Syndiospecific polymerization of styrene by half-titanocene catalysts with the sulfur-containing donor ligand

1 Introduction

Syndiotactic polystyrene (sPS) is a novel synthetic material with the characteristics of low specific gravity, low dielectric constant, high modulus of elasticity and excellent resistance to water and chemicals (1). Since Ishihara succeeded in the synthesis of sPS with a half-titanocene catalyst (Cp′TiX₃) (2, 3), many efforts were devoted to the development of suitable catalytic systems for the synthesis of sPS (4–8). As the importance of Cp′ ligand on the syndiospecific polymerization of styrene was fully realized, the influence of X ligand was also taken into account. For example, the half-titanocene fluoride exhibited higher activity than the half-titanocene chloride, resulting in an sPS with higher molecular weight (9, 10). A series of reports showed that the complex of the type Cp′TiX₂ L (L=donor ligand, e.g., phenoxy, amido) is a promising catalyst for olefin polymerization (11, 12). The catalyst is also efficient for the syndiospecific polymerization of styrene. It has been concluded that the structure of donor ligand strongly affects not only the catalytic activity but also the molecular weight of the resulting sPS (13–18).

The synthesis of the half-metallocene complex containing a sulfide has been well described in previous researches (19, 20), and the use of this type of compound as a catalyst for ethylene polymerization is patented (21). Herein, we present our design of a series of sulfur-containing half-titanocenes and the preliminary studies on the syndiospecific styrene polymerization.

2 Experimental

2.2 Syntheses

Syntheses of the half-titanocene complexes are illustrated in Scheme 1.

1. Synthesis of complex 1: In a 250-ml three-neck flask equipped with a magnetic stirring bar under a nitrogen atmosphere, 1.80 g of pentamethylcyclopentadienyltitanium trimethoxide (Strem, MA, USA) [Cp*Ti(OCH₃)₃, 6.52 mmol], 50 ml of toluene and 1.30 g of pentafluorobenzenethiol (TCI, Tokyo, Japan) (C₆ F₅ SH, 6.52 mmol) were introduced in this sequence. The reaction mixture was stirred overnight at room temperature. The solvent was removed under reduced pressure. A yellow powder (2.1 g) was obtained with a yield of 72%. Some of this product was dissolved in a small amount of toluene. After freezing, a yellow prism crystallized. ¹H NMR (CDCl₃, 25°C): δ=2.12(15H), 3.96(6H). ¹³C NMR (CDCl₃, 25°C): δ=162, 148, 140, ₁ 37, 105[C₅(CH₃)₅ and C₆ F5], 38[OC_H3), 1₃(C₅(CH₃)₅]. Elemental analysis: calcd. C 48.66, H, 4.76; found C 48.68, H 4.81.

2. Synthesis of complex 2: In a 50-ml Schlenk flask equipped with a magnetic stirring bar under a nitrogen atmosphere, 0.75 g of complex 1 was dissolved in 5 ml of toluene, and 0.7 ml of chlorotrimethylsilane (TCI, Tokyo, Japan) was added to the solution. The reaction mixture was stirred for 3 days at room temperature, and the reaction was allowed to proceed at 70°C for 4 h. The solvent was removed in vacuum, and the residue was dissolved with a mixture of toluene/hexane. After freezing, orange crystals were obtained with a yield of 82%. ¹H NMR (CDCl₃, 25°C): δ=2.28. ¹³C NMR (CDCl₃, 25°C): δ=161, 148, 140, 134, 105[C₅(CH₃)₅ and C₆ F5], 14[C₅(CH₃)₅]_.

3. Synthesis of complex 3: In a 250-ml three-neck flask equipped with a magnetic stirring bar under a nitrogen atmosphere, 1.0 g of pentamethylcyclopentadienyltitanium trimethoxide [Cp*Ti(OCH₃)₃, 3.62 mmol], 50 ml of toluene, and 0.50 g of 2,6-dimethylbenzenethiol (TCI, Tokyo, Japan) (2,6-Me₂-C₆ H₃ SH, 3.62 mmol) were introduced. The reaction mixture was stirred overnight at room temperature. The solvent was removed under reduced pressure. A yellow powder (1.33 g) was obtained with a yield of 96%. ¹H NMR (CDCl₃, 25°C): δ=2.10(15H), 2.31(6H), 3.77(6H), 6.88–7.02(3H).

Scheme 1

Syntheses of the complexes.

All NMR spectra of the complexes are available in the Supplementary Material.

2.3 Polymerization

Styrene polymerization was carried out in a 250-ml glass reactor equipped with a magnetic stirring bar. Calculated amounts of toluene, styrene and MAO solution were charged in the reactor under a nitrogen atmosphere. The mixture was heated to the predetermined temperature in an oil bath. The polymerization reaction started after the catalyst solution was injected. After the needed polymerization time, the reaction was terminated by adding approximately 200 ml of acidified ethanol. The polymer was isolated after the mixture was stirred for 6 h and dried in vacuum at 60°C. The dried polymer was treated in boiling butanone for 2 h to remove the atactic polystyrene produced by MAO. The hot mixture was filtered to isolate the polymer. The polymer was dried in vacuum at 60°C for at least 24 h to obtain sPS.

¹³C NMR experiments for some typical polymer samples were performed on a Bruker AVANCE III 400-MHz spectrometer (Bruker, Germany) using d₂-1,1,2,2,-tetrachloroethane as the solvent at 125°C, δ=43.3, 46.5, 127.6, 129.9, 147.5. The ¹³C NMR spectrum of polymer sample 7 is shown in Figure 1.

Figure 1

¹³C NMR spectrum of sample 7.

The melting temperature and the glass transition temperature of polymer were determined on a TAQ 100 (TA, DE, USA). Approximately 2 mg of polymer sample was heated from room temperature to 300°C at a heating rate of 10°C/min under a nitrogen atmosphere. After keeping the temperature for 1 min, the sample was cooled down to room temperature, and the temperature was again kept for 1 min. Then, the sample was heated to 300°C at a heating rate of 10°C/min and the data were recorded.

The molecular weight was determined by GPC on a Waters Alliance GPCV2000 (Waters, MA, USA) at 150°C with 1,2,4-trichlorobenzene as the eluent. GPC curves of some selected polymer samples are shown in the Supplementary Material.

3 Results and discussion

The complexes were easily synthesized using the procedures shown in Scheme 1, with high yields. The complexes Cp*Ti(C₆ F₅ S)(OCH₃)₂ (1), Cp*Ti(C₆ F₅ S)Cl₂ (2) and Cp*Ti(2,6-Me₂ C₆ H₃ S) (OCH₃)₂ (3) were used as the catalysts combined with MAO for styrene polymerization and the results are collected in Table 1.

All polymerization runs showed high activities for styrene syndiospecific polymerization when complexes 1, 2 and 3 were used as catalysts combined with MAO, resulting in polymer samples with high molecular weight and narrow MWD. The effects of Al/Ti ratio, polymerization temperature and polymerization time on the activities and molecular weight were investigated.

It was found that the Al/Ti ratio highly affected the activities when complexes 1 and 2 were used as the catalysts. When the Al/Ti ratio increased from 500 to 2000, the activity of complex 1 was enhanced almost three times (run 1 vs. 3: from 0.72×10⁵ to 2.04×10⁵ g sPS/mol Ti per hour). Further increase in Al/Ti ratio gave a decreased activity (run 4: 1.74×10⁵ g sPS/mol Ti per hour). Similar results were observed in the polymerization runs with complex 2 (run 8 vs. 9). However, when complex 3 was used as the catalyst, higher activity was observed at lower Al/Ti ratio (run 11 vs. 12). The results seem to be unusual. In studies on the syndiospecific polymerization of styrene by CpTiCl₂ X (X=OR) catalyst, similar results were obtained. The probable reason is that anionic OR participates in the formation of the active center, and under the condition of the low concentration of MAO, the function of the alkoxyl group is much stronger than that of the high MAO concentration (22).

Higher activity was achieved at a higher polymerization temperature for all catalyst systems (complex 1: runs 3 and 7; complex 2: runs 9 and 10; complex 3: runs 12 and 13). The activities with different polymerization times were observed as changing a little for complex 1 (runs 5, 3 and 6), indicating that the catalytic species remain stable under these conditions.

Under the same polymerization conditions, complex 3 exhibited the highest activities regardless of the Al/Ti ratios and polymerization temperatures used. Complex 2 gave the lowest activities under any conditions. The reason for this observation still remains unclear, but it could be concluded that both the donor ligand (complex 1 vs. 3) and the anion ligand (complex 1 vs. 2) affect the polymerization activity.

MWDs of the polymer samples were observed from 1.57 to 2.32, which indicated the nature of the single-site catalyst. High molecular weight of the resultant polymer was obtained at high Al/Ti ratio and low polymerization temperature, which may indicate that β-H elimination is the main transfer reaction rather than the transfer reaction to aluminum. Polymer synthesized by complexes 1 and 3 showed higher molecular weight than that by complex 2, indicating the influence of catalyst structure on the molecular weight. It should be noted that the molecular weight of a polymer sample obtained under the polymerization time of 20 min by complex 1 (run 3) is almost double that of the sample obtained with half polymerization time (run 5). It is a pity that the continuous increase of the molecular weight under a longer polymerization time was not observed (run 6), and all the MWDs were exhibited to be approximately 2.

The melting temperature and the glass-transition temperature of polymer were determined and are listed in Table 1. Complexes 1 and 3 give polymers with similar T_m and T_g; at the same time, both the melting temperature and the glass transition temperature of polymer produced by complex 2 are lower. It may indicate that complexes 1 and 3 have stronger syndiospecifity than complex 2.

4 Conclusion

We have shown for the first time that monocyclopentadienyl titanium complexes containing a sulfide donor ligand are efficient catalysts for the syndiospecific polymerization of styrene. Styrene polymerization catalyzed by these complexes exhibits high activity, affording a syndiotactic product with high molecular weight and narrow MWD. We also find the dependency of molecular weight of polymer on the polymerization time in some experiments; thus we have the interest to explore this in more detail in the future, and we will also focus on ethylene polymerization and copolymerization using this type of catalyst.