2 h, 30 mL of CH2Cl2. bDetermined by 1H NMR spectroscopy. cMn and PDI were determined by gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C using a light scattering detector. dBranching density, branches per 1000 carbon atoms determined by 1H NMR spectroscopy. e50 mL of CH2Cl2.

Figure 4. Plots of Mn (▲) and MA incorporation (●) as a function of polymerization time using 2 at 25 °C and GPC traces at different times. Figure 5. Dependency of intrinsic viscosity on molecular weight of EMA copolymers obtained by 2 at different ethylene pressures.

α-diimine palladium catalyst bearing bulky 8-p-tolynaphthyl substituents,38 catalyst 2 produced narrowly dispersed EMA copolymers without MA homopolymers in the absence of galvinoxyl as a radical inhibitor, and no palladium-mediated radical species formed in the copolymerization mixtures. 1 H NMR spectroscopy analysis showed that the obtained EMA copolymers were also highly branched (90−99/1000C). As in previous reports, copolymer topology could be tuned by changing ethylene pressure using the standard α-diimine palladium catalyst 3 although living characteristics were not observed.22 Similarly, the branching density of copolymers catalyzed by the dibenzobarrelene derived catalyst 2 was nearly independent of ethylene pressure (entries 2, 7, and 8 in Table 2). Despite the mild change of total branching density in the ethylene pressure range from 3 to 300 psig, branching distributions of the obtained PEs determined by 13C NMR spectroscopy revealed that the branch-on-branch structures generated through tertiary carbons (sec-butyl group) (19.44 and 11.72 ppm) increased with reducing ethylene pressure (Table S5 in Suporting Information), suggesting a more hyperbranched topology at low ethylene polymerization pressure.38 The change of branching topology with variation in ethylene pressure was further reflected by the dependency of intrinsic viscosity on polymer molecular weight detected by high temperature gel permeation chromatography with a viscosity detector. As shown in Figure 5, the intrinsic viscosity of the obtained copolymer increased with increasing ethylene pressure under the same molecular weight, indicating that the copolymer topology changed. The more linear EMA copolymer was formed at high pressure of 300 psig whereas the hyperbranched EMA copolymer was produced at low pressure of 3 psig using

catalyst 2. Compared with the branching topology control in ethylene homopolymerization (Figure S5), the polymer branching topology control was similar in ethylene−MA copolymerization using catalyst 2. It is noteworthy that copolymerizations of ethylene and MA at different ethylene pressures using catalyst 2 afforded narrowly dispersed copolymers (PDI < 1.2), suggesting living nature. Therefore, it is possible to synthesize EMA copolymers with precise molecular weight, monomer composition, and branching topology using catalyst 2. Copolymerizations of ethylene with other polar monomers including butyl acrylate (BA) and dodecyl acrylate (DA) were also conducted. Copolymerization results (entries 9 and 10 in Table 2) showed that catalyst 2 was also capable of catalyzing copolymerizaions of ethylene with other acrylates in a living fashion. With increased bulk of the acrylate substituent, copolymerization yield and copolymer molecular weight decreased while the incorporation of polar monomers increased. This could be attributed to electronic factor of the acrylate substituent.35,36 The bulky acrylate binds more strongly to the electrophilic palladium center because of increased electron-donating effect, thus improving the incorporation of polar monomers. Besides, the six-membered chelates with bulky substituents open difficultly in the presence of ethylene because of electron-donating effect, thus decreasing copolymerization yield and copolymer molecular weight. 13 C NMR spectroscopies of all amorphous ethylene−acrylate copolymers showed that the acrylate functionality was D

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Macromolecules completely located at the end of branches (see Figures S9− S14), and no characteristic signals of the α-CH carbon atom on the polyethylene main chain adjacent to ester groups were observed at 40−50 ppm.83 This result is consistent with previous observations regarding the standard α-diimine palladium catalyst 3, which is reasonably interpreted by the mechanistic model presented by Brookhart.35,36 Acrylate monomers bind palladium center by π-coordination mode and selectively insert into a Pd−Me bond in a 2,1-mode to form a six-membered ester chelate complexes. Further chain growth yields highly branched polymers with terminal ester functionality by coordination and insertion of ethylene. Mechanistic Studies. To gain a deep insight into living copolymerization nature, the MA chelate palladium complex was prepared by treatment of chloromethylpalladium complex 1 with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBArF) and MA at room temperature (Scheme 2). This

Figure 7. Crystal structure of cationic α-diimine palladium complex 4. Hydrogen atoms and the anion BArF− are omitted for clarity.

(∼95% selectivity).35,36 Presumably, the rigid dibenzobarrelene backbone can help to keep the steric bulk at the active palladium center for 2,1-insertion of MA.84 The high selectivity of 2,1-insertion is crucial for achieving living copolymerization because stable five-membered palladacycle intermediates formed by 1,2-insertion of acrylate have been reported to retard the copolymerization.85,86 Overall, high selectivity of 2,1insertion of acrylates, high thermal stability of dibenzobarrelene palladium catalyst, and suppression of chain transfer are cooperatively responsible for the living copolymerization of ethylene and acrylate monomers.

Scheme 2. Reaction Path of α-Diimine Palladium Complex 4

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CONCLUSIONS In summary, our work has addressed the important challenge that is to precisely synthesize functionalized polyolefins by living coordination copolymerization of ethylene and polar monomers under mild reaction conditions. The strategy of increasing steric bulk of α-diimine ligand backbone that we describe here provides a viable access to efficiently enhancing the thermal stability of the palladium catalyst and improving the selectivity of 2,1-insertion of acrylate monomers to prevent polar groups from poisoning palladium centers by stable fivemembered palladacycle intermediates formed by 1,2-insertion mode. The dibenzobarrelene-derived palladium catalyst 2 showed living nature and improved thermal stability up to 55 °C for ethylene polymerization. Living copolymerizations of ethylene and MA were successfully achieved using catalyst 2, which is contrast to general knowledge that polar monomers are a typical poison to the transition metal catalysts. In addition to control over the molecular weight, polydispersity, and composition of the EMA copolymers produced, the copolymerization of ethylene with MA catalyzed by the dibenzobarrelene-derived palladium catalyst enabled the precise tuning of the branching topology by changing ethylene pressure. The palladium-catalyzed CIP process tolerates a variety of acrylate monomers, which provides a basis for the precision design of functionalized polyolefins. Future work is focused on copolymerizations of ethylene with various polar monomers.

reaction path was used to simulate the insertion of MA into the propagation chain if the methyl connected on the pallladium metal was regard as a short polymer chain. 1H NMR analysis of the isolated palladium product showed that only one chelate palladium complex 4 chelating six-membered ester [(L1)Pd(CH2)3C(O)OMe]+BArF− was detected under the determined conditions (Figure 6).35,36 X-ray crystal structure of cationic

Figure 6. 1H NMR spectrum of cationic α-diimine palladium complex 4.

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palladium complex 4 further proved a six-membered ester ancillary ligand structure (Figure 7). These results strongly suggest that MA inserts into a Pd−Me bond in a 2,1-mode with an ∼100% selectivity, and then the primary insertion product rearranges to a six-membered chelate 4 by rapid isomerization (Scheme 2). For comparison, the α-diimine palladium catalyst bearing dibenzobarrelene showed the higher migratory insertion selectivity of MA (∼100% selectivity for 2,1-insertion of MA) than the standard α-diimine palladium catalyst 3

EXPERIMENTAL SECTION

General Procedures. All manipulations involving air- and moisture-sensitive compounds were carried out under an atmosphere of dried and purified nitrogen using standard vacuum-line, Schlenk, or glovebox techniques. Materials. Dichloromethane was distilled from CaH2 under nitrogen, and toluene and hexane were from Na/K alloy. Anthracene, vinylene carbonate, and trifluoroacetic acid were purchased from E

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into the autoclave. The ethylene pressure was raised to the specified value, and the reaction was carried out for a certain time. Polymerization was terminated by addition of triethylsilane after releasing ethylene pressure. After solvents were removed on a rotary evaporator, the obtained polymer was dissolved in hexane. The solution of polymer was filtered through a plug of silica gel to remove palladium black before precipitating in methanol. The resulting polymers were collected and treated by filtering, washing with methanol several times, and drying under vacuum at 40 °C to a constant weight. Synthesis of α-Diimine Compounds. α-Diimine ligand L1 was synthesized according to the literature.69 L1 was fully confirmed by 1H and 13C NMR spectroscopies. L1, Ar−NC(An)−(An)CN−Ar (An = dibenzobarrelene, Ar = 2,6-diisopropylphenyl): 1H NMR (CDCl3, 400 MHz), δ (ppm): 7.25−7.05 (m, 14H, Ph), 4.978 (s, 2H, CH), 2.49 (m, 4H, CH), 1.15 (d, 12H, CH3), 1.02 (d, 12H, CH3). 13C NMR (CDCl3, 100 MHz), δ (ppm): 158.45, 145.56, 138.57, 136.38, 127.27, 125.40, 124.12, 122.79, 51.10, 23.29, 22.49. Anal. Calcd for C40H44N2: C, 86.91; H, 8.02; N, 5.07. Found: C, 86.95; H, 7.93; N, 5.12. Synthesis of Neutral α-Diimine Palladium Complex 1. The neutral α-diimine palladium complex 1 was synthesized by the reaction of (COD)PdMeCl with the corresponding α-diimine ligand L1 in dichloromethane. A typical synthetic procedure for 1 was described as follows: 1.2 mmol of ligand L1 and 1.0 mmol of (COD)PdMeCl were added to a Schlenk tube together with 20 mL of dichloromethane, and the reaction mixture was then stirred for 12 h at room temperature. The solution was evaporated under vacuum to 5 mL, and then 30 mL of hexane was added. Complex 1 was obtained as an orange-red powder in 76% yield. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.30− 7.22 (m, 14H, Ar−H), 4.95 (s, 1H, CH), 4.90 (s, 1H, CH), 2.79 (m, 4H, CH), 1.47−1.38 (d, 12H, CH3), 1.16−1.12 (d, 12H, CH3), 0.52 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3), δ (ppm): 171.89, 167.03, 140.39, 140.00, 139.35, 137.97, 137.23, 128.66, 128.49, 128.28, 127.55, 126.26, 126.22, 124.10, 123.43, 51.35, 50.48, 29.45, 29.07, 28.66, 24.31, 23.88, 23.39, 23.28, 3.95. Anal. Calcd for C41H47ClN2Pd: C, 69.39; H, 6.68; N, 3.95; Found: C, 69.34; H, 6.66; N, 4.01. Synthesis of Cationic α-Diimine Palladium Complex 2. The cationic α-diimine palladium complex 2 was synthesized by the reaction of NaBArF and acetonitrile with neutral palladium complex 1 in dichloromethane. A typical synthetic procedure for 2 can be described as follows: 1.0 mmol of palladium complex 1, 1.2 mmol of NaBArF, and 2 mL of acetonitrile were added to a Schlenk tube together with 20 mL of dichloromethane, and the reaction mixture was then stirred for 12 h at room temperature. The solution was filtered by Celite and then evaporated under vacuum, to which 5 and 30 mL of hexane were added. The palladium complex 2 as a yellow powder was obtained in 83% yield. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.72 (s, 8H, Ar−H in BArF−), 7.54 (s, 4H, BArF−), 7.40−7.28 (m, 14H, Ar− H), 5.01 (d, 2H, CH), 2.62 (m, 4H, CH(CH3)2), 1.72 (s, 3H, CH3), 1.23−1.18 (d, 12H, CH3), 1.28−1.23 (d, 12H, CH3), 0.55 (s, 3H, PdCH3). 13C NMR (100 MHz, CDCl3), δ (ppm): 176.58, 169.13, 162.27, 161. 84, 161.61, 161.28, 141.19, 141.09, 135.85, 134.89, 129.97, 128.92, 128.68, 127.59, 125.98, 123.73, 117.59, 51.35, 50.48, 29.45, 29.07, 28.66, 24.31, 23.88, 23.39, 23.28, 3.95, 1.64. Anal. Calcd for C75H62BF24N3Pd: C, 57.07; H, 3.96; N, 2.66; Found: C, 56.92; H, 3.91; N, 2.69. Synthesis of Cationic α-Diimine Palladium Complex 4. The cationic α-diimine palladium complex 4 was synthesized by the reaction of NaBArF and MA with neutral palladium complex 1 in Et2O. A typical synthetic procedure for 4 can be described as follows: 1.0 mmol of palladium complex 1, 1.2 mmol of NaBArF, and 0.11 mL of MA were added to a Schlenk tube together with 20 mL of Et2O, and the reaction mixture was then stirred for 12 h at room temperature. The solution was filtered by Celite and then evaporated under vacuum, to which 5 and 30 mL of hexane were added. The palladium complex 4 as orange powders was obtained in 85% yield. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.70 (s, 8H, Ar−H in BArF−), 7.51 (s, 4H, Ar−H in BArF−), 5.06−4.98 (d, 2H, CH), 2.98 (s, 3H, OCH3), 2.65 (m, 4H, CH(CH3)2), 2.33 (t, 2H, CH2C(O)), 1.36 (t, 2H, PdCH2), 1.30−1.15

Energy Chemical and used as received. 2,6-Diisopropylaniline was purchased from Aldrich Chemical and was distilled under reduced pressure before use. Ethylene (99.99%) was purified by passing through Agilent moisture and oxygen traps. Acrylate monomers including methyl acrylate (MA), butyl acrylate (BA), and dodecyl acrylate (DA) were dried over CaH2 and then freshly distilled under vacuum prior to use in polymerization. Other commercially available reagents were purchased and used without purification. Measurements. Elemental analyses were performed on a Vario EL microanalyzer. NMR spectra of compounds were carried out on Bruker 400 or 500 MHz instruments in CDCl3 using TMS as a reference. The total branching density per 1000 carbon atoms of polymers was determined by integrating methyl proton signals with respect to signals of all protons in the 1H NMR spectrum. GPC analyses of the molecular weights and molecular weight distributions (PDI = Mw/Mn) of the polymers at 150 °C were performed on a PLGPC 220 high-temperature chromatograph equipped with a tripledetection array, including a differential refractive-index detector, a twoangle light-scattering detector (15° and 90°), and a four-bridge capillary viscometer. The laser wavelength was 658 nm. One guard column (PLgel 10 μm 50 × 7.5 mm) and three 30 cm columns (PLgel 10 μm MIXED-B 300 × 7.5 mm) were used. 1,2,4-Trichlorobenzene (TCB) was used as the eluent at a flow rate of 1.0 mL/min. Cirrus Multi Online GPC/SEC Software from Polymer Laboratories was used for data collection and analysis. The GPC was calibrated with narrow polystyrene standards (Polymer Laboratories). The DRI increment (dn/dc) value of 0.078 mL/g was used for all polyethylenes, and the value of 0.185 mL/g was used for polystyrene standards. Two polystyrene standards (30 000 and 100 000 g/mol) were measured to have a typical Mw value of 30.1 and 102.4 kg/mol in this system, respectively, with a PDI value of 1.00 for both, which are in good agreement with the data provided from the supplier. Crystal Structure Determination. The crystals were mounted on a glass fiber and transferred to a Bruker CCD platform diffractometer. Data obtained with the ω−2θ scan mode were collected on a Bruker SMART 1000 CCD diffractometer with graphite-monochromated Cu Kα radiation (λ= 1.541 84 Å) or Mo Kα radiation (λ = 0.710 73 Å). The structures were solved using direct methods, while further refinement with full-matrix least-squares on F2 was obtained with the SHELXTL program package. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced in calculated positions with the displacement factors of the host carbon atoms. Homo- and Copolymerization of Ethylene at Atmosphere Pressure. A round-bottom Schlenk flask with stirring bar was heated for 3 h at 150 °C under vacuum and then cooled to room temperature. The flask was pressurized to 3 psig of ethylene and vented three times. The appropriate solvent (toluene or CH2Cl2) and polar monomers for copolymerization were added into the glass reactor under 3 psig of ethylene. The system was continuously stirred for 5 min, and then 2 mL of a solution of palladium complex in CH2Cl2 was added by syringe to the well-stirred solution. The ethylene pressure was kept constant at 3 psig by continuous feeding of gaseous ethylene throughout the reaction. The other reaction temperatures were controlled with an external oil bath or a cooler in polymerization experiments. Polymerization was terminated by addition of triethylsilane after the reaction was carried out for a certain time. After solvents were removed on a rotary evaporator, the obtained polymer was dissolved in hexane. The solution of polymer was filtered through a plug of silica gel to remove palladium black before precipitating in methanol. The resulting polymers were collected and treated by filtering, washing with methanol several times, and drying under vacuum at 40 °C to a constant weight. Homo- and Copolymerization of Ethylene at High Pressure. A mechanically stirred 100 mL Parr reactor was heated to 150 °C for 2 h under vacuum and then cooled to room temperature. The autoclave was pressurized with ethylene and vented three times. The autoclave was then charged with appropriate solvent (toluene or CH2Cl2) and polar monomers for copolymerization at initialization temperature. The system was maintained by continuously stirring for 5 min, and then 2 mL of solution of palladium catalyst in CH2Cl2 was charged F

DOI: 10.1021/acs.macromol.7b01132 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (m, 24H, CH(CH3)2), 0.50 (pentet, 2H, PdCH2CH2CH2C(O)). 13C NMR (100 MHz, CDCl3), δ (ppm): 182.50 (CO), 175.13, 168.10, 162.45, 162.05, 161.65, 161.25, 139.50, 138.69, 136.57, 135.98, 134.97, 129.63, 129.53, 129.43, 129.16, 128.92, 128.38, 126.55, 126.20, 125.79, 124.66, 123.99, 123.62, 117.60, 54.53, 51.15, 50.14, 35.24, 30.84, 29.04, 28.80, 24.06, 23.69, 22.92, 22.71. Anal. Calcd for C75H62BF24N3Pd: C, 56.96; H, 4.04; N, 1.73; Found: C, 56.90; H, 4.01; N, 1.69.

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(10) Liu, S.; Elyashiv, S.; Sen, A. Copper-mediated controlled copolymerization of methyl acrylate with 1-alkenes under mild conditions. J. Am. Chem. Soc. 2001, 123, 12738. (11) Venkatesh, R.; Klumperman, B. Olefin copolymerization via controlled radical polymerization: copolymerization of methyl methacrylate and 1-octene. Macromolecules 2004, 37, 1226. (12) Venkatesh, R.; Harrisson, S.; Haddleton, D. M.; Klumperman, B. Olefin copolymerization via controlled radical polymerization: copolymerization of acrylate and 1-octene. Macromolecules 2004, 37, 4406. (13) Venkatesh, R.; Vergouwen, F.; Klumperman, B. Atom transfer radical copolymerization of α-olefins with methyl acrylate: determination of activation rate parameters. Macromol. Chem. Phys. 2005, 206, 547. (14) Tanaka, K.; Matyjaszewski, K. Controlled copolymerization of n-butyl acrylate with nonpolar 1-alkenes using activators regenerated by electron transfer for atom-transfer radical polymerization. Macromolecules 2007, 40, 5255. (15) Liu, S.; Gu, B.; Rowlands, H. A.; Sen, A. Controlled random and alternating copolymerization of methyl acrylate with 1-alkenes. Macromolecules 2004, 37, 7924. (16) Venkatesh, R.; Staal, B. B. P.; Klumperman, B. Olefin copolymerization via reversible addition-fragmentation chain transfer. Chem. Commun. 2004, 1554. (17) Mishima, E.; Tamura, T.; Yamago, S. Controlled copolymerization of 1-octene and (meth)acrylates via organotellurium-mediated living radical polymerization (TERP). Macromolecules 2012, 45, 8998. (18) Kermagoret, A.; Debuigne, A.; Jerome, C.; Detrembleur, C. Precision design of ethylene- and polar-monomer-based copolymers by organometallic-mediated radical polymerization. Nat. Chem. 2014, 6, 179. (19) Berkefeld, A.; Mecking, S. Coordination copolymerization of polar vinyl monomers H2CCHX. Angew. Chem., Int. Ed. 2008, 47, 2538. (20) Carrow, B. P.; Nozaki, K. Transition-metal-catalyzed functional polyolefin synthesis: effecting control through chelating ancillary ligand design and mechanistic insights. Macromolecules 2014, 47, 2541. (21) Chen, E. Y. X. Coordination polymerization of polar vinyl monomers by single-site metal catalysts. Chem. Rev. 2009, 109, 5157. (22) Chen, G.; Ma, X.; Guan, Z. Synthesis of functional olefin copolymers with controllable topologies using a chain-walking catalyst. J. Am. Chem. Soc. 2003, 125, 6697. (23) Shi, X.; Zhao, Y.; Gao, H.; Zhang, L.; Zhu, F.; Wu, Q. Synthesis of hyperbranched polyethylene amphiphiles by chain walking polymerization in tandem with RAFT polymerization and supramolecular self-assembly vesicles. Macromol. Rapid Commun. 2012, 33, 374. (24) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Neutral, single-component nickel (II) polyolefin catalysts that tolerate heteroatoms. Science 2000, 287, 460. (25) Zhao, Y.; Gao, H.; Liang, G.; Zhu, F.; Wu, Q. Synthesis of welldefined amphiphilic branched polyethylene-graft-poly (N-isopropylacrylamide) copolymers by coordination copolymerization in tandem with RAFT polymerization and their self-assembled vesicles. Polym. Chem. 2014, 5, 962. (26) Nakamura, A.; Anselment, T. M. J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.; van Leeuwen, P. W. N. M.; Nozaki, K. Ortho-phosphinobenzenesulfonate: a superb ligand for palladium-catalyzed coordination−insertion copolymerization of polar vinyl monomers. Acc. Chem. Res. 2013, 46, 1438. (27) Hu, H.; Chen, D.; Gao, H.; Zhong, L.; Wu, Q. Amine-imine palladium catalysts for living polymerization of ethylene and copolymerization of ethylene with methyl acrylate: incorporation of acrylate units into the main chain and branch end. Polym. Chem. 2016, 7, 529. (28) Tao, W.; Nakano, R.; Ito, S.; Nozaki, K. Copolymerization of ethylene and polar monomers by using Ni/IzQO catalysts. Angew. Chem., Int. Ed. 2016, 55, 2835.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01132. NMR spectra of palladium complexes, 1H and 13C NMR of polymers and assignments, crystallographic data, results of olefin polymerization, and GPC traces (PDF) Crystallographic data for palladium complex 1 (CIF) Crystallographic data for palladium complex 2 (CIF) Crystallographic data for palladium complex 4 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*(H.G.) Fax +86-20-84114033; Tel +86-20-84113250; e-mail [email protected]. ORCID

Haiyang Gao: 0000-0002-7865-3787 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from National Natural Science Foundation of China (NSFC) (Projects 21374134 and 21674130), Natural Science Foundation of Guangdong Province (1414050000552), and the Fundamental Research Funds for the Central Universities (17lgjc02).

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01132 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01132 Macromolecules XXXX, XXX, XXX−XXX