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Pressure Sensitive Supported FI Catalyst for the Precise Synthesis of Uni/Bimodal Polyethylene Feng Yu, Yanqiong Yang, Dengfeng He, Dirong Gong, and Zhong-Ren Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00083 • Publication Date (Web): 29 Mar 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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Industrial & Engineering Chemistry Research

Pressure Sensitive Supported FI Catalyst for the Precise Synthesis of Uni/Bimodal Polyethylene Feng Yu,† Yanqiong Yang,§ Dengfeng He,† Dirong Gong,† and Zhong-Ren Chen*,‡ †

Department of Polymer Science and Engineering, School of Material Science and Chemical Engineering, Ningbo University, Ningbo 315211, P. R. China ‡

Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, P. R. China Department of Macromolecular Science, Graduate School of Science, Osaka University, Osaka 560-0043, Japan Supporting Information Placeholder §

ABSTRACT: Pressure sensitive phenoxy-imine titanium catalyst (FI-catalyst) was successfully synthesized by covalently immobilizing it on the surface of functionalized polystyrene (PS) nanoparticles, which acted as a support and a giant ligand simultaneously. The supported catalyst shows high catalytic activities in ethylene polymerization. Utilizing this catalyst, a serious of unimodal polyethylenes with tailored molecular weights ranging from 2,000,000 to 300,000 were produced under various reaction pressures and temperatures. Particularly, via shifting the polymerization pressure during reaction, the ethylene polymer with bimodal molecular weight distribution was successfully synthesized just in a single-reactor. DSC analysis of the resultant polyethylene revealed a large difference between crystallinities at the first and the second heating cycles, indicating that the synthesized polyethylene is of low entanglement.

1. INTRODUCTION Polyethylene (PE), one of largest synthetic resin in the world, is a very attractive commercial product due to its outstanding physical and mechanical properties. However, the topological constraints caused by chain entanglements limit the processability of unimodal PE with very high molecular weight (MW).1 PE, with a broad/bimodal molecular weight distribution (MWD), has been obtained an increasing popularity over its unimodal counterparts due to its preferred balance of processing or rheological characteristics and mechanical properties.2-3 The high molecular weight fraction of bimodal PE provides good strength, toughness and environmental stress cracking resistance, whereas the low molecular weight fraction of bimodal PE acts as lubricants to improve the processability of the resin.4 There are several approaches to produce bimodal PE, such as melt mixing method, series reactor method and single reactor method.5 Among them, the series reactor method is the most commonly used for bimodal PE production. However, building such multiple reactors needs large energy consumption and capital with complicated operation procedures. Single reactor method, instead of the existing series-reactor technology, attracts increasing attention. The focus of the study was mainly on the design of the catalyst in a single reactor, such as the hybrid catalyst,6-10 single-core multi-activity center and chain-shuttling polymerization.11-12 López-Linares et al.13 obtained a bimodal PE, using combined catalysts (Ziegler-Natta catalyst and metallocene catalyst (CpZrCl3)) with methylaluminoxane (MAO) activator. Reb et al.12 synthesized diastereomeric amido functionalized ansa half-sandwich dichloride complexes of titanium and zirconium, which exhibit distinct catalytic characteristics in preparing bimodal PE. Fluorinated

FI catalyst coupled with ZnEt2 is also reported to prepare bimodal poly-ethylene since ZnEt2 acts as a transfer agent during ethylene polymerization.11 However, the catalysts used in industry are generally heterogeneous. So the catalyst load is necessary. In industry, supported catalysts are used to control the PE particle morphology. The most common support is inorganic materials, such as high surface area silica or alumina.14 However, these acidic supports could lead to the deactivation of metallocene catalyst or FI catalyst.15-17 Furthermore, the remaining inorganic particles in the product often damage the optical property and chemical stability of polymer products. For example, these inorganic impurities could reduce the transparency of the materials.18-19 For these reasons, the development of new support systems become meaningful. Polystyrene (PS) nanoparticle, which is increasing used as an excellent organic support, has advantages of chemical stability,18 well-controlled size, and desirable functionalization capability.20 In this study, we report a pressure-sensitive FI catalyst supported by PS nanoparticle. The unique feature of this catalyst is the pressure tunability for producing PE with desired molecular weight, especially with designed bimodal molecular weight distribution. The high molecular weight PE is synthesized under low polymerization pressure, while the low molecular weight PE is achieved at high polymerization pressure. The bimodal PE is successfully prepared in one pot by shifting the polymerization pressure. Since polymerization temperature and pressure are easily controlled for a single reactor in industry, the pressure-sensitive FI catalyst represents a new promising technology for commercial production of PE with bal-

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anced processing and mechanical properties in polyolefin industry.

2. EXPERIMENTAL SECTION 2.1 General Procedure and Materials. All air and water sensitive reactions are carried out in the glovebox. Dried solvents (n-heptane, Tetrahydrofuran, n-hexane, toluene) used for reaction were purchased from TCI. An n-butyllithium (1.6mol/L) in n-hexane solution, 3-tert-butylsalicylaldehyde, and TiCl4 (1mol/L) in n-hexane solution were purchased from TCI. Styrene (Acros) was passed through a column of aluminum oxide and then distilled under reduced pressure in a distilling flask to which CaH2 was added. Anaerobic deionized water was used in all miniemulsion polymerizations. The cocatalyst, methylalumoxane (1.2 mol/L) in toluene solution, was purchased from Acros. 2.2 Preparation and modification of polystyrene nanoparticles. Prewashed styrene (20 g, 192 mmol), potassium persulfate (0.08 g, 0.3 mmol), sodium dodecylbenzenesulphonate (0.96 g, 2.8 mmol), and sodium carbonate (0.125 g, 0.0015 mmol) were added to the deionized water (60 mL) and mixed at room temperature. The solution was stirred at 1500 rpm for 2 h to form a miniemulsion. The emulsion was boiled under high purity argon atmosphere for 20 min. Polymerization condition was set at 350 rpm and 70 °C. After 5 h, the mixture was added into a 500 mL flask and put in the fridge (15 °C) for 8 h. As the frozen mixture was melted at room temperature, polymer particles were separated from water phase by separating funnel, and then further isolated by Sand core glass filter. To remove unreacted styrene and moisture, the polystyrene nanoparticles were dried at 60 °C in a vacuum oven for 24 h and yielded a fine white powder. Nitration of the polystyrene nanoparticles was performed in the mixed acid. Polystyrene nanoparticles (20 g), nitric acid (80 mL, 69 wt.%) were mixed at room temperature and then sonicated with a Sonifier (700 W, ice-water) for 5 min. Sulfuric acid（30 mL，98 wt.%）was added to the miniemulsion dropwise for 10 min under ice-water bath. The above mixture was reacted at 45 °C under moderately stirring. After 8 h, the nanoparticles was isolated by filtration using glass filter and washed to neutral with deionized water. To remove moisture, the nanoparticles were dried in a vacuum oven at 60 °C for 24 h and yielded a light yellow powder. For the preparation of nanoparticle containing amino groups, nitrated-nanoparticles (10 g), anhydrous acetic acid (20 mL, 99.8 % (v/v))，and SnCl2·2H2O (30 g) were mixed and added to HCl (60 mL, 6 M). The hydrogenation reaction mixture was stirred at 100 °C for 12 h. The aminated-nanoparticles were isolated by filtration using glass filter and washed with 0.1 M HCl. To remove moisture and HCl, the aminated-nanoparticles were dried in a vacuum oven at room temperature for 24 h and yielded a yellow powder. 2.3 Preparation of catalyst. P-toluenesulfonic acid (ca. 20 mg), aminated-nanoparticle (2 g, 6.9 mmol amine groups) and 3-tert-butylsalicylaldehyde (1.2 g, 98 % purity, 6.9 mmol) were mixed and added to n-heptane (20 mL) at room temperature. The mixture was stirred at reflux temperature for 5 h, and filtered with glass filter and washed with heptane. To remove heptane, the product was dried in a vacuum oven at room temperature for 24 h to constant weight and yielded a luminous yellow powder.

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A 1.6 mol/L n-butyllithium in n-hexane solution (0.267 mL, 0.427 mmol) was added to a stirred mixture of nanoparticle ligand (0.2 g, 0.427 mmol) with dried n-hexane (20 mL) at 78 °C dropwise for 10 min. The mixture was stirred at room temperature for 2 h. And then a 1mol/L n-heptane solution of TiCl4 (0.427 mL, 0.427 mmol) in dried tetrahydrofuran (5 mL) was added to the mixture dropwise for 10 min at -78 °C. The mixture was further stirred at room temperature for 18 h. The mixture was filtered with glass filter and washed three times with dried n-hexane. 2.4 Ethylene Polymerization. The polymerization of ethylene was carried out in a high pressure reactor equipped with a U-formed stirrer. The reactor was heated to 130 °C under vacuum for 3 h. And then the temperature was lowered to the desired temperature. Toluene (500 mL) was added to the reactor through a solvent purification apparatus. The catalyst was injected into the reactor under argon by a syringe. The cocatalyst and the catalyst were injected into the reactor under nitrogen by a syringe. An ethylene pressure of 1-10 bar was applied. A polymerization temperature of 30-50 °C was applied. The unreacted ethylene in the reactor was released after the desired time of polymerization. And then the acidic methanol (400 mL) was added to terminate the reaction. The PE product was filtered, and dried at 70 °C in a vacuum oven. 2.5 Characterization of polymer. Polymer melting points (Tm) and heat of fusion were determined on a differential scanning calorimeter (DSC). The PE sample was first heated to 160 °C at a rate of 10 °C/min. After 5 min, it was cooled to 50 °C at 10 °C/min. Subsequently, the PE sample was heated to 160 °C at a rate of 10 °C/min again. Crystallinity was calculated by comparison with the heat of fusion of a perfectly crystalline PE, namely, 289 J/g. The molecular weights and molecular weight distributions of polymer were measured with a PL GPC-220 at 150 °C, with 1,2,4-trichlorobenzene as the mobile phase. The detector of GPC is a differential refractive index (DRI) detector. The morphology of PE, PS nanoparticle and aminated-PS nanoparticle were monitored by SEM (emission scanning electron microscopy, Zeiss Merlin). Samples were coated with Pt for 3 min. The distribution of PS nanoparticles is measured by laser light scattering (Brookhaven, BI-200SM).

3. RESULTS AND DISCUSSION 3.1 Characterization of catalyst. Scheme 1 shows the preparation route of the catalyst. PS nanoparticles were prepared by miniemulsion polymerization of styrene, and then the prepared PS nanoparticles were used to prepare aminatednanoparticles. The SEM of PS nanoparticles and aminated PS nanoparticles are shown as in figure 1. The particle size distribution of PS nanoparticles detailed in supporting parts. In figure 2, compared with PS nanoparticles, the infrared spectra of nitrated PS nanoparticles appeared the characteristic absorption peaks of 1,520 cm-1 and 1,347 cm-1, respectively, corresponding to the antisymmetric and symmetrical stretching vibration absorption peaks of the nitro group. It was proved that the nitro group was attached to the surface of the PS nanoparticles after nitrification. Comparing the infrared spectra of nitrated PS nanoparticles and aminated PS nanoparticles, it can be seen that the two peaks of the nitro group characteristic peaks on l, 520 cm-1 and 1,347 cm-1 of aminatednanoparticle are obviously reduced, and 3,422 cm-1, the intensity of absorption peak of the N-H stretching vibration region


2

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is obviously increased, which indicates that the nitro group is reduced to amino group on the surface of nitrated PS nanoparticles.

1347 1520

3422

Figure 1. The SEM micrographs of (a) PS nanoparticles and (b) aminated PS nanoparticles.

Finally the catalyst was covalently immobilized on the surface of the functionalized PS nanoparticles via condensation reaction and coordination reaction. Amino group content of aminated-nanoparticles was determined. Nitrogen content is 3.5 mmol/g which was determined by elemental analyzer, and amine content is 3.45 mmol/g by conductance titration. This result indicates that most of the nitrogen is present in the form of amine groups. Conductance titration procedure detailed in supporting parts. The ligand structure of the catalyst was verified by NMR. The NMR results of the phenoxy-imine structure are as follows：1H NMR (DMSO-D6, ppm) δ 1.38 (s, 9H, tBu), 7.04 (t, 1H, aromatic-H), 7.56-7.66 (m, 1H, aromatic-H), 7.95 (dd, 1H, aromatic-H), 9.98 (s, 1H, CH=N), 11.85 (s, 1H, OH). The titanium content of the catalyst was determined by ICP method. The content of Ti is 3.2 mmol/g. The results show that molar ratio of titanium and nitrogen is 1:1.07.

1347

PS PS-NO2 PS-NH2

3500

3000

2500 2000 ν/cm-1

1500

1000

Figure 2. The infrared spectra of PS, nitrated PS nanoparticles and aminated PS nanoparticles.

Scheme 1. The synthetic route for catalyst

Table 1. Polymerization Conditions and Results. Crystallinitye (%)

Tm (°C) Item

P(bar)

Mwc

PDIc

Activityd

ScanⅠ

ScanⅡ

ScanⅠ

ScanⅡ

1a

1

210

3.32

48.0

143.4

135.0

76.0

46.7

2a

3

87.7

4.20

150

141.4

133.1

69.4

43.6

3a

5

43.8

3.19

200

141.5

132.7

66.9

42.4

4a

10

49.0

5.69

480

145.9

136.3

68.1

42.6

b

1

185

3.14

96.0

141.2

132.7

62.0

45.7

6b

3

65.1

3.50

180

145.5

136.3

69.4

47.4

7b

5

32.0

3.70

250

141.4

133.0

68.8

46.7

8b

10

26.7

3.15

500

142.7

134.3

72.1

41.1

5

Polymerization conditions: 400 mL of toluene, [Al]/[Ti]=1000, 40 mg of catalyst, polymerization temperature at 30 °Ca and 50 °Cb, at various polymerization pressure, for 60min of reaction time. cMw in unit of 104 g/mol, the GPC shown in figure 3. dActivity in unit of kg/mol•h. eDetermined according to ΔHm from DSC.


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(a)

(b) 1bar 3bar 5bar 10bar

1bar 3bar 5bar 10bar

102 103 104 105 106 107 108 Mw(g/mol)

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102 103 104 105 106 107 108 Mw(g/mol)

Figure 3. Mw and MWD of PE obtained under various polymerization pressures at temperatures of (a) 30 °C and (b) 50 °C.

3.2 Melting Point and Crystallinity. In order to evaluate the performance of the catalyst in producing PE, a series of PEs were prepared under different polymerization conditions, as shown in Table 1. The degree of crystallinity of the PE was analyzed by DSC. Since the first scan reflects the nature of the resultant PE chain, while the second scan reveals those of the melt-crystallized PE, both the first and the second scans were analyzed. For the first scan, we can see that the crystallinity is about 70 %, and first melting temperature is around 143 °C, which seems slightly higher than those of the homogeneous catalytic system.21 This result suggests the existence of the oriented crystalline structure.22-24 PS nanoparticle as a part of the ligand, forms a confined environment. Consequently, the crystallization kinetic and thermodynamically metastable oriented crystals are expected to be formed during the polymerization.22, 25-26 For the second scan, both the melting point (Tm=132-137 °C) and heat of fusion (ΔHm=118-137 J/g) are significantly reduced as shown in table 1. These differences between the first and second heating cycles could be attributed to the formation of chain entanglement in melt which hindered recrystallization. This phenomenon indicates the low entanglement of the nascent PE.27-28 3.3 Molecular Weight. Table 1 shows the effect of polymerization condition on the molecular weight and molecular weight distribution of prepared PE. At 30 °C, a high molecular weight of above 2,000,000 has achieved at 1 bar. FI catalysts with similar structures can also reach molecular weight of millions, indicating that our prepared catalyst has similar characteristics with classical FI catalysts.29 The molecular weight shown in figure 3 decreases drastically to below 1,000,000 at elevated pressure of 3 bar, and reaches bottom of below 500,000 at 5 bar and 10 bar. At 50 °C, the molecular weight is reduced to below 2,000,000 at 1 bar. With the similar pressure dependence, the molecular weight is dramatically decreased to 650,000 at 3 bar, and reached to about 300,000 at 5 bar and 10 bar. This reduction of molecular weight is attributed to the increased chain transfer under higher polymerization pressure.29 However, polymerization temperature from 30 °C to 50 °C (a, b) does not alter measurable catalytic activities and molecular weight distribution (MWD).30 3.4 Preparation of bimodal PE. For the molecular weight dependence on the pressure, we prepared bimodal PE by shifting from a low pressure of 2 bar to a higher pressure of 10 bar in one-pot. The polymerization was first conducted under 2 bar 30 minutes. Then, the polymerization pressure was increased to 10 bar for 60 minutes. DSC data of bimodal polyethylene shows the melting point (Tm=142.4 °C) and heat of fusion (ΔHm=195 J/g) in the first scan. But the melting point becomes 133.9 °C and the heat of fusion is 125.7 J/g in the second scan. This indicates that the nascent bimodal polyethylene is the low entanglement PE.27-28 GPC analysis revealed bimodal molecular weight distribution as shown in Figure 4, with Mn of 15.2k and PDI of 47.3, respectively. The SEM

micrographs of bimodal PE show uniform particles with the size of about 500 nm as shown in Figure 5. This means that the fragmentation behavior of PS nanoparticle support for FI catalyst follows a multi-grain mode.31

1

Mn=15,216 PDI=47.3 2

102

103

104

105

106

107

108

Mw(g/mol) Figure 4. Molecular size of bimodal PE.

Figure 5. (a, b) SEM images of bimodal PE prepared under 2 bar for 30 min and subsequently under 10 bar for 60 min at 50 °C. (b) PE particles in figure (a) at high magnification.

3.5 Plausible mechanism of polymerization. A plausible mechanism of ethylene polymerization is presented in Scheme 2, which involves with Ti (II) and Ti (IV) species.32-34 The molecular weight of the polymer is mainly controlled by the R3 group when the polymerization is catalyzed by FI catalyst.35-37 At low polymerization pressure of 1 bar, the R3 group would inhibit the β-hydride elimination/reduction elimination reaction, forms PE chains with very high molecular weight. Indeed our prepared catalyst uses the giant R 3 group, which is an attached polystyrene particle. At an elevated polymerization pressure of 5 bar or higher, the (a) and (b) processes of β-hydride elimination /reduction elimination reaction increases drastically , resulting in the reduced molecular weight of PE chains via chain transfer occurrences.


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Scheme 2. Proposed mechanism for the formation of PEs with different molecular size.

4. CONCLUSIONS We have found a Pressure Sensitive FI Catalyst Supported by PS Nanoparticles. When other conditions keep the same, the greater the pressure is, the lower the molecular weight of PE is obtained. We successfully prepared PE with bimodal molecular weight distribution by shifting the polymerization pressure from 2 bar to 10 bar in one-pot. The produced PE has high melting temperature (141-146 °C) and heat of fusion (ΔHm =190-210 J/g) in the first heating cycle. It implies that the PE forms low entanglement during polymerization process.

ASSOCIATED CONTENT Supporting Information Full experimental details and additional characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel: +86 13757447975. Fax: +86 755 88018328. E-mail: [email protected]. Author Contributions The manuscript is written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors greatly acknowledge the financial support for this work provided by the Project of Natural Science Foundation of China (No. 21274070), the Start-up fund of SUSTC (Y01216121), and the key scientific and technological innovation team of Zhejiang province (2011R50001). The authors also greatly acknowledge for the experimental support provided by Ying Chen in Ningbo University.

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