Efficient Synthesis of Hydroxylated Polyethylene via Copolymerization

ARTICLE pubs.acs.org/Organometallics

Efficient Synthesis of Hydroxylated Polyethylene via Copolymerization of Ethylene with 5-Norbornene-2-methanol using Bis(β-enaminoketonato)titanium Catalysts Miao Hong,†,‡ Yong-Xia Wang,† Hong-Liang Mu,†,‡ and Yue-Sheng Li*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ Graduate School of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China

bS Supporting Information ABSTRACT: The copolymerizations of ethylene with 5-norbornene-2-methanol (NBM) using bis(β-enaminoketonato)titanium complexes [RNC(CH3)CHCO(CF3)]2TiCl2 [1a: R = Ph, 1b: R = C6H4Me(p), 1c: R = C6H4tBu(p), 1d: R = C6H4F(p), 1e: R = C6H4Me(o)] and [R2NC(CF3)CHCO(R1)]2TiCl2 [2a: R1 = Ph, R2 = Ph; 2b: R1 = Ph, R2 = C6H4Me(p); 2c: R1 = Ph, R2 = C6H4tBu(p); 2d: R1 = Ph, R2 = C6H4F(p); 2e: R1 = C6H4Me(p), R2 = Ph] have been investigated. By optimizing the catalyst structures and polymerization conditions, outstanding catalytic activity and efficient polar comonomer incorporation have been achieved. Catalysts 1b,c and 2b,c with electron-donating groups (Me and tBu) display improved tolerance toward polar group, exhibiting higher catalytic activities. Among these catalysts, 1b produced the copolymer with the highest NBM incorporation. Using Et2AlCl as the pretreated reagent, the high NBM incorporation up to 22.0 mol % with a catalytic activity of 700 kg/molTi 3 h has been achieved by catalyst 1b. DFT calculations have been utilized to explain the high catalytic activity and efficient comonomer incorporation in the present catalyst system. The alternating ethylene
NBM sequence can be detected at high NBM incorporation, as demonstrated by 13C NMR (DEPT) spectra.

’ INTRODUCTION Since the discovery of Ziegler
Natta catalysts, it has been a long scientific interest and technologically important subject to investigate the routes to incorporate functional (polar) groups into a polyolefin chain, because the polyolefins with functional groups possess improved performance such as excellent dyeability, adhesion, and wettability.1 Conventionally, functional polyethylenes are produced by free radical copolymerizations of ethylene and polar comonomers at high temperature and extremely high pressure.2 In recent years, much attention has been paid to the copolymerization of ethylene with polar comonomers initiated by transition metal catalysts, because it provides an easy and low-cost access to functional polyethylene. Significant progress has been made in developing late transition metal catalysts that can copolymerize ethylene with a polar comonomer under mild conditions.3
5 Conversely, early transition metal catalysts suitable for the copolymerization of ethylene with polar comonomers are scarce due to their high oxophilicity, although these catalysts have offered remarkable opportunities for the synthesis of novel olefin-based materials.6
9 Especially, the titanium catalysts, which are extremely oxophilic and deactivated easily in the presence of polar monomers, are normally regarded as unpromising for such a copolymerization. To date, only two successful r 2011 American Chemical Society

examples have been achieved in titanium-catalyzed copolymerization of ethylene with a polar monomer.7,8 Fujita and coworkers have demonstrated that bis(phenoxyimine)titanium complexes (Ti-FI) are potent catalysts for ethylene/5-hexene1-yl-acetate copolymerization, and this result opened the door to polar monomer copolymerization with titanium catalysts.7 Recently, a class of titanium complexes bearing tridentate ligands ([O
NSR]TiCl3) developed by Tang and his colleagues has been proved to be efficient catalysts for copolymerization of ethylene with ω-alkenol or ω-alkenoic acid with comonomer incorporation up to 11.2 mol %.8 However, the synthesis of functional polyethylene that has the efficient polar comonomer incorporated with alternating sequence has not been reported yet in titanium-catalyzed copolymerization. Additionally, the polar comonomers used in titanium-catalyzed copolymerizations were limited to polar R-olefins. Compared with polar R-olefins, polar norbornene derivatives such as 5-norbornene-2-methanol9 are more convenient for easy synthesis via Diels
Alder reaction.10 Moreover, the copolymerization of ethylene with a polar norbornene derivative would provide a functional cyclic olefin copolymer with improved performance.6e,9 To date, few studies Received: June 18, 2011 Published: August 18, 2011 4678

dx.doi.org/10.1021/om200526n | Organometallics 2011, 30, 4678–4686

Organometallics Scheme 1. Synthetic Route for Hydroxylated Polyethylene and Structures of Catalysts Used

have been reported on the use of a polar norbornene derivative as the comonomer in titanium-catalyzed copolymerization. Recently, we demonstrated that the bis(β-enaminoketonato)titanium catalysts [PhNC(CF3)CHCO(Ph)]2TiCl2 (1a) and [PhNC(CH3)CHCO(CF3)]2TiCl2 (1b) (Scheme 1) are very effective for the copolymerization of ethylene with norbornene or norbornene derivatives such as dicyclopentadiene.11 However, these catalysts are extremely oxophilic and easily deactivated in the presence of a polar monomer. In order to improve the tolerance of the catalyst toward polar groups, we modified the electronic and steric properties of the titanium active site by introducing different substituents at different positions of the ligands. The trifluoromethyl group on the framework has been retained in the modification because it plays a crucial role in determining catalytic activity in the bis(β-enaminoketonato)titanium system.11a Thus, a series of catalysts 1b
e and 2b
e (Scheme 1) bearing a trifluoromethyl group on the framework were designed, and a detailed, systematic investigation regarding the copolymerization of ethylene with 5-norbornene-2-methanol (NBM) was carried out in this study.

’ EXPERIMENTAL SECTION General Procedure and Materials. All work involving air- and/ or moisture-sensitive compounds was carried out in an MBraun glovebox or under an argon atmosphere by using standard Schlenk techniques. The molecular weights (MWs) and polydispersity indices (PDIs) of the polymer samples were determined at 150 °C by a PL-GPC 220type high-temperature gel permeation chromatography. 1,2,4-Trichlorobenzene was employed as the solvent at a flow rate of 1.0 mL/min. The calibration was made by polystyrene standard Easi-Cal PS-1 (PL Ltd.). The differential scanning calorimetry (DSC) measurements were performed on a Perkin-Elmer Pyris 1 DSC instrument under N2 atmosphere. The samples were heated at a rate of 20 °C/min and cooled at a rate of 20 °C/min. The 1H NMR spectra of the ligands and catalysts were recorded on a Varian Unity-300 MHz spectrometer. The 13 C NMR spectra of the catalysts were recorded on a Varian Unity600 MHz spectrometer. The 1H and 13C NMR spectra of the polymers were recorded on a Varian Unity-400 MHz spectrometer. All deuterated NMR solvents were stored over molecular sieves under a nitrogen atmosphere, and all chemical shifts are given in ppm and referenced to Me4Si. The NBM content of copolymer was determined by 1H NMR spectra and calculated according to the formula NBM mol

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% = {[4(I3.6 ppm + I3.4 ppm)]/[2I0.7
2.3 ppm
5(I3.6 ppm + I3.4 ppm)]}  100%, where I3.6 ppm and I3.4 ppm are the peak areas of protons at 3.6 and 3.4 ppm, and I0.7
2.3 ppm is the total peak area of protons at 0.7
2.3 ppm. NBM was purchased from Aldrich, dried over molecular sieves for 3 days, and vacuum-distilled before use. Anhydrous toluene was purified by a solvent purification system purchased from MBraun. Modified methylaluminoxane (MMAO, 7% aluminum in heptane solution) and diethylchloroaluminum (Et2AlCl) were purchased from Akzo Nobel Chemical Inc. and Albemarle Corporation, respectively, and used without further purification. Synthesis of Ligands. p-MeC6H4NHC(CH3)CHCOCF3. To a stirred solution of 1,1,1-trifluoro-2,4-pentanedione (3.14 g, 20.0 mmol) in dried toluene (20 mL) were added 4-methylaniline (2.68 g, 25 mmol) and p-toluenesulfonic acid (ca. 20 mg) as a catalyst at room temperature. The mixture was heated and refluxed, and the water formed was removed azotropically using a Dean
Stark apparatus for 7 h. During the stirring period, a red solution was formed. Evaporation of toluene gave a deep red oil. The crude product was purified by column chromatography on silica gel with petroleum ether
ethyl acetate (200:1) as an eluent, affording product as colorless crystals in 85% yield. Ligands [RNHC(CF3)CHCOPh] were prepared via a procedure similar to that for [RNHC(CH3)CHCO(CF3)] except with refluxing for 24 h. 1H NMR (CDCl3): δ 12.52 (s, 1H, N-H), 7.21
7.18 (d, J = 9 Hz, 2H, Ph-H), 7.04
7.01 (d, J = 9 Hz, 2H, Ph-H), 5.50 (s, 1H, dCH), 2.36 (s, 3H, p-CH3), 2.07 (s, 3H, CH3). Anal. Calcd for C12H12F3NO: C, 59.26; H, 4.94; N, 5.76. Found: C, 59.00; H, 4.83; N, 5.67. p-tBuC6H4NHC(CH3)CHCOCF3. Yield: 90%, colorless crystal. 1H NMR (CDCl3): δ 12.55 (s, 1H, N-H), 7.42
7.39 (d, J = 9 Hz, 2H, Ph-H), 7.09
7.06 (d, J = 9 Hz, 2H, Ph-H), 5.51 (s, 1H, dCH), 2.10 (s, 3H, CH3), 1.31 (s, 9H, tBu). Anal. Calcd for C15H18F3NO: C, 63.16; H, 6.32; N, 4.91. Found: C, 63.28; H, 6.33; N, 4.90. p-FC6H4NHC(CH3)CHCOCF3. Yield: 86%, colorless crystal. 1H NMR (CDCl3): δ 12.48 (s, 1H, N-H), 7.26
7.09 (m, 4H, Ph-H), 5.55 (s, 1H, CH), 2.12 (s, 3H,
CH3). Anal. Calcd for C11H9F4NO: C, 54.35; H, 3.67; N, 5.67. Found: C, 54.21; H, 3.72; N, 5.64. o-MeC6H4NHC(CH3)CHCOCF3. Yield: 56%, yellow oil. 1H NMR (CDCl3): δ 12.43 (s, 1H, N-H), 7.30
7.21 (m, 3H, Ph-H), 7.10
7.07 (m, 1H, Ph-H), 5.54 (s, 1H, dCH), 2.27 (s, 3H, o-CH3), 1.98 (s, 3H, CH3). Anal. Calcd for C12H12F3NO: C, 59.26; H, 4.94; N, 5.76. Found: C, 59.17; H, 4.89; N, 5.50. p-MeC6H4NHC(CF3)CHCOPh. Yield: 58%, yellow oil. 1H NMR (CDCl3): δ 12.27 (s, 1H, N-H), 7.95
7.93 (m, 2H, Ph-H), 7.56
7.43 (m, 3H, Ph-H), 7.17
7.11 (m, 4H, Ph-H), 6.40 (s, 1H, dCH), 2.35 (s, 3H, CH3). Anal. Calcd for C17H14F3NO: C, 66.88; H, 4.62; N, 4.59. Found: C, 66.71; H, 4.58; N, 4.69. p-tBuC6H4NHC(CF3)CHCOPh. Yield: 65%, yellow crystal. 1H NMR (CDCl3): δ 12.30 (s, 1H, N-H), 7.95
7.92 (m, 2H, Ph-H), 7.53
7.44 (m, 3H, Ph-H), 7.37
7.35 (d, J = 6 Hz, 2H, Ph-H), 7.17
7.15 (d, J = 6 Hz, 2H, Ph-H), 6.39 (s, 1H, dCH), 1.31 (s, 9H, tBu). Anal. Calcd for C20H20F3NO: C, 69.16; H, 5.76; N, 4.03. Found: C, 69.71; H, 5.58; N, 4.19. p-FC6H4NHC(CF3)CHCOPh. Yield: 72%, orange crystal. 1H NMR (CDCl3): δ 12.17 (s, 1H, N-H), 7.95
6.80 (m, 9H, Ph-H) 6.38 (s, 1H, dCH). Anal. Calcd for C17H14F3NO2: C, 62.13; H, 3.56; N, 4.53. Found: C, 63.25; H, 3.33; N, 4.30. PhNHC(CF3)CHCO(p-MeC6H4). Yield: 68%, orange oil. 1H NMR (CDCl3): δ 12.30 (s, 1H, N-H), 7.61 (m, 4H, Ar-H), 7.35
7.22 (m, 5H, Ph-H), 6.41 (s, 1H, dCH), 2.41 (s, 3H, CH3). Anal. Calcd for C17H14F3NO: C, 66.82; H, 4.58; N, 4.58. Found: C, 66.02; H, 4.80; N, 4.67. Synthesis of Titanium Complexes 1b
e and 2b
e. [pMeC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1b). To a stirred solution of compound p-MeC6H4NHC(CH3)CHCOCF3 (2 mmol) in dried diethyl ether (20 mL) at
78 °C was added dropwise a 2.5 M n-butyllithium 4679

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Organometallics hexane solution (0.84 mL) over 5 min. The mixture was allowed to warm to room temperature and stirred for 4 h. Then the mixture was added dropwise to TiCl4 (1 mmol) in dried diethyl ether (10 mL) at
78 °C with stirring over 30 min. The mixture was allowed to warm to room temperature and stirred overnight. The evaporation of the solvent under vacuum yielded a crude product. To the crude product was added 30 mL of dried dichloromethane, and the mixture was stirred for 10 min and then filtered. The filtrate was evaporated under vacuum to afford a solid residue. Dried dichloromethane and dried n-hexane were added to the solid residue, and the mixture was stirred for 10 min. The filtration of the mixture gave complex 1b as a red crystal in 70% yield. 1H NMR (CDCl3): δ 7.24
7.21 (d, J = 9 Hz, 2H, Ph-H), 7.09
7.03 (m, 4H, Ph-H), 6.45
6.42 (d, J = 9 Hz, 2H, Ph-H), 5.78 (s, 2H, dCH), 2.32 (s, 6H, p-CH3), 1.85 (s, 6H, CH3). 13C NMR (CDCl3): δ 172.40, 146.50, 136.61, 134.38, 130.54, 130.03, 128.86, 125.10, 120.21, 106.32, 90.85, 24.88, 20.98, 20.80, 20.22. Anal. Calcd for C24H22Cl2F6N2O2Ti: C, 47.77; H, 3.65; N, 4.64. Found: C, 46.62; H, 3.64; N, 4.70. Other complexes were prepared via a similar procedure. [p-tBuC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1c). Yield: 85%, red crystal. 1 H NMR (CDCl3): δ 7.45
7.42 (d, J = 9 Hz, 2H, Ph-H), 7.26
7.23 (d, J = 9 Hz, 2H, Ph-H), 7.12
7.09 (d, J = 9 Hz, 2H, Ph-H), 6.48
6.45 (d, J = 9 Hz, 2H, Ph-H), 5.75 (s, 2H, dCH), 1.85 (s, 6H, CH3), 1.29 (s, 18H, tBu). 13C NMR (CDCl3): δ 172.27, 156.46, 156.22, 149.88, 146.39, 126.95, 126.37, 125.74, 125.24, 124.43, 120.00, 106.49, 34.59, 31.21, 25.09. Anal. Calcd for C30H34Cl2F6N2O2Ti: C, 52.41; H, 4.95; N, 4.08. Found: C, 52.52; H, 4.74; N, 4.10. [p-FC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1d). Yield: 68%, red crystal. 1H NMR (CDCl3): δ 7.26
6.98 (m, 16H, Ph-H), 5.30 (s, 2H, CH), 2.07 (s, 6H, -CH3). 13C NMR (CDCl3): δ 176.90, 168.10, 162.42, 160.76, 132.81, 127.26, 116.39, 90.91, 20.15. Anal. Calcd for C22H16Cl2F8N2O2Ti: C, 43.24; H, 2.64; N, 4.58. Found: C, 43.41; H, 2.60; N, 4.54. [o-MeC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1e). Yield: 62%, red crystal. 1 H NMR (CDCl3): δ 7.23
6.89 (m, 16H, Ph-H), 6.27
6.09 (m, 2H, dCH), 2.27
2.17 (m, 6H, o-CH3), 1.89
1.84 (m, 6H, CH3). 13C NMR (CDCl3): δ 173.77, 149.93, 146.10, 135.85, 133.92, 131.30, 131.18, 130.97, 130.75, 130.64, 128.10, 127.33, 127.19, 126.82, 126.39, 126.31, 125.13, 107.49, 23.94, 18.55, 18.22, 17.86. Anal. Calcd for C24H22Cl2F6N2O2Ti: C, 47.77; H, 3.65; N, 4.64. Found: C, 46.90; H, 3.72; N, 4.61. [p-MePhNHC(CF3)CHCO(Ph)]2TiCl2 (2b). Yield: 70%, brown crystal. 1 H NMR (CDCl3): δ 7.53
7.50 (d, J = 9 Hz, 4H, Ph-H), 7.34
7.31 (d, J = 9 Hz, 2H, Ph-H), 7.16
7.13 (d, J = 9 Hz, 4H, Ph-H), 7.06
6.93 (m, 4H, Ph-H), 6.80
6.77 (d, J = 9 Hz, 4H, Ph-H), 6.47 (s, 2H, dCH), 2.40 (s, 6H, CH3). 13C NMR (CDCl3): δ 175.08, 157.10, 145.51, 136.49, 132.89, 132.62, 128.83, 128.45, 128.07, 127.96, 126.35, 121.60, 99.20, 20.50. Anal. Calcd for C34H26Cl2F6N2O2Ti: C, 56.14; H, 3.60; N, 3.85. Found: C, 56.32; H, 3.54; N, 3.80. [p-tBuC6H4NC(CF3)CHCO(Ph)]2TiCl2 (2c). Yield: 90%, black crystal. 1 H NMR (CDCl3): δ 7.70
7.67 (d, J = 9 Hz, 4H, Ph-H), 7.49
7.45 (t, J = 6 Hz, 2H, Ph-H), 7.38
7.32 (t, J = 9 Hz, 4H, Ph-H), 7.27
7.24 (d, J = 9 Hz, 2H, Ph-H), 7.05
6.99 (t, J = 9 Hz, 4H, Ph-H), 6.72
6.70 (d, J = 6 Hz, 2H, Ph-H), 6.52 (s, 1H, dCH), 0.92 (s, 9H, tBu). 13C NMR (CDCl3): δ 174.82, 157.24, 157.06, 149.30, 145.08, 132.95, 132.53, 128.65, 128.05, 125.99, 124.99, 124.43, 121.51, 98.77, 34.06, 30.82. Anal. Calcd for C40H38Cl2F6N2O2Ti: C, 59.19; H, 4.69; N, 3.45. Found: C, 59.22; H, 4.58; N, 3.40. [p-FC6H4NC(CF3)CHCO(Ph)]2TiCl2 (2d). Yield: 76%, black crystal. 1H NMR (CDCl3): δ 7.70
6.54 (m, 18H, Ph-H), 6.40 (s, 2H, dCH). 13C NMR (CDCl 3 ): δ 177.00, 168.30, 162.48, 160.72, 132.95, 132.81, 132.53, 128.65, 127.26, 124.43, 116.31, 92.91. Anal. Calcd for C32H20Cl2F8N2O2Ti: C, 52.25; H, 2.72; N, 3.81. Found: C, 51.75; H, 2.92; N, 3.62. [PhNC(CF3)CHCO(p-MeC6H4)]2TiCl2 (2e). Yield: 71%, black crystal. 1 H NMR (CDCl3): δ 7.49
6.86 (m, 18H, Ph-H), 6.58 (s, 2H, dCH),

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2.02 (s, 6H, CH3). 13C NMR (CDCl3): δ 174.50, 156.64, 148.60, 143.50, 138.34, 136.43, 129.75, 129.40, 128.00, 127.36, 126.34, 120.66, 99.32, 20.67. Anal. Calcd for C34H26Cl2F6N2O2Ti: C, 60.09; H, 3.83; N, 4.12. Found: C, 60.00; H, 3.92; N, 4.32. Crystallographic Studies. Single crystals of complexes 1b
d suitable for X-ray analyses were grown from a saturated CH2Cl2
hexane solution. The intensity data were collected with the ω scan mode (186 K) on a Bruker Smart APEX diffractometer with a CCD detector using Mo KR radiation (λ = 0.71073 Å). Lorentz and polarization factors were made for the intensity data, and absorption corrections were performed using the SADABS program. The crystal structures were solved using the SHELXTL program and refined using full matrix least squares. The positions of hydrogen atoms were calculated theoretically and included in the final cycles of refinement in a riding model along with attached carbons. DFT Calculations. All density functional theory (DFT) calculations were performed by using the Amsterdam Density Functional (ADF) program package. The structures and energies are obtained based on the local density approximation augmented with Becke’s nonlocal exchange correction and Perdew’s nonlocal correlation correction. A triple STO basis set was employed for Ti, while all other atoms were described by a double-ζ plus polarization STO basis. The frozen core approximation was employed for the 1s electrons of the C, N, O, and F atoms, as well as the 1s
2p electrons of the Ti atom. Finally, firstorder scalar relativistic corrections were added to the total energy of the system.

General Procedure for Pretreating Comonomer with Et2AlCl. To a solution of comonomer (0.04 mol) in toluene (20 mL)

was added carefully Et2AlCl (1.2 equiv for NBM) at 0 °C. The resulting mixture was allowed to warm to room temperature. Then the desired amount of toluene was added until the total volume of the solution was 40 mL, and the obtained solution was stored under nitrogen for further use. Typical Copolymerization Procedure. Copolymerizations were carried out under atmospheric pressure in toluene in a 150 mL glass reactor equipped with a mechanical stirrer. The total volume of the copolymerization solution was 50 mL. The reactor was charged with a prescribed volume of toluene and the described comonomer under argon atmosphere, and then the ethylene gas feed was started, followed by the addition of MMAO to the reactor. After equilibration at the desired polymerization temperature for 5 min, the polymerization was initiated by the toluene solution of the catalyst. After a desired period of time, the reactor was vented. The resultant copolymers were precipitated from hydrochloric acid
ethanol (2%V), filtered, washed three times with ethanol and acetone, then immersed in ethanol for 12 h to remove the unreacted comonomer, and then dried in vacuo at 40 °C to a constant weight.

’ RESULTS AND DISCUSSION Synthesis and Characterization of Bis(β-enaminoketonato)titanium Complexes. The desired titanium complexes were

obtained in good yields (62
90%) by the reaction of TiCl4 with 2 equiv of the relevant lithium salts of β-enaminoketonates in dried diethyl ether. These complexes were fully identified by 1H NMR spectra and elemental analyses. The crystals of complexes 1b
d suitable for crystallographic study were grown from the CH2Cl2
hexane mixture solution, and their molecular structures were further confirmed by X-ray crystallographic analyses. The crystallographic data, collection parameters, and refinement parameters for the X-ray diffraction analyses of complexes 1b
d are summarized in Table S1 (see Supporting Information). As shown in Figure 1, complexes 1b
d adopt a distorted octahedral geometry around the titanium center with the trans oxygen atoms 4680

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Figure 1. Molecular structures of complexes 1a
d with thermal ellipsoids at the 30% probability level (H atoms are omitted for clarity).

Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for Complexes 1a
d 1a

1b

1c(1)

1c(2)

1d

Ti
Cl(1) Ti
Cl(2)

2.2777(9) 2.2777(9)

2.3035(15) 2.2697(15)

2.282(2) 2.276(2)

2.2780(19) 2.267(2)

2.2804(9) 2.2804(9)

Ti1
O(1)

1.900 (2)

1.893(3)

1.873(4)

1.872(4)

1.889(2)

Ti1
O(2)

1.900 (2)

1.881(3)

1.878(4)

1.883(4)

1.889(2)

Ti1
N(1)

2.198(2)

2.209(4)

2.180(5)

2.176(5)

2.198(2)

Ti1
N(2)

2.198(2)

2.174(4)

2.192(5)

2.204(5)

2.198(2)

O(1)
Ti
O(2)

170.82(13)

172.43(13)

171.08(17)

172.61(18)

173.50 (13)

N(1)
Ti
N(2)

84.00(13)

82.03(13)

83.41(18)

82.54(17)

82.22 (12)

Cl(1)
Ti
Cl(2) O(1)
Ti
N(1)

98.18(5) 83.22(9)

102.71(5) 83.09(13)

100.07(8) 83.55(17)

101.92(7) 82.73(19)

100.42 (5) 82.92(8)

O(2)
Ti
N(1)

89.95(9)

90.24(13)

89.23(18)

91.07(18)

92.17(9)

O(1)
Ti
N(2)

89.95(9)

91.52(13)

90.55(18)

90.94(18)

82.92 (9)

O(1)
Ti
Cl(1)

93.64(7)

92.46(10)

93.06(14)

92.61(14)

90.22(7)

O(2)
Ti
Cl(2)

93.27(7)

94.48(11)

91.92(14)

91.20(13)

93.95 (7)

N(1)
Ti
Cl(1)

89.06(7)

87.19(10)

171.58(15)

169.00(13)

89.08(6)

N(2)
Ti
Cl(1)

171.77(7)

167.98(11)

88.94(14)

87.60(13)

168.677(6)

Bond Distances (Å)

Bond Angles (deg)

and cis chloride atoms as well as cis nitrogen atoms similar to nonsubstituted complex 1a.12 Selected bond distances and angles for complexes 1a
d are summarized in Table 1.12 Complex 1c crystallizes two independent molecules in the asymmetric unit cell with slightly different bond lengths and angles, and the average values of bond lengths and angles are used for comparison. Although the Ti
O, Ti
N, and Ti
Cl bond distances for complexes 1a
d are very similar, the bond angles (e.g., O(1)
Ti
O(2), N(1)
Ti
N(2), and Cl(1)
Ti
Cl(2)) for these complexes are significantly different. Especially, there is an evident discrepancy in the Cl(1)
Ti
Cl(2) angle which would provide the vacant coordination site at the active center in the presence of MMAO (1a: 98.18°, 1b: 102.71°, 1c: 101.00, 1d: 100.42). The wider Cl(1)
Ti
Cl(2) angle of complex 1b indicates that this complex may have a sterically more open

active site and is expected to produce copolymer with higher comonomer incorporation. Ethylene/NBM Copolymerization. Upon treatment with MMAO, catalysts 1b
d and 2b
e exhibit high catalytic activities toward ethylene polymerization (except catalyst 1e, discussed later), affording linear polyethylenes with high molecular weights and unimodal molecular weight distributions (Table S2, see Supporting Information). The copolymerization behavior of these catalysts (1b
e and 2b
e) took our attention and was explored in depth. For comparison, the copolymerization promoted by nonsubstituted catalysts 1a and 2a was also investigated. Representative results are summarized in Table 2. Overall, for the catalysts bearing the same substituents at the para-position of the N-aryl moiety of the ligands, catalysts 1a
d, with a methyl on the framework, exhibited higher 4681

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Table 2. Ethylene/NBM Copolymerization by Bis(β-enaminoketonato)titanium Catalystsa yield (mg)

activityb

incorpc (mol %)

Mwd (kg/mol)

Mw/Mnd

Tme (°C)

entry

cat.

comon (mmol)

Al/Ti

1

1a

4.5

2500

70

53

5.5

22.8

1.9

81.4

2

1b

2.0

1500

390

300

2.8

89.0

2.6

102.7

3

1b

3.0

2000

213

160

90.0

4

1b

4.5

2500

160

120

5

1c

2.0

1500

450

337

6

1c

3.0

2000

267

200

7

1c

4.5

2500

200

8 9

1d 2a

4.5 4.5

2500 2500

10

2b

2.0

11

2b

3.0

12

2b

13 14

5.0

53.4

2.5

36.5

1.9

1.6

95.1

1.7

120.6

3.4

51.9

2.2

93.0

150

6.0

31.7

1.9

87.0

40 55

30 41

6.8 5.2

7.61 18.7

2.1 1.9

93.2 97.4

1500

320

240

1.8

84.5

1.7

105.0

2000

173

130

4.6

50.4

2.1

96.0

4.5

2500

107

80

9.2

28.0

1.8

2c

2.0

1500

360

270

1.5

102.0

1.6

120.0

2c

3.0

2000

200

150

3.1

60.0

1.9

91.2

15

2c

4.5

2500

133

100

5.6

36.2

1.7

89.0

16 17

2d 2e

4.5 4.5

2500 2500

25 47

15 35

6.5 1.2

32.5

1.8

110.0

10

Conditions: catalyst, 8 μmol; cocatalyst, MMAO; ethylene, 1 atm; Vtotal = 50 mL; T = 25 °C; time =10 min. b Catalytic activity: kg polymer/molTi 3 h. c Comonomer incorporation (mol %) established by 1H NMR spectra. d Weight-average molecular weights and polydispersity indices determined by GPC at 150 °C in 1,2,4-C6Cl3H3 vs narrow polystyrene standards. e Melting temperatures were measured by DSC. a

Figure 2. Energy difference (ΔE) between ethylene-coordinated and oxygen-coordinated cationic complexes (NB-OCH3 was used in DFT calculations for simplicity).

activity than those (2a
d) with a phenyl group on the framework (1a vs 2a, 1b vs 2b, 1c vs 2c, 1d vs 2d). Compared with nonsubstituted catalysts 1a and 2a, 1b,c and 2b,c, bearing electron-donating groups (Me and tBu) at the para-position of the N-aryl moiety, displayed much higher activities, while the introduction of electron-withdrawing groups (F, 1d and 2d) led to lower activities. Additionally, the activities of catalysts 1c and 2c, with tert-butyl groups, are higher than those of catalysts 1b and 2b, with methyl groups (Table 2, entry 3 vs 6 and entry 11 vs 14). Since the introduction of an electrondonating group can enhance the catalytic activity, we thus tried to synthesize the catalyst with a methyl group adjacent to the imine moiety (1e). However, only a trace amount of polyethylene was obtained, which may be caused by the steric effect of ortho-methyl groups hindering ethylene from accessing the titanium center (Table S2, see Supporting Information). Moreover, complex 2e, with a methyl group at the para-position of the phenyl group on the framework, was also synthesized and utilized in ethylene/ NBM copolymerization. However, lower activity and NBM incorporation were observed compared with that of catalyst 2a (Table 2, entry 9 vs 17).

To gain more insight into the ligand effect on catalytic activity in polar monomer copolymerization, DFT calculations were used to estimate the energy difference (ΔE = EE
EO) between ethylene-coordinated (EE) and oxygen-coordinated (EO) cationic complexes, which is an indication of functional group tolerance (Figure 2).7 The calculated structures of ethylene-coordinated and oxygen-coordinated species are provided in Figures S1 and S2 (see Supporting Information). The oxygen-coordinated cationic methyl complexes are more stable than ethylene-coordinated cationic methyl complexes. The ΔE values for catalysts 1a
d are 43.47, 36.16, 31.77, and 64.37 kJ/mol, respectively. The lower ΔE value of a catalyst suggests that it has a less oxophilic titanium center and will display better tolerance toward a functional group. Accordingly, among these catalysts, 1b,c, with electron-donating groups (tBu and Me), have much lower oxophilic titanium centers, while 1d, with an electron-withdrawing group (F), has the highest oxophilic titanium center. Consistent with the order of experimental catalytic activity, the order of functional group tolerance for catalysts 1a
d predicted by DFT calculations is 1c > 1b > 1a > 1d. These calculation results provide a reasonable demonstration that a less oxophilic Ti center, generated by a more electron-donating ligand, exhibits better tolerance toward functional groups, giving rise to an enhanced catalytic activity. The comonomer incorporation is strongly influenced by not only the catalyst structures but also the reaction conditions. Overall, relative to nonsubstituted catalysts 1a and 2a, catalysts with substituents at the para-position of the N-aryl moiety (1b
d and 2b
d) produced the copolymer with higher NBM incorporation. Among these catalysts with substituents at the para-position of the N-aryl moiety (1b
d and 2b
d), the NBM incorporation increases in the sequence 1c ≈ 2c (tBu) < 1d ≈ 2d (F) < 1b ≈ 2b (Me). Remarkably, catalysts 1b and 2b, with methyl groups, produce the copolymers with significantly enhanced NBM incorporations, which are nearly twice as high as those produced by nonsubstituted catalysts (Table 2, entry 1 4682

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Figure 3. Calculated structures of cationic methyl complexes derived from complexes 1a
d (H atoms are omitted for clarity).

Table 3. Ethylene/Pretreated NBM Copolymerization by Bis(β-enaminoketonato)titanium Catalystsa comonb (mmol)

incorpd (mol %)

Mwe (kg/mol)

Mw/Mne

Tmf (°C)

78.0

1.9

80.0

60.0 51.8

1.9 1.8

cat.

1

1b

2 3

1b 1b

4

1c

667

2500

93.4

1.6

5

1c

10

523

1960

12

72.0

2.3

6

1c

18

360

1350

16

68.0

2.0

4.5 10 18 4.5

yield (mg)

activityc

entry

507

1900

320 187

1200 700

7.0 15 22 5.3

86.5

Conditions: catalyst, 8 μmol; cocatalyst, MMAO; ethylene, 1 atm; Vtotal = 50 mL; T = 25 °C; time = 2 min. b Comonomer was pretreated by 1.2 equiv of Et2AlCl. c Catalytic activity: kg polymer/molTi 3 h. d Comonomer incorporation (mol %) established by 1H NMR spectra. e Weight-average molecular weights and polydispersity indices determined by GPC at 150 °C in 1,2,4-C6Cl3H3 vs narrow polystyrene standards. f Melting temperatures were measured by DSC. a

vs 4 and entry 9 vs 12). As the NBM feed rises gradually from 2.0 to 4.5 mmol, the NBM incorporation for catalyst 1b increases significantly from 2.8 to 10.0 mol % (Table 2, entries 2
4). Generally, high comonomer incorporation is associated with the sterically open nature of the active site and high electrophilicity of the metal center achieved by introducing the electron-withdrawing groups.13 However, in this work, the catalysts 1b and 2b, with electron-donating groups (Me), produced the copolymer with even higher NBM incorporation than those of the catalysts 1d and 2d, with electron-withdrawing groups (F), which is probably caused by the more open active site of the catalysts 1b and 2b. In order to obtain the structures of real active species after the initiation reaction, DFT calculations were carried out. As the active species derived from a group 4 transition metal complex is now widely accepted to be a cationic alkyl complex,13,14 the cationic methyl species (for simple calculation) derived from complexes 1a
d were calculated. The resultant structures of active species are shown in Figure 3. As observed, the alkene approaches the titanium center on the side of either the N
Ti
C or O
Ti
C angle. In fact, the coordination reactions

for both ethylene and NBM all occur in the space between the N
Ti
C angles (Figures S1 and S3, see Supporting Information). It can be clearly explained by more open environment (120.59
124.95°) of the N
Ti
C angle relative to the O
Ti
C angle (1a: 85.33°, 1b: 83.96°, 1c: 85.41°, 1d: 86.04°), which is propitious for the alkene to access the titanium center with less steric congestion. Accordingly, the cationic methyl species with a wider N
Ti
C angle has more open coordination space. The N
Ti
C angles of active species derived from 1a
d are 120.59°, 124.95°, 121.37°, and 121.35°, respectively, indicating the coordination space has a sequence of 1b > 1d ≈ 1c > 1a. The highest comonomer incorporation of catalyst 1b can be clearly explained by its widest coordination space. Catalyst 1d displays higher NBM incorporation than 1c though both catalysts have similar coordination space, which may be due to the higher electrophilicity of 1d originating from the electron-withdrawing groups. In our work, the catalysts with improved functional group tolerance (1b,c) exhibited high activity even without the pretreatment procedure (120
337 kg polymer/molTi 3 h, Table 2, entries 2
7). In order to further increase the catalytic activity, 4683

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Figure 5. 1H NMR spectrum of ethylene/NBM copolymer (NBM mol % = 16.0 mol %, Table 3, entry 6) in o-C6D4Cl2 at 125 °C.

Figure 4. Comparison of the 13C NMR (DEPT) spectra of ethylene/ NBM copolymers in o-C6D4Cl2 at 125 °C: (a) NBM mol % = 7.0 mol % (Table 3, entry 1); (b) NBM mol % = 16 mol % (Table 3, entry 6); (c) NBM mol % = 22.0 mol % (Table 3, entry 3); (d) 13C NMR (DEPT) spectra (NBM mol % = 16 mol %). The detailed 13C NMR spectra in the range 38
50 ppm are provided at the bottom.

the copolymerizations of ethylene with NBM using Et2AlCl as the pretreated reagent were also carried out by catalysts 1b,c, and the results are summarized in Table 3. The influence of catalyst structure on catalytic activity and comonomer incorporation in ethylene/Et2AlCl-pretreated NBM copolymerizations was similar to those in nonpretreated copolymerizations. The highest catalytic activity, up to 2500 kg/molTi 3 h, has been achieved by catalyst 1c with NBM incorporation of 5.3 mol % (Table 3, entry 4). Increasing the NMB feed resulted in higher comonomer incorporation, and the highest incorporation, up to 22.0 mol %, has been achieved by catalyst 1b with an activity of 700 kg/ molTi 3 h (Table 3, entry 3). As far as we know, the present catalyst system is the first example of successful titaniumcatalyzed ethylene/polar comonomer copolymerization with efficient polar comonomer incorporation. All the copolymers obtained by nonpretreated or pretreated copolymerizations display single melting temperatures (Tm) in the measurement of DSC, indicating the component of copolymer is homogeneous (typical DSC curves are provided in

Figures S4
6, Supporting Information). With increasing comonomer incorporation, Tm values of copolymers decrease. Moreover, as expected for a polymer generated from a chemically homogeneous catalyst, all the copolymers possess narrow molecular weight distributions (Mw/Mn = 1.6
2.6; typical GPC curves are provided in Figure S7, Supporting Information). The molecular weights of copolymers vary with catalyst structures and reaction conditions. Overall, the electron-donating substituted catalysts produce copolymers with higher molecular weights than those nonsubstituted and electron-withdrawing substituted catalysts. Copolymers obtained by pretreatment procedure have higher molecular weights than those obtained without pretreatment procedure. Microstructure of Ethylene/NBM Copolymer. The microstructure of ethylene/NBM copolymer is established by 1H and 13 C NMR (DEPT) spectra. The typical 13C NMR (DEPT) spectra for ethylene/NBM copolymers are presented in Figure 4. The copolymer with low NBM incorporation (7.0 mol %) shows a similar 13C NMR spectrum (Figure 4a) to that previously reported.9d The analysis of spectra indicated that the bis(βenaminoketonato)titanium catalysts promote addition-type polymerization of NBM, which contains both exo and endo enchainment. The peaks at 67.4 and 64.8 ppm are assigned to C8a (exo) and C8b (endo) in the CH2OH substituent, respectively. Those peaks in the region of 29.60
49.00 ppm are ascribed to the carbons in the norbornenyl moiety and successive ethylene units. Additionally, the 13C NMR spectra for the copolymer with low NBM incorporation also reveal that NBM is randomly incorporated into the polymer with the isolated NBM unit present in the polyethylene sequences, while the continuous NBM
NBM sequence and alternating ethylene
NBM sequence are not detectable. The negligible NBM-NBM sequence can be clearly explained by steric congestion of titanium active species associated with the successive insertion of NBM units, which is consistent with the fact that these catalysts exhibit no catalytic activity in an attempted homopolymerization of NBM. With NBM incorporation increasing from 7.0 mol % to 16.0 mol %, the 13C NMR spectrum (Figure 4b) is different from the previous report.9d The new peaks at 56.35 [2a0 (exo)], 51.75 [2b0 (endo)], 48.96 [3b0 (endo)], 47.52 [3a0 (exo)], 46.27 [1b0 (endo)], 44.92 [4a0 (exo)], 43.20 [4b0 (endo)], 42.48 [1a0 (exo)], 40.80 [5a0 (exo)], 40.45 [5b0 (endo)], 39.46 [7a0 (exo)], 37.15 [7b0 (endo)], and 32.29 ppm [6a0 (exo)] can be tracked, which are attributed to the alternating ethylene
NBM sequence. These peaks are assigned on the basis of the 13C NMR spectra with low NBM incorporation, the molar ratio of exo and endo in 4684

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Organometallics the copolymer, and a comparison of 13C NMR and 13C NMR (DEPT) spectra. When NBM incorporation increases to 22.0 mol % (Figure 4c), the intensities of these peaks corresponding to the alternating ethylene
NBM sequence are further enhanced. As far as we are aware, the present catalyst system based on 1b,c represents the first example of successful titaniumcatalyzed polar monomer copolymerization that has efficient polar comonomer incorporation with alternating sequence. A typical 1H NMR spectrum of E/NBM copolymer is provided in Figure 5. The peaks at 3.6 and 3.4 ppm are assigned to the protons of methylene in CH2OH corresponding to endo and exo enchainments, respectively. 1H and 13C NMR spectra all indicate both endo and exo isomers are incorporated into the polymer chain and the intensity ratio is almost 1:1. Regarding NBM comonomer used in our work composed of exo and endo isomers with the molar ratio of 0.3:1, the present catalytic system is considered to prefer copolymerizing the exo isomer to the endo isomer.

’ CONCLUSIONS A series of hydroxylated polyethylenes with high hydroxyl group contents have been efficiently synthesized via copolymerizations of ethylene with 5-norbornene-2-methanol using newly designed bis(β-enaminoketonato)titanium complexes. The catalysts with electron-donating groups (Me and tBu, 1b,c and 2b,c), possessing less oxophilic Ti centers as confirmed by DFT calculations, have improved tolerance toward the functional groups, exhibiting higher catalytic activities. Among these catalysts, 1b produced the copolymer with the highest NBM incorporation on account of its widest coordination space as illuminated by DFT calculations. Using Et2AlCl as the pretreated reagent, the high NBM incorporation up to 22.0 mol % with a catalytic activity of 700 kg/molTi 3 h has been achieved by catalyst 1b. 13C NMR (DEPT) spectra revealed that the alternating ethylene
NBM sequence can be detected at high NBM incorporation. As far as we know, the present catalyst system is the first example of successful titanium-catalyzed ethylene/polar comonomer copolymerization that has efficient polar comonomer incorporation with alternating sequence. ’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic data, collection parameters, and refinement parameters for the X-ray diffraction analyses of complexes 1b
d, selective 1H NMR spectra for complexes, ethylene polymerization, calculated structures for ethylene-coordinated, oxygen-coordinated, and NBMcoordinated species, and typical DSC and GPC curves for copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: +86-431-85262039. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors are grateful for a subsidy provided by the National Natural Science Foundation of China (Nos. 20734002 and 20923003).

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