Multidentate Electron Donors for Ziegler–Natta ... - ACS Publications

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Polyethylenimines: Multidentate Electron Donors for Ziegler−Natta Catalysts Ville H. Nissinen,† Mikko Linnolahti,† Andrey S. Bazhenov,†,∥ Tuula T. Pakkanen,*,† Tapani A. Pakkanen,† Peter Denifl,‡ Timo Leinonen,§ Kumudini Jayaratne,§ and Anneli Pakkanen§ †

Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland Borealis Polyolefine GmbH, St.-Peter Strasse 25, 4021 Linz, Austria § Borealis Polymers Oy, P.O. Box 330, FI-06101 Porvoo, Finland ‡

S Supporting Information *

ABSTRACT: Polyethylenimines, polymers bearing amino functionalities, are studied for the first time as internal electron donors for Ziegler−Natta catalysts. An advantage of polyethylenimines (PEIs) compared to the conventional phthalate electron donors is their relative harmlessness. Interaction of PEI with MgCl2 support was studied using computational (DFT; M06-2X) and experimental (PXRD, DRIFT, CP/MAS 13 C NMR) methods. Quantum chemical calculations suggest that the structural variations in PEIs significantly affect their ability to stabilize the catalytically relevant MgCl2 surfaces. Coordination on the (104) surface seems to be favored upon consideration of the layered structure of MgCl2. The surface stabilization energies of branched PEIs are of the same magnitude with a phthalate electron donor reference. Experimental results indicate, in agreement with theoretical results, a strong coordination ability of branched PEI through nitrogen atoms to MgCl2. Based on spectroscopic data, nitrogen atoms of primary, secondary, and tertiary amino groups can participate in coordination to MgCl2. Calculations indicate that the strongest coordination of branched PEI occurs through primary amino groups. A Ziegler−Natta catalyst containing branched PEI as an internal electron donor (MgCl2/PEI/TiCl4) showed a reasonably high activity in ethylene/1-butene copolymerization. Overall, the combined computational and experimental results provide detailed information about coordination of nitrogen-containing polymeric electron donors to MgCl2 support and indicate their potential as a new type of internal electron donors for Ziegler−Natta catalysts.

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predominating.7,10,18,22,24,28 Electron donors are also known to affect the catalytic properties of active polymerization sites (e.g., activity, stereoselectivity, hydrogen response), although the exact role of electron donors is still not fully understood.2,20,25,29−32 Electron donors have been proposed to control the amount and distribution of active titanium species in the catalyst and to modify the local environment of active Ti species by coadsorbing to their proximity.2,30,32−34 Modern Ziegler−Natta catalysts exhibit high activities and good stereoregulating properties.2 However, work with electron donors still continues in order to obtain catalysts with enhanced performance and polyolefins with new features.2,29 In recent years, phthalates, which are widely used electron donors in commercial propylene polymerization catalysts, have aroused increasing health concerns.35,36 The stricter chemical regulations have led to a need to find new less harmful electron donors.37

INTRODUCTION Ziegler−Natta catalysts are responsible for a vast majority of the world’s annual polyolefin production, thus having a substantial commercial significance.1 The essential components of a modern Ziegler−Natta catalyst are an active Ti species, in most cases MgCl2 -supported, and an alkyl aluminum cocatalyst.2 Ziegler−Natta catalysts usually contain also Lewis bases (internal and external electron donors), which are known to play a key role in the polymerization process.2 Interaction of electron donors with MgCl2 has been extensively studied using both experimental and computational approaches.3−24 The role of electron donors in the formation of MgCl2 crystallites is important as they can stabilize MgCl2 crystals by coordinating on the unsaturated lateral surfaces, most importantly the catalytically relevant (104) and (110) (or alike) surfaces.10,11,19,25,26 In some cases, electron donors can also direct the growth of MgCl2 crystals by selectively coordinating on certain lateral surfaces.3,4,6,7,11,22,27 In the absence of an electron donor, formation of the (104) surfaces with 5-coordinated Mg atoms is preferred, but the presence of an electron donor can make the (110) surfaces with 4-coordinated Mg atoms © XXXX American Chemical Society

Received: June 1, 2017 Revised: October 5, 2017 Published: October 6, 2017 A

DOI: 10.1021/acs.jpcc.7b05343 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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under vacuum at 120 °C overnight. 1-Chlorobutane and solvents were dried and stored over activated 3 Å molecular sieves. MgCl2-EtOH support53−55 containing approximately 40 wt % of MgCl2 and 60 wt % of EtOH and a MgCl2-supported reference catalyst containing bis(2-ethylhexyl) phthalate as internal electron donor were provided by Borealis Polymers. The reference catalyst had been prepared otherwise according to a method described in the patent literature (Example 8 in ref 56), except that diethylaluminum chloride had been used as an aluminum compound instead of triethylaluminum. Preparation of MgCl2/PEI Complexes. MgCl2/PEI complexes were prepared by addition of branched polyethylenimine to δ-MgCl2, which had been synthesized using a method reported by Di Noto et al. with minor modifications.57 Magnesium, 1-chlorobutane (in a molar ratio of 1:3), and a few small crystals of iodine and octane (solvent) were packed into an autoclave, which was heated at 130 °C for 2 h. The solid product formed in the reaction was washed with octane and dried under vacuum at room temperature. According to a PXRD study, the solid product possesses characteristics of δMgCl2. Next, branched PEI was added to the synthesized δMgCl2 using different Mg/donor molar ratios and toluene as the solvent medium. The reagents were packed into an autoclave, which was heated at 130 °C for 2 h. The solid product was isolated with filtration, washed with toluene (3 times with 10 mL), and dried under vacuum at room temperature. Preparation of MgCl2/PEI/TiCl4 Precatalyst. MgCl2/ PEI/TiCl4 precatalyst was prepared by a sequential addition of branched PEI and TiCl4 to MgCl2-EtOH support. PEI was first added to the MgCl2-EtOH adduct53−55 in an autoclave using toluene as the solvent medium (90 °C/22 h). A PEI/Mg molar ratio of 0.9 was used. The product formed was separated with filtration and washed with toluene. The addition of TiCl4 to MgCl2/EtOH/PEI support was conducted in a glass reactor. A slurry of support (2.6 g) and heptane (12 mL) was cooled to 10 °C with an immersion cooler and a propanol bath. TiCl4 (16 mL; n(Ti):n(EtOH) = 5.5) was added slowly. After 30 min of mixing, temperature of the system was slowly raised until 110 °C was reached. After 60 min of mixing at 110 °C, the liquid phase of the reaction mixture was removed. The product was washed twice with the TiCl4/toluene mixture (1:1) (110 °C), once with toluene (90 °C), and 3 times with heptane (90 °C). To remove remaining impurities, the precatalyst was further washed twice with TiCl4 (110 °C), 4 times with toluene (110 °C), and twice with heptane (room temperature). The precatalyst obtained was dried under vacuum at room temperature. Characterization of MgCl2/PEI Complexes and MgCl2/ PEI/TiCl4 Precatalyst. A Bruker AXS D8 ADVANCE diffractometer was utilized in recording the X-ray diffractograms of the products. The following parameters were used: Cu Kα radiation (λ = 1.5418 Å), a measurement range (2θ) of 4.0− 70.0°, a step size of 0.05°, and a time per step of 8 s. The samples were placed on a custom-made steel sample holder and protected from air by Mylar film. A Hitachi S-4800 scanning electron microscope (SEM) was utilized in morphology analysis of the products (acceleration voltage of electrons of 3.0 kV). The samples were in contact with air for a short period of time when transferred into the loading chamber. The IR spectra of the solid products were recorded with a Nicolet Impact 400D spectrometer by using a DRIFT (diffuse reflectance infrared Fourier transform) unit mounted inside a

Polymeric compounds bearing suitable functional groups are interesting and potential alternatives for conventional electron donors. It is expected that the binding properties of a polymeric electron donor are strongly affected by structural features, e.g., the distance between electron-donating atoms and polymer chain length. Thus, adjustment of a polymeric donor structure can offer a way to tailor the electron-donating properties. The polymeric electron donors have not yet drawn much attention, and only a few studies concerning the use of polyethers (e.g., polyethylene glycols and polytetrahydrofurans) have been published in the context of Ziegler−Natta catalysis.38−40 Preparation and properties of MgCl2 complexes with polyethylene glycols have been more widely reported in the field of electrolytes.41−43 In our previous study, we have demonstrated the use of polyethers as electron donors in the preparation of Ziegler− Natta type polymerization catalysts.40 Herein we introduce a new type of multidentate electron donor bearing an amino functionality, namely, polyethylenimine (PEI). Use of nitrogenbased polymers as electron donors in Ziegler−Natta catalysis has not been previously reported. However, polyethylenimine has been widely studied in biochemical and pharmaceutical applications, particularly in gene/drug delivery.44,45 The relative harmlessness of PEI is a major advantage compared to conventional phthalate electron donors. In this study, computational and experimental methods are used to study the interaction of PEI with a MgCl2 support. Several different models of PEI are utilized in theoretical studies in order to explore the general principles of PEI coordination to MgCl2. On the basis of computational results, commercially available low molar mass branched PEI is used in the experimental study. Furthermore, preparation of a MgCl2/TiCl4 catalyst containing branched PEI as an internal electron donor and its performance in ethylene/1-butene copolymerization are also reported.

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EXPERIMENTAL SECTION Computational Details. The magnesium chloride−electron donor complexes were fully optimized by periodic DFT methods using the M06-2X meta-hybrid GGA functional,46 which has been shown as a cost-effective choice for systems that contain halides bridging between main group metals.47 Optimized triple-ζ-valence + polarization basis sets (TZVP) were used in all calculations.48 The basis sets have been derived from the def-TZVP basis sets of Ahlrics and co-workers.49 The catalytically relevant (104) and (110) surfaces of MgCl2 were presented by one-dimensional ribbons having thickness of five atomic layers (see Figure S1 in Supporting Information). The model choice was based on our previous computational studies of MgCl2−electron donor complexes.33,48,50,51 The PBC module of the Gaussian09 program package52 was employed in all calculations, with the default k-space integration method and PBC cells range of 100 Bohr. Materials and General Considerations. A glovebox and standard Schlenk techniques were used for the preparation of the supports and precatalysts and the manipulation of the samples. All the equipment used in the experiments was dried and stored at 110 °C. Magnesium turnings (Acros Organics, 99.9+%) were also dried at 110 °C overnight before the synthesis. n-Octane (reagent grade, 98%), 1-chlorobutane (ReagentPlus, 99%), toluene (anhydrous, 99.8%), branched polyethylenimine (Mw = 800 g/mol), and titanium tetrachloride (ReagentPlus, 99%) were purchased from SigmaAldrich and n-heptane (for analysis) from Merck. PEI was dried B

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Figure 1. Fragments of infinitely long polymeric donors employed in the periodic calculations and their coordination on the (104) and (110) surfaces of MgCl2. (a) Isotactic linear PEI, (b) syndiotactic linear PEI, (c) isotactic branched PEI, and (d) syndiotactic branched PEI.

glovebox (number of scans of 32, resolution of 2 cm−1). The reference IR spectrum of branched PEI was recorded as a thin film between two KBr pellets using a Bruker Vertex 70 spectrometer (number of scans of 32, resolution of 2 cm−1). Solid-state 13C NMR spectra were recorded with a Bruker AMX-400 spectrometer using cross-polarization and magic angle spinning (CP/MAS) and the following parameters: a spin rate of 4500 Hz, a relaxation delay of 5 s, a contact time of 3.0 ms, and a number of scans of 10 000. Glycine was used as an external standard. The reference 13C NMR spectrum of branched PEI was recorded with the same spectrometer equipped with a liquid-state probe. PEI was dissolved in D2O, and sodium acetate was used as an internal reference. An inverse gated pulse program with a relaxation delay of 60 s and a number of scans of 1000 was employed in order to ensure quantitativity of the spectrum. Magnesium contents of the products were determined by a complexometric EDTA (ethylenediaminetetraacetic acid) titration. 1H NMR spectroscopy (Bruker Avance 400 spectrometer) was used to determine the amounts of organic compounds in the complexes. For the analysis, solid products were dissolved in 10% (V/V) D2SO4/D2O solution. Number of scans was 32, with a relaxation delay of 10 s. Sodium acetate was used as an internal standard. Titanium contents of the precatalysts were determined by a spectrophotometric method, in which the solids were dissolved in H2SO4 solution and addition of H2O2 gave solutions of a yellow complex.58 A Shimadzu UVmini1240 spectrophotometer was used to measure absorbances of the solutions at 410 nm wavelength.

Polymerizations. Copolymerization of ethylene and 1butene using the MgCl2/PEI/TiCl4 catalyst was conducted in a 3 L semibatch reactor using triethylaluminum (TEA) as a cocatalyst. First, 55 mL of 1-butene was added into the reactor, followed by 1250 mL of propane and hydrogen gas (0.75 bar). The reactor was heated to the reaction temperature (85 °C), and a batch of ethylene (3.7 bar) was introduced into the reactor, giving a C4/C2 molar ratio of 770 (mol/kmol). The catalyst (approximately 15 mg) and TEA cocatalyst (an Al/Ti molar ratio of 15 (mol/mol)) were added to the reactor simultaneously after a few seconds of precontacting, alongside with an additional 100 mL of propane. The total reactor pressure was maintained at 38.3 ± 0.2 bar by a continuous ethylene feed. Polymerization was stopped after 60 min by venting off the monomers and H2. The polymer product was dried in a fume hood at least overnight. In the case of the phthalate-containing reference catalyst,56 a C4/C2 molar ratio of 400 (mol/kmol) was employed in the copolymerization in order to obtain a polymer product with comparable comonomer content to that of copolymer produced with the MgCl2/PEI/TiCl4 catalyst. Characterization of Polymers. The melt flow rates (MFRs) of ethylene/1-butene copolymers were measured according to an ISO 1133 standard method at 190 °C. Loads of 2.16 and 21 kg were employed for MFR2 and MFR21, respectively. The flow rate ratio (FRR21/2) is given as a ratio between MFR21 and MFR2. Melting points (Tm) of the polymers were determined according to ISO 11357 using a Mettler TA820 differential scanning calorimeter. The sample size was 3.0 ± 0.5 mg. C

DOI: 10.1021/acs.jpcc.7b05343 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Molar masses of the polymer products were determined using a high-temperature GPC (gel permeation chromatography) instrument, equipped with either an infrared detector (IR4 or IR5 from PolymerChar) or a differential refractometer from Agilent Technologies, and with 3 × Agilent-PLgel Olexis and 1 × Agilent-PLgel Olexis Guard columns. 1,2,4Trichlorobenzene stabilized with 2,6-di-tert-butyl-4-methylphenol (250 mg/L) was used as the solvent and as the mobile phase. The chromatographic system was operated at 160 °C and at a constant flow rate of 1 mL/min. For each analysis, 200 μL of sample solution was injected. Data collection was performed using either Agilent Cirrus software version 3.3 or PolymerChar GPC-IR control software. The column set was calibrated using universal calibration (according to ISO 16014− 2:2003) with 19 narrow MWD polystyrene standards in the range of 0.5 kg/mol to 11 500 kg/mol. The amount of 1-butene in ethylene/1-butene copolymers was measured using FTIR spectroscopy (Bruker Tensor 37 spectrometer) according to ASTM D6645-01. IR spectra of polymers were recorded in the form of thin film, and the comonomer content was estimated from the ratio between the absorbance of the signal at 1378 cm−1 and the area of the band at 2019 cm−1. The calibration was carried out by using polymer standards of known comonomer contents. The metal plate used in polymer plaque pressing was covered with a silicone paper.

different references for the binding on the (104) and (110) surfaces, the present approach accounts for the relative stabilities of donor-coordinated surfaces and enables comparison between different lateral surfaces.50,59 A conventional bidentate electron donor, dimethyl phthalate, was included as a molecular reference,60 together with molecular diamines, which are shown in Figure 2. We first

RESULTS AND DISCUSSION Theoretical Study on Coordination of PEIs to MgCl2. In order to explore the general structural and energetic features of MgCl2/PEI complexes, coordination of PEIs on the catalytically relevant MgCl2 (104) and (110) surfaces bearing 5- and 4-coordinated Mg atoms, respectively, was studied by the means of periodic quantum chemical calculations. Computations are based on idealized MgCl2 and PEI model systems, which do not consider possible defects in the MgCl2 crystallites and folding of the polymer chains. In the beginning, simplified models of PEI were employed, as depicted in Figure 1, to simultaneously account for the effect of branching and for stereochemical configuration of PEI (isotactic and syndiotactic forms). The optimized structures of the PEIs coordinated on the MgCl2 (104) and (110) surfaces are presented in Figure 1 with surface stabilization energies given in Table 1. The electronic energies are reported with respect to a fully saturated basal MgCl2 layer and free adsorbates per surface length of a nanometer. In contrast to donor binding energies, which have

Figure 2. Coordination of the employed (a) primary, (b) secondary, and (c) tertiary molecular diamines on the (104) and (110) surfaces of MgCl2.

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discuss the molecular systems. Dimethyl phthalate and the diamines bind the surfaces similarly: on (104) in the bridging mode and on (110) in the chelate mode. In the absence of an electron donor, the (104) surface of MgCl2 is more stable than the (110) surface.60 Binding of dimethyl phthalate brings the stability of the surfaces below the reference, i.e., the fully saturated basal MgCl2 layer, by −110.5 and −139.3 kJ/mol per a surface length of a nanometer, respectively, thus reversing the stability order of the surfaces in favor of (110). For comparison, the corresponding binding energies per Mg are −177.1 kJ/mol for (104) and −195.0 kJ/mol for (110). The diamines similarly favor the (110) surface but with an enhanced stability of the donor bound surface in comparison to dimethyl phthalate. The relative stabilities decrease from primary to secondary to tertiary diamine. Because the Lewis basicity of a NH group (secondary amine) is expected to be higher than that of a NH2 group (primary amine) due to the electron-donating effect of alkyl groups,61,62 the stability order is likely dominated by steric effects. This interpretation is supported by corresponding results obtained for the (104) surface, where the steric interference of adjacent donors is stronger, and in the case of NMe2 groups produces an incomplete surface coverage of donors (see Figure 2c). The resulting unsaturated surface magnesium atoms lead to a strong relative destabilization of the surface. Turning to the polymeric donors, structural variations significantly affect the ability of PEI to stabilize the (104) and (110) surfaces of MgCl2. We first discuss the results in terms of the (104) surface (Figure 1). The linear PEIs bind to the surface via secondary amino groups and show decreased stabilization of the surfaces relative to the secondary molecular diamine. This indicates that the distance between adjacent nitrogen atoms of the polymer chain does not perfectly match

Table 1. Stabilities of Electron-Donor-Coordinated MgCl2 Surfaces Relative to Crystalline Monolayer and Free Electron Donorsa electron donor b

dimethyl phthalate ethylenediamine (primary diamine) N,N′-dimethylethylenediamine (secondary diamine) N,N,N′,N′-tetramethylethylenediamine (tertiary diamine) isotactic linear PEI syndiotactic linear PEI isotactic branched PEI syndiotactic branched PEI

(104)

(110)

−110.5 −145.9 −103.0 −46.2

−139.3 −230.5 −212.6 −170.4

−27.7 −74.2 −123.3 −129.5

−55.5 −16.5 −45.5 −63.0

a

The energies are reported in kJ/mol per a surface length of a nanometer. bReference 60. D

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same method.60 The layering provides further stabilization due to dispersive interactions, and the same applies also for the unsaturated (110) surface bound by one polymer chain. Overall, because of layering of MgCl2, the polymeric amine electron donors favor the (104) surface. Experimental Study on Coordination of Branched PEI to δ-MgCl2. The theoretical results indicated a strong coordination of simplified branched PEI models on the idealized MgCl2 surfaces. An experimental approach was utilized alongside the calculations in order to study coordination of low molar mass branched polyethylenimine (structure presented in Figure 4) to MgCl2 under realistic

the Mg−Mg distances on the (104) surface, leading to strain, which is stronger in isotactic than in syndiotactic configuration. As opposed to linear PEI, which coordinates through NH groups, the coordination of branched PEI stabilizes the MgCl2 surface significantly more. Coordination of the branched PEI takes place via NH2 groups and allows for higher flexibility and hence reduced strain. These together bring the relative stabilities of the branched PEI-coordinated surfaces to the level of the reference molecular donors. On discussion of the coordination of the polymeric donors on the (110) surface, it is essential to pay attention to the layered structure of MgCl2. The linear and branched PEIs show behavior illustrated in Figure 3 for syndiotactic branched PEI. A

Figure 4. Typical repeating unit of branched polyethylenimine.

support preparation conditions. Interaction of PEI with magnesium dichloride was studied by addition of branched PEI to δ-MgCl2 using three different Mg/donor molar ratios and toluene as the solvent medium. Chemical compositions of the MgCl2/PEI complexes obtained are presented in Table 2. Table 2. Chemical Compositions of MgCl2/PEI Complexes reaction mixture

product

n(Mg):n(PEI)a

n(Mg):n(PEI)a

wt % (Mg)

wt % (PEI)

1:0.5 1:1 1:5

1:0.47 1:0.85 1:1.23

19.6 15.5 12.9

16.4 23.2 28.2

a

Mg/PEI molar ratios are given with respect to repeating units of PEI (CH2−CH2−NH).

In the cases of 1:0.5 and 1:1 molar ratios, most of the polyethylenimine added to the reaction mixture was found to be present in the products, indicating, in accordance with computational results, a strong coordination ability of branched PEI to MgCl2. The highest amount of PEI used in the reaction (molar ratio 1:5) did not result in a complex having a significantly higher PEI content, indicating saturation of the MgCl2 surface with PEI. The high amount of PEI present in the complexes in the cases of 1:1 and 1:5 molar ratios suggests either that only part of the amino groups of branched PEI are coordinated to MgCl2 or a possible presence of physisorbed PEI due to strong intermolecular dispersive interactions between PEI chains found in the computational study. According to PXRD results presented in Figure 5, addition of branched PEI did not affect notably the structure of δ-MgCl2. All the products were highly disordered, and the X-ray patterns possessed characteristics of δ-MgCl2. However, the reflection at 2θ ≈ 14° associated with the crystal plane (003) was observed to be slightly broader in the MgCl2/PEI complexes, indicating variations in the spacing between adjacent Cl−Mg−Cl triple layers due to coordination of PEI. The signal is especially broad in the case of 1:5 molar ratio, possibly due to physisorbed PEI present in the complex. Large variations in the spacing between adjacent Cl−Mg−Cl layers may be attributed to various coordination possibilities of the flexible polymeric donor, e.g.,

Figure 3. Layering of MgCl2 surfaces upon coordination of the syndiotactic branched PEI, accompanied by relative stabilities (kJ/mol per nm) of layered structures and interlayer distances (Å) of bilayer systems.

single layer of MgCl2 would allow for simultaneous binding of two polymeric donors that would saturate the (110) surface and would lead to significant further stabilization over the molecular donors due to intermolecular dispersive interactions between the adjacent polymer chains. However, since the interlayer distance in the crystal structure of MgCl2 is 5.9 Å,60 only one polymer chain fits to bind the (110) surface, leaving the (110) surface unsaturated. The (104) surface, on the contrary, allows saturation of the surface with polymeric donors upon layering. The optimized interlayer distance of the MgCl2 bilayer bound by syndiotactic branched PEI is 5.6 Å, which is equal to the interlayer distance of bulk MgCl2 calculated by the E

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Figure 7. IR spectra of unbound PEI and the MgCl2/PEI complex (n(Mg):n(PEI) = 1:0.5).

Figure 5. Powder X-ray diffractograms of MgCl2/PEI complexes. The reference product (n(Mg):n(PEI) = 1:0) refers to δ-MgCl2, which was heated in toluene in the absence of an electron donor. Reflections of the sample holder are marked with asterisks (*).

in the region 35−65 ppm. The broad signals indicate multiple possible chemical environments of amino groups in the complexes. Broadening of all the NMR signals suggests that the nitrogen atoms of primary, secondary, and tertiary amino groups can coordinate to the MgCl2 surface, supporting the IR result. However, it must be noted that the broad and partly overlapping IR and 13C NMR signals of MgCl2/PEI complexes complicate detailed interpretation of the spectroscopic data. Quantum chemical calculations were utilized to obtain a further insight into binding of branched PEI to MgCl2. In evaluation of the binding properties of the PEI used in the experiments (real PEI), we focused on the favored (104) surface of MgCl2. First, we modified the structure of the syndiotactic branched PEI to introduce primary, secondary, and tertiary amino end groups, which are all present in the real PEI. In accordance with the studies on molecular diamines reported above, the relative stabilities are by far the highest for the NH2 group, followed by NHMe and NMe2, whose steric environment does not allow full saturation of the surface magnesium atoms (Figure 8). These findings allowed determination of the preferred binding mode of the real PEI, which rearranges to bind the surface primarily via NH2 groups. The binding preferably takes place via four NH2 groups and one NHMe group in a unit cell consisting of five surface magnesium atoms, fully saturating the surface. The relative stability of the real PEIbound (104) surface is therefore close to that of the surface coordinated by simple branched PEI containing only primary amino end groups (Figure 8a), which thus serves as a useful practical model system for the real PEI. Ziegler−Natta Catalysts with PEI as an Internal Electron Donor. To study the effect of the branched PEI internal electron donor on performance of a Ziegler−Nattatype polymerization catalyst, a MgCl2/PEI/TiCl4 precatalyst was prepared by a sequential addition of branched PEI and TiCl4 to a MgCl2-EtOH support.53−55 The precatalyst contained 24.4 wt % of PEI, 12.6 wt % of Mg, 4.2 wt % of Ti, and 1.5 wt % of EtO. Virtually all PEI added to MgCl2EtOH support was also present in the final precatalyst, indicating that even extensive washing cannot remove PEI from the precatalyst. The 13C NMR and IR spectroscopic data (Figures S4 and S5) indicated also in the case of MgCl2/PEI/ TiCl4 precatalyst coordination of PEI to MgCl2 through nitrogen atoms. Analytical data of ethylene/1-butene copolymer produced with the MgCl2/PEI/TiCl4 catalyst are presented in Table 3, including comparison to a reference catalyst containing bis(2ethylhexyl) phthalate as an internal electron donor (MgCl2/

zip coordination25 of PEI chains to Mg atoms of adjacent MgCl2 layers. In addition, the relative intensity of the signal at 2θ ≈ 50° corresponding to the crystal plane (110) decreases as PEI is introduced to MgCl2, in accordance with the theoretical results, which showed preferable coordination of PEI to the (104) surface. The SEM analyses (Figure S2) showed that there were no major changes in the morphologies of δ-MgCl2 particles upon addition of PEI. The branched polyethylenimine donor containing primary, secondary, and tertiary amino groups (see Figure 4) gives rise to multiple signals in both IR and 13C NMR spectra. According to the 13C NMR study, the branched PEI used contained the three types of amino groups approximately in a molar ratio of 1.8:1.5:1, respectively (see Figure 6).45 The infrared study of

Figure 6. 13C NMR spectra of unbound PEI (dissolved in D2O) and the MgCl2/PEI complex (n(Mg):n(PEI) = 1:5) (solid state). In the case of free PEI, sodium acetate (NaOAc) was used as an internal reference for calibrating the chemical shifts. Note that a minor amount of entrapped toluene (solvent) is present in the product.

MgCl2/PEI complexes (Figures 7 and S3) indicates coordination of branched PEI to MgCl2 through nitrogen atoms as the C−N stretching vibrations of MgCl2/PEI complexes at 870− 1180 cm−1 have shifted to lower wavenumbers compared to unbound PEI (1000−1200 cm−1). Broadening and shifting of the C−N stretching band suggest that all types of the nitrogen atoms are able to coordinate to the MgCl2 surface, not only the primary end groups of branched PEI. NH2 and NH wagging vibrations of PEI in the region 750−900 cm−1 were not observed in the infrared spectra of MgCl2/PEI complexes. The CP/MAS 13C NMR spectrum of the MgCl2/PEI complex (Figure 6) shows at least three overlapping signals F

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modifications allow thus tailoring of binding properties of PEI toward MgCl2. Taking into consideration layering of MgCl2, coordination of PEIs on the (104) surface of MgCl2 is favored. Recent computational studies, including dispersion, indicate that from a thermodynamic point of view both the (104) and (110) surfaces are catalytically relevant.59 Branched polyethylenimines stabilize catalytically relevant MgCl2 surfaces more than linear polyethylenimines. In the case of branched PEIs, the stabilization energies are of the same magnitude with the conventional phthalate electron donor. The distance between coordinating nitrogen atoms, the flexibility of PEI structure, and the nature of coordinating amino groups (primary, secondary, or tertiary) were found to be important factors in the stabilization of MgCl2 surfaces by polyethylenimines. Also the experimental results show a strong coordination ability of branched PEI to MgCl2. The IR and CP/MAS 13 C NMR spectroscopic data clearly indicate coordination of branched PEI through its nitrogen atoms to MgCl 2 . Furthermore, the data suggest that coordination of branched PEI can occur through primary, secondary, and tertiary amino groups, in accordance with the calculations, which also show that coordination through primary amino groups is favored. The MgCl2-supported Ziegler−Natta catalyst possessing branched PEI as internal electron donor gave a relatively high activity in the copolymerization of ethylene and 1-butene. Overall, the results obtained indicate the potential of polyethylenimines as a new type of internal electron donors for Ziegler−Natta polymerization catalysts.

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Figure 8. Coordination of the employed (a) primary PEI, (b) secondary PEI, (c) tertiary PEI, and (d) real PEI on the (104) surface of MgCl2, accompanied by the corresponding surface stabilization energies.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05343. Details of the quantum chemical calculations concerning the interaction of PEIs with MgCl2, SEM image, and IR spectra of MgCl2/PEI complexes, and characterization of MgCl2/PEI/TiCl4 catalyst (PDF)

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phthalate/TiCl4). The MgCl2/PEI/TiCl4 catalyst performed well in the copolymerization of ethylene and 1-butene as activity of the MgCl2/PEI/TiCl4 catalyst was over 40% higher than that of the phthalate-containing reference catalyst.56 However, the comonomer response of the MgCl2/PEI/TiCl4 catalyst was somewhat lower than that of the MgCl2/phthalate/ TiCl4 catalyst,56 as higher C4/C2 ratio was needed in the case of MgCl2/PEI/TiCl4 catalyst to obtain polymers with comparable comonomer contents. The molar mass (Mw) and dispersity (Mw/Mn) of the copolymer produced with the MgCl2/PEI/ TiCl4 catalyst were comparable to those of the copolymer obtained with the phthalate-containing reference catalyst.56 Overall, the polymerization results indicate the potential of polyethylenimines as internal electron donors for Ziegler− Natta catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: tuula.pakkanen@uef.fi. Tel.: +358 504 354 379. ORCID

Ville H. Nissinen: 0000-0002-3709-4421 Mikko Linnolahti: 0000-0003-0056-2698 Tuula T. Pakkanen: 0000-0002-8196-392X

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CONCLUSIONS The study revealed interesting aspects of coordination of nitrogen-containing polymeric electron donors to the MgCl2 support. The quantum chemical calculations indicated that the structural variations in PEI significantly affect its ability to stabilize the lateral surfaces of MgCl2. The structural

Present Address ∥

Nanoscience Center, Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland. Notes

The authors declare no competing financial interest.

Table 3. Analytical Data of Ethylene/1-Butene Copolymers

a

catalyst

activity (kgpo/gcat/h)

1-butene (wt %)

Mw (g/mol)

Đa

MFR2 (g/10 min)

MFR21 (g/10 min)

FRR21/2

Tm (°C)

MgCl2/PEI/TiCl4 MgCl2/phthalate/TiCl4b

9.5 6.7

3.8 4.4

130500 102000

4.4 3.3

0.91 2.0

24.5 43.4

27.0 22.1

125.0 125.1

Dispersity (Mw/Mn). bReference 56. G

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ACKNOWLEDGMENTS The authors acknowledge grants of computer capacity from the Finnish Grid and Cloud Infrastructure (urn:nbn:fi:researchinfras-2016072533). Furthermore, the authors gratefully acknowledge the polymerization team in Borealis Polymers for performing ethylene/1-butene copolymerizations.

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