Journal Pre-proof Isobutene (co)polymerization initiated by rare-earth metal cationic catalysts Yang Jiang, Zhen Zhang, Shihui Li, Dongmei Cui PII:
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Please cite this article as: Jiang Y, Zhang Z, Li S, Cui D, Isobutene (co)polymerization initiated by rareearth metal cationic catalysts, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2019.122105. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Isobutene (co)polymerization initiated by rare-earth metal cationic catalysts Yang Jiang,a,b Zhen Zhang,c Shihui Li*,a,b Dongmei Cuia,b a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China.
University of Science and Technology of China, Hefei 230026, China. Department of Materials Science and Engineering, Jilin University, Changchun 130022, China
E-mail: [email protected]
Highlights: Isobutene (co)polymerizations were studied by various well-defined rare-earth metal complexes. The activity of isobutene polymerization strongly depends on the ligand, metal type of rare-earth metal complexes and aluminium alkyls, organoborate. The molecular weights of resultant polyisobutenes mainly depend on the reaction temperature and less rest with monomer to initiator ratios. Monocationic scandium active species could not efficiently initiate the isobutene copolymerization with isoprene. Isobutene copolymerization with isoprene could be realized by the dicationic half-sandwich scandium active species.
Abstract Isobutene (co)polymerizations by rare-earth metal cationic complexes were studied. The half-sandwich scandium and lutetium complexes 1a and 1c activated by one 1
equivalent of [Ph3C][B(C6F5)4], displayed high activity for isobutene polymerization at –30 ºC. Its yttrium analogue 1b, the constrained geometry complexes 3 and 4 and the yttrium metallocene 5 showed lower activities. Another half-sandwich scandium complex 2 and non-Cp complex 6 were nearly inert. The resultant molecular weights produced by cationic initiator 1a increased from 2.45 × 104 to 27.8 × 104 g/mol with temperature declining from 20 ºC to –50 ºC. Despite increment of isobutene/1a mole ratio from 3000:1 to 30000:1, the resultant molecular weights still fell within the scope of 20.0×104 – 25.0×104 g/mol. The cationic species 1a/[Ph3C][B(C6F5)4] is not an efficient initiator for isobutene copolymerization with isoprene, but if the cationic species is first treated by isobutene or the dicationic species 1a/2[Ph3C][B(C6F5)4] is applied, the copolymerization can reach high conversion. Keywords: isobutylene, rare-earth metal, cationic polymerization, polyisobutene, butyl rubber 1. Introduction Polyisobutene (PIB) and butyl rubber (IIR) with a unique set of properties such as excellent air-barrier performance, good flex fatigue and vibration damping etc. have found numerous commercial applications, rubbers, sealants, lubricants, and oil additives. Commercially, PIB and IIR are synthesized by means of cationic polymerization with inorganic Lewis acid initiators in chloromethane [1-3] and high-molecular-weight products require costly cryogenic temperatures of about –100 to –90 ºC. This kind of industrial process leads to seriously environment taxing. In order to increase energy efficiency and eliminate toxic solvents and waste streams, a large amount of research has been done and achieved significant improvements [4-11]. Baird and coworkers first found that the transition metal cationic catalyst Cp*TiMe3/B(C6F5)3 could initiate isobutene (IB) cationic polymerization in CH2Cl2/toluene at temperatures of –70 to –20 ºC, which gave high molecular weight PIB of 106 g/mol with molecular weight distributions within the range of 1.9–3.2 . Soon after that, group 4 based metallocene compounds are extensively exploited. For 2
instance, zirconocene compounds (Cp2ZrMe2, Cp*2ZrMe2 or [Cp′2ZrH(µ-H)]2) activated by B(C6F5)3 or [Ph3C][B(C6F5)4] exhibited good catalytic performance for IB polymerization even at temperature up to –30 ºC [12, 13]. On the other hand, Bochmann and coworkers reported that the cationic organoaluminum compound [Cp2Al][B(C6F5)3Me] was a highly active initiator for IB polymerization, which readily gave high molecular weight PIB of 3.2×105 g/mol with molecular weight distribution of 1.8 in CH2Cl2 at – 30 ºC. Interestingly, changing the Cp (C5H5) ligand to bulkier Cp rings C5Me4H and C5Me5 really depressed the polymerization activity and even led to be completely inactive. This phenomenon is different from that of the analogous zirconocene system, where the bulkier Cp rings lead to better results [10, 12]. These results demonstrate that the ligands of transition metal complexes play important role in IB cationic polymerization. In the past decade, various rare-earth metal cationic alkyl catalysts have been developed and demonstrated excellent catalytic performances in (co)polymerization of olefins, conjugated dienes, and polar olefins [15-19]. In contrast, to date, only two well-defined rare-earth metal complexes were used to initiate IB (co) polymerizations. Cationic metallocene complex [(C5H4SiMe3)2YMe]/[Ph3C][B(C6F5)4] provided high molecular weight PIB (Mw = 1.38×105 g/mol) in a low yield of 6% . Recently, [(C5Me4SiMe3)Sc(CH2SiMe3)2THF]/[Ph3C][B(C6F5)4] system was reported to convert 2680 equivalents of IB into a high molecular weight PIB (Mw = 8.4×105 g/mol) at – 55 ºC [21, 22]. In this contribution, we explored the effects of supported ligands and rare-earth metal elements on cationic polymerization behaviors of IB in a wide temperature range from 20 to –75 ºC. the half-sandwich fluorenyl scandium complex 1a combined with organoborate [Ph3C][B(C6F5)4] exhibited extremely high activity for IB polymerization, readily providing high molecular weight PIB (Mw = 3.6×105 g/mol). Treated by IB in advance or using dicationic species 1a/2[Ph3C][B(C6F5)4] can efficiently enhance the yield of IB copolymerization with isoprene (IP). 2. Experimental Section 2.1 General considerations All manipulations were performed under a dry and oxygen-free nitrogen 3
atmosphere using standard high-vacuum Schlenk techniques or in a glovebox. All solvents were purified via a SPS system. Isobutene (gas products, purify grade, 99.5%) was purified by passage through columns of 3A molecular sieves and potassium hydroxide and condensed into a flask with purified toluene in which a 32 wt% toluene solution of IB was obtained. All chemicals, excepted as noted, were purchased from Energy Chemical Company. Complex 1-4 and 6 were prepared according to the reported methods [23-27]. [Ph3C][B(C6F5)4], [PhNMe2][B(C6F5)4] and B(C6F5)3 were purchased from Energy Chemical Company. 1
H and 13C NMR spectra were recorded on a Bruker AV400 (400 MHz for 1H
NMR; 100 MHz for
C NMR) spectrometer. Crystallographic data were
collected at -86 °C on a Bruker SMART APEX diffractometer with a CCD area detector (Mo Kα, λ = 0.71073 Å). The determination of crystal class and unit cell parameters was carried out by the SMART program package. The structures were solved by using the SHELXTL program. Elemental analyses of carbon, and hydrogen in the solid samples were performed with a VarioEL analyzer. The molecular weight and molecular weight distribution were measured by TOSOH HLC-8220 GPC at 40 °C using THF as eluent (the flow rate is 0.35 mL/min) against polystyrene standards. The glass transition temperature (Tg) was measured through differential scanning calorimetry (DSC) analysis, which was carried out on a METTLER TOPEM DSC instrument under a nitrogen atmosphere, with a heating rate of 10 oC /min in a range of – 100 to 100 °C. 2.2 Synthesis of complex 5 Under a nitrogen atmosphere, two equivalents of LiCH2SiMe3 (0.565 g，6 mmol) was added into a THF solution of [IndCMe2Ind]H2(0.820, 3 mmol) at room temperature and stirred for 2 h to afford a red solution. Then, the THF solution of the dilithium salt reacted with a THF suspension of ScCl3(THF)3 (1.103 g, 3 mmol) for 3 h at room temperature. Next, the solution of LiCH2SiMe3 (0.2825 g，3 mmol) was dropwise added and the mixture was stirred for 2 h at room temperature. All volatiles were removed at vacuo and extracted with hexane. The yellow crystalline product was 4
obtained in the concentrated hexane solution at –30 °C (0.35 g, 76%).
(500 MHz, Benzene-d6, 25 °C) δ 7.76 (d, J = 8.7 Hz, 2H), 7.51 (d, J = 8.5 Hz, 1H), 7.26 (d, J = 8.6 Hz, 1H), 7.08 – 6.97 (m, 2H), 6.92 (t, J = 7.8 Hz, 1H), 6.65 – 6.47 (m, 2H), 6.16 (s, 1H), 5.97 (s, 1H), 5.82 (s, 1H)，2.77 (d, J = 114.2 Hz, 4H), 2.22 (d, J = 62.4 Hz, 6H), 0.88 (s, 4H), 0.24 (s, 9H), -1.81 (d, J = 12.1 Hz, 1H), –2.19 (d, J = 12.1 Hz, 1H). Anal. Calcd for C29H37OSi Sc (%): C, 73.38; H, 7.86. Found: C, 73.61; H, 7.89. 2.3 The typical polymerization procedure of IB Polymerizations were carried out using a 100 mL glass reactor equipped with a magnetic stirred bar. The pressure bottle with the toluene solution of isobutene (17.53 g, 30 wt%) was immersed in cold ethanol at –45 oC under nitrogen atmosphere for 10 min, and then a toluene solution composed of complex 1a (10 µmol, 5.3 mg) and [Ph3C][B(C6F5)4] (10µmol, 9.2 mg) was added into the reactor with a syringe to initiate polymerization. After the mixture was stirred at –45 C for 1 hour, the reaction solution was terminated o
by 5 mL ethanol. The resulting mixtures were poured into a 100 mL beaker containing 30 mL of ethanol and stirred for 30 min, after which the polymers were collected and dried to constant weight at 50 oC in vacuo. 3. Results and Discussion 3.1 Effects of cocatalysts on IB polymerization As reported previously, the reaction of well-defined rare-earth metal dialkyl complexes with one equivalent of organoborate [Ph3C][B(C6F5)4]/[PhNMe2][B(C6F5)4] afforded the electronically unsaturated cationic rare-earth metal alkyl species, which could behave as an excellent catalyst for coordination polymerization process[28, 29]. Herein, the half-sandwich scandium complex 1a, activated by one equivalent of [Ph3C][B(C6F5)4], was used to initiate IB cationic polymerization at –30 ºC in a toluene solution and gave high molecular weight polyisobutene (PIB) (Mw = 13.8×104 g/mol) with molecular weight distribution of 1.85 in a yield of 76% (Table 1, run 1). Generally, alkylaluminum as cocatalyst, molecular chain transfer regent, and 5
impurities removal, plays important role in coordination polymerizations of α-olefins and conjugated dienes, and is helpful to enhance polymerization activity[30-34]. However, the presence of AliBu3 intensively inhibited the catalyst’s activity (Table 1, run 2), although the AliBu3/[Ph3C][B(C6F5)4] adduct can initiate IB polymerization in a low polymerization activity under same condition (Table 1, run 3). This probably is caused
[η5-FluSiMe3Sc(µ-CH2SiMe3)(µ-iBu)AliBu2][B(C6F5)4], which inhibits IB monomer access to the cationic metal center[33, 34]. In addition, alkylaluminum AliBu3 was completely inert for IB polymerization (Table 1, run 4). In contrast, organoborate [Ph3C][B(C6F5)4] solely converted 22% IB into low molecular weight product (Table 1,
[PhNMe2H][B(C6F5)4], the catalyst system 1a/[PhNMe2H][B(C6F5)4] remained inert for IB polymerization (Table 1, run 6), because the released N,N’-dimethylaniline is a strong carbocationic poison. Unexpectedly, although the borane B(C6F5)3 is good cocatalyst for group 4 metal based complexes, and, however, the adduct of complex 1a/B(C6F5)3 cannot initiate IB polymerization (Table 1, runs 7 and 8).
Figure 1. Rare-earth metal alkyl complexes. Table 1. IB polymerization initiated by various rare-earth metal cationic complexes. a
1 2000:1 76 13.8 1.85 -65 1a+A 2b 2000:1 – 1a+A 2000:1 15 2.8 2.01 -65 3c Al+A d 4 200:1 – – – Al 5 2000:1 22 2.2 2.00 -74 A 6 2000:1 – – – – 1a+B 7 2000:1 – – – – 1a+C 8 2000:1 – – – – 1a+2C 9 2000:1 70 11.1 1.92 -66 1b 10 2000:1 9 9.3 1.64 – 1c 11 2000:1 – – – – 2 12 2000:1 41 17.2 1.72 -66 3 13 2000:1 15 18.3 1.81 – 4 14 2000:1 8 15.8 1.67 – 5 15 2000:1 trace – – – 6 [a] Conditions: cat. 10 µmol; [Ph3C][B(C6F5)4] (A), [PhNMe2H][B(C6F5)4] (B), B(C6F5)3 (C), 10 µmol; [IB] 30 wt% in toluene; polymerization time, 1 hour; polymerization temperature, -30 ºC [b] [1a]: [Ph3C][B(C6F5)4]:[AliBu3] = 1:1:10. [c] AliBu3 = 10 µmol; [d] AliBu3 = 100 µmol; [e] Determined by GPC in THF at 40 °C against polystyrene standard.
3.2. IB polymerization initiated by various rare-earth metal complexes. The performance of transition metal catalysts for coordination polymerizations strongly depends on the coordination environment around metal center and the type of metal elements. Similarly, these factors also intensely influence the carbocation polymerization of IB. For instance, lutetium complex 1b, in combination with [Ph3C][B(C6F5)4], is an efficient initiator for IB polymerization at -30 ºC and gave PIB with Mw up to 11.1×104 g/mol and PDI of 1.92 (Table 1, run 9). In contrast, the IB polymerization initiated by its yttrium analogue 1b became sluggish with 9% conversion (Table 1, run 10). The half-sandwich scandium complex 2, in presence of [Ph3C][B(C6F5)4], was completely inert for IB polymerization (Table 1, run 11), mainly attributed to the release of 2-Ph3CCH2C6H4NMe2, a strong carbocationic poison. Under same conditions, the constrained geometry complexes 3 and 4 displayed decreased activities with the increment of steric hindrance (Table 1, runs 12 and 13), probably because the reduced available vacancy around metal center impedes IB monomer access to the cationic metal center. The sluggish activity also was observed in IB polymerization initiated by the scandium metallocene 5 (Table 1, run 7
14). The non-cyclopentadienyl ligand supported scandium complex 6 was inert for IB polymerization (Table 1, run 15), probably because the weakened Lewis acidity of the cationic metal center makes it difficult to capture IB monomer. The molecular weights of the resultant PIB mainly rest with polymerization temperature and are not dependent on monomer to initiator ratios. As showed in Table 2, while the polymerization temperature dropped from 20 to –50 ºC, the resultant molecular weights increased from 2.45×104 to 27.7×104 with relatively constant molecular weight distributions (1.72–2.12) (Table 2, runs 1-6). Unexpectedly, when the polymerization reactions were carried out at –60 ºC and –75 ºC, the resultant PIB formed a spheroid separated from the toluene solution and their molecular weights obviously decreased to 7.34×104 g/mol and 4.89×104 g/mol, respectively (Table 2, runs 7 and 8). A possible explanation for these abnormal results comes from the fact that the product polymer is poorly soluble in a chilled toluene and readily aggregates into a solvent-swelled micelle under a steady stirring effect, which impedes diffusion of monomer into the micelle and the β-hydrogen elimination reaction gains an advantage over chain propagation. In coordination polymerization initiated by transition metal catalysts, high molecular weight product could be obtained by increasing monomer loading amount [36, 37]. For this case, while the monomer to initiator ratios were boosted from 3000:1 to 30000:1, the resultant molecular weights almost remained constant (Mw = 20.0 – 25.0×104 ) at –45 ºC (Table 2, runs 9-15). This result is accordance with the characteristic of cationic polymerization, in which the molecular weight of product polymer mainly depends on polymerization temperature . Table 2. IB polymerization initiated by complex 1a/[Ph3C][B(C6F5)4].a Run
1 2 3 4 5 6
10000:1 10000:1 10000:1 10000:1 10000:1 10000:1
20 0 -15 -30 -45 -50
1 1 1 1 1 7
76 79 81 77 87 84
2.45 3.99 11.6 15.0 23.5 27.7
1.72 2.03 1.88 1.92 2.12 1.96
-68 -67 -66 -66 -65 -65 8
7 10000:1 -60 24 71 7.34 8 10000:1 -75 24 74 4.89 9 3000:1 -45 1 79 24.4 10 5000:1 -45 1 67 20.0 11 7200:1 -45 1 88 21.6 12 12000:1 -45 1.5 71 25.0 12 15000:1 -45 2.5 83 24.8 14 20000:1 -45 3 65 24.4 15 30000:1 -45 6 76 24.7 [a] Conditions: 1a. 10 µmol; [Ph3C][B(C6F5)4], 10 µmol; [IB] by GPC in THF at 40 °C against polystyrene standard.
1.53 -66 1.62 -66 1.95 -65 2.41 -65 2.37 -64 2.17 -64 1.76 -65 1.79 -66 1.95 -64 30 wt% in toluene. [b] Determined
3.3. Copolymerization of IB and IP Incorporation
copolymerization of IB and IP enable isobutene-isoprene copolymer to be readily vulcanized into aimed products by ordinary sulfur vulcanization systems. Therefore, cationic copolymerization of IB and IP initiated by complex 1a/[Ph3C][B(C6F5)4] was carried out in order to evaluate the potential of rare-earth metal initiator system for the synthesis of isobutene-isoprene copolymer. As presented in Table 3, the polymerization activity was intensely suppressed by the addition of only 3 mol% IP loading content with the conversion declining from 87% to 15% (Table 2, run 5; Table 3, run 1). The phenomenon is consistent with that reported by Li etc. This is probably caused by the formation of cationic η3-allyl scandium species (A-I) stemming from insertion reaction of isoprene monomer into cationic alkyl scandium initiator (A), which inhibits the access of isobutene to cationic metal center. To assess the hypothesis, controlled experiments were carried out. 500 equivalents of IB first reacted with the initiator complex 1a/[Ph3C][B(C6F5)4] (A) at –35 °C for 1 min before the mixture of IB and IP was injected into the reaction vessel. In this way, polymerization activity was less affected except broadening of molecular weight distribution (Table 3, runs 2 and 3). For eliminating the effect of insertion reaction of IP into metal alkyl species, the molar ratio of complex 1a/[Ph3C][B(C6F5)4] declined from 1:1 to 1:2 to form a dicationic scandium species (B) and with this active species the polymerization activity remained at a high level (Table 3, run 4). But the presence of isoprene substantially resulted in lower copolymer molecular weight than those of 9
homopolymers obtained under identical conditions, because the incorporation of isoprene accelerated the chain transfer processes such as β-H eliminations (Table 3, runs 4 and 5). This result is consistent with previous observation that isoprene normally functions as a chain transfer agent in conventional cationic copolymerization [12, 38].
Figure 2. Isobutene copolymerization mechanism by complex 1a/[Ph3C][B(C6F5)4] and complex 1a/2[Ph3C][B(C6F5)4]. Table 3. Cationic copolymerization of IB and IP initiated by complex 1a/[Ph3C][B(C6F5)4]. Run
1 1a+1B 1:300:9700 -45 1 15 1.2 5.64 1.64 -66 2 1a+1B 1:100:3233 -35 1 20 1.1 3.20 1.77 -67 d 3 1a+1B 1:100:3233 -35 1 82 1.5 3.12 3.90 -68 4 1a+2B 1:300:9700 -45 1 67 1.6 3.72 1.70 -66 5 1a+2B 1:1000:10000 -45 6 79 6.6 1.37 1.61 -64 a Conditions: 1a. 10 µmol; [Ph3C][B(C6F5)4], 10 µmol; [IB+IP] 30 wt% in toluene. bCalculated by 1 H NMR in CDCl3. cDetermined by GPC in THF at 40 °C against polystyrene standard. d500 equiv. of IB was firstly added into the reaction vessel at -35°C, after 1 min, the mixture of the residual IB and IP was added into the reaction vessel.
4. Conclusion We have demonstrated that isobutene polymerization and its copolymerization with isoprene could be achieved utilizing rare-earth metal cationic complexes. The polymerization activities strongly depend on the natures of the ligated ligand, metal center, aluminum alkyls and organoborate. The half-sandwich fluorenyl ligands supported scandium and Lutetium cationic alkyl complexes showed more high 10
initiation efficiency, which should be attributed to less steric hindrance around the metal center and stronger Lewis acidity. The polymer molecular weights mainly rest with polymerization temperature and were less affected by isobutene to initiator ratio, indicating that the polymerization process features a cationic polymerization mechanism. In copolymerization of isobutene and isoprene initiated by rare-earth metal cationic alkyl initiator, isoprene is apt to act as a strong retardant, due to the formation of η3-allyl-metal species after insertion of isoprene into metal-carbon bond. Addition of two equivalents of organoborate to eliminate two alkyl groups from rare-earth metal dialkyl complex efficiently gets rid of the disadvantageous effect on polymerization activity. Author contribution sections Yang Jiang contributes to the isobutene polymerization, the characterization of the resulting polymers. Zhen Zhang contributes to synthesis of most of rare-earth metal complexes. Shihui Li contributes to the writing of the manuscript. Dongmei Cui and Shihui Li contribute to the design of this work and the revise of the manuscript. All authors contribute to the discussion and the processing of all data. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is partially supported by the NSFC (project No.s 51773193, 21634007), department of science and technology of Jilin province for project (20180101171JC), the “973” project (2015CB654702).
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Author contribution sections Yang Jiang contributes to the isobutene polymerization, the characterization of the resulting polymers. Zhen Zhang contributes to synthesis of most of rare-earth metal complexes. Shihui Li contributes to the writing of the manuscript. Dongmei Cui and Shihui Li contribute to the design of this work and the revise of the manuscript. All authors contribute to the discussion and the processing of all data.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: