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Access to High-Molecular-Weight Polyethylenes through High Temperature Ethylene Polymerization Catalysed by Ethylene-Bridged ansa-(3-R-Cyclopentadienyl)(Fluorenyl) Zirconocene Complexes
RESEARCH ARTICLE | 更新时间：2023-12-26
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    • Access to High-Molecular-Weight Polyethylenes through High Temperature Ethylene Polymerization Catalysed by Ethylene-Bridged ansa-(3-R-Cyclopentadienyl)(Fluorenyl) Zirconocene Complexes

    • Access to High-Molecular-Weight Polyethylenes through High Temperature Ethylene Polymerization Catalysed by Ethylene-Bridged ansa-(3-R-Cyclopentadienyl)(Fluorenyl) Zirconocene Complexes

    • Li Bo

      ，  

      Ma Haiyan

      ，  

      Huang Jiling

      ，  
    • 高分子科学（英文版）   2024年42卷第1期 页码：42-51

    • DOI：10.1007/s10118-023-3034-z    

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  • Li, B.; Ma, H.; Huang, J. Access to high-molecular-weight polyethylenes through high temperature ethylene polymerization catalysed by ethylene-bridged ansa-(3-R-cyclopentadienyl)(fluorenyl) zirconocene complexes. Chinese J. Polym. Sci. 2024, 42, 42–51 DOI： 10.1007/s10118-023-3034-z.

    Bo Li, Haiyan Ma, Jiling Huang. Access to High-Molecular-Weight Polyethylenes through High Temperature Ethylene Polymerization Catalysed by Ethylene-Bridged ansa-(3-R-Cyclopentadienyl)(Fluorenyl) Zirconocene Complexes[J]. Chinese Journal of Polymer Science, 2024,42(1):42-51. DOI： 10.1007/s10118-023-3034-z.

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    Abstract

    A series of C1-symmetric ethylene-bridged ansa-(3-R-cyclopentadienyl)(fluorenyl) metallocene complexes (Zr: 1−5; Hf: 6) have been synthesized, characterized and investigated as catalyst precursors for the high temperature ethylene polymerization. Using methylaluminoxane (MAO) as the cocatalyst, zirconium complexes 1−5 bearing a bulky substituent on the 3-position of the cyclopendienyl ring showed high catalytic activities up to 1.48×107 gPE·molZr−1·h−1 toward the polymerization of ethylene and afforded polyethylenes with high molecular weights (1.49×105−6.31×105 g/mol), meanwhile exhibting great thermal stability at high temperatures up to 120 °C together with a long catalytic life time up to 2 h. By adopting low Al/Zr ratios, such as 125, polyethylenes with ultra high molecular weights up to 2.86×106 g/mol were obtained. It is worthy of noting that zirconium complexes 1−4 bearing a substituent with an aryl pendant showed temperature-dependent activities, which increased rapidly with the increase of polymerization temperature, thus weak interaction of the pendent aryl group with the cationic active center is proposed to account for the very low activities displayed at low temperatures. In contrast to zirconocene complexes 1−5, hafnocene complex 6 only displayed very low catalytic activities toward the polymerization of ethylene and afforded polyethylenes with molecular weights ten times smaller than those obtained by zirconocene complexes 1−5. Zirconocene complexes 1−5 were also able to catalyse the polymerization of propylene at high temperatures, but only afforded waxes with low molecular weights.

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    摘要

    Novel ethylene-bridged ansa-(3-R-cyclopentadienyl)(fluorenyl) zirconocene and hafnocene complexes have been synthesized and characterized. Among them zirconocene complexes showed high catalytic activities toward the polymerization of ethylene at high temperatures. By adopting low Al/Zr ratios such as 125, polyethylenes with ultra-high molecular weights up to 2.86×106 g/mol were obtained.

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    Keywords

    ansa-Metallocene ; Ethylene; Propylene; Polymerization; High temperature

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    INTRODUCTION

    Over the past decades, numerous efforts have been paid to the research of homogeneous olefin polymerization both academically and industrially since the breakthrough in homogeneous Group 4 metallocene catalysts that exhibit excellent catalytic performance in combination with methylaluminoxane (MAO).[

    1−5] It has been shown that the structures of metallocene catalysts exert dominant influences on the physical properties of the resultant polyolefins, thus structure modifications of metallocene complexes aiming to achieve high catalytic activities and polyolefins with enhanced properties have attracted broad interests.
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    Not only the ligated five-membered π-systems such as cyclopentadienyl (Cp), indenyl (Ind), fluorenyl (Flu) and their analogues, but also the substituents on them can affect the polymerization characteristics by changing the environment around the metal centre sterically and electronically. In 2001, Hessen’s group[

    6] developed a series of half-sandwich titanocene complexes bearing Cp ligands with a pendent aryl ring (A, Chart 1), which showed to be excellent catalysts for ethylene trimerization in combination with MAO. The authors attributed such a distinguishing selectivity of the catalysts to a likely hemilabile behavior of the pendent aryl group during the polymerization process. Thereafter, a lot of soft donors or groups such as S-donor, P-donor, alkenyl and aryl were introduced as pendants to the framework of half-sandwich titanocenes.[7−11] Such a strategy was also adopted in non-metallocene ethylene polymerization catalysts, such as nickel catalysts,[12−15] cobalt catalysts,[16,17] iron catalysts[18−20] and the others.[21−24]
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    In contrary to the half-sandwich titanocene complexes with an aryl pendant exhibiting high activities in the trimerization of ethylene, the zirconium and hafnium analogues tended to catalyze ethylene polymerization with low to moderate activities.[

    25] To further understand the effect of similar pendent groups on the catalytic behaviour of metallocene complexes, titanocene complexes with a pendent thienyl group (B, Chart 1),[26] zirconocene complexes with an alkenyl substituent (C, Chart 1)[27−29] and zirconocene complexes with an ether functionalized substituent (D, Chart 1)[30] were also evaluated in the polymerization of ethylene and other α-olefin, and enhanced thermal stability and longer life time of the catalyst were generally observed, implying the remarkable effects of the pendent donors.
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    fig

    Fig 1  Representative metallocene complexes with pendant groups.

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    A few examples of pendent groups were introduced into the ansa-metallocene complexes. In 2010, Carpentier and co-workers[

    31] introduced a pendent aryl group into the diphenylmethyl-bridged cyclopentadienyl-fluorenyl zirconium complex (E, Chart 1), which however was inactive in propylene polymerization. The authors proposed a C―H activation process occurring on the aryl group upon the activation with MAO, leading to inactive species. In 2019, our group[32] obtained a series of ethylene-bridge indenyl-fluorenyl zirconium complexes (F, Chart 1) with a 3-benzyl substituted indenyl moiety, which showed high activities in catalysing propylene oligomerization/polymerization to afford products dominantly with allyl terminals via selective β-methyl elimination transfer. To further understand the influences of pendent groups on the polymerization behaviour of ansa-metallocene complexes, herein we report the synthesis and characterization of a series of ethylene-bridged ansa-cyclopentadienyl-fluorenyl zirconium and hafnium complexes possessing different pendent groups on the cyclopentadienyl unit, and explore their catalytic behavior toward ethylene and propylene polymerization in the presence of MAO.
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    EXPERIMENTAL

    General Considerations

    All manipulations were carried out under a dry argon atmosphere using standard Schlenk techniques unless otherwise indicated. Tetrahydrofuran, diethyl ether, n-hexane, petroleum ether and toluene were distilled under argon from sodium-benzophenone prior to use. Dichloromethane was distilled from calcium hydride. Chloroform-d was dried over calcium hydride under argon and stored in the presence of activated 4 Å molecular sieves. n-BuLi (2.4 mol/L in n-hexane) was purchased from Acros Organics. 9-(2-Bromoethyl)-fluorene,[

    33] 6,6-dimethyl- fulvene[34] were prepared according to literature procedures. Zirconium tetrachloride and hafnium tetrachloride were purchased from Aldrich and freshly sublimed under vacuum prior to use. The cocatalyst methylaluminoxane (MAO, 1.53 mol/L in toluene) was purchased from Akoz Nobel. Polymer grade ethylene was purchased from Shanghai Chunyu Special Gas Co., Ltd. and used directly for polymerization. 1H- and 13C-NMR spectroscopies of complexes were obtained on a Bruker-Advance-400 spectrometer in CDCl3 at ambient temperature. 13C-NMR spectroscopy of polyethylene was obtained on a Bruker-Advance-500 spectrometer using 1,2-dichlorobenzene-d4 as solvent at 100 °C. Chemical shifts were referenced internally using the residual solvent resonances and reported relative to tetramethylsilane (TMS). Elemental analyses were carried out on an Elementar III Vario EI analyser.
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    Synthesis of Complexes

    Synthesis of zirconocene complex 1

    To a mixture of bromobenzene (12.0 mL, 114 mmol) and petroleum ether (30 mL) cooled to 0 °C, was added dropwise a solution of n-BuLi in n-hexane (54.0 mL, 2.30 mol/L, 124 mmol). The reaction mixture was then warmed to room temperature and stirred overnight. White precipitates separated out gradually. After filtration, the solid residue was washed with 10 mL of petroleum ether and dried under vacuum. The obtained white solids were dissolved in 30 mL of tetrahydrofuran and the solution was titrated with standard hydrochloric acid (yield 68.5%). The obtained solution was cooled to 0 °C, then a solution of 6,6-dimethylfulvene (8.31 g, 78.0 mmol) in tetrahydrofuran (20 mL) was added dropwise. The resultant orange solution was stirred overnight and then hydrolysed with 50 mL of aqueous ammonium chloride. The organic phase was separated and the aqueous layer was extracted with Et2O. The combined organic phases were dried over anhydrous MgSO4 and evaporated. The residue was purified by column chromatography (eluent: petroleum ether) to afford Cp1 as a pale-yellow oil (10.10 g, 70.0%). The product was a mixture of two isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.36−7.27 (m, 4H, Ph- H), 7.24−7.18 (m, 1H, Ph- H), 6.50−6.16 (m, 3H, Cp-C H), 3.09−2.78 (two singlets, 2H, Cp-C H2), 1.63−1.58 (two singlets, 6H, ―C H3).

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    To a solution of Cp1 (2.78 g, 15.0 mmol) in 30 mL of tetrahydrofuran at −78 °C was added dropwise a solution of n-BuLi in n-hexane (6.1 mL, 2.30 mol/L, 15 mmol). The solution was stirred at room temperature overnight and then added via cannula to a solution of 9-(2-bromoethyl)-fluorene (3.82 g, 15.0 mmol) in 20 mL of tetrahydrofuran. The reaction mixture was stirred overnight at room temperature and then hydrolysed with 50 mL of aqueous ammonium chloride. The organic phase was separated, and the aqueous layer was extracted with diethyl ether. The combined organic phases were dried over anhydrous MgSO4 and evaporated. The residue was purified by column chromatography (eluent: petroleum ether) to afford L1H2 as a pale-yellow oil (1.67 g, 28.4%), which was used directly in the next step without further purification. The product was a mixture of several isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.83−7.62 (m, 2H, Flu- H), 7.56−7.43 (m, 2H, Flu- H), 7.33−7.04 (m, 9H, Ph- H, Flu- H), 6.09−5.62 (m, 2H, Cp-C H), 4.01−3.86 (m, 1H, Cp-C H), 2.87−2.44 (m, 2H, Cp-C H2), 2.23−1.94 (m, 4H, Cp-C H2C H2-Ful), 1.56−1.43 (m, 6H, ―C H3).

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    To a solution of L1H2 (1.52 g, 3.9 mmol) in 30 mL of Et2O at 0 °C was added dropwise a solution of n-BuLi in n-hexane (4.9 mL, 1.60 mol/L, 7.8 mmol). The resulting suspension was stirred for 48 h at room temperature. ZrCl4 (0.91 g, 3.9 mmol) was added to the suspension as solid. The resultant orange mixture was stirred for 48 h at room temperature, and the solvents were removed under vacuum. The residue was dissolved with 30 mL of CH2Cl2. After filtration, the solution was concentrated and stored at −20 °C to give 1 as orange crystalline solids (0.32 g, 16%). 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 8.03 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 8.02 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.71 (dt, 3J=8.5 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.65 (dt, 3J=8.5 Hz, 1H, Flu- H), 7.62 (td, 3J=8.5 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.59 (td,3J=8.5 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.45−7.39 (m, 2H, Flu- H), 7.22−7.18 (m, 2H, Ph- H), 7.13−7.09 (m, 1H, Ph- H), 7.05−7.03 (m, 2H, Ph- H), 6.19 (t, 3J=3.0 Hz, 1H, Cp- H), 6.08 (t, 3J=2.6 Hz, 1H, Cp- H), 6.05 (t, 3J=2.6 Hz, 1H, Cp- H), 5.30 (s, 0.17 × 2H, 0.17 C H2Cl2), 4.01−3.88 (m, 1H, Cp-C H2C H2-Ful), 3.81−3.72 (m, 2H, Cp-C H2C H2-Ful), 3.58−3.50 (m, 1H, Cp-C H2C H2-Ful), 1.65 (s, 3H, ―C H3), 1.57 (s, 3H, ―C H3), 13C{1H} NMR (100 MHz, 25 °C, CDCl3, δ, ppm): 150.4, 140.1, 133.7, 127.9, 127.5, 126.9, 124.89, 124.85, 124.7, 124.6, 124.5, 123.9, 123.74, 123.70, 120.3, 119.9, 119.3, 110.3, 106.1, 101.9, 39.4, 30.5, 27.2, 26.9, 26.3. Anal. Calcd. for C29H26ZrCl2·0.17 CH2Cl2: C, 63.58; H, 4.82; found: C, 63.00; H, 5.05%.

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    Synthesis of zirconocene complex 2

    A similar method described for Cp1 was adopted, except that 4-methylbromobenzene (8.00 g, 47.0 mmol), n-butyllithium (19.5 mL, 2.30 mol/L in n-hexane, 47.0 mmol), 6,6-dimethylfulvene (3.52 g, 33.0 mmol) were used. Cp2 was obtained as a pale-yellow oil (6.51 g, 69.0%). The product was a mixture of two isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.22−7.05 (m, 4H, Ph- H), 6.48−6.14 (m, 3H, Cp-C H), 3.06−2.78 (two singlets, 2H, Cp-C H2), 2.33−2.31 (two singlets, 3H, Ar-C H3), 1.62−1.50 (two singlets, 6H, ―C H3).

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    To a solution of Cp2 (3.00 g, 15.0 mmol) in 30 mL of tetrahydrofuran at −78 °C were added dropwise a solution of n-BuLi in n-hexane (6.1 mL, 2.30 mol/L, 15 mmol) and hexamethylphosphoramide (HMPA, 5 mL). The mixture was stirred at room temperature overnight and then added via cannula to a solution of 9-(2-bromoethyl)-fluorene (3.82 g, 15.0 mmol) in 20 mL of tetrahydrofuran. The reaction mixture was stirred overnight at room temperature and then hydrolysed with 50 mL of aqueous ammonium chloride. The organic phase was separated, and the aqueous layer was extracted with diethyl ether. The combined organic phases were dried over anhydrous MgSO4 and evaporated. The residue was purified by column chromatography (eluent: petroleum ether) to afford L2H2 as a pale-yellow oil (0.96 g, 16.4%), which was used directly in the next step without further purification. The product was a mixture of several isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.79−7.71 (m, 2H, Flu- H), 7.60−7.47 (m, 2H, Flu- H), 7.41−7.25 (m, 4H, Ph- H, Flu- H), 7.23−7.05 (m, 4H, Ph- H, Flu- H), 6.14−5.77 (m, 2H, Cp-C H), 4.15−3.92 (m, 1H, Cp-C H), 2.89−2.56 (m, 2H, Cp-C H2), 2.50−2.04 (m, 7H, Cp-C H2C H2-Ful, Ar-C H3), 1.52−1.39 (m, 6H, ―C H3).

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    A similar method described for complex 1 was adopted, except that L2H2 (0.66 g, 1.2 mmol), n-BuLi (1.0 mL, 1.60 mol/L in n-hexane, 2.4 mmol), ZrCl4 (0.28 g, 1.2 mmol) were used.Complex 2 was isolated as orange crystalline solids (0.28 g, 41%). 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 8.03 (dt, 3J=8.5 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 8.02 (dt, 3J=8.5 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.70 (dt, 3J=8.5 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.65 (dt, 3J=8.5 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.61 (td, 3J=8.5 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.58 (td, 3J=8.5 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.44−7.35 (m, 2H, Flu- H), 6.96 (d, 3J=8.2 Hz, 2H, Ph- H), 6.83 (d, 3J=8.2 Hz, 2H, Ph- H), 6.18 (t, 3J=3.0 Hz, 1H, Cp- H), 6.05(t, 3J=2.6 Hz, 1H, Cp- H), 6.03 (t, 3J=2.8 Hz, 1H, Cp- H), 5.30 (s, 0.15 × 2H, 0.15 C H2Cl2), 4.03−3.82 (m, 1H, Cp-C H2C H2-Ful), 3.83−3.70 (m, 2H, Cp-C H2C H2-Ful), 3.62−3.54 (m, 1H, Cp-C H2C H2-Ful), 2.26 (s, 3H, Ar-C H3), 1.57 (s, 3H, ―C H3), 1.52 (s, 3H, ―C H3). 13C{1H} NMR (100 MHz, 25 °C, CDCl3, δ, ppm): 147.6, 140.5, 134.0, 133.7, 127.8, 127.6, 127.5, 124.9, 124.7, 124.5, 124.4, 123.9, 123.75, 123.71, 122.6, 120.2, 119.9, 119.1, 110.2, 106.2, 101.8, 39.1, 30.5, 27.2, 26.8, 26.4, 19.8. Anal. Calcd. for C30H28ZrCl2·0.15 CH2Cl2: C, 64.27; H, 5.06; found: C, 63.91; H, 5.27%.

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    Synthesis of zirconocene complex 3

    A similar method described for Cp1 was adopted, except that 3,5-dimethylbromobenzene (6.80 g, 37.0 mmol), n-BuLi (15.7 mL, 2.30 mol/L in n-hexane, 37.0 mmol), 6,6-dimethylfulvene (2.65 g, 25.0 mmol) were used. Cp3 was obtained as a pale-yellow oil (3.85g, 72.0%). The product was a mixture of several isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 6.92−6.78 (m, 3H, Ph- H), 6.48−6.14 (m, 3H, Cp-C H), 3.02−2.75 (two singlets, 2H, Cp-C H2), 2.3−2.26 (two singlets, 6H, Ar-C H3), 1.60−1.53 (two singlets, 6H, ―C H3).

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    A similar method described for L2H2 was adopted, except that Cp3 (3.80 g, 17.9 mmol), n-butyllithium (7.8 mL, 2.30 mol/L in n-hexane, 18.0 mmol), HMPA (5 mL), 9-(2-bromoethyl)-fluorene (3.92 g, 14.3 mmol) were used. L3H2 was obtained as a pale yellow oil (3.27 g, 56.6%). The product was a mixture of several isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.81−7.66 (m, 2H, Flu- H), 7.62−7.48 (m, 2H, Flu- H), 7.40−7.28 (m, 4H, Flu- H), 6.95−6.78 (m, 3H, Ph- H), 6.20−5.78 (m, 2H, Cp-C H), 4.10−3.95 (m, 1H, Ful- H), 2.95−2.57 (m, 2H, Cp-C H), 2. 52−2.09 (m, 10H, Cp-C H2C H2-Ful, Ar-C H3), 1.52−1.39 (m, 6H, ―C H3).

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    A similar method described for complex 1 was adopted, except that L3H2 (1.68 g, 4.2 mmol), n-BuLi (5.2 mL, 1.60 mol/L in n-hexane, 8.4 mmol), ZrCl4 (0.97 g, 4.2 mmol) were used. Complex 3 was isolated as orange crystalline solids (0.40 g, 17%). 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 8.03 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 8.02 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.71 (dt, 3J=8.6 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.65 (dt, 3J=8.6 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.61 (td,3J=8.6 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.58 (td, 3J=8.6 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.44−7.35 (m, 2H, Flu- H), 6.73 (s, 1H, Ph- H), 6.63 (s, 2H, Ph- H), 6.18 (t, 3J=3.0 Hz, 1H, Cp- H), 6.06 (t, 3J=2.6 Hz, 1H, Cp- H), 6.03 (t, 3J=2.6 Hz, 1H, Cp- H), 5.30 (s, 0.16 × 2H, 0.16 C H2Cl2), 3.97−3.88 (m, 1H, Cp-C H2C H2-Ful), 3.83−3.71 (m, 2H, Cp-C H2C H2-Ful), 3.57−3.50 (m, 1H, Cp-C H2C H2-Ful), 2.24 (s, 6H, Ar-C H3), 1.58 (s, 3H, ―C H3), 1.52 (s, 3H, ―C H3). 13C{1H} NMR (100 MHz, 25 °C, CDCl3, δ, ppm): 151.5, 141.5, 137.2, 134.6, 128.8, 128.5, 127.2, 125.9, 125.7, 125.5, 124.9, 124.7, 123.6, 123.4, 121.3, 120.9, 120.2, 111.3, 107.2, 102.8, 40.2, 31.6, 28.2, 27.7, 27.4, 21.5. Anal. Calcd. for C31H30ZrCl2·0.16 CH2Cl2: C, 64.72; H, 5.28; found: C, 64.23; H, 5.43%.

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    Synthesis of zirconocene complex 4

    Under the protection of argon, a tetrahydrofuran (50 mL) solution of benzyl bromide (26.0 g, 152 mmol) was added slowly to a 100 mL three-neck flask loaded with dried magnesium strips (4.00 g, 167 mmol). After addition, the reaction mixture was heated to reflux overnight. The unreacted magnesium strips in the reaction mixture were removed by filtration; 1 mL of the filtrate was subjected to hydrolysis and titrated with a standard hydrochloric acid solution. 50 mL of the filtrate (1.20 mol/L, 61.0 mmol) was added to a solution of 6,6-dimethylfulvene (6.44 g, 61.0 mmol) in tetrahydrofuran. The resultant orange solution was stirred overnight and then hydrolysed with 50 mL of aqueous ammonium chloride. The organic phase was separated and the aqueous layer was extracted with Et2O. The combined organic phases were dried over anhydrous MgSO4 and evaporated. The residue was purified by column chromatography (eluent: petroleum ether) to afford Cp4 as a pale-yellow oil (3.15 g, 47.50%). The product was a mixture of several isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.37−7.23 (m, 5H, Ph- H), 6.49−5.87 (m, 3H, Cp-C H), 2.87−2.81 (m, 4H, Cp-C H2, ―C H2-Ph), 1.26−1.23 (two singlets, 6H, ―C H3).

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    A similar method described for L2H2 was adopted, except that Cp4 (2.80 g, 14.2 mmol), n-BuLi (5.6 mL, 2.30 mol/L in n-hexane, 14.0 mmol), hexamethylphosphoramide (HMPA, 5 mL), 9-(2-bromoethyl)-fluorene (3.85 g, 14.2 mmol) were used. L4H2 was obtained as a pale-yellow oil (1.81 g, 32.6%). The product was a mixture of several isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.80−7.77 (m, 2H, Flu- H), 7.59−7.54 (m, 2H, Flu- H), 7.42−7.31 (m, 4H, Flu- H), 7.20−7.13 (m, 3H, Ph- H), 6.98−6.93 (m, 2H, Ph- H), 6.26−5.61 (m, 2H, Cp-C H), 4.10−4.04 (m, 1H, Ful- H), 2.89−2.62 (m, 4H, Cp-C H2C H2-Ful), 2.42−2.16 (m, 4H, ―C H2-Ph, Cp- H), 1.12−1.10 (m, 6H, ―C H3).

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    A similar method described for complex 1 was adopted, except that L4H2 (0.85 g, 2.2 mmol), n-BuLi (2.7 mL, 1.60 mol/L in n-hexane, 4.4 mmol), ZrCl4 (0.51 g, 2.2 mmol) were used. Complex 4 was isolated as orange crystalline solids (0.30 g, 27%). 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 8.00 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 2H, Flu- H), 7.67 (d, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.64 (dt, 3J=8.6 Hz, 4J≈5J≈0.8 Hz, 1H, Flu-H), 7.58 (td, 3J=8.4 Hz, 4J=0.8 Hz, 2H, Flu-H), 7.41−7.33 (m, 2H, Flu- H), 7.16−7.10 (m, 3H, Ph- H), 6.74−6.68 (m, 2H, Ph- H), 6.16 (t, 3J=3.0 Hz, 1H, Cp- H), 6.03 (t, 3J=2.8 Hz, 1H, Cp- H), 5.69 (t, 3J=2.6 Hz, 1H, Cp- H), 5.30 (s, 0.09 × 2H, 0.09 C H2Cl2), 3.95−3.87 (m, 1H, Cp-C H2C H2-Ful), 3.78−3.71 (m, 1H, Cp-C H2C H2-Ful), 3.66−3.58 (m, 1H, Cp-C H2C H2-Ful), 3.53−3.46 (m, 1H, Cp-C H2C H2-Ful), 2.63 (d, 2J=12.8 Hz, 1H, -C H2-Ph), 2.54 (d, 2J=12.8 Hz, 1H, -C H2-Ph), 1.20 (s, 3H, ―C H3), 1.10 (s, 3H, ―C H3). 13C{1H} NMR (100 MHz, 25 °C, CDCl3, δ, ppm): 142.1, 138.2, 135.3, 130.7, 128.8, 128.5, 127.3, 126.0, 125.8, 125.77, 125.74, 125.5, 124.9, 124.8, 124.7, 123.6, 121.1, 121.0, 118.4, 110.2, 107.5, 102.7, 52.6, 37.8, 31.3, 28.4, 27.0, 25.0. Anal. Calcd. for C30H28ZrCl2·0.09 CH2Cl2: C, 64.73; H, 5.09; found: C, 64.23; H, 5.52%.

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    Synthesis of zirconocene complex 5

    Under the protection of argon, a solution of n-BuLi in n-hexane (2.5 mol/L, 15.8 mL, 40.0 mmol) was added dropwise to a solution of 6,6-dimethylfulvene (4.20 g, 40.0 mmol) in tetrahydrofuran. The resultant orange solution was stirred overnight and then hydrolysed with 50 mL of aqueous ammonium chloride. The organic phase was separated and the aqueous layer was extracted with Et2O. The combined organic phases were dried over anhydrous MgSO4 and evaporated. The residue was purified by column chromatography (eluent: petroleum ether) to afford Cp5 as a pale-yellow oil (2.40 g, 36.9%). The product was a mixture of two isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 6.49−5.87 (m, 3H, Cp-C H), 2.87−2.81 (m, 2H, Cp-C H2), 1. 38−1.29 (m, 2H, Bu-C H2), 1.18−1.14 (m, 2H, Bu-C H2), 1.04−0.98 (m, 8H, ―C H3, Bu-C H2). 0.80−0.76 (m, 3H, Bu-C H3).

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    A similar method described for L2H2, was adopted, except that Cp5 (2.10 g, 12.8 mmol), n-BuLi (5.1 mL, 2.30 mol/L in n-hexane, 13.0 mmol), HMPA (5 mL), 9-(2-bromoethyl)-fluorene (3.50 g, 12.8 mmol) were used. L5H2 was obtained as a pale-yellow oil (1.63 g, 35.8%). The product was a mixture of several isomers. 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 7.78−7.73 (m, 2H, Flu- H), 7.59−7.52 (m, 2H, Flu- H), 7.38−7.28 (m, 4H, Flu- H), 6.26−5.61 (m, 2H, Cp-C H), 4.10−4.04 (m, 1H, Ful- H), 2.81−2.70 (m, 2H, Cp-C H), 2.36−2.12 (m, 4H, Cp-C H2C H2-Ful), 1.39−1.31 (m, 2H, Bu-C H2), 1.26−1.18 (m, 2H, Bu-C H2), 1.08−1.04 (m, 8H, ―C H3, Bu-C H2). 0.89−0.82 (m, 3H, Bu-C H3).

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    A similar method described for complex 1 was adopted, except that L5H2 (0.86 g, 2.4 mmol), n-BuLi (3.0 mL, 1.60 mol/L in n-hexane, 4.8 mmol), ZrCl4 (0.56 g, 2.4 mmol) were used. Complex 5 was isolated as orange crystalline solids (0.36 g, 29%). 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 8.00 (ddd,3J=8.5 Hz, 4J =1.1 Hz, 5J=0.8 Hz, 2H, Flu- H), 7.68 (td, 3J=8.4 Hz, 4J=1.1 Hz, 2H, Flu- H), 7.61−7.55 (m, 2H, Flu- H), 7.38 (td, 3J=8.4 Hz, 4J=1.1 Hz, 2H, Flu- H), 6.17 (t, 3J=3.0 Hz, 1H, Cp- H), 6.06 (t, 3J=2.8 Hz, 1H, Cp- H), 5.88 (t, 3J=2.6 Hz, 1H, Cp- H), 5.30 (s, 0.15 × 2H, 0.15 C H2Cl2), 3.97−3.89 (m, 1H, Cp-C H2C H2-Ful), 3.79−3.65 (m, 2H, Cp-C H2C H2-Ful), 3.54−3.47 (m, 1H, Cp-C H2C H2-Ful), 1.29−1.23 (m, 2H, ―C H2―), 1.14−1.08 (m, 8H, ―C H3, ―C H2―), 1.05−0.94 (m, 1H, ―C H2―) 0.75−0.79 (m, 4H, ―C H2―, ―C H3). 13C{1H} NMR (100 MHz, 25 °C, CDCl3, δ, ppm): 142.2, 133.8, 127.7, 127.4, 124.8, 124.7, 124.65, 123.8, 123.7, 123.6, 122.6, 120.2, 120.0, 117.3, 109.5, 106.2, 101.7, 45.8, 35.2, 30.4, 27.3, 26.3, 25.5, 23.7, 22.1, 13.0. Anal. Calcd. for C27H30ZrCl2·0.15 CH2Cl2: C, 61.60; H, 5.77; found: C, 61.07; H, 5.81%.

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    Synthesis of hafnocene complex 6

    A similar method described for complex 1 was adopted, except that L1H2 (1.10 g, 2.9 mmol), n-BuLi (3.7 mL, 1.60 mol/L in n-hexane, 5.9 mmol), HfCl4 (0.94 g, 2.9 mmol) were used. Complex 6 was isolated as yellow solids (0.45 g, 25%). 1H-NMR (400 MHz, 25 °C, CDCl3, δ, ppm): 8.02 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 8.01 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.67 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu-H), 7.63 (dt, 3J=8.4 Hz, 4J≈5J≈0.8 Hz, 1H, Flu- H), 7.58 (td, 3J=8.6 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.54 (td, 3J=8.6 Hz, 4J=0.8 Hz, 1H, Flu- H), 7.39−7.31 (m, 2H, Flu- H), 7.20−7.14 (m, 2H, Ph- H), 7.11−7.05 (m, 1H, Ph- H), 7.02−6.98 (m, 2H, Ph- H), 6.06 (t, 3J=3.0 Hz, 1H, Cp- H), 5.96 (d, 3J=3.0 Hz, 2H, Cp- H), 5.30 (s, 0.03 × 2H, 0.03 C H2Cl2), 3.87−3.83 (m, 2H, Cp-C H2C H2-Ful), 3.69−3.53 (m, 2H, Cp-C H2C H2-Ful), 1.55 (s, 3H, ―C H3), 1.46 (s, 3H, ―C H3). 13C{1H} NMR (100 MHz, 25 °C, CDCl3, δ, ppm): 151.7, 139.5, 133.0, 128.6, 128.3, 128.0, 125.64, 125.60, 125.5, 125.4, 125.20, 125.18, 124.8, 124.7, 123.8, 122.7, 121.0, 120.8, 118.7, 109.5, 105.6, 98.5, 40.4, 31.0, 27.9, 27.7, 27.3. Anal. Calcd. for C29H26HfCl2·0.03 CH2Cl2: C, 55.66; H, 4.19; found: C, 55.51; H, 4.29%.

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    General Procedure for Ethylene Polymerization

    Ethylene polymerization was carried out in a 100 mL autoclave equipped with a mechanical stirrer. The autoclave was heated at 100 °C under vacuum for 30 min and then thermos-stated to the desired temperature and filled with ethylene. An appropriate amount of MAO solution and toluene were added to the autoclave and the solution was stirred under ethylene atmosphere for 15 min. After an appropriate amount of toluene solution of the catalyst was injected into the reactor, ethylene at the desired pressure was introduced to start the polymerization. The reaction mixture was stirred vigorously for a desired time and then the ethylene pressure in the autoclave was slowly vented. 10 mL of ethanol was added to terminate the polymerization. The resulting mixture was poured into 3% HCl in ethanol (50 mL). The polymer was collected by filtration, washed with ethanol (30 mL × 2), and then dried for 16 h in a vacuum oven at 60 °C to constant weight.

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    X-Ray Crystallographic Studies

    Single-crystal X-ray diffraction studies for complexes 2, 4 and 6 were carried out on a Bruker AXSD8 diffractometer with graphite monochromated Mo-Kα radiation (λ=0.71073 Å). All data were collected at −133 °C (2) and −143 °C (4 and 6) using the ω-scan technique. Unit cell dimensions were obtained with least-squares refinements. Intensities were corrected for Lorentz and polarization effects and empirical absorption. The structures were solved by direct methods and refined by full-matrix least square on F2. All the calculations were carried out with the SHELXTL program. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were included in idealized position.

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    RESULTS AND DISCUSSION

    Synthesis of Ethylene-Bridged Group 4 Metallocenes

    As illustrated in Scheme 1, substituted cyclopentadienes Cp1−Cp5 with a pendent aryl or alkyl group were prepared as mixtures of two isomers via the reactions of 6,6-dimethylfulvene with proper lithium or magnesium agents respectively. Then the lithium salts of these substituted cyclopentadienes were reacted with 9-(2-bromoethyl)- fluorene to give the proligands L1H2−L5H2 again as double-bond isomers in low to high yields of 31%–82% as pale yellow or colourless oil. Metallocene complexes 1–6 were prepared according to the standard salt metathesis reactions between ZrCl4 or HfCl4 and dilithium salts of the proligands as shown in Scheme 1, and were obtained as orange crystalline solids after recrystallization from their dichloromethane solutions. All these complexes were characterized by1H- and 13C-NMR as well as elemental analysis, which also showed the inevitable inclusion of small amount of solvent residue in the samples. The 1H-NMR spectra of 1–6 exhibit a single set of resonances, indicating exclusively the formation of racemates in considering their C1-symmetric structures.

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    fig

    Fig 1  Synthetic routes to metallocene complexes 1−6.

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    Single crystals of zirconocene complexes 2, 4 and hafnocene complex 6 suitable for X-ray diffraction analysis were obtained at −20 °C from their saturated CH 2Cl2 solutions. All the complexes are composed of a pair of enantiomers. Their molecular structures are shown in Figs.1−3.

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    fig

    Fig 1  ORTEP view of the molecular structure of zirconocene complex 2 (Thermal ellipsoids are drawn at the 50% probability level with hydrogen atoms omitted for clarity); Selected bond lengths (Å) and angles (°): Zr–Cl1, 2.449(5); Zr–Cl2, 2.427(8); Zr–C1, 2.544(7); Zr–C6, 2.669(7); Zr–C7, 2.690(0); Zr–C12, 2.549(7); Zr–C13, 2.435(2); Zr–C16, 2.494(8); Zr–C17, 2.475(9); Zr–C18, 2.525(0); Zr–C19, 2.601(7); Zr–C20, 2.510(9); Cl1–Zr–Cl2, 93.52(5).

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    fig

    Fig 2  ORTEP view of the molecular structure of zirconocene complex 4 (Thermal ellipsoids are drawn at the 50% probability level with hydrogen atoms omitted for clarity); Selected bond lengths (Å) and angles (°): Zr–Cl1, 2.436(0); Zr–Cl2, 2.427(1); Zr–C1, 2.558(1); Zr–C6, 2.720(4); Zr–C7, 2.709(4); Zr–C12, 2.555(7); Zr–C13, 2.434(6); Zr–C16, 2.501(2); Zr–C17, 2.471(6); Zr–C18, 2.512(0); Zr–C19, 2.590(3); Zr–C20, 2.516(7); Cl1–Zr–Cl2, 96.22(2).

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    fig

    Fig 3  ORTEP view of the molecular structure of hafnocene complex 6 (Thermal ellipsoids are drawn at the 50% probability level with hydrogen atoms omitted for clarity); Selected bond lengths (Å) and angles (°): Zr–Cl1, 2.392(4); Zr–Cl2, 2.398(1); Zr–C1, 2.563(7); Zr–C6, 2.733(8); Zr–C7, 2.720(8); Zr–C12, 2.528(8); Zr–C13, 2.425(7); Zr–C16, 2.491(7); Zr–C17, 2.520(7); Zr–C18, 2.557(8); Zr–C19, 2.499(8); Zr–C20, 2.445(7); Cl1–Zr–Cl2, 97.07(7).

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    The solid-state molecular structures of 2, 4 and 6 exhibit essentially similar geometrical parameters. Correspondingly, the metal-CentCp and metal-CentFlu distances, α and β angles are almost identical in these molecules. However, by comparing the data listed in Table 1, we do find that the substituent on the cyclopentadienyl ring and the type of metal centre have somewhat influence on the solid-state structure of the complex. In comparison with zirconocene complex 2 having a 2-(p-methylphenyl)-2-propyl group on the 3-position of the cyclopentadienyl ring, complex 4 with a 2-benzyl-2-propyl group possesses a slightly longer Zr-CentFlu distance and a bigger α angle, indicative of a less compact structure. Meanwhile, for hafnocene complex 6 bearing 2-phenyl-2-propyl group, slightly longer metal-CentFlu and shorter metal-CentCp distances and a bigger β angle are observed, indicative of somewhat approaching of the metal centre toward the cyclopentadienyl ring. For all the complexes, the bond lengths between metal centre and carbon atoms of the π-bonding five-membered ring in the fluorenyl unit vary to a certain degree (distances between the shortest to the longest bond lengths, 2: 0.28 Å; 4: 0.28 Å, 6: 0.28 Å), suggesting a slip of the coordination mode of the central five-membered ring from η5 to η3.

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    Table 1  Selected geometric parameters of complexes 2, 4 and 6.
    intablefig
    Complexα (°) β (°) M-CentCp (Å) M-CentFul (Å)
    2 62.14 126.38 2.213 2.271
    4 62.98 126.33 2.211 2.291
    6 62.70 128.00 2.191 2.289
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    Ethylene Polymerization

    The catalytic behavior of metallocene complexes 1−6 toward the ethylene polymerization were evaluated at different temperatures using MAO as the cocatalyst. The polymerization results are summarized in Table 2.

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    Table 2  Ethylene polymerization catalysed by complexes 1−6/MAO. a
    RunCat.Tp (°C) Al/MYield (g) Activity (106 g·mol·M−1·h−1) Mηb(105 g/mol)
    1 1 40 2000 0.51 1.02 6.02
    2 60 2000 1.32 2.64 4.29
    3 80 2000 2.71 5.42 3.54
    4 100 2000 3.62 7.24 2.86
    5 120 2000 6.41 12.8 2.09
    6 120 c 2000 4.32 8.64 4.03
    7 140 c 2000 2.16 4.32 2.69
    8 2 40 2000 0.50 1.00 6.13d
    9 60 2000 1.05 2.10 4.42
    10 80 2000 2.35 4.70 3.54
    11 100 2000 3.11 6.22 2.91
    12 120 2000 5.46 10.9 2.14
    13 3 40 2000 0.20 0.40 6.31
    14 60 2000 0.95 1.90 4.56
    15 80 2000 2.10 4.20 3.73
    16 100 2000 3.35 6.70 2.85
    17 120 2000 5.03 10.1 2.12d
    18 4 40 2000 1.21 2.42 5.83
    19 60 2000 1.82 3.64 4.18
    20 80 2000 3.34 6.68 3.47
    21 100 2000 4.16 8.32 2.54
    22 120 2000 7.05 14.1 1.63
    23 5 40 2000 6.37 12.7 5.67
    24 60 2000 6.72 13.4 4.08
    25 80 2000 7.40 14.8 3.42
    26 100 2000 7.03 14.1 2.76
    27 120 2000 6.52 13.0 1.49
    28 6 40 2000 0.45 0.90 0.67
    29 60 2000 0.52 1.04 0.58
    30 80 2000 0.78 1.56 0.32
    31 100 2000 1.32 2.64 0.16
    32 120 2000 0.84 1.68 0.09

    a Conditions: in toluene, V=100 mL, [Cat.]=0.01 mmol/L, Pethylene=1 MPa, t=30 min;b Intrinsic viscosity was determined in decahydronaphthalene at 135 °C using Ubbelohde calibrated viscometer technique and viscosity average molecular weight was calculated using the relation: [η]=6.67×10−4Mη0.67; c In xylene; d Run 8: Mw=1.4×106 g/mol, Mw/Mn=3.1; Run 17: Mw=3.8×105 g/mol, Mw/Mn=5.6, Mw and Mw/Mn were determined by GPC, using 1,3,5-trichlorobenzene as solvent at 160 °C.

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    For an easier understanding on the influence of polymerization temperature on the catalytic activities of these metallocene complexes, their activity data at different temperatures were further illustrated in Fig. 4. It is found that, at relatively low temperatures, zirconocene complexes 1−4 bearing a substituent with a pendent aryl group on the 3-position of the cyclopentadienyl ring showed much lower catalytic activities of 0.40×106−2.42×106 g·molZr−1·h−1 in comparison with zirconocene complex 5 bearing an alkyl substituent (1.27×107 g·molZr−1·h−1). Nevertheless, with the increase of temperature, the catalytic activities of complexes 1−4 increased significantly and reached the highest values of 1.01×107−1.41×107 g·molZr−1·h−1 at 120 °C, while the activity of complex 5 only showed a slight increase and then decreased at higher temperatures over 80 °C. Although complexes 1−4 seem to possess bulkier substituents than complex 5, such a surge in catalytic activity for complexes 1−4 with the increase of temperature from 40 °C to 120 °C may not be simply attributed to the steric bulkiness of the substituent on the 3-position of Cp ring, which brings certain steric hindrance to the coordination/insertion of ethylene monomer at relatively lower temperatures. An additional factor is the possible intramolecular interaction of the pendent group of the ligand with the zirconium centre. Although no direct coordination between the aryl group and the zirconium centre could be observed in the molecular structures of complexes 2 and 4, such an interaction of the pendent aryl group with the cationic metal centre generated upon the activation with MAO is however possible as suggested in literature.[

    26] It is conceivable that such a weak interaction would be favored at relatively low temperatures, thus leading to a dormant state of the active species, thereafter low catalytic activities were obtained. When the polymerization runs were carried out at high temperatures, for example at 120 °C, which should be in favour of the dissociation and lead to the release of active species, thereafter the activity increased dramatically. Nevertheless, all these zirconocene complexes displayed good thermal stability at high polymerization temperatures.
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    fig

    Fig 4  Influence of polymerization temperature on the catalytic activities of complexes 1−6/MAO toward ethylene polymerization (Conditions: [Cat.]=0.01 mmol/L, Al/M=2000, Pethylene=1 MPa, t=30 min, V=100 mL).

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    By comparing the activities of complexes 1−4, a reversed order is generally found with the increase of the steric bulkiness of the pendent aryl group, that is 4 (Bn) > 1 (Ph) > 2 (4-MePh) ≥ 3 (3,5-Me2Ph). Among them, complex 4 having a pendent benzyl group displayed the highest activities in the whole investigated temperature range. It seems that the elongation of one additional methylene unit between the pendent phenyl ring and the quaternary carbon bridge in complex 4 tends to bring less hindrance to the active metal centre either via weakening the coordination interaction or by introducing more flexibility of the pendent group, thus leading to higher catalytic activities.

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    As shown in Table 2, the introduction of pendent aryl groups also brings some influences on the molecular weights of resultant polymers. Although polyethylenes with nearly the same molecular weights of 5.67×105−6.31×105 g/mol were obtained at 40 °C by zirconium complexes 1−5, the most significant decrease of molecular weight at 120 °C is observed for the polymer sample obtained by using complex 5 as the catalyst, then is the one obtained by complex 4. While, complexes 1−3 with bulkier pendent aryl groups afforded polyethylenes with relatively higher molecular weights.

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    From Table 2 and Fig. 4, it is also found that, in contrary to the high activities obtained by the zirconocene complexes 1−4, the hafnium complex 6 only showed moderate activities toward ethylene polymerization in the temperature range of 40−120 °C. Meanwhile the effect of the pendent aryl group is also not significant, although a summit of the activity was reached at 100 °C. Moreover, hafnium complex 6 only produced polymers with relatively lower molecular weights of 1.6×104−6.7×104 g/mol. Obviously, the nature of metal centre played a dominant effect on the polymerization behavior of the catalyst.

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    To further study the thermal stability of this series of zirconocene complexes, complex 1 was chosen to polymerize ethylene in xylene at higher temperatures. Under the same conditions, complex 1 showed a lower catalytic activity in xylene than in toluene, but polyethylene with a higher molecular weight was obtained (runs 5 and 6 at 120 °C). Further increasing the temperature to 140 °C caused a decrease of the catalytic activity, which may be attributed to the partial decomposition of the active species or the poor solubility of ethylene in xylene at a higher temperature. Nevertheless, these data provide a solid support to the good thermal stability of this series of zirconocene complexes.

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    In addition to temperature, the influences of other factors, such as the molar ratio of Al/Zr, polymerization time and ethylene pressure were also studied by using complex 1 as the catalyst, and the results are summarized in Table 3.

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    Table 3  Ethylene polymerization catalyzed by complex 1/MAO. a
    Run Al/Zr Tp(°C) Time (min) Pressure (MPa) Yield (g) Activity (107 g·molZr−1·h−1) Mη b(105 g/mol)
    1 125 120 30 1.0 0.50 0.10 28.6
    2 250 120 30 1.0 1.05 0.21 19.3
    3 500 120 30 1.0 2.65 0.53 6.42
    4 1000 120 30 1.0 5.10 1.02 3.41
    5 2000 120 30 1.0 6.41 1.28 2.09
    6 4000 120 30 1.0 7.82 1.56 1.82
    7 2000 120 15 1.0 2.31 0.92 1.28
    8 2000 120 60 1.0 9.62 0.96 2.42
    9 2000 120 120 1.0 10.34 0.52 2.67
    10 2000 120 30 0.2 3.06 0.61 1.28
    11 2000 120 30 0.4 3.69 0.74 1.32
    12 2000 120 30 0.6 4.16 0.83 1.46
    13 2000 120 30 0.8 5.01 1.02 1.82

    a Conditions: in toluene, V=100 mL, [Cat.]=0.01 μmol/mL; b Intrinsic viscosity was determined in decahydronaphthalene at 135 °C by Ubbelohde calibrated viscometer technique and viscosity average molecular weight was calculated using the relation: [η] = 6.67×10−4Mη0.67.

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    The variation of the Al/Zr molar ratio had a significant influence on the catalytic activity and the molecular weight of the resultant polyethylene. As the Al/Zr molar ratio increased from 125 to 4000, the catalytic activity rapidly increased from 0.10×107 g·molZr−1·h−1 to 1.56×107 g·molZr−1·h−1. Meanwhile, the Mη value of the resultant polyethylene decreased when the Al/Zr molar ratio increased, due to the enhanced chain transfer to aluminium for the termination. It is worthy of noting, polyethylene with an ultra-high molecular weight of Mη=2.86×106 g/mol was obtained by adopting a very low Al/Zr molar ratio of 125, meanwhile the catalytic activity of the catalyst was still maintained at a moderately high value of 1.00×106 g·molZr−1·h−1.

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    Complex 1 was then conducted to polymerize ethylene within different time. With the increase of polymerization time from 15 min to 30 min, we found that not only the amount of polymeric product increased gradually, at the same time, a significant increase of the catalytic activity was also observed. We consider that due to the presence of a pendent aryl group on the 3-position of the cyclopentadienyl moiety, the activation of the metal centre might be somewhat hindered, resulting in a higher activation energy and thus slower activation of the catalyst. At the beginning of the reaction, the relatively lower concentration of active sites would lead to a lower catalytic activity; with the progress of the polymerization process, the catalyst should be extensively activated and thus showed an enhanced catalytic activity.

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    As shown in Table 3, further prolonging the reaction time to two hours resulted in a somewhat decrease of the catalytic activity, but the amount of resultant polymer continued to increase, indicative of good thermal stabilities of the active species generated from complex 1 and a long-life time of 120 min maintained at 120 °C. Moreover, the molecular weight of the polyethylene obtained increased slightly with the prolongation of the polymerization time.

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    Besides, the ethylene pressure showed a positive influence on the catalytic activity and the molecular weight of the resultant polyethylene. When the ethylene pressure was increased, both the catalytic activity and the molecular weight increased consistently.

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    13C-NMR analysis of a typical polymer sample (Fig. S13 in the electronic supplementary information, ESI, Table 1, run 5) indicated that the resultant polyethylene is highly linear with no detectable branches.

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    Propylene Polymerization

    In our previous studies, ethylene-bridged ansa-(indenyl) (fluorenyl) zirconocene complexes[

    28] with a pendent aryl group in the indenyl moiety could catalyse the dimerization of propylene to afford 2-methylpentene in good selectivities. Complexes 1−6 were then evaluated to catalyse the oligomerization/polymerization of propylene. Unfortunately, no dimers or other oligomers could be identified; instead, in most of the cases waxes were obtained by zirconocene complexes 1−5 when the polymerization runs were carried out at high temperatures over 80 °C and hafnocene complex 6 hardly displayed catalytic activity toward the oligomerization/ polymerization of propylene. Thus, it is suggested that the steric bulky substituents in these complexes block the coordination site considerably, which is unfavourable for the coordination/insertion of propylene monomer even at high temperatures.
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    CONCLUSIONS

    In summary, a series of ethylene-bridged ansa-(3-R- cyclopentadienyl)(fluorenyl) zirconocene and hafnocene complexes bearing a bulky substituent with a pendent aryl or alkyl group on the cyclopentadienyl ring were synthesized. Single-crystal X-ray diffraction studies on zirconocene complexes 2, 4 and hafnocene complex 6 reveal their similar molecular structures in the solid state and slight flip of the coordination mode of the fluorenyl from η5 to η3. In the presence of MAO, zirconocene complexes 1−4 bearing a pendent aryl group showed much higher catalytic activities toward ethylene polymerization at 120 °C when compared with those of the polymerization runs carried at lower temperatures. In contrast, zirconocene complex 5 nearly displayed constant activities within the studied temperature range. The surge of activity observed for complex 1−4 with the increase of polymerization temperature is suggested to be attributed to the pendent aryl group on the 3-position of the cyclopentadienyl ring, which might interact with the metal centre and thus hinder the coordination/insertion of ethylene monomer at low temperatures. All zirconocene complexes 1−5 showed high catalytic activities and great thermal stability at high temperatures up to 120 °C toward the polymerization of ethylene, affording polyethylenes with high molecular weights. A long catalytic life time up to 2 h was also evidenced by using typical complex 1 in combination with MAO. By adopting low Al/Zr ratios, polyethylenes with ultra-high molecular weights were obtained. The nature of metal centre exhibits crucial influence, and in comparison with zirconocene complexes 1−5, complex 6 only displayed very low catalytic activities toward the polymerization of ethylene and afforded polyethylenes with molecular weights ten times smaller than those obtained by 1−5. Moreover, zirconocene complexes 1−5 were also able to catalyse the polymerization of propylene at high temperatures, but only afforded waxes with low molecular weights.

    译
     ZH

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