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. 2022 Aug 1;2(5):386–391. doi: 10.1021/acsorginorgau.2c00020

Effect of Borate Cocatalysts toward Activity and Comonomer Incorporation in Ethylene Copolymerization by Half-Titanocene Catalysts in Methylcyclohexane

Suphitchaya Kitphaitun †, Takuya Fujimoto ‡, Yosuke Ochi ‡, Kotohiro Nomura †,*
PMCID: PMC9955119  PMID: 36855669

Abstract

graphic file with name gg2c00020_0003.jpg

Ethylene copolymerizations with 2-methyl-1-pentene, 1-dodecene (DD), vinylcyclohexane (VCH), [Me2Si(C5Me4)(NtBu)]TiCl2 (1), Cp*TiMe2(O-2,6-iPr2-4-RC6H2) [R = H (2), SiEt3 (3)]–borate, and [A(H)]+[BAr4]− [Ar = C6F5; A(H)+ = N+(H)Me(n-C18H37)2, N+(H)(CH2CF3)(n-C18H37)2, HO+(n-C14H29)2·O(n-C14H29)2, HO+(n-C16H33)2·O(n-C16H33)2; Ar = C10F7, A(H)+ = HO+(n-C14H29)2·O(n-C14H29)2 (B5), N+(H)(CH2CF3)(n-C18H37)2] catalyst systems conducted in methylcyclohexane (MCH) exhibited better comonomer incorporation than those conducted in toluene (in the presence of methylaluminoxane (MAO) or borate cocatalysts). The activity was affected by the borate cocatalyst and 1,3–B5 catalyst systems in MCH and showed the highest activity in the ethylene copolymerizations with VCH and DD.

Keywords: polymerization, borate cocatalyst, titanium, ethylene copolymerization, cation−anion interaction, half-titanocene

Olefin polymerization is the core technology for production of polyolefins (such as linear high/low density polyethylene, isotactic polypropylene), widely used synthetic polymers in our daily life. It has been recognized that the catalyst development provides a new possibility for synthesis of new polymers and/or the more efficient process.1−10 It has been proposed that the cationic metal alkyl species, generated from group 4 transition metal complexes such as metallocenes (Cp′2MX2; Cp′ = cyclopentadienyl; M = Ti, Zr, Hf; X = Cl, Me, etc.),1−3 linked half-titanocenes exemplified as [Me2Si(C5Me4)(NtBu)]TiCl2 (1),4 or modified half-titanocenes [Cp′TiX2(Y), Y = anionic ancillary donor]5−8 by treatment with methylaluminoxane (MAO) or borate cocatalysts, play an essential role in this catalysis.1−13

The cation/anion interaction,14−22 called the catalyst–cocatalyst nuclearity effect,19−21 has been known to affect the activity and the comonomer incorporation in the ethylene copolymerization. Remarkable improvements in the comonomer incorporation were first demonstrated by the bimetallic catalyst system,16−18,21 ethylene bridged linked half-titanocene, (CH2CH2)[Me2Si(indenyl)(NtBu)]2Ti2Me4 (bimetallic CGC, Chart 1), activated with [Ph3C]2[1,4-(C6F5)3BC6F4B(C6F5)3] in the copolymerization with 1-octene,16 isobutene,16 methylenecyclopentane,17 and with styrene in toluene.18 It was reported later that the ethylene/1-hexene copolymerization by the ethyl-indenyl zirconium analogue (Et-Ind CGC, Chart 1) activated with [{4-(n-C8H17)C6H4}3C][B(C6F5)] conducted in methylcyclohexane (MCH) showed higher catalytic activity and better 1-hexene incorporation than that conducted in toluene.22 The better catalyst performance observed in the latter catalyst system was explained as due to the successful utilization of a weak interaction between the cation and the borate anion without coordination of toluene to the formed cationic alkyl species (Chart 1, right).22−25 The effect has been considered as an important factor for design of the efficient catalyst system as well as better understanding of the reaction mechanism.26−32 However, the effect of borates including countercations [B(C6F5)4– vs B(C10F7)4–; A(H)+ (ammonium salts, oxonium salts), Scheme 1] in the ethylene copolymerization, especially in alkane solvents, has not yet been well studied.

Chart 1. Catalyst–Cocatalyst Nuclearity Effect in Ethylene Copolymerization by a Bimetallic (Bimetallic CGC) System in Toluene16−18 or Monomeric (Et-Ind CGC) System in Methylcyclohexane (MCH)22 and the Proposed Catalytically Active Cationic Alkyl Species (Right).

Chart 1

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Scheme 1. Ethylene Copolymerization with 2-Methyl-1-pentene (2M1P), 1-Dodecene (DD), and Vinylcyclohexane (VCH) in Methylcyclohexane (MCH) by [Me2Si(C5Me4)(NtBu)]TiCl2 (1) and Cp*TiMe2(O-2,6-iPr2-4-R′C6H2) [R′ = H (2), SiEt3 (3)] in the Presence of Borate (B1–B6) Cocatalyst.

Scheme 1

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We thus conducted an ethylene copolymerization study using half-titanocene catalysts in the presence of six alkane soluble borate compounds (B1–B6, Scheme 1), [A(H)]+[B(C6F5)4]− [A(H)+ = N+(H)Me(n-C18H37)2 (B1),33 N+(H)(CH2CF3)(n-C18H37)2 (B2), HO+(n-C14H29)2·O(n-C14H29)2 (B3), HO+(n-C16H33)2·O(n-C16H33)2 (B4)] or [A(H)]+[B(C10F7)4]− [A(H)+ = HO+(n-C14H29)2·O(n-C14H29)2 (B5), N+(H)(CH2CF3)(n-C18H37)2 (B6)], in MCH. Through this study, we wish to communicate an important role/effect of borate anion toward both the activity and the comonomer incorporation including the fact that drastic improvements in comonomer incorporation were observed by using these borate cocatalysts in MCH compared to those conducted in toluene.

Linked half-titanocene called constrained geometry type, [Me2Si(C5Me4)(NtBu)]TiCl2 (1), have been chosen in this study because 1 shows efficient comonomer incorporation especially of long chain α-olefins in the ethylene copolymerization.4,34−36 The phenoxide modified half-titanocenes, Cp*TiMe2(O-2,6-iPr2-4-R′C6H2) [R′ = H (2), SiEt3 (3)], have also been chosen because these catalysts display efficient comonomer incorporations in the ethylene (E) copolymerizations not only with 1-dodecene (DD)35−37 but also with 2-methyl-1-pentene (2M1P)38,39 and vinylcyclohexane (VCH),40 which are not incorporated by the ordinary (metallocene) catalysts. The dimethyl complex (3) was newly prepared by treating Cp*TiMe3 with 2,6-iPr2-4-SiEt3C6H2OH in Et2O according to the published method for synthesis of 2(41) and was identified by NMR spectra and elemental analysis (shown in the Supporting Information, SI). Borate compounds (B1–B6, Scheme 1) have been chosen to explore the effect of the borate anion, B(C6F5)4– or B(C10F7)4–, as well as of the counter cations, ammonium salts or oxonium salts, that would interact (as amine or ether) with the assumed cationic alkyl titanium species in situ.42,43

Table 1 summarizes selected results in the E/2M1P copolymerization (ethylene 4 atm in MCH at 25 °C), and additional results are shown in Table S1 in the SI.44 These copolymerizations in the presence of MAO or borate cocatalysts were conducted under the optimized conditions (amount of Al cocatalyst).36,39,45 MCH has also been chosen due to better solubility of borate compounds (B1–B6).

Table 1. Copolymerization of Ethylene (E) with 2-Methyl-1-pentene (2M1P) by [Me2Si(C5Me4)(NtBu)]TiCl2 (1) or Cp*TiMe2(O-2,6-iPr2-4-R′-C6H2) [R = H (2), SiEt3 (3)]–Borate Cocatalyst Systemsa.

run cat. (μmol) solvent Al cocat. borate yield/mg activityb Mnc × 10–4 Mw/Mnc Tmd/°C 2M1Pe/mol %
1 1 (1.0) MCH AliBu3f B3 1135 6810 44.9 6.42 120 0.4
2 1 (0.1) toluene MAO --- 84.8 5090 32.4 3.66 129  
3 2 (1.0) MCH AliBu3 B3 702 4210 6.28 1.93 99.0 5.5
4 2 (1.0) MCH AliBu3 B5 710 4260 5.76 1.76 93.6 6.8
5 2 (0.05) toluene MAO --- 93.3 11200 8.41 2.25 111 2.6
6 3 (1.0) MCH AliBu3 B2 446 2680 3.82 1.96 98.7  
7 3 (1.0) MCH AliBu3 B3 673 4040 4.11 1.90 101 5.0
8 3 (1.0) MCH AliBu3 B4 664 3980 3.21 2.41 98.4  
9 3 (1.0) MCH AliBu3 B5 843 5060 5.41 1.84 97.9 6.0
10 3 (0.05) toluene MAO --- 155 18600 8.34 2.05 109 3.1
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a

Conditions: MCH (methylcyclohexane) or toluene and 2-methyl-1-pentene (2M1P) 1.35 M (5.0 mL) total 30.0 mL, AliBu3 [0.55 mmol/L hexane, Al/Ti = 1000 (molar ratio)] or MAO 3.0 mmol (Al/Ti = 60000, molar ratio), borate (borate/Ti molar ratio = 1.0), ethylene 4 atm, 25 °C, 10 min.

b

Activity = kg-polymer/mol-Ti·h.

c

GPC data in o-dichlorobenzene vs polystyrene standards (Mn in g/mol).

d

By DSC thermograms.44

e

2M1P content (mol %) estimated by 13C NMR spectra.44

f

Al/Ti = 500, molar ratio.

It was revealed that the activity by 1 was affected by the borate cocatalyst employed and increased in the following order: activity = 149 kg-polymer/mol-Ti·h (B1, run S1) < 768 (B6, run S7) < 2660 (B5, run S6) < 3770 (B2, run S2) < 6810 (B3, run 1). The activity in the presence of B3 was rather higher than that conducted in toluene in the presence of MAO cocatalyst (5090 kg-polymer/mol-Ti·h; more data for comparison are shown in the SI), although we do not have a clear explanation for the effect of borate cocatalyst toward the activity. It was also revealed that the 1–B3 catalyst system showed 2M1P incorporation [0.4 mol %, as observed by the decrease in the melting temperature (Tm) in the DSC thermogram (Figure S54) and by the 13C NMR spectrum (Figure S23a)],44 whereas the 1–MAO catalyst system showed negligible 2M1P incorporation under the same conditions (estimated by the DSC thermogram, Tm = 129 °C).44 It also seems that the 2M1P incorporations (estimated by the Tm values) were also affected by the borate cocatalyst employed (B1 and B6 showed rather low 2M1P incorporations compared to the others), whereas the resultant copolymers possessed sole Tm values (suggesting uniform composition, Figure S54).44 The rather large PDI (Mw/Mn) values by 1 would be due to the heterogeneous nature of the reaction solution. It was also revealed that the activities of 2 and 3 were affected by the borate cocatalyst (B1–B6) employed. The 3–B5 catalyst system showed the highest activity (run 9, 5060 kg-polymer/mol-Ti·h), and the resultant copolymer possessed higher 2M1P content than that conducted in toluene by the 1–MAO catalyst system (6.0 mol % vs 3.1 mol %). In contrast, the activities in the presence of B1 were low, affording polymers containing a composition with rather high Tm values (runs S19 and S35 in Table S1).44 Moreover, B5 showed better 2M1P incorporation than B3 [2M1P 5.5 mol % (B3, run 3) vs 6.8 mol % (B5, run 4), 5.0 mol % (B3, run 7) vs 6.0 mol % (B5, run 9)]. It was shown that conducting these copolymerizations in MCH in the presence of borate cocatalysts (B2–B6) showed better 2M1P incorporation than conducting them in toluene in the presence of MAO. As reported previously (runs S30–S34 in Table S1),39 2M1P incorporations in the copolymerization using 2 or Cp*TiCl2(O-2,6-iPr2C6H3) (2-Cl) in toluene were not affected by cocatalysts such as MAO, modified MAO, or AliBu3–borate systems (more data are shown in Table S1, runs S22–S30).44 It was also revealed that, in the copolymerization by 2, a critical optimization of Al/Ti molar ratios was required for obtainment of the copolymers with uniform compositions (without contamination of polymer with negligible 2M1P incorporation) (runs 3, 4, and S12–S22 in Table S1);44,45 the SiEt3 analogue (3), thus, is suited in terms of the copolymer synthesis with uniform compositions for the following study.

Table 2 summarizes selected results in the E/DD copolymerization (ethylene 6 atm, at 25 °C) by 1 and 3 in MCH in the presence of borate cocatalyst, and additional results are shown in Table S2.44 It should be noted that the 1–B5 catalyst system in MCH showed higher activity with better DD incorporation than the 1–MAO catalyst system in toluene (run 13 vs 14; activity 48500 vs 38000 kg-polymer/mol-Ti·h; DD content 14.0 mol % vs 7.2 mol %). Notable improvement in the DD incorporation was observed when these copolymerizations by 1 were conducted in MCH in the presence of borate cocatalysts (runs 11–13, S40, and S41 in Table S2), although the 1–B1 catalyst system showed inferior catalyst performance to the other systems.44

Table 2. Copolymerization of Ethylene (E) with 1-Dodecene (DD) by [Me2Si(C5Me4)(NtBu)]TiCl2 (1) or Cp*TiMe2(O-2,6-iPr2-4-SiEt3-C6H2) (3)–Cocatalyst Systemsa.

run cat. (μmol) solvent Al cocat. borate yield/mg activityb Mnc × 10–4 Mw/Mnc Tmd/°C DDe/mol %
11 1 (0.1) MCH AliBu3 B2 343 34300 17.6 2.72 –35.6 14.1
12 1 (0.1) MCH AliBu3 B3 241 24100 14.3 2.87 –35.3 14.1
13 1 (0.1) MCH AliBu3 B5 485 48500 15.9 2.53 –36.4 14.0 (14.7)f
14 1 (0.05) toluene MAO --- 190 38000 73.5 4.51 31.8 7.2
15 3 (0.05) MCH AliBu3 B2 123 24600 8.79 1.86 –40.7 13.7
16 3 (0.05) MCH AliBu3 B3 232 46400 10.2 1.93 –40.6 13.7
17 3 (0.05) MCH AliBu3 B5 311 62200 11.6 1.95 –38.7 13.8
18 3 (0.001) toluene MAO --- 242 2420000 20.0 2.60 37.3 6.8
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a

Conditions: MCH (methylcyclohexane) or toluene and 1-dodecene (DD) 0.45 M (3.0 mL) total 30.0 mL, AliBu3 [0.55 mmol/L hexane, Al/Ti = 1000 (molar ratio)] or MAO 2.0 mmol (Al/Ti 2000, molar ratio), borate (borate/Ti molar ratio = 1.0), ethylene 6 atm, 25 °C, 6 min.

b

Activity = kg-polymer/mol-Ti·h.

c

GPC data in o-dichlorobenzene vs polystyrene standards (Mn in g/mol).

d

By DSC thermograms.

e

DD content (mol %) estimated by 1H NMR spectra.44

f

DD content (mol %) estimated by 13C NMR spectra.44

Similarly, notable improvement in the DD incorporation was observed by 3 in MCH in the presence of borate cocatalyst (B2, B3, and B5, runs 15–17, and S43 in Table S2),44 although the observed activities were apparently low compared to that for the incorporation conducted in toluene in the presence of MAO (run 18 in Table S2). The resultant copolymers prepared by the 1–borate and 3–borate catalyst systems possessed rather low Mn values probably due to the high DD content with unimodal molecular weight distributions (Mn = 8.06–17.6 × 104, Mw/Mn = 1.84–2.87) as well as uniform compositions confirmed by DSC thermograms (Figures S59–S61).44 As observed in Table 1, the countercation in the borate [expressed as A(H)+ in Scheme 1] seems to especially affect the activity rather than the DD incorporation, the Mn value in the resultant copolymers. As reported previously (runs 18 and S44–S47 in Table S2),35−37 DD incorporation in toluene was not affected by the cocatalyst system.

On the basis of the copolymerization results with 2M1P and DD, E/VCH copolymerizations by 1 and 3 were conducted in MCH in the presence of B1–B5. The selected results are summarized in Table 3, and additional results (including the results by B1) are shown in Table S3.44 It should be noted that the 1–B5 catalyst system exhibited much higher catalytic activity than the 1–MAO catalyst system; the activity reached 152000 kg-polymer/mol-Ti·h under the optimized borate/Ti molar ratio (run 27 in Table 3). Moreover, as observed in the copolymerizations with 2M1P and DD, the VCH contents in the resultant copolymers prepared by the 1–B5 catalyst system were high (9.1–9.7 mol %) compared to that prepared by the 1–MAO catalyst system (6.0 mol %). The activity by 1 was affected by the borate cocatalyst employed, and the VCH content in the copolymer increased upon increasing the VCH concentration charged accompanied with a decrease in both the activity and the Mn values in the resultant copolymers [runs 19 and 20 (B2), runs 21 and 22 (B3), and runs 25, 28, and 29 (B5) in Table 3]. Also note that no significant differences in the VCH contents in the copolymers were observed when these polymerizations by 1–borate (B1–B5) catalyst systems were conducted in toluene (Table S3, runs S50–S54).44 As demonstrated in the copolymerization with 2M1P,39 the VCH content by the 1–MAO catalyst system was close to those prepared by the 1–borate catalyst systems in toluene; the VCH incorporation was thus affected by the solvent (toluene vs MCH). The results thus suggest that the observed difference in MCH would be due to a weak cation–anion interaction without coordination of toluene (and amine or ether, exhibited A, as shown in Scheme 1, formed after treatment of 1 with borate) to the assumed cationic alkyl species.22,42,43

Table 3. Ethylene Copolymerization with Vinylcyclohexane (VCH) by [Me2Si(C5Me4)(NtBu)]TiCl2 (1) or Cp*TiMe2(O-2,6-iPr2-4-SiEt3-C6H2) (3)]–Cocatalyst Systemsa.

run cat. (μmol) borate (B/Ti)b VCHc/M yield/mg activityd Mne×10–4 Mw/Mne Tm,(Tg)f/°C VCHg/mol %
19 1 (0.05) B2 (1.0) 0.73 154 30800 10.4 1.82 95.4 5.4
20 1 (0.05) B2 (1.0) 1.22 84 16800 7.70 1.70 87.5 8.8
21 1 (0.05) B3 (1.0) 0.73 254 50800 9.65 1.90 97.7  
22 1 (0.05) B3 (1.0) 1.22 220 44000 6.31 1.65 86.0 9.0
23 1 (0.25) B4 (1.0) 1.22 264 10600 5.93 1.75 87.0 8.7
24 1 (0.25) B4 (3.0) 1.22 565 22600 5.87 1.91 86.2 9.0
25 1 (0.05) B5 (1.0) 1.22 345 69000 8.65 1.98 81.4 9.7
26 1 (0.05) B5 (2.0) 1.22 474 94800 9.32 2.00 83.1 9.4
27 1 (0.05) B5 (3.0) 1.22 762 152000 8.50 1.98 85.3 9.1
28 1 (0.05) B5 (1.0) 1.83 147 29400 6.43 1.85 73.8 (−17.0) 12.7
29 1 (0.05) B5 (1.0) 2.44 89 17800 5.63 1.73 62.9 (−16.9) 15.9
30h 1 (0.05) --- 1.22 197 39400 21.4 3.38 92.5 6.0
31 3 (0.05) B2 (1.0) 1.22 170 34000 9.71 1.78 (−6.4)  
32 3 (0.05) B2 (3.0) 1.22 654 131000 7.41 1.79 (−0.8)  
33 3 (0.05) B3 (3.0) 1.22 727 145400 7.82 1.90 (−0.1) 32.2
34 3 (0.05) B4 (3.0) 1.22 749 150000 6.41 1.94 (−7.8) 29.4
35 3 (0.05) B5 (3.0) 1.22 818 164000 11.4 1.89 (1.6) 33.0
36h 3 (0.01) --- 1.22 224 224000 17.5 2.43 (−15.1) 24.1
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a

Conditions: MCH (methylcyclohexane) and vinylcyclohexane (VCH) total 30.0 mL, AliBu3 (0.55 mmol/L n-hexane, Al/Ti molar ratio = 1000), ethylene 6 atm, 25 °C, 6 min.

b

Molar ratio of borate (B) to Ti.

c

Initial VCH concentration in mmol/mL.

d

Activity = kg-polymer/mol-Ti·h.

e

GPC data in o-dichlorobenzene vs polystyrene standards (Mn in g/mol).

f

By DSC thermograms.44

g

VCH content (mol %) estimated by 13C NMR spectra.44

h

MAO (Al/Ti = 2000, molar ratio) was used in place of AliBu3 – borate cocatalyst, and the polymerization was conducted in toluene.

It was also revealed that 3–borate catalysts systems (B2–B4, runs 31–35 in Table 3) showed better VCH incorporations than the 3–MAO catalyst system (run 36 in Table 3), whereas the activities by the borate catalyst systems were low compared to that by the 3–MAO catalyst system. The VCH incorporation seemed somewhat affected by the borate employed (runs 31–35 in Table 3 and runs S57–S59 in Table S3). The resultant copolymers prepared by the 1–borate and 3–borate catalyst systems (B2–B5) possessed high Mn values with unimodal molecular weight distributions (Mn = 5.63–11.4 × 104, Mw/Mn = 1.65–2.00), and their compositions were uniform, as confirmed by DSC thermograms [observed sole melting temperature (by 1) or glass transition temperature (by 3), Figures S62–68],44 suggesting that the reactions proceed with uniform (single) catalytically active species.

We have shown that conducting the ethylene copolymerizations with DD and VCH by half-titanocenes (1 and 3) in the presence of alkane soluble borate cocatalysts (B2–B5) in MCH enabled efficient syntheses of the copolymers with better comonomer incorporations than those in the presence of MAO cocatalyst in toluene. The activities in MCH were affected by the borate compounds employed. Use of the perfluorinated naphthyl borate, B(C10F7)4–, exhibited higher catalytic activity than the pentafluorophenyl borate, B(C6F5)4– (probably due to better delocalization of the counteranion).44 Noncoordinating oxonium ion, especially HO+(n-C14H29)2·O(n-C14H29)2 (B5) containing long alkyl chains, was preferred compared to the ammonium salts (probably due to the poor coordination ability of O(n-C14H29)2 to the assumed cationic species). The information presented here should be helpful not only for the design of an efficient catalyst strategy for synthesis of new copolymers (with sterically encumbered olefins38,39 including biobased disubstituted olefin,46 low strain cyclic olefins,47 etc.) and/or more active catalysts48 but also for better understanding of the catalysis mechanism.

Acknowledgments

This project was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS, Grant No. 18H01982, 21H01942). K.N. and S.K. express their heartfelt thanks to Tosoh Finechem Co. for donating MAO, and S.K. expresses her thanks to the Tokyo Metropolitan government (Tokyo Human Resources Fund for City Diplomacy) for a predoctoral fellowship.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsorginorgau.2c00020.

  • General experimental procedure, syntheses of Cp*TiMe2(O-2,6-iPr2-4-SiEt3C6H2) (3) and borates [A(H)]+[B(C6F5)4]− [A(H)+ = N+(H)(CH2CF3)(n-C18H37)2 (B2), HO+(n-C14H29)2·O(n-C14H29)2 (B3), and HO+(n-C16H33)2·O(n-C16H33)2 (B4)] or [A(H)]+[B(C10F7)4]− [A(H)+ = HO+(n-C14H29)2·O(n-C14H29)2 (B5) and N+(H)(CH2CF3)(n-C18H37)2 (B6)], additional results in ethylene copolymerization with 2-methyl-1-pentene, 1-dodecene, and vinylcyclohexane, selected NMR spectra for complex 3, borates, and the copolymers, selected DSC thermograms of the copolymers, and selected GPC charts of the copolymers (PDF)

Author Contributions

CRediT: Suphitchaya Kitphaitun data curation (lead), formal analysis (lead), investigation (lead), writing-original draft (supporting); Takuya Fujimoto conceptualization (supporting), formal analysis (supporting), investigation (supporting), project administration (supporting), writing-original draft (supporting); Yosuke Ochi data curation (supporting), formal analysis (supporting), investigation (supporting); Kotohiro Nomura conceptualization (lead), data curation (supporting), formal analysis (supporting), funding acquisition (lead), investigation (lead), methodology (lead), project administration (lead), supervision (lead), writing-original draft (lead), writing-review & editing (lead).

The authors declare no competing financial interest.

Supplementary Material

gg2c00020_si_001.pdf (5.9MB, pdf)

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