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Catalysts 2017, 7(10), 284; doi:10.3390/catal7100284

Article
Synthesis of Stereodiblock Polybutadiene Using Cp*Nd(BH4)2(thf)2 as a Catalyst
Ryo Tanaka *, Yuto Shinto, Yuushou NakayamaOrcid and Takeshi Shiono
Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-hiroshima 739-8527, Japan
*
Correspondence: Tel.: +81-82-424-7729
Received: 7 September 2017 / Accepted: 19 September 2017 / Published: 25 September 2017

Abstract

:
Butadiene polymerization, in both a highly cis- and trans-specific manner, was achieved by using a Cp*Nd(BH4)2(thf)2–Bu2Mg system as an initiator. The cis-/trans- ratio can be tuned by the amount of trialkylaluminum-depleted modified methylaluminoxane (dMMAO). The cis-regularity of the polymer was much higher than those obtained by Nd(BH4)3(thf)3. The molecular weight of cis-regular polymer was increased according to polymer yield, showing that there was no termination or chain transfer reaction during the polymerization. Synthesis of stereodiblock polybutadiene, which showed a high melting temperature (Tm) compared with stereodiblock polyisoprene, was also performed by the addition dMMAO during the polymerization.
Keywords:
coordination polymerization; neodymium catalyst; polyconjugated dienes; stereoblock polymer; rubber

1. Introduction

Polyconjugated dienes change their thermal and mechanical properties according to the stereoregularity. Generally, cis-1,4-regular polymer shows typical elastomeric property with a very low glass transition temperature (Tg) and trans-1,4-regular polymer has a melting point (Tm). Based on these properties, synthetic cis-1,4 polymer is widely applied as an alternative material to natural rubber, especially in tire industries [1]. Trans-1,4 polymer has great potential for shape-memory rubber and blend in cis-rich polymer which improves mechanical properties such as the rolling resistance of tires. Stereoblock polyconjugated dienes, which possess both cis- and trans-sequences in a single polymer chain, would be a promising material showing unique thermal and mechanical properties.
Some synthetic examples of stereoblock polymers are reported using Ni [2], Co [3,4] and Fe [5] catalysts with additional ligands, or rare-earth metal catalysts with chain transfer reagent [6]. However, these stereoblock polymers always contain atactic blocks or short stereoregular blocks consist of less than 10 repeating units. Highly stereoregular long blocks are difficult to synthesize, probably because of the difficulty of changing the stereoselectivity during the polymerization whilst keeping the living manner.
Previously, we reported the synthesis of stereodiblock polyisoprene which consists of cis-1,4- and trans-1,4-sequences using a Nd/Mg/Al combined catalyst system (Scheme 1) [7]. This is based on the idea that a simple neodymium catalyst system such as Nd(BH4)3(thf)3/Bu2Mg and (C3H5)NdCl2/methylaluminoxane (MAO) can promote trans- and cis-specific polymerization of isoprene and butadiene in a living manner, respectively (Figure 1) [8,9,10,11,12,13,14]. The stereospecificity is basically strongly affected by the metal charge; namely, neutral systems give trans-polymer and cationic systems give cis-polymer, and the change of the metal charge from neutral to cationic during the polymerization with Lewis acid gave stereodiblock polymer. The strategy would be applied to the other conjugated diene monomers, and as an extensive example, we attempted to synthesize stereoblock polybutadiene herein. The Tm of the polymer from the trans-1,4 polybutadiene block, which is an important property for the application to the thermoplastic elastomer, was significantly higher than that of stereodiblock polyisoprene.

2. Results and Discussion

Butadiene polymerization using a combination of Cp*Nd(BH4)2(thf)2 and Bu2Mg was performed in the presence of excess dMMAO (Table 1). In the previous research, iBu3Al was an efficient chain transfer agent of borohydrido-neodymium catalyzed polymerization [15,16], and we therefore removed free trialkylaluminums from MMAO to prevent chain transfer reaction, which is a critical problem for block polymer synthesis. It is already reported that Cp*Nd(BH4)2(thf)2–Bu2Mg promotes trans-1,4-specific polymerization of butadiene with narrow molecular weight distribution, and we successfully reproduced it (Run 1). The addition of 25 equivalents of dMMAO accelerated the reaction so that an almost quantitative amount of polymer was obtained at room temperature within an hour (Run 2). To reproduce the polymerization well, pre-activation of Cp*Nd(BH4)2(thf)2 with Bu2Mg in the presence of a small amount of butadiene was required. Precipitation of the catalyst occurred without the addition of monomer, which indicated that formation of allylneodymium species was important. The molecular weight distribution was larger than the polymer obtained without dMMAO, but it is probably because of the high viscosity of the reaction mixture. The trans-specificity was greatly reduced according to the amount of dMMAO and finally cis-specificity reached 93% (Runs 2, 4, and 5), although the activity was greatly reduced with a high Al/Nd ratio. The increase of the Bu2Mg amount slightly lowered cis-specificity (Runs 3 and 4). These tendencies of stereospecificity were similar with those observed in isoprene polymerization using the Nd–Mg/Al system, and use of Cp*Nd(BH4)2(thf)2 showed much higher cis-specificity and narrower molecular weight distribution than the Nd(BH4)3(thf)3–Bu2Mg system (Runs 4 and 6).
The 13C NMR spectrum of the polymer showed four distinct signals around 130 ppm, which is assigned to stereodyad sequences (Figure 2, see also the Supplementary Materials) [17]. From the integral ratios of these four signals, the ratio of stereodyad (tt:tc:ct:cc = 17:21:22:40) was close to the calculated statistical distribution (tt:tc:ct:cc = 14:24:24:38) supposing that trans/cis selectivity was 38% and 62%, which was the same as the polymerization result, respectively. This result showed randomly distributed trans and cis sequences in the obtained polymer and the occurrence of interconversion between cis-specific and trans-specific active species via disproportionation (Figure 3). The increase of cis-specificity along with the Al/Mg ratio showed that the neutral active species with trans-specificity was gradually converted to a cationic cis-specific one by the increasing amount of dMMAO, similar to Nd(BH4)3(thf)3-catalyzed isoprene polymerization previously reported.
Next, the relationship between the initial butadiene feed and molecular weight of the polymer was investigated (Table 2). Each run did not reach full conversion because of the high viscosity of the reaction mixture at the end of the polymerization and gave relatively broad molecular weight distribution. However, a constant N value, which is the number of polymer chains per Nd catalyst, showed that the molecular weight of the polybutadiene was linearly increased according to the amount of consumed monomer. These results indicated that no chain transfer reaction exists during the polymerization, which is a critical problem for synthesizing the block polymer.
Synthesis of stereodiblock polybutadiene was performed by the addition of dMMAO during trans-specific polymerization using the Cp*Nd(BH4)2(thf)2–Bu2Mg system (Scheme 2). The cis/trans ratio was not proportional to the molecular weight, probably because the generation of cis-specific active species is slow, but the stereodiblock polymer with a narrow molecular weight distribution was obtained. The whole GPC trace of the trans-prepolymer was shifted to a higher molecular weight region in the stereodiblock polymer, indicating high block efficiency (Figure 4). The trans–trans and ciscis stereodyad ratio calculated from the 13C NMR spectrum was much higher than the ratio of the others (Figure 5). Moreover, the polymer was not soluble in Et2O, showing that there was no cis-polybutadiene included in the obtained polymer. The difference of Tm and Tg of the polymer measured by DSC (Tm = 50 °C, Tg = −103 °C, Figure 6) was broader than that of stereodiblock polyisoprene [7] (Tm = 32 °C, Tg = −67 °C), showing potential as a building block for the thermoplastic elastomer.

3. Experimental Section

3.1. General

All manipulations were performed under an atmosphere of nitrogen using standard Schlenk line techniques. Trialkylaluminum-depleted MMAO (dMMAO) was prepared by the treatment of modified methylaluminoxane (MMAO), which was generously donated by Tosoh-Finechem Co. (Shunan, Japan), with SiO2 according to the literature [18]. Dry toluene was purchased from Kanto Chemical Co. Inc. (Tokyo, Japan), and a trace of residual water was removed by reaction with sodium metal. Butadiene solution in toluene were purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan) and used as received. Nd(BH4)3(thf)3 and Cp*Nd(BH4)2(thf)2 was prepared according to the literature and used immediately after the preparation [19]. Other materials were used without further purifications. 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded in chloroform-d on a Varian 500 NMR spectrometer. The obtained spectra were referenced to the signal of residual protonated solvent [1H: δ = 7.26 ppm] or the signal of solvent [13C: δ = 77.16 ppm]. Molecular weights of polymers were determined by Tosoh HLC-8320 gel permiation chromatography (GPC) system (Tokyo, Japan), calibrated with polystyrene standard (T = 40 °C; eluent: THF). Tg, Tm and melting enthalpy (ΔHm) of the polymer were measured by differential scanning calorimetry (DSC) analyses performed by a SII EXSTAR6000 system (Tokyo, Japan).

3.2. Polymerization Procedure

3.2.1. Butadiene Polymerization Using the Cp*Nd(BH4)2(thf)2–Bu2Mg/dMMAO System

To a 20 mL Schlenk flask, Cp*Nd(BH4)2(thf)2 (11 mg, 25 μmol) was charged and dissolved into butadiene solution in toluene (0.50 mL, containing 1.6 mmol butadiene). To this light blue solution, Bu2Mg in heptane (50 μL, 1.0 M, 50 μmol) was added and the resulting yellow-green solution was stirred at room temperature for 30 min. dMMAO solution in toluene (1.10 mL, 1.25 mmol) was added to the solution and further stirred at room temperature for 30 min. To the resulting light-yellow solution, butadiene in toluene (3.2 M, 4.5 mL, 14.4 mmol) was added to start the polymerization. After stirring at room temperature for 30 min, the reaction mixture was poured into acidic methanol containing 2% of hydrochloric acid and the precipitated solid was recovered. The polymer was dried under vacuum overnight until constant weight. An amount of 735 mg (85%) of colorless viscous polymer was obtained.

3.2.2. Synthesis of Stereoblock Polybutadiene Using Cp*Nd(BH4)2(thf)2 as a Catalyst

To a 20 mL Schlenk flask, Cp*Nd(BH4)2(thf)2 (11 mg, 25 μmol), butadiene solution in toluene (3.2 M, 5.0 mL, 16 mmol) and Bu2Mg in heptane (50 μmol, 1.0 M, 50 μL) was charged and stirred for 1 h at 40 °C. The resulting mixture was cooled to room temperature and a toluene solution of dMMAO (3.75 mL, 3.75 mmol) was added. The mixture was further stirred for 1 h and poured into acidic methanol containing 2% of hydrochloric acid. The polymer was recovered and dried under vacuum overnight until constant weight. An amount of 862 mg (99%) of colorless polymer was obtained.

3.2.3. Determination of Stereoregularity of Polybutadiene

The ratio of 1,4- and 1,2-unit was calculated from 1H NMR spectra according to the following equation [17]:
R 1 , 2 ( % ) = 200 I ter / ( 2 I int + I ter )
where R1,2 represents the ratio of 1,2-unit, Iint represents the integral ratio of internal olefinic protons observed at 5.4 ppm, and Iter represents the integral ratio of terminal vinyl protons observed at 5.0 ppm. The ratio of trans-1,4 and cis-1,4 unit was calculated from the integral ratio of the signals at 27.4 (cis-1,4) and 32.7 ppm (trans-1,4) on 13C NMR spectra.

4. Conclusions

Stereodiblock polybutadiene was successfully synthesized by using Cp*Nd(BH4)2(thf)2 as a catalyst. Unlike the previously reported synthesis of stereodiblock polyisoprene using Nd(BH4)3(thf)3, there was no need to add a chloride source such as tBuCl and Me2SiCl2. The obtained stereodiblock polybutadiene showed high Tm compared with the corresponding stereodiblock polyisoprene. This result showed the great potential of stereoblock polydienes for application to the thermoplastic elastomer.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4344/7/10/284/s1. Figure S1. 1H NMR spectrum of polybutadiene obtained in Table 1, Run 1 (500 MHz, in CDCl3), Figure S2. 13C NMR spectrum of polybutadiene obtained in Table 1, Run 1 (125 MHz, in CDCl3), Figure S3. 1H NMR spectrum of polybutadiene obtained in Table 1, Run 2 (500 MHz, in CDCl3), Figure S4. 13C NMR spectrum of polybutadiene obtained in Table 1, Run 2 (125 MHz, in CDCl3), Figure S5. 1H NMR spectrum of polybutadiene obtained in Table 1, Run 3 (500 MHz, in CDCl3), Figure S6. 13C NMR spectrum of polybutadiene obtained in Table 1, Run 3 (125 MHz, in CDCl3), Figure S7. 1H NMR spectrum of polybutadiene obtained in Table 1, Run 4 (500 MHz, in CDCl3), Figure S8. 13C NMR spectrum of polybutadiene obtained in Table 1, Run 4 (125 MHz, in CDCl3), Figure S9. 1H NMR spectrum of polybutadiene obtained in Table 1, Run 5 (500 MHz, in CDCl3), Figure S10. 13C NMR spectrum of polybutadiene obtained in Table 1, Run 5 (125 MHz, in CDCl3), Figure S11. 1H NMR spectrum of polybutadiene obtained in Table 1, Run 6 (500 MHz, in CDCl3), Figure S12. 13C NMR spectrum of polybutadiene obtained in Table 1, Run 6 (125 MHz, in CDCl3), Figure S13. 1H NMR spectrum of polybutadiene obtained in Table 2, Run 7 (500 MHz, in CDCl3), Figure S14. 13C NMR spectrum of polybutadiene obtained in Table 2, Run 7 (125 MHz, in CDCl3), Figure S15. 1H NMR spectrum of polybutadiene obtained in Table 2, Run 9 (500 MHz, in CDCl3), Figure S16. 13C NMR spectrum of polybutadiene obtained in Table 2, Run 3 (125 MHz, in CDCl3), Figure S17. 1H NMR spectrum of stereoblock polybutadiene (500 MHz, in CDCl3), Figure S18. 13C NMR spectrum of stereoblock polybutadiene (125 MHz, in CDCl3).

Acknowledgments

The authors thank Tosoh-Finechem Co. for generous donation of MMAO.

Author Contributions

Ryo Tanaka conceived and designed the experiments; Yuto Shinto performed the experiments and a part of them were reproduced by Ryo Tanaka; Ryo Tanaka, Yuushou Nakayama and Takeshi Shiono analyzed the data; Ryo Tanaka wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Synthesis of stereoblock polyisoprene using Nd(BH4)3(thf)3 as a catalyst precursor. dMMAO: trialkylaluminum-depleted modified methylaluminoxane.
Scheme 1. Synthesis of stereoblock polyisoprene using Nd(BH4)3(thf)3 as a catalyst precursor. dMMAO: trialkylaluminum-depleted modified methylaluminoxane.
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Figure 1. Examples of living butadiene/isoprene polymerization using simple neodymium catalysts. MAO: methylaluminoxane.
Figure 1. Examples of living butadiene/isoprene polymerization using simple neodymium catalysts. MAO: methylaluminoxane.
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Figure 2. 13C NMR spectrum of polybutadiene (Table 1, Run 2, 125 MHz, in CDCl3).
Figure 2. 13C NMR spectrum of polybutadiene (Table 1, Run 2, 125 MHz, in CDCl3).
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Figure 3. Plausible mechanism of stereospecificity change of butadiene polymerization at low dMMAO concentration.
Figure 3. Plausible mechanism of stereospecificity change of butadiene polymerization at low dMMAO concentration.
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Scheme 2. Synthesis of stereoblock polybutadiene using Cp*Nd(BH4)2(thf)2 (Run 10).
Scheme 2. Synthesis of stereoblock polybutadiene using Cp*Nd(BH4)2(thf)2 (Run 10).
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Figure 4. GPC traces of trans-1,4 prepolymer and stereodiblock polymer synthesized in Run 10.
Figure 4. GPC traces of trans-1,4 prepolymer and stereodiblock polymer synthesized in Run 10.
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Figure 5. 13C NMR spectrum (125 MHz, in CDCl3) of trans-1,4 prepolymer (above) and stereodiblock polybutadiene synthesized in Scheme 2 (below).
Figure 5. 13C NMR spectrum (125 MHz, in CDCl3) of trans-1,4 prepolymer (above) and stereodiblock polybutadiene synthesized in Scheme 2 (below).
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Figure 6. Differential scanning calorimetry (DSC) trace of stereoblock polybutadiene synthesized in Run 10.
Figure 6. Differential scanning calorimetry (DSC) trace of stereoblock polybutadiene synthesized in Run 10.
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Table 1. Butadiene polymerization using the Cp*Nd(BH4)2(thf)2–Mg/Al catalyst system.
Table 1. Butadiene polymerization using the Cp*Nd(BH4)2(thf)2–Mg/Al catalyst system.
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RunNd (μmol)Al/Mg (mol/mol)Time (h)Yield (%)Mna (×104)Mw/Mnatrans:cis:vinyl b (mol %)
1 c2501200.81.393:3:4
225251858.21.534:61:5
3 d25250.5707.91.715:81:4
4255018410.21.713:84:3
52510012282.71.76:92:2
6 e50251.5935.22.559:38:3
a Determined by GPC (gel permiation chromatography) calibrated with PS standard. b Determined by 1H and 13C NMR. c Performed at 40 °C. d 4 equivalents of Bu2Mg was used. e Nd(BH4)3(thf)3 was used instead of Cp*Nd(BH4)2(thf)2.
Table 2. Effect of initial monomer feed on the molecular weight in the butadiene polymerization using the Cp*Nd(BH4)2(thf)2–Mg/Al catalyst system.
Table 2. Effect of initial monomer feed on the molecular weight in the butadiene polymerization using the Cp*Nd(BH4)2(thf)2–Mg/Al catalyst system.
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RunBd Total (mmol)Bd/Nd (mol/mol)Yield (%)Mna (×104)PDIaTrans:Cis:Vinyl b (mol %)N c
78320984.91.914:83:30.35
8166408410.21.713:84:30.28
93212807316.01.720:76:40.32
a Determined by GPC; calibrated with PS standard. b Determined by 1H and 13C NMR. c Number of polymer chains per Nd calculated from Mn and yield. Bd: butadiene.
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