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Systematic Investigations of Ligand Steric Effects on α‑Diimine Palladium Catalyzed Olefin Polymerization and Copolymerization Shengyu Dai, Shixin Zhou, Wen Zhang, and Changle Chen* CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: In the Brookhart type α-diimine palladium catalyst system, it is highly challenging to tune the polymer branching densities through ligand modifications or polymerization conditions. In this contribution, we describe the synthesis and characterization of a series of α-diimine ligands and the corresponding palladium catalysts bearing both the dibenzhydryl moiety and with systematically varied ligand sterics. In ethylene polymerization, it is possible to tune the catalytic activities ((0.77−8.85) × 105 g/ (mol Pd·h)), polymer molecular weights (Mn: (0.2−164.7) × 104), branching densities (25−116/1000C), and polymer melting temperatures (amorphous to 98 °C) over a very wide range. In ethylene−methyl acrylate (E−MA) copolymerization, it is possible to tune the catalytic activities ((0.3−8.8) × 103 g/(mol Pd·h)), copolymer molecular weights (1.1 × 103−79.8 × 103), branching densities (30−119/1000C), and MA incorporation ratio (0.4−13.8%) over a very wide range. The molecular weights and branching densities could also be tuned in α-olefin polymerization. The tuning in polymer microstructures leads to significant tuning in polyethylene mechanical properties and the surface properties of the E− MA copolymer.
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parameters would be the modifications of the α-diimine ligands. However, this has also proven to be highly challenging to achieve. Brookhart et al. showed that the polyethylene branching density could only be slightly decreased by increasing the steric bulkiness of the ortho substituents on the α-diimine ligands (Chart 1, at 11 atm ethylene pressure and 25 °C, 100/1000C for Ia with iPr; 106/1000C for Ib with Me).31 Wu et al. investigated some α-diimine Pd(II) catalysts (Chart 1, II) with various backbone structures such as camphyl, phenyl, 4-fluorophenyl, and 4-methylphenyl.32 These Pd(II) complexes generated polyethylene with branching densities ranging from 84 to 107 per 1000 carbon atoms. Guan et al. studied a α-diimine Pd(II) catalyst (Chart 1, IIIa) bearing a macrocyclic ligand, which showed much higher thermal stability than the acyclic analogue.33 This catalyst generated polymers with significantly high branching density (>100/1000C). Interestingly, the fluorinated counterpart (Chart 1, IIIb) led to the formation of polyethylene with high molecular weight and very low branching density (51−60/1000C).34 The direct interaction between the fluorine atom and the metal center was proposed to be responsible. Catalyst IV bearing C6 hydrocarbon bridges exhibited very low activities (TON < 10) in ethylene polymerization, which was attributed to C−H activation reactions.35 The Pd(II) catalyst with a pendant pyridine group (Chart 1, V) only showed moderate activity in ethylene
INTRODUCTION
Since the Nobel Prize winning discovery of Ziegler and Natta catalysts, transition metal catalyzed olefin polymerization has enjoyed great successes both in industry and in academia. A milestone discovery in this field is the study of the α-diimine Pd(II) and Ni(II) catalysts by Brookhart et al. in the 1990s.1,2 Since then, numerous efforts have been directed to this area, leading to the developments of various α-diimine-based metal catalysts and a huge amount of functional polymers and copolymers.3−20 The most distinguishing feature of these catalysts is the chain walking mechanism, leading to the formation of highly branched polymeric materials. The branching density is an important parameter that greatly affects the polymer physical properties. A major factor determining the branching density is the relative rate of insertion of ethylene into a primary metal alkyl species versus insertion into a secondary metal alkyl species. The ratio of the rate of chain walking to the insertion rate also plays an important role. In the α-diimine Ni(II) catalysts, the polymer microstructures can be modulated from highly linear to highly branched by varying the polymerization conditions, such as ethylene pressures or polymerization temperatures.1,2,21−29 In the Pd(II) system, the topology and long chain branching distribution of the polymers can be controlled using ethylene pressure.3−12,30 However, the polymer molecular weight, branching density, and the distribution of short-chain branches are relatively independent of polymerization conditions such as ethylene pressures or polymerization temperatures. As a result, the only strategy to tune these © XXXX American Chemical Society
Received: September 28, 2016 Revised: November 9, 2016
A
DOI: 10.1021/acs.macromol.6b02104 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Chart 1. Selected Examples of Previously Reported α-Diimine Pd(II) Catalysts
Scheme 1. Synthesis of Ligands L1−L6 and Palladium Complexes 1−6
understood. In the α-diimine Pd(II) system, the very high polymer branching density is largely independent of simple ligand modifications with steric or electronic perturbations (Chart 1, VII), or the polymerization conditions. As a result, totally amorphous polymers and copolymers are usually generated, thus limiting the potential applications of these materials only in very specific areas. In addition, the regio- and stereo-controlled copolymerization of α-olefins with polar monomers became highly unlikely because of the chain walking properties of these catalysts. This represents a big limitation for the α-diimine type Pd(II) catalysts. Recently, we demonstrated the synthesis of polyethylene with very low branching densities (23−29/1000C) using some αdiimine Pd(II) complexes (Chart 1, IX) bearing a dibenzhydryl moiety.40 A similar feature was observed in ethylene−methyl acrylate copolymerization. Furthermore, the branching density of the resulting polyethylene could be lowered to 6/1000C by using the naphthalene or benzothiophene substituted α-diimine Pd(II) complexes.41 Despite these interesting properties, the dibenzhydryl-based α-diimine Pd(II) catalysts shared the same limitation with the classic α-diimine Pd(II) catalysts: the
polymerization with the addition of AlMe2Cl, affording polyethylene with high molecular weight and very low branching density (2−5/1000C).36 Unfortunately, the active species were not identified, and whether the very low branching density and the high molecular weight could be maintained in ethylene− methyl acrylate (MA) copolymerization was not studied. Brookhart et al. showed that highly branched polyethylene (102−117/1000C) could be generated using “sandwich” αdiimine palladium complexes VI.37 Guan et al. also showed that the α-diimine Pd(II) catalysts (Chart 1, VII) with electrondonating substituents led to higher polymer molecular weight and more linear topology in ethylene polymerization.38,39 However, the branching density was largely unaffected (94− 100/1000C). Takeuchi et al. showed that a dinuclear α-diimine Pd(II) complex bearing double-decker ligand (Chart 1, VIII) led to moderately branched polyethylene (55−72/1000C). Very limited examples including catalysts IIIb, V, and VIII could significantly reduce the polymer branching density in the α-diimine Pd(II) system. In these systems, a potential hemilabile interaction or a metal−metal cooperativity effect might be responsible and the exact operative mechanism was not fully B
DOI: 10.1021/acs.macromol.6b02104 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Molecular structure of the palladium complexes. Hydrogen atoms have been omitted for clarity. Atoms are drawn at 30% probability level. Selected bond lengths (Å) and angles (deg): (a) Complex 1: Pd1−N1 2.043(6), Pd1−C44 2.077(6), Pd1−N2 2.129(5), Pd1−Cl1 2.264(2), N1−Pd1− C44 97.7(3), N1−Pd1−N2 77.0(2), C44−Pd1−Cl1 87.6(2), N2−Pd1−Cl1 97.64(16). (b) Complex 2: Pd1−N1 2.060(2), Pd1−C46 2.097(3), Pd1− N2 2.133(2), Pd1−Cl1 2.2589(9), N1−Pd1−C46 98.02(12), N1−Pd1−N2 77.37(9), C46−Pd1−Cl1 86.61(10), N2−Pd1−Cl1 98.28(7). (c) Complex 3: Pd1−N2 2.0636(15), Pd1−C51 2.0819(18), Pd1−N1 2.1354(15), Pd1−Cl1 2.2759(7), N2−Pd1−C51 98.49(7), N2−Pd1−N1 76.68(6), C51−Pd1−Cl1 85.81(6), N1−Pd1−Cl1 99.07(5). (d) Complex 4: Pd1−N1 2.079(3), Pd1−N2 2.116(3), Pd1−C48 2.132(4), Pd1−Cl1 2.2740(13), N1−Pd1−N2 77.43(13), N1−Pd1−C48 98.67(14), N2−Pd1−Cl1 96.39(10), C48−Pd1−Cl1 87.55(11). (e) Complex 5: Pd1−N2 2.070(5), Pd1− C38 2.151(4), Pd1−N1 2.116(4), Pd1−Cl1 2.240(2), N2−Pd1−C38 98.66(18), N2−Pd1−N1 77.53(18), C38−Pd1−Cl1 86.12(14), N1−Pd1−Cl1 98.04(13). (f) Complex 6: Pd1−N1 2.059(2), Pd1−C71 2.104(5), Pd1−N2 2.115(2), Pd1−Cl1 2.2728(13), N1−Pd1−C71 92.72(15), N1−Pd1−N2 77.39(9), C71−Pd1−Cl1 92.17(14), N2−Pd1−Cl1 98.12(7).
Because of the unsymmetric nature of complexes 1−5, at least two Pd−Me signals were observed corresponding to the cis and trans isomers. For the cases of complexes 4 and 5, the situation is more complicated because of the presence of carbon stereocenter. For all the cases, 1H−13C HSQC analysis was carried out for the assignment of the Pd−Me signals. Interestingly, the polymerization and copolymerization were not influenced by the existence of isomers, since unimodal GPC curves and narrow polydispersity index were observed for all the polymers and copolymers generated using these palladium complexes. Probably, these isomers possess similar catalytic properties. Previously, it was demonstrated that Ni(II) complexes based on mixtures of the rac and meso α-diimine ligands led to the formation of polyethylene with bimodal molecular weight distribution in ethylene polymerization.48−50 Recently, we studied the properties of some Pd(II) complexes based on mixtures of the rac and meso α-diimine ligands. Interestingly, a narrow polymer molecular weight distribution was observed in ethylene polymerization and copolymerization with methyl acrylate, which is similar to the results observed in the current study. The molecular structures of all six palladium complexes (1−6) were determined by X-ray diffraction analysis (Figure 1). For all the cases, the geometry at the Pd center is square planar. The bond distances and bond angles are typical comparing with previously reported α-diimine Pd(II) complexes.51−53 The systematic tuning of the ligand steric environment and the blocking of the axial position of the metal center from the
branching density of the resulting polymer products is relatively independent of polymerization conditions such as temperatures or ethylene pressures. The ability of the α-diimine palladium system to gradually tune the branching density of the polymer or copolymer products has not been demonstrated previously. Herein, we report a strategy to address this issue by synthesizing a series of α-diimine Pd(II) complexes bearing both dibenzhydryl moiety and systematically varied ligand steric environment. The ligand steric effect was found to dramatically affect the catalytic properties in ethylene polymerization, ethylene−methyl acrylate copolymerization, and α-olefin polymerization. Specifically, the branching density of the resulting polymeric materials can be adjusted over a very wide range.
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RESULTS AND DISCUSSION Synthesis and Characterization of the Palladium Complexes. The monoimine ligand (2,6-dibenzhydryl-4methylphenylimino)butanone was prepared from the reaction of 2,6-bis(diphenylmethyl)-4-methylaniline with 2 equiv of 2,3butadione at 80% yield on multigram scale (Scheme 1). Subsequently, the reaction with 1 equiv of the corresponding anilines led to the formation of the α-diimine ligands L1−L6 at 61−95% yields. It should be noted that this kind of stepwise synthesis of unsymmetric α-diimine or pyridine−diimine ligands has been widely studied previously.42−47 The reaction of ligands L1−L6 with 1 equiv of (COD)PdMeCl in CH2Cl2 afforded the desired palladium complexes 1−6 at 60−94% yields. These palladium complexes were characterized by 1H, 13C NMR spectroscopy, elemental analysis, and mass spectrometry. C
DOI: 10.1021/acs.macromol.6b02104 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Effect of Catalyst and Temperature on Ethylene Polymerizationa entry
cat.
T (°C)
yield (g)
act.b
Mnc (×10−4)
PDI
Bd
Tge (°C)
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 6 1 2 3 4 5 6
20 20 20 20 20 20 40 40 40 40 40 40
0.230 0.497 0.824 1.024 1.584 1.728 0.398 0.782 1.241 1.475 1.684 2.654
0.77 1.66 2.74 3.41 5.28 5.76 1.32 2.61 4.13 4.91 5.61 8.85
0.42 27.39 38.44 47.55 80.19 147.13 0.23 40.39 52.84 65.36 117.31 164.66
1.40 1.51 1.37 1.32 1.32 1.61 1.98 1.76 1.80 1.85 1.89 1.61
116 94 77 70 51 25 117 96 76 71 48 27
−76 −68 −68
Tme (°C) −26 −6 37 62 98
−75 −69 −66
−30 −2 52 68 96
Conditions: 1 μmol of precatalyst, 1.2 equiv of NaBAF, 2 mL of CH2Cl2, 48 mL of toluene, 8 atm, 3 h. bActivity (act.) = 105 g/(mol Pd·h). Molecular weight was determined by GPC in trichlorobenzene at 150 °C using polystyrene standards. dB = branches per 1000 carbons, determined by 1H NMR spectroscopy. eDetermined by differential scanning calorimetry (DSC). a c
Figure 2. (a) Polymer yield for catalysts 1−6 (Table 1, entries 1−6). (b) Branching density of the polyethylene generated using catalysts 1−6 (Table 1, entries 1−6).
Table 2. Ethylene−MA Copolymerizationa entry
cat.
P (atm)
[MA] (M)
yield (g)
act.b
XMAc (%)
Mnd (×10−3)
PDI
Be
1 2 3f 4 5 6f 7 8 9f 10 11 12f 13 14 15f 16 17 18f
1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6
1 1 8 1 1 8 1 1 8 1 1 8 1 1 8 1 1 8
1 2 5 1 2 5 1 2 5 1 2 5 1 2 5 1 2 5
0.094 0.053 0.047 0.143 0.093 0.078 0.265 0.126 0.166 0.323 0.193 0.314 0.424 0.263 0.841 0.567 0.320 1.32
0.63 0.35 0.31 0.95 0.62 0.52 1.77 0.84 1.11 2.15 1.29 2.09 2.83 1.75 5.61 3.78 2.13 8.80
3.9 6.9 13.8 3.2 5.8 6.9 2.6 3.4 0.93 2.1 2.7 0.76 1.2 1.9 0.56 0.54 1.1 0.38
3.78 1.13 1.32 4.94 1.97 2.31 21.43 9.25 35.70 32.56 9.21 45.42 68.32 5.63 58.68 47.2 7.49 79.80
2.13 2.36 1.21 2.19 2.15 1.26 2.14 1.84 1.64 2.04 1.96 1.40 2.15 2.26 1.86 1.78 1.60 2.27
116 119 117 99 102 95 78 79 76 74 71 70 54 53 51 31 30 31
a Conditions: 0.010 mmol of precatalyst, 1.2 equiv of NaBAF, total volume of toluene, CH2Cl2 and MA: 25 mL, 15 h, 30 °C. bActivity (act.) = 103 g/ (mol Pd·h). cXMA= MA incorp. (mol %). dMolecular weight was determined by GPC. eB = branches per 1000 carbons; branching numbers were determined using 1H NMR spectroscopy; the branches ending with functional groups are added to the total branches. f0.050 mmol of galvinoxyl was used to prevent radical polymerization of MA.
Ethylene Polymerization Studies. In the polymerization studies, an in situ activation procedure with the addition of 1.2
dibenzhydryl moiety can be clearly observed from these molecular structures. D
DOI: 10.1021/acs.macromol.6b02104 Macromolecules XXXX, XXX, XXX−XXX
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of galvinoxyl does not affect the MA incorporation (Figures S46 and S48). When galvinoxyl was not used at 8 ethylene pressure and 5 mol/L MA concentration, MA radical polymerization indeed happened. In this case, the characteristic peak for radical MA homopolymer was observed at ca. δ 2.4 corresponding to CH2CH(COOMe)CH2 (the integration ratio versus COOMe at ca. 3.6 ppm is close to 1/3; see Figure S44a). In all of our cases, this peak was not observed, suggesting the absence of MA radical polymerization. In addition, the NMR analysis of the MA homopolymer from free radical polymerization was carried out for comparison (Figure S44b). By changing the catalyst structures and the polymerization conditions, it is possible to tune the MA incorporation ratio (0.4−13.8%), the copolymer molecular weight (Mn: 1.1 × 103− 79.8 × 103), and the branching density (B: 30−119/1000C) over a very wide range. Similarly to previously reported α-diimine palladium catalysts,3−20 the MA unit was incorporated at the end of the polymer branches based on NMR analysis. It should be noted that the copolymers generated using catalysts 5 and 6 are semicrystalline solid in contrast to the oily products generated using catalysts 1−4. Melting temperatures of 53 °C (Table 2, entry 15) and 72 °C (Table 2, entry 18) were observed. α-Olefin Polymerization Studies. In α-olefin polymerization catalyzed by α-diimine palladium and nickel catalyst, the resulting polymers contain fewer branches than expected because of a significant fraction of 2,1 insertion followed by chain walking to the primary carbon and insertion. As such, the α-olefin is enchained in a 1,ω fashion, and longer α-olefin leads to more linear polymer microstructure (Scheme 2).55−58
equiv of tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF) was employed. At 20 °C and 8 atm of ethylene pressure, the polymerization activity was increased gradually with ligand sterics from complex 1 to complex 6 (Table 1 and Figure 2a). Meanwhile, the polyethylene molecular weight (Mn) was increased from 4.2 × 103 to 1.47 × 106. The PDI of the polyethylene product is larger than 1.3, suggesting that the system is not living under the current conditions. Most interestingly, the polymer branching density was decreased gradually from 116 to 25 per 1000 carbon atoms (Figure 2b). The polyethylene product could be tuned from amorphous materials with very low glass transition temperature (−76 °C for catalyst 1) gradually to semicrystalline materials with high melting temperature (98 °C for catalyst 6). The polyethylene microstructure analysis based on 13C NMR showed that methyl, ethyl, n-propyl, n-butyl, >C4 branches, and sec-butyl (branch on branch structure) exist for the products generated using catalysts 1−3 (Table S1). In contrast, only methyl and long chain (>C4) branches were observed for catalysts 5 and 6.54 The microstructure difference and the lower degree of branching density for the sterically bulkier catalysts are probable due to the relatively greater preference of insertion of ethylene into a primary metal alkyl species versus insertion into a secondary metal alkyl species. In addition, the greater ratio of insertion rate versus chain walking rate may also contribute. At 40 °C and 8 atm of ethylene pressure, the catalytic activities of all six palladium complexes were increased (Table 1, entries 7−12). The molecular weight was also increased significantly except complex 1, which might be due to catalyst decomposition. The branching density was generally not affected by polymerization temperature, which agrees with previous studies.3−20 The ligand electronic effect may play a role in this system since the dibenzhydryl group is electronically more withdrawing than the isopropyl group. However, this is highly unlikely in this system. Guan et al. and our group have investigated the ligand electronic effect in the α-diimine palladium catalyst system,38−41 which showed that the branching density of the polymer product was not affected by the ligand electronic effect. In this system, the polymer branching density is dramatically different, which is probably mainly due to the ligand steric effect. Ethylene−Methyl Acrylate (E−MA) Copolymerization Studies. Similar trends in catalytic activity, copolymer molecular weight, and copolymer branching density were observed in the EMA copolymerization (Table 2). In addition, the MA incorporation ratio was decreased gradually with ligand sterics from catalysts 1 to 6. For all the catalysts at 1 atm of ethylene pressure and 2 mol/L of MA concentration, the MA incorporation ratio was increased from that under 1 atm of ethylene pressure and 1 mol/L of MA concentration. An interesting effect was observed at high ethylene pressure (8 atm) and high MA concentration (5 mol/L). For catalysts 3−6, the MA incorporation ratio was decreased comparing with those under 1 atm of ethylene pressure and 2 mol/L of MA concentration. In contrast, an opposite trend was observed for catalysts 1 and 2. Probably, the sterically open catalysts 1 and 2 possessed higher reactivity toward the bulky MA monomer, making them much more suitable for MA incorporation under these conditions. Galvinoxyl was used to prevent MA radical polymerization in all the runs at 8 atm ethylene pressure and 5 mol/L MA concentration. At lower ethylene pressure and MA concentration (1 and 2 mol/L), no MA radical polymerization was observed without the addition of galvinoxyl. For catalysts 4 and 5 at 2 mol/L MA concentration, the addition of 0.050 mmol
Scheme 2. Modes of Monomer Enchainment in α-Diimine Palladium and Nickel Catalyzed α-Olefin Polymerization
In 1-hexene polymerization, the TOFs were decreased with increasing ligand sterics (Table 3, entries 1−6). Catalyst 3 showed the highest polymer molecular weight (Mn = 2.9 × 105) among these catalysts. The branching density could be only tuned over a narrow range (77−101/1000C) comparing with that in ethylene polymerization. This is probably due to the low coordination capability of α-olefin monomer. In ethylene polymerization, secondary Pd−alkyl species could be trapped by ethylene monomer. However, it is much more difficult for the α-olefin monomer to trap the secondary Pd−alkyl species. The polymer branching density is gradually decreased with the increasing ligand sterics, suggesting higher degree of 2,1 insertion and chain straightening for sterically bulky catalysts. The microstructure analysis of the polymer samples based on 13C NMR showed the presence of only methyl, butyl, and long chain branches for all the catalysts (Table S2). The absence of ethyl, propyl, and adjacent methyl branches indicates no occurrence of 1-hexene insertion into secondary Pd−alkyl bond. Similarly to previous reports,56 the isomerization of 1-hexene to internal hexenes is competitive with the polymerization (17−55% of 1hexene was isomerized to internal hexenes, Table S2). E
DOI: 10.1021/acs.macromol.6b02104 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 3. Polymerizations of α-Olefinsa entry
cat.
monomer
[M] (mol/L)
T (°C)
yield (g)
TOFb
Mnc (×10−4)
PDI
Bd
Tme
1 2 3 4 5 6 7 8 9 10 11 12 13
1 2 3 4 5 6 6 6 6 6 6 6 6
1-hexene 1-hexene 1-hexene 1-hexene 1-hexene 1-hexene 1-hexene 1-hexene 1-hexene 1-hexene 1-octene 1-decene 1-hexadecene
3.3 3.3 3.3 3.3 3.3 3.3 0.66 6.6 3.3 3.3 3.3 3.3 3.3
20 20 20 20 20 20 20 20 40 60 20 20 20
1.82 1.61 1.47 0.80 0.40 0.26 0.13 0.56 0.29 0.21 0.42 0.54 0.67
721 638 582 317 159 105 51 222 114 83 125 129 100
0.42 17.21 28.92 8.97 7.58 6.59 3.33 7.75 4.57 4.94 7.68 8.94 7.40
1.20 1.62 1.43 1.20 1.66 1.57 1.16 1.61 1.66 1.58 1.43 1.58 1.66
101 95 91 89 82 77 79 79 79 79 54 44 32
−f −f −f −f −f −f −f −f −f −f 69.8 81.2 97.9
a Conditions: 0.010 mmol of precatalyst, 1.2 equiv of NaBAF, total volume of toluene, CH2Cl2 and monomer: 15 mL, 3 h. bTurnover frequency = moles of substrate converted per mole of catalyst per hour. cMolecular weight was determined by GPC in trichlorobenzene at 150 °C using polystyrene standards. dB = branches per 1000 carbons; branching numbers were determined using 1H NMR spectroscopy. eDetermined by differential scanning calorimetry (DSC). fAmorphous polymers.
Polyolefin is a very important class of polymer with wide applications and huge annual production. However, one of its biggest disadvantages is the nonpolar nature. The introduction of polar functional groups could dramatically improve its many properties such as adhesion, toughness, barrier properties, surface properties (paintability, printability, etc.), and compatibility with polymer additives and other polymers. In this work, we demonstrate that the incorporation of MA unit could indeed greatly improve the surface properties of the polyolefin materials. As can be seen from Figure 4 (Figure S1), the water contact angle
Higher activity and higher polymer molecular weight were observed at higher 1-hexene concentration (Table 3, entries 7 and 8). Catalyst 6 reached highest activity at 40 °C in the temperature range of 20−60 °C (Table 3, entries 9 and 10). When longer chain α-olefins were used, similar activities were observed (Table 3, entries 11−13 versus 6). Most interestingly, the branching density could be dramatically reduced (32−54/ 1000C) with greatly increase polymer melting temperatures (70−98 °C). The longer chain α-olefin monomers led to the formation of more linear, less branched polymers through 2,1 insertion and 1,ω enchainment.22,55−58 Properties of the Polymer Products. The polyethylene products generated using catalysts 1−3 are totally amorphous oil (Table 1, entries 1−3). In contrast, the polyethylene products generated using catalysts 4−6 showed nice mechanic properties (Table 1, entries 4−6). The polyethylene from catalyst 5 with moderate molecular weight and moderate branching density showed smaller tensile strength (7.1 MPa versus 15.9 MPa) and much better elastic properties (strain at break of 670% versus 230%) than that from catalyst 6. The molecular weight and branching density have been shown to be crucial to determine the polyethylene mechanical properties.59 In this system, the ability to systematically tune the molecular weight and branching density enables the tuning of the polymer mechanical properties over a wide range.
Figure 4. Water contact angle of the polyethylene (sample from Table 1, entry 3) and E−MA copolymer (samples from Table 2, entries 2, 5, 8, 11, 14, and 17) versus the MA incorporation ratio.
(WCA) was decreased from 104° for pure PE gradually to 54° for copolymer with 6.9% of MA incorporation. When the 6.9% EMA copolymer was hydrolyzed to COOH containing copolymer, the WCA was further decreased to 40° (Figure S1). Clearly, the incorporation of even a small amount of MA comonomer could induce dramatically improved surface properties. For comparison, the WCA for polystyrene is 92°, which is a paintable polymer.60
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CONCLUSIONS To conclude, the olefin polymerization and copolymerization properties of a series of α-diimine palladium catalysts were investigated. The tuning in ligand sterics enables the tuning of the polymer microstructures such as molecular weight, branching density, and comonomer incorporation. This translates into the
Figure 3. Stress versus strain for polyethylene generated using complexes 4−6 at 20 °C (samples from Table 1, entries 4−6). F
DOI: 10.1021/acs.macromol.6b02104 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
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tuning of the properties of the resulting polymeric products such as melting temperatures, mechanical properties, and surface properties. The Brookhart α-diimine palladium catalysts have been extensively studied and have been applied for the synthesis of various polymers and copolymers with novel microstructures. However, the systematic tuning of the branching densities of the polyethylene or copolymers has not been realized previously. In this work, it is demonstrated that this class of catalysts is capable of realizing such kind of tuning through careful ligand modifications. This could potentially lead to a lot more flexibilities to design and synthesize polymeric materials with unique microstructures.
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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.6b02104. Experimental procedures, NMR spectra for ligands, palladium complexes, polyethylene, and copolymers (PDF) CIF files for the palladium complexes 1−6 (ZIP)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (C.C.). ORCID
Wen Zhang: 0000-0001-8577-2613 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, 21374108, 51522306, 21960071), Anhui Provincial Natural Science Foundation (1408085QB28, 1608085MB29), the Fundamental Research Funds for the Central Universities (WK3450000001), and the Recruitment Program of Global Experts.
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