Synthesis of New Phenoxide-Modified Half-Titanocene Catalysts for Ethylene Polymerization
1
Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji 192-0397, Tokyo, Japan
2
Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(9), 840; https://doi.org/10.3390/catal15090840 (registering DOI)
Submission received: 19 July 2025
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Revised: 19 August 2025
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Accepted: 29 August 2025
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Published: 2 September 2025
(This article belongs to the Special Issue Innovative Catalytic Approaches in Polymerization)
Abstract
A series of half-titanocenes containing different trialkylsilyl para-phenoxy substituents, Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [Cp* = C5Me5; R = Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)], were prepared and identified. Catalytic activity in ethylene polymerization by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), 5–7]–MAO (methylaluminoxane) catalysts increased in the following order (in toluene at 25 °C, ethylene 4 atm): R = H (1) < SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4) < SiMe2(t-Bu) (7) < SiMe2(n-C8H17) (6) < Si(n-Bu)3 (5, activity = 6.56 × 104 kg-PE/mol-Ti·h). The results thus suggest that the introduction of an alkyl group into a silyl substituent led to an increase in activity. The activities of 5 were affected by the Al/Ti molar ratio (amount of MAO charged), and the highest activity (7.00 × 105 kg-PE/mol-Ti·h) was observed under optimized conditions at 50 °C, whereas the activity decreased at 80 °C. In ethylene copolymerization with 1-dodecene, the Si(n-Bu)3 analog (5) exhibited remarkable catalytic activity (4.32 × 106 kg-polymer/mol-Ti·h at 25 °C), which was higher than those of the reported catalysts (1–3), affording poly(ethylene-co-1-dodecene)s with efficient comonomer incorporation as observed in 3 [rE = 3.77 (5) vs. 3.58 (3)].
1. Introduction
Polyolefins [such as high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (PP), ethylene propylene diene terpolymer (EPDM), etc.] are commodity plastics in daily life, and transition metal-catalyzed olefin polymerization is a core technology. Development of new polymers exhibiting specified properties is an important subject in the fields of catalysis and polymer chemistry [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. The copolymerization approach, which allows for the incorporation of monomers that cannot be incorporated by conventional catalysts (e.g., Ziegler–Natta or metallocene catalysts), has been employed as an efficient approach because their material properties (physical, mechanical, chemical, etc.) can be modified by individual components (containing two or three monomers) [2,3,4,5,7,8,9,12]. The design of molecular catalysts, especially those exhibiting high catalytic activities with better comonomer incorporations, thus attracts considerable attention in the field of catalysis as well as polymer chemistry [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16].
It has been demonstrated that nonbridged half-titanocenes (half-sandwich titanium complexes) modified with anionic ancillary donor ligands (Y) of type Cp’TiX2(Y) (Cp’ = cyclopentadienyl, X = halogen or alkyl, Scheme 1) display promise as catalysts for the synthesis of new copolymers by incorporating various olefins (sterically encumbered olefins, cyclic olefins, aromatic vinyl monomers, and others) [7,8,9,16]. Phenoxide (1, 3, A) [7,8,9,17,18,19], ketimide (B) [7,8,9,20,21,22], and phosphinimide (C) [7,8,9,23,24,25] analogs have been known to be successful examples [26,27,28], and the η1-amidinate analog (D) has been used in industrial production of chlorine-free synthetic EPDM rubber without deep cooling, which is commonly employed in conventional (Ziegler type) catalyst systems in industry [12].
Recently, we communicated that phenoxide-modified half-titanocenes containing the SiEt3 para-substituent Cp*TiCl2(O-2,6-iPr2-4-SiEt3-C6H2) (3, Cp* = C5Me5) exhibited a remarkable increase in catalytic activity compared to the original catalyst, 1, in ethylene copolymerizations with 2-methyl-1-pentene, 1-decene, 1-dodecene (DD) at 50 °C in the presence of a methylaluminoxane (MAO) cocatalyst. The efficient synthesis of high molecular weight ethylene copolymers containing 9-decene-1-ol was also demonstrated in this catalysis with high catalytic activities [17]. Moreover, we reported later that both the SiEt3 (3) and Si(i-Pr)3 (4) catalysts were effective for the synthesis of ethylene copolymers with 2-allylphenol [29]. In order to explore the effect of trialkyl substituents on the para-phenoxide ligand in ethylene polymerization, we prepared and identified various half-titanocenes containing different para-phenoxy substituents, Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)], and studied the ligand effect in ethylene polymerization and ethylene copolymerization with 1-dodecene by using 1–7–MAO catalysts (Scheme 2). We thus wish to present our preliminary data that Si(n-Bu)3 (5) displays promising capabilities in terms of catalytic activity and comonomer incorporation.
2. Results and Discussion
2.1. Synthesis of Half-Titanocenes Containing Different Trialkylsilyl Para-Phenoxy Substituents, Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) (5–7)
A series of phenoxide-modified half-titanocenes containing different trialkylsilyl para-phenoxy substituents, Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)], were prepared by treating Cp*TiCl3 with the corresponding phenol, 2,6-iPr2-4-R-C6H2OH, in toluene, in the presence of NEt3, according to the procedure for the synthesis of the SiEt3 analog (3) [17]. The reaction conditions for the synthesis of 2,6-iPr2-4-R-C6H2OH had to be optimized, as conducted in the synthesis of 2,6-iPr2-4-Si(i-Pr)3-C6H2OH [28], especially the ratio of solvent and the reaction temperature and time; these phenols could be obtained as moderate yields (51–86%). The resultant titanium complexes were identified by NMR spectra and elemental analysis (shown below in the Section 3).
2.2. Ethylene Polymerization by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) (1–7)
Ethylene polymerizations using Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [1–7; R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)] were conducted in toluene (30 mL) in the presence of a MAO cocatalyst at 25 or 50 °C. Select results conducted under the same conditions (catalyst 0.015 μmol, MAO 3.0 mmol, ethylene 4 atm, 10 min) are summarized in Table 1. MAO employed in this study was used as a white solid (expressed as dried MAO) formed by the removal of AlMe3 and toluene from the commercially available MAO [TMAO-S, 9.5 wt% (Al) toluene solution, Tosoh Finechem Co.], since the use of this MAO effectively afforded ethylene copolymers with uniform compositions [7,8,9,17,18,19,20,22]. Polymerization did not take place in the absence of titanium catalysts or MAO cocatalyst.
It was revealed that the catalytic activities were affected by the trialkylsilyl para-phenoxy substituent employed, and the activities of Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) (1–7)–MAO catalysts increased in the following order (25 °C, ethylene 4 atm): R = H (1, activity 49,600 kg-PE/mol-Ti·h, run 1) < SiMe3 (2, 54,400, run 3), SiEt3 (3, 55,600, run 5), Si(i-Pr)3 (4, 56,400, run 7) < SiMe2(t-Bu) (7, 58,800, run 15) < SiMe2(n-C8H17) (6, 62,800, run 13) < Si(n-Bu)3 (5, 65,600, run 9). It seems likely that the introduction of an alkyl group into the silyl group led to an increase in activity probably due to better π-donation to titanium, as discussed previously [7,8,9,17]. It was also revealed that the activities at 50 °C of 1, 4, and 5 were low compared to those at 25 °C, whereas the activities of 2, 3, 6, and 7 increased at 50 °C. The resultant polymers prepared at 25 °C possessed ultrahigh molecular weights (Mn = 0.78–1.64 × 106) with unimodal molecular weight distributions (Mw/Mn = 2.43–3.84), although the PDI (Mw/Mn) values were rather high. The Mn values in the resultant polymers prepared at 50 °C were low (Mn = 4.3–6.7 × 105) with low PDI values (Mw/Mn = 2.31–2.96) compared to those prepared at 25 °C, suggesting that a certain degree of chain transfer increased with an increase in the polymerization temperature.
Table 2 summarizes the results for the effects of MAO (Al/Ti molar ratio) on the catalytic activities in ethylene polymerization of the Si(i-Pr)3 (4) and the Si(n-Bu)3 (5) analogs at 25–80 °C, since the activity of the SiEt3 analog (3) was affected by the amount of MAO charged at various temperatures [17]. It was revealed that, as observed in ethylene polymerization by 2,3–MAO catalysts (under the same conditions), the activities of 4, 5 conducted at 25 °C increased upon an increase in the amount of MAO charged (runs 17–22). Moreover, the activity of 5 at 50 °C initially increased upon an increase in the MAO charged (runs 23–25) but decreased upon further additions (run 26); a similar trend was seen in the activity of 5 conducted at 80 °C (runs 27–31). Since the Mn values decreased upon an increase in the MAO charged, as well as upon an increase in the polymerization temperature, it thus seems likely that a certain degree of the chain transfer to Al occurred during this catalysis, especially at 50–80 °C.
2.3. Ethylene Copolymerization with 1-Dodecene by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) (4–7)
Ethylene copolymerizations with 1-dodecene (DD) by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = Si(i-Pr)3 (4), Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)] were conducted at 25 or 50 °C in the presence of a MAO cocatalyst [ethylene 6 atm, DD 5.0 mL, 0.75 M in toluene (total 30 mL), 6 min, Scheme 3] in order to study the effect of trialkylsilyl para-phenoxy substituents on activity and DD incorporation. The results are summarized in Table 3, and the results for 1–3 are also placed there for comparison.
It was revealed that the catalytic activities in the copolymerizations conducted at 25 °C increased in the following order: 1 (activity = 1.60 × 105 kg-polymer/mol-Ti·h, run 32) < 2 (8.50 × 105, run 34) < 3 (1.09 × 106, run 36) < 7 (1.56 × 106, run 52) < 4 (2.07 × 106, run 38) < 6 (3.10 × 106, run 46) < 5 (3.40 × 106, run 41; 4.32 × 106, run 42); the Si(n-Bu)3 analog (5) thus showed the highest activity under the same conditions. The resultant polymers were poly(ethylene-co-DD)s possessing rather high molecular weights (Mn = 1.50–2.54 × 105) with unimodal molecular weight distributions (Mw/Mn = 1.54–1.90); no significant differences in the DD contents of the resultant copolymers were seen [30]. It should be noted that the activities of 1–3 increased at 50 °C (with increases in the DD contents), whereas no significant differences in the activities were seen for 5, 6 conducted under the optimized amount of MAO charged (runs 44, 45, 50); the activities, in contrast, decreased for 4 and 7 (runs 39, 40, 53, 54). Among these catalysts, the Si(n-Bu)3 analog (5) showed the highest catalytic activities (runs 44, 45). The Mn values in the resultant copolymers prepared at 50 °C (Mn = 1.31–1.68 × 105) were somewhat low compared to those prepared at 25 °C (Mn = 1.50–2.54 × 105); no significant differences in the DD contents were seen, as observed in 1–3 [17]. This probably suggests that trialkylsilyl substituents only contribute to increasing the catalytic activity (through π-donation to catalytically active Ti), leading to a higher catalytic activity [7,8,9].
Table 4 summarizes the triad sequence distribution, the dyads, rE, rD, and rE·rD values [E = ethylene, D = 1-dodecene (DD)] by microstructural analysis of poly(ethylene-co-1-dodecene)s prepared by 4–7–MAO catalysts estimated by 13C NMR spectra (shown in Supplementary Materials) [31,32,33]. The rE·rD values are 0.44–0.57, clearly suggesting that these copolymerizations proceeded in a random manner (DD incorporation is random), whereas the DD incorporation was instead alternating. The monomer reactivity ratios, defined as rE, for 4–7 are 3.34–4.40. These values were not affected by the polymerization temperature employed, whereas it is known that the rE value is affected by the polymerization temperature in ethylene copolymerization using ordinary metallocene catalysts [34,35]. As commented on previously [7,8,9], this is one of the unique characteristics in this catalysis. The rE values are close to those of the original catalyst 1 (rE= 3.92) [36] and the SiEt3 analog (3, rE = 3.58) [37]. The values produced by the Si(n-Bu)3 analog 5 (3.77, 4.09) exhibited the highest catalytic activity with efficient DD incorporation, but were still rather small compared to those produced by the linked half-titanocene catalyst (called constrained geometry catalysis) [Me2Si(C5Me4)(NtBu)]TiCl2 (CGC, rE = 4.31) [36], known as the most efficient catalyst for ethylene copolymerization [4,5].
3. Materials and Methods
All experiments were carried out under a nitrogen atmosphere in a Vacuum Atmospheres drybox unless otherwise specified. All chemicals used, including 1-dodecene (DD, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan), were of reagent grade and were purified by standard purification procedures. Anhydrous-grade toluene (Kanto Kagaku Co. Ltd., Tokyo, Japan) was transferred into a bottle containing molecular sieves (mixture of 3A, 4A 1/16, and 13X) in the drybox and used without further purification. Ethylene for polymerization was of polymerization grade (purity > 99.9%; Sumitomo Seika Co., Ltd., Tokyo, Japan) and was used as received. Methylaluminoxane (MAO) [TMAO-S, 9.5 wt% (Al) toluene solution, Tosoh Finechem Co., Yamaguchi, Japan] was used as a white solid prepared by removing toluene and Me3Al from the commercially available samples in the drybox [17,18,19,20,22]. Cp*TiCl2(O-2,6-iPr2-4-R-C6H3) [R = H (1) [38], SiMe3 (2) [17], SiEt3 (3) [17], Si(i-Pr)3 (4) [29]] were prepared according to the reported procedure.
All 1H NMR and 13C NMR spectra were recorded on a Bruker AV500 spectrometer (500.13 MHz, 1H; 125.77 MHz, 13C) at 25 °C unless otherwise noted. Chemical shifts are given in ppm and are referenced with SiMe4 (δ 0.00 ppm, 1H, 13C). Elemental analyses were performed by using EAI CE-440 CHN/O/S Elemental Analyzer (Exeter Analytical, Inc., Yokohama, Japan). Molecular weights and molecular weight distributions for the resultant polymers were measured by gel permeation chromatography (Tosoh HLC-8121GPC/HT) using a RI-8022 detector (for high temperature; Tosoh Co., Tokyo, Japan) with a polystyrene gel column (TSK gel GMHHR-H HT × 2, 30 cm × 7.8 mm i.d.), ranging from <102 to < 2.8 × 108 MW) at 140 °C, using o-dichlorobenzene containing 0.05 wt/v% 2,6-di-tert-butylp-cresol as the solvent. The molecular weight was calculated by a standard procedure based on calibration with standard polystyrene samples.
Synthesis of Cp*TiCl2(O-2,6-iPr2-4-SiBu3-C6H2) (5). (i) Synthesis of 4-SiBu3-2,6-iPr2C6H2OH. To a solution of 4-Br-2,6-iPr2C6H2OH (2.57 g, 10 mmol) in 15 mL of Et2O, TMSCl (1.35 g, 12 mmol) and Et3N (1262 mg, 12 mmol) were added at −30 °C. The mixture was then warmed to room temperature and stirred overnight. The reaction mixture was filtered through a Celite pad and washed with Et2O. The filtrate was dried under reduced pressure to produce a yellow liquid product (4-Br-2,6-iPr2C6H2OTMS, 3.0 g). Yield: 92%. 4.0 mL n-BuLi (1.6 M, 6.2 mmol) was added at −78 °C to a solution of 4-Br-2,6-iPr2C6H2OTMS (1.66 g, 5.0 mmol) in 6.0 mL hexane, and 10 mL THF was added at −78 °C. The mixture was stirred for 1.5 h at −78 °C, and Bu3SiCl (5.87 g, 25 mmol) was then added. The mixture was stirred for 6 h at −78 °C. The reaction mixture was then warmed consecutively to 0 °C and stirred for 18 h, then to room temperature and stirred for 2 h. The reaction mixture was filtered through a Celite pad and washed with hexane. The filtrate was dried under reduced pressure to produce a green oily product. 10 wt% HCl aq. (10 mL) was added to a solution of the green oily product in dissolved in 10 mL of THF, and then the solution was stirred for 20 min at 0 °C. Water was added to quench the reaction and was then extracted with hexane. The organic phase was collected and dried over MgSO4. The mixture was filtered to remove the MgSO4, and the organic layer was collected and dried under reduced pressure to produce a liquid crude product. The crude product was purified by silica gel column chromatography (hexane/EtOAc = 30:2) to produce a slightly orange liquid product (1.25 g, yield: 66%). 1H NMR (CDCl3): δ 7.15 (s, 2H), 4.82 (s, 1H), 3.11–3.19 (m, 2H), 1.32–1.33 (m, 12H), 1.28 (d, J = 8.2 Hz, 12H), 0.88 (t, J = 7.5 Hz, 9H), 0.74–0.77 (m, 6H). 13C NMR (CDCl3): δ 151.7, 133.5, 130.4, 129.5, 28.1, 27.8, 27.1, 23.8, 14.8, 13.4.
(ii) Synthesis of Cp*TiCl2(O-4-SiBu3-2,6-iPr2C6H2) (5). A solution of 4-SiBu3-2,6-iPr2C6H2OH (412 mg, 1.1 mmol) in 10 mL of toluene and Et3N (0.15 g, 1.5 mmol) was added at −30 °C to a solution of Cp*TiCl3 (289 mg, 1.0 mmol) in toluene (20 mL). The mixture was warmed to room temperature and stirred overnight. The reaction mixture was filtered through a Celite pad and washed with toluene. The filtrate was dried under reduced pressure to produce a red oily product. The oily product was then dissolved in hexane and placed in the freezer (−30 °C) to produce red crystals. Yield: 497 mg (81.3%). 1H NMR (CDCl3): δ 7.16 (s, 2H), 3.15–3.19 (m, 2H), 2.18 (s, 15H), 1.31–1.33 (m, 12H), 1.21 (d, J = 8.2 Hz, 12H), 0.88 (t, J = 7.5 Hz, 9H), 0.74–0.78 (m, 6H). 13C NMR (CDCl3): δ 161.1, 138.9, 133.5, 133.3, 129.9, 27.7, 27.5, 27.2, 25.0, 14.8, 13.9, 13.3. Anal. Calcd. C34H58Cl2OSiTi: C, 64.85; H, 9.28; found: C, 64.84; H, 9.20.
Synthesis of Cp*TiCl2(O-2,6-iPr2-4-SiMe2(n-C8H17)-C6H2) (6). (i) Synthesis of 4-SiMe2(n-C8H17)-2,6-iPr2C6H2OH. NaH (150 mg, 6.2 mmol) was added at −30 °C to a solution of 4-Br-2,6-iPr2C6H2OH (1290 mg, 5.0 mmol) in 6.0 mL hexane and 10 mL THF. The mixture was warmed to room temperature and stirred for 1 h. Then it was cooled down to −78 °C. 4.0 mL n-BuLi (1.6 M, 6.2 mmol) was added to the reaction mixture and stirred for 1.5 h at −78 °C. SiMe2(n-C8H17)Cl (5.15 g, 25 mmol) was then added and stirred for 4 h at −78 °C. The reaction mixture was then warmed consecutively to 0 °C and stirred for 14 h, then warmed to room temperature and stirred for 2 h. The reaction mixture was treated with a similar purification procedure as that for the synthesis of 4-SiBu3-2,6-iPr2C6H2OH (described above) to produce a liquid crude product. Further purification by silica gel column chromatography (hexane/EtOAc = 30:2) produced an orange liquid product (0.93 g, yield: 51.2%). 1H NMR (CDCl3): δ 7.15 (s, 2H), 4.82 (s, 1H), 3.09–3.15 (m, 2H), 1.22–1.35 (m, 24H), 0.84 (t, J = 2.7 Hz, 3H), 0.69 (t, J = 7.5 Hz, 2H), 0.20 (s, 6H). 13C NMR (CDCl3): δ 151.9, 133.6, 131.2, 129.8, 34.6, 32.9, 30.3, 30.2, 28.3, 24.9, 23.7, 23.6, 16.9, 15.1.
(ii) Synthesis of Cp*TiCl2(O-4-SiMe2(n-C8H17)-2,6-iPr2C6H2) (6). The procedure for the synthesis of 6 was conducted using the same procedure for the synthesis of catalyst 5 using 4-SiMe2(n-C8H17)-2,6-iPr2C6H2OH (381.7 mg, 1.1 mmol). Yield: 573 mg (89.3%). 1H NMR (CDCl3): δ 7.18 (s, 2H), 3.13–3.20 (m, 2H), 2.18 (s, 15H), 1.24–1.30 (m, 12H), 1.21 (d, J = 8.2 Hz, 12H), 0.85–0.90 (m, 3H), 0.71 (d, J = 6.9 Hz, 2H), 0.24 (s, 6H). 13C NMR (CDCl3): δ 161.2, 139.0, 135.0, 133.3, 129.3, 34.5, 32.9, 30.3, 30.2, 27.5, 25.0, 24.9, 23.7, 16.9, 15.1, 13.9, 13.9. Anal. Calcd. C32H54Cl2OSiTi: C, 63.88; H, 9.05; found: C, 64.10; H, 9.08.
Synthesis of Cp*TiCl2(O-2,6-iPr2-4-SiMe2(t-Bu)-C6H2) (7). (i) Synthesis of 4-SiMe2(t-Bu)-2,6-iPr2C6H2OH. NaH (150 mg, 6.2 mmol) was added at −30 °C to a solution of 4-Br-2,6-iPr2C6H2OH (1.29 g, 5.0 mmol) in 6.0 mL hexane and 10 mL THF. The mixture was warmed to room temperature and stirred for 1 h. Then it was cooled down to −78 °C. 4.0 mL n-BuLi (1.6 M, 6.2 mmol) was added to the reaction mixture and stirred for 1.5 h at −78 °C. SiMe2(t-Bu)Cl (5.25 g, 35 mmol) was added and stirred for 3 h at −78 °C. The reaction mixture was then warmed consecutively to 0 °C and stirred for 10 h, then warmed to room temperature and stirred for 2 h. The reaction mixture was treated with a similar purification procedure as the procedure for the synthesis of 4-SiBu3-2,6-iPr2C6H2OH (described above) to produce a liquid crude product. Further purification by silica gel column chromatography (hexane/EtOAc = 30:2) produced an orange liquid product (1.0 g, yield: 86%). 1H NMR (CDCl3): δ 7.19 (s, 2H), 4.85 (s, 1H), 3.12–3.19 (m, 2H), 1.28 (d, J = 6.9 Hz, 12H), 0.86 (s, 9H), 0.25 (s, 6H). 13C NMR (CDCl3): δ 151.9, 133.4, 130.7, 129.1, 28.2, 27.5, 23.7, 17.8, 15.1.
(ii) Synthesis of Cp*TiCl2(O-4-SiMe2(t-Bu)-2,6-iPr2C6H2) (7). The procedure for the synthesis of 5 was conducted using the same procedure for the synthesis of catalyst 4, using 4-SiMe2(t-Bu)-2,6-iPr2C6H2OH (320.1 mg, 1.1 mmol). Yield: 448 mg (65.0%). 1H NMR (CDCl3): δ 7.19 (s, 2H), 3.12–3.23 (m, 2H), 2.18 (s, 15H), 1.21 (d, J = 6.9 Hz, 12H), 0.84 (s, 9H), 0.27 (s, 6H). 13C NMR (CDCl3): δ 160.2, 137.9, 133.7, 132.5, 129.5, 35.0, 26.6, 24.2, 24.1, 21.5, 18.8, 13.05. Anal. Calcd. C32H54Cl2OSiTi: C, 61.65; H, 8.50; found: C, 61.65; H, 8.46.
Ethylene Polymerization. Reactions with ethylene were conducted as follows. In the drybox, Toluene (29.0 mL) and the prescribed amount of MAO were charged to a 100 mL scale stainless steel autoclave. The reaction apparatus was then filled with ethylene (1 atm), and the prescribed amount of complex in 1.0 mL of toluene was added. The autoclave was then pressurized with ethylene to 3 atm (total ethylene pressure is 4 atm), and the mixture was stirred for 5 or 10 min with constant ethylene pressure. The resultant polymers were collected as white precipitates by precipitation in HCl-acidified MeOH and by filtration and were adequately washed with MeOH. The resultant polymer was then dried in vacuo at 60 °C for 2 h.
Ethylene copolymerization with 1-dodecene (DD). The copolymerization reactions were conducted as follows. In the drybox, the prescribed amounts of 1-hexene or 1-dodecene, MAO, and toluene (total 29 mL) were added into a 100 mL scale stainless steel autoclave. A toluene solution containing a prescribed amount of complex (1.0 mL) was then added to the reaction apparatus under ethylene atmosphere (1 atm), and the autoclave was immediately pressurized with ethylene to 5 atm (total pressure is 6 atm). The mixture was stirred for 6 min with constant ethylene pressure at the prescribed temperature. After the reaction, the autoclave was placed in an ice bath to release the ethylene that remained. The resultant polymers were collected as white precipitates by precipitation in MeOH containing HCl through filtration and were adequately washed with MeOH; the resultant polymer was then dried in vacuo at 60 °C for 6 h.
4. Conclusions
In this study, the effects of trialkylsilyl para-phenoxy substituent on ethylene polymerization and ethylene/1-dodecene copolymerization using Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [1–7; R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)] were studied. For this purpose, complexes 4–7 were prepared and identified by NMR spectra and elemental analysis. The activities during ethylene polymerization of a series of (1–7)–MAO catalysts increased in the following order (25 °C, ethylene 4 atm): R = H (1, activity 49,600 kg-PE/mol-Ti·h, run 1) < SiMe3 (2, 54,400, run 3), SiEt3 (3, 55,600, run 5), Si(i-Pr)3 (4, 56,400, run 7) < SiMe2(t-Bu) (7, 58,800, run 15) < SiMe2(n-C8H17) (6, 62,800, run 13) < Si(n-Bu)3 (5, 65,600, run 9). And this activity was also affected by the amount of MAO charged, where an increase in activity was observed when the polymerizations by 5 were conducted at 50 °C. Moreover, the catalytic activities in the ethylene/1-dodecene copolymerizations (ethylene 6 atm, DD 0.75M, 25 °C) increased in the following order: 1 (activity = 1.60 × 105 kg-polymer/mol-Ti·h, run 32) < 2 (8.50 × 105, run 34) < 3 (1.09 × 106, run 36) < 7 (1.56 × 106, run 52) < 4 (2.07 × 106, run 38) < 6 (3.10 × 106, run 46) < 5 (3.40 × 106, run 41; 4.32 × 106, run 42); the Si(n-Bu)3 analog (5) thus showed the highest activity under the same conditions. Microstructural analysis revealed that the rE values of 5 (3.77, 4.09) are close to those of the original catalyst 1 (rE = 3.92) [36] and the SiEt3 analog (3, rE = 3.58) [37]. The values of the Si(n-Bu)3 analog (5) clearly suggest that copolymerization by 5 proceeds with efficient comonomer incorporation. It is clear that the present catalysts, especially 5, display promising capabilities as catalysts for ethylene copolymerization; further studies that include copolymerizations with sterically encumbered olefins and alken-1-ol [17] will be introduced in the future [39].
Supplementary Materials
The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15090840/s1: select NMR spectra of the ligands and catalysts, select 13C NMR spectra for the resultant ethylene/1-dodecene copolymers, and GPC charts of the resultant polymers.
Author Contributions
Conceptualization, K.N.; methodology, K.N.; supervision, K.N. and W.-H.S.; validation, formal analysis, J.G.; investigation, data curation, K.N. and J.G.; resources, K.N.; writing—original draft preparation, J.G., W.-H.S., and K.N.; writing—review and editing, visualization, project administration, funding acquisition, K.N. All authors have read and agreed to the published version of the manuscript.
Funding
This project was partly supported by Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS, grant number 21H01942, 25K01583).
Data Availability Statement
The data are contained within the article and the Supplementary Materials (NMR spectra, GPC traces).
Acknowledgments
J.G. expresses his heartfelt thanks to Mohamed M. Abdellatif (Tokyo Univ. Metropolitan University) and the laboratory members for their fruitful support and to Tokyo Metropolitan government (Tokyo Human Resources Fund for City Diplomacy) for its pre-doctoral fellowship. K.N. thanks Tosoh Finechem Co. for donating MAO.
Conflicts of Interest
The authors declare no conflicts of interest.
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Scheme 2.
Effects of trialkysilyl para-phenoxy substituents in ethylene (co)polymerization using Cp*TiCl2(O-2,6-iPr2-4-R-C6H2)–MAO catalysts.
Scheme 2.
Effects of trialkysilyl para-phenoxy substituents in ethylene (co)polymerization using Cp*TiCl2(O-2,6-iPr2-4-R-C6H2)–MAO catalysts.
Scheme 3.
Effect of trialkysilyl para-phenoxy substituents in ethylene copolymerization with 1-dodecene (DD) using Cp*TiCl2(O-2,6-iPr2-4-R-C6H2)–MAO catalysts.
Scheme 3.
Effect of trialkysilyl para-phenoxy substituents in ethylene copolymerization with 1-dodecene (DD) using Cp*TiCl2(O-2,6-iPr2-4-R-C6H2)–MAO catalysts.
Table 1.
Ethylene polymerization by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)]–MAO catalysts (in toluene, ethylene 4 atm, 10 min) 1.
Table 1.
Ethylene polymerization by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)]–MAO catalysts (in toluene, ethylene 4 atm, 10 min) 1.
| Run | Cat. | Temp /°C | Yield /mg | Activity /kg-PE/mol-Ti·h | Mn 2 ×10−6 | Mw/Mn 2 |
|---|---|---|---|---|---|---|
| 1 | 1 | 25 | 124 | 49,600 | 1.06 | 3.18 |
| 2 | 1 | 50 | 120 | 48,000 | 0.67 | 2.41 |
| 3 | 2 | 25 | 136 | 54,400 | ||
| 4 | 2 | 50 | 147 | 58,800 | ||
| 5 | 3 | 25 | 139 | 55,600 | 1.64 | 3.84 |
| 6 | 3 | 50 | 161 | 64,400 | 0.55 | 2.96 |
| 7 | 4 | 25 | 141 | 56,400 | 1.17 | 2.67 |
| 8 | 4 | 50 | 127 | 50,800 | 0.43 | 2.31 |
| 9 | 5 | 25 | 164 | 65,600 | 0.88 | 2.84 |
| 10 | 5 | 25 | 170 | 68,000 | 1.21 | 2.78 |
| 11 | 5 | 50 | 157 | 62,800 | 0.53 | 2.44 |
| 12 | 6 | 25 | 154 | 61,600 | 0.78 | 2.89 |
| 13 | 6 | 25 | 157 | 62,800 | 0.67 | 2.49 |
| 14 | 6 | 50 | 179 | 71,600 | ||
| 15 | 7 | 25 | 147 | 58,800 | 1.06 | 2.43 |
| 16 | 7 | 50 | 157 | 62,800 | 0.44 | 2.72 |
1 Conditions: Catalyst 0.015 μmol, toluene total 30 mL, ethylene 4 atm, d-MAO 3.0 mmol, 10 min. 2 GPC data in o-dichlorobenzene vs. polystyrene standards.
Table 2.
Ethylene polymerization by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = Si(i-Pr)3 (4), SinBu3 (5)]–MAO catalysts (in toluene, ethylene 4 atm): effect of MAO and temperature 1.
Table 2.
Ethylene polymerization by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = Si(i-Pr)3 (4), SinBu3 (5)]–MAO catalysts (in toluene, ethylene 4 atm): effect of MAO and temperature 1.
| Run | Cat. /μmol | Temp /C | MAO /mol | Yield /g | Activity /g-PE/mol-Ti·h | Mn 2 ×10−6 | Mw/ Mn 2 |
|---|---|---|---|---|---|---|---|
| 17 | 4 (0.005) | 25 | 2.0 | 40 | 48,000 | 1.23 | 2.98 |
| 18 | 4 (0.005) | 25 | 3.0 | 60 | 72,000 | ||
| 19 | 4 (0.005) | 25 | 4.0 | 73 | 87,600 | 0.65 | 2.18 |
| 20 | 5 (0.005) | 25 | 2.0 | 80 | 96,000 | 0.65 | 2.81 |
| 21 | 5 (0.005) | 25 | 3.0 | 108 | 129,600 | ||
| 22 | 5 (0.005) | 25 | 4.0 | 120 | 144,000 | 0.55 | 2.20 |
| 23 | 5 (0.015) | 50 | 3.0 | 157 | 628,000 | 0.53 | 2.44 |
| 24 | 5 (0.015) | 50 | 4.0 | 161 | 644,000 | 0.37 | 1.95 |
| 25 | 5 (0.015) | 50 | 5.0 | 175 | 700,000 | 0.29 | 1.75 |
| 26 | 5 (0.015) | 50 | 6.0 | 161 | 644,000 | 0.24 | 2.28 |
| 27 | 5 (0.015) | 80 | 2.0 | 107 | 428,000 | 0.28 | 2.85 |
| 28 | 5 (0.015) | 80 | 3.0 | 124 | 496,000 | 0.27 | 1.87 |
| 29 | 5 (0.015) | 80 | 4.0 | 132 | 528,000 | 0.23 | 2.14 |
| 30 | 5 (0.015) | 80 | 5.0 | 151 | 604,000 | 0.17 | 2.05 |
| 31 | 5 (0.015) | 80 | 6.0 | 144 | 576,000 | 0.15 | 2.22 |
1 Conditions: Catalyst 0.005 or 0.015 μmol, toluene total 30 mL, ethylene 4 atm, 10 min. 2 GPC data in o-dichlorobenzene vs. polystyrene standards.
Table 3.
Ethylene copolymerization with 1-dodecene (DD) by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)]–MAO catalysts (in toluene, DD 0.75 M, ethylene 6 atm, 6 min) 1.
Table 3.
Ethylene copolymerization with 1-dodecene (DD) by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)]–MAO catalysts (in toluene, DD 0.75 M, ethylene 6 atm, 6 min) 1.
| Run | Cat. | Temp /°C | MAO /mmol | Yield /mg | Activity 2 /kg-Polymer/mol-Ti·h | Mn 3 ×10−5 | Mw/ Mn 3 | Cont. 4 /mol% |
|---|---|---|---|---|---|---|---|---|
| 32 | 1 | 25 | 2.0 | 16 | 160,000 | 2.54 | 1.90 | |
| 33 | 1 | 50 | 2.0 | 89 | 890,000 | |||
| 34 | 2 | 25 | 2.0 | 85 | 850,000 | 1.50 | 1.54 | |
| 35 | 2 | 50 | 2.0 | 105 | 1,050,000 | |||
| 36 | 3 | 25 | 2.0 | 109 | 1,090,000 | 1.67 | 1.62 | |
| 37 | 3 | 50 | 2.0 | 155 | 1,550,000 | |||
| 38 | 4 | 25 | 2.0 | 207 | 2,070,000 | 1.95 | 1.86 | 19.8 |
| 39 | 4 | 50 | 2.0 | 60 | 600,000 | |||
| 40 | 4 | 50 | 3.5 | 72 | 720,000 | 1.59 | 2.03 | 25.2 |
| 41 | 5 | 25 | 2.0 | 340 | 3,400,000 | 2.29 | 1.72 | 18.9 |
| 42 5 | 5 | 25 | 2.0 | 108 | 4,320,000 | |||
| 43 | 5 | 50 | 2.0 | 298 | 2,980,000 | |||
| 44 | 5 | 50 | 3.5 | 350 | 3,500,000 | 1.46 | 2.07 | 25.9 |
| 45 | 5 | 50 | 3.5 | 355 | 3,550,000 | |||
| 46 | 6 | 25 | 2.0 | 310 | 3,100,000 | 1.99 | 1.84 | 19.8 |
| 47 | 6 | 50 | 2.0 | 40 | 400,000 | |||
| 48 | 6 | 50 | 2.5 | 75 | 750,000 | |||
| 49 | 6 | 50 | 3.0 | 309 | 3,090,000 | |||
| 50 | 6 | 50 | 3.5 | 328 | 3,280,000 | 1.31 | 2.01 | 21.8 |
| 51 | 6 | 50 | 4.0 | 225 | 2,250,000 | |||
| 52 | 7 | 25 | 2.0 | 156 | 1,560,000 | 1.96 | 1.84 | 18.3 |
| 53 | 7 | 50 | 2.0 | 90 | 900,000 | |||
| 54 | 7 | 50 | 3.5 | 120 | 1,200,000 | 1.68 | 2.07 | 23.2 |
1 Conditions: Catalyst 0.001 μmol (run 42, 0.00025 μmol), toluene and 1-dodecene (5.0 mL, DD 0.75 M) total 30.0 mL, ethylene 6 atm, 6 min. 2 Activity in kg-polymer/mol-Ti·h. 3 GPC data in o-dichlorobenzene vs. polystyrene standards. 4 1-Dodecene (DD) content (mol%) estimated by 13C NMR spectra. 5 Catalyst 0.00025 μmol.
Table 4.
Microstructural analysis for Poly(ethylene-co-1-dodecene) by 13C NMR spectra 1.
| Run | Cat. | Temp. | DD 2 | Triad Sequence Distribution 3/% | Dyads 4/% | rE 5 | rD 5 | rE·rD 6 | rE·rD 7 | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| /°C | /mol% | EEE | EED + DEE | DED | EDE | DDE + EDD | DDD | EE | ED + DE | DD | ||||||
| 38 | 4 | 25 | 19.8 | 46.4 | 27.5 | 6.2 | 15.3 | 3.6 | 0.9 | 60.1 | 37.1 | 2.7 | 3.34 | 0.14 | 0.48 | 0.48 |
| 40 | 4 | 50 | 25.2 | 48.1 | 20.6 | 6.0 | 19.6 | 5.6 | trace | 58.5 | 38.7 | 2.8 | 4.05 | 0.11 | 0.44 | 0.44 |
| 41 | 5 | 25 | 18.9 | 52.5 | 25.3 | 3.3 | 15.4 | 2.8 | 0.7 | 65.1 | 32.8 | 2.1 | 4.09 | 0.12 | 0.50 | 0.50 |
| 44 | 5 | 50 | 25.9 | 42.9 | 26.7 | 4.4 | 20.4 | 3.6 | 1.9 | 56.3 | 40.0 | 3.7 | 3.77 | 0.14 | 0.51 | 0.51 |
| 46 | 6 | 25 | 19.7 | 47.3 | 28.7 | 4.3 | 16.3 | 1.1 | 2.3 | 61.6 | 35.5 | 2.9 | 3.57 | 0.16 | 0.56 | 0.57 |
| 50 | 6 | 50 | 21.8 | 46.7 | 27.7 | 3.7 | 17.3 | 4.0 | 0.5 | 60.6 | 36.9 | 2.5 | 4.40 | 0.10 | 0.44 | 0.45 |
| 52 | 7 | 25 | 18.3 | 54.5 | 23.9 | 3.2 | 15.4 | 2.4 | 0.5 | 66.5 | 31.7 | 1.7 | 4.32 | 0.11 | 0.46 | 0.46 |
| 54 | 7 | 50 | 23.2 | 48.2 | 22.9 | 5.6 | 17.3 | 5.4 | 0.5 | 59.7 | 37.1 | 3.2 | 4.31 | 0.13 | 0.55 | 0.56 |
1 Detailed polymerization data, see Table 3. 2 1-Dodecene (DD or D) contents estimated by 13C NMR spectra, [DD] = [EDE] + [DDE + EDD] + [DDD]. 3 Estimated by 13C NMR spectra, E = ethylene, D = 1-dodecene. 4 [EE] = [EEE] + 1/2[EED + DEE], [ED + DE] = [DED] + [EDE] + 1/2{[EED + DEE] + [DDE + EDD]}, [DD] = [DDD] + 1/2[DDE + EDD]. 5 rE = [D]0/[E]0 × 2[EE]/[ED + DE], rC = [E]0/[C]0 × 2[DD]/[ED + DE], [E]0, [D]0 corresponds to the initial concentration. 6 rE·rD = 4[EE][DD]/[ED + DE]2. 7 rE·rD = rE × rD.
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Gao, J.; Sun, W.-H.; Nomura, K. Synthesis of New Phenoxide-Modified Half-Titanocene Catalysts for Ethylene Polymerization. Catalysts 2025, 15, 840. https://doi.org/10.3390/catal15090840
AMA Style
Gao J, Sun W-H, Nomura K. Synthesis of New Phenoxide-Modified Half-Titanocene Catalysts for Ethylene Polymerization. Catalysts. 2025; 15(9):840. https://doi.org/10.3390/catal15090840
Chicago/Turabian StyleGao, Jiahao, Wen-Hua Sun, and Kotohiro Nomura. 2025. "Synthesis of New Phenoxide-Modified Half-Titanocene Catalysts for Ethylene Polymerization" Catalysts 15, no. 9: 840. https://doi.org/10.3390/catal15090840
APA StyleGao, J., Sun, W.-H., & Nomura, K. (2025). Synthesis of New Phenoxide-Modified Half-Titanocene Catalysts for Ethylene Polymerization. Catalysts, 15(9), 840. https://doi.org/10.3390/catal15090840
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