Substituent Effects of Phenyl Group on Silylene Bridge in Stereospecific Polymerization of Propylene with C1-Symmetric Ansa-Silylene(fluorenyl)(amido) Dimethyl Titanium Complexes

A C1-symmetric (methylphenyl)silylene-bridged (fluorenyl)(naphthylamido) titanium complex (1) and (diphenyl)silylene-bridged (fluorenyl)(naphthylamido) titanium complex (2) were synthesized and characterized by 1H NMR, element analysis, and X-ray crystal analysis. The coordination mode of the fluorenyl ligand to the titanium metal is an η3 manner in each complex. These complexes were applied for propylene polymerization using dried modified methyaluminoxane (dMMAO) as a cocatalyst under different propylene pressures in a semi batch-type method. The catalytic activity was strongly dependent on the structure of the complex and the propylene pressure, where complex 1 exhibited the highest activity (600 kg mol−1·h−1) under a propylene pressure of 8.0 atm to produce high molecular weight polypropylene. The polypropylenes obtained were syndiotactic-rich with an rr value of 0.50, indicating that the silylene bridge was not efficient for the isospecificity of a constrained geometry catalyst (CGC). The mechanical properties of the resulting polymers depended on their microstructure.

propylene [35]. We supposed that two diastereotopically distinct sites, which are necessary for high isospecificity, were not provided in these C1-symmetric complexes by the fast rotation of the amido ligand. In this study, we thus synthesized (methylphenyl)silylene or (diphenyl)silylene-bridged C1-symmetric fluorenyl-naphthylamido dimethyltitanium complexes (1 and 2, Scheme 1) to investigate the substituent effects on the silylene bridge in the stereospecific polymerization of propylene. Scheme 1. Chemical diagrams of mono-metallocene group 4 complexes.

Materials
All operations were performed using standard Schlenk techniques. All solvents were distilled or purified by the PS-MD-5 (Innovative Technology (China) Ltd., HongKong, China) solvent purification system. Research-grade propylene was purified by a dehydration column of ZHD-20 and deoxidation column of ZHD-20A. Modified methylaluminoxane (MMAO) solution in toluene was donated by Tosoh-Finechem Co. (Shunan, Japan) Trialkylaluminium-free dried modified methylaluminoxane (dMMAO) was prepared from the toluene solution of MMAO by vacuum-drying followed by washing with hexane, as reported previously [32]. All the other reagents were purchased and used as received.

Synthesis of Titanium Complexes
2.2.1. Synthesis of (Methylphenyl)silylene-Bridged Ligand L1 Step A. n-BuLi (2.5 M in hexane 2.8 mL, 7 mmol) was added dropwise at 0 °C into a solution of 1.3 g (7 mmol) of (dimethyl)(naphthyl)amine in 50 mL of diethylether, and the reaction mixture was stirred for 3 h at r.t. The substituent effects on the amido ligand were also investigated. C 1 -symmetric complexes (D, Scheme 1) bearing sterically bulkier naphthyl amido ligand yielded isospecific-rich polypropylene (PP) with an isotactic triad (mm) of 0.61 [33,34]. In contrast to C s -symmetric CGC complexes, fluorenyl-ligated C 1 -symmetric complexes (E and F, Scheme 1) produced statistically atactic PP, indicating the different substituent effect of the fluorenyl ligand in the isospecific polymerization of propylene [35]. We supposed that two diastereotopically distinct sites, which are necessary for high isospecificity, were not provided in these C 1 -symmetric complexes by the fast rotation of the amido ligand. In this study, we thus synthesized (methylphenyl)silylene or (diphenyl)silylene-bridged C 1 -symmetric fluorenyl-naphthylamido dimethyltitanium complexes (1 and 2, Scheme 1) to investigate the substituent effects on the silylene bridge in the stereospecific polymerization of propylene.

Materials
All operations were performed using standard Schlenk techniques. All solvents were distilled or purified by the PS-MD-5 (Innovative Technology (China) Ltd., HongKong, China) solvent purification system. Research-grade propylene was purified by a dehydration column of ZHD-20 and deoxidation column of ZHD-20A. Modified methylaluminoxane (MMAO) solution in toluene was donated by Tosoh-Finechem Co. (Shunan, Japan) Trialkylaluminium-free dried modified methylaluminoxane (dMMAO) was prepared from the toluene solution of MMAO by vacuum-drying followed by washing with hexane, as reported previously [32]. All the other reagents were purchased and used as received.

Synthesis of (Methylphenyl)silylene-Bridged Ligand L1
Step A. n-BuLi (2.5 M in hexane 2.8 mL, 7 mmol) was added dropwise at 0 • C into a solution of 1.3 g (7 mmol) of (dimethyl)(naphthyl)amine in 50 mL of diethylether, and the reaction mixture was stirred for 3 h at r.t.
Step B. The product obtained in step A was added at r.t. to a solution of 3.0 g (7 mmol) of (2,7-di-tBuFlu)(MePh)SiCl in 60 mL of diethylether, and the resultant yellow suspension was stirred overnight at r.t. The solvent was removed in vacuo, and the residue was extracted with hexane. The removal of the hexane gave (2,7-di-tBuFlu)(MePh)Si-naphthylamido as an orange solid in an 82.6% yield (3.4 g). 1  MeLi (1.6 M in ether, 15 mL, 24 mmol) was added dropwise at 0 • C into a solution of ansa-(MePh)silylene-(2,7-di-tBu)fluorenyl-naphthylamido ligand (3.49 g, 6 mmol) in 60 mL of diethylether. The resultant orange solution was stirred at r.t. for 4 h, then was added to a solution of TiCl 4 (0.66 mL, 6 mmol) in 30 mL hexane at room temperature in the stirring condition for 2 h. The solvent was removed, and the residue was extracted with hexane (150 mL). The hexane solution was concentrated to 50 mL and cooled at −30 • C for several days to get complex 1 as red crystals (1.58 g, 40.1%). 1

Polymerization Procedure
The atmospheric pressure polymerization of propylene was performed in a 100 mL glass reactor equipped with a magnetic stirrer, and the high-pressure polymerization of propylene was carried out in a Parr Instrument Company autoclave (Moline, IL, USA). The reactor was vacuumed and charged with nitrogen three times before the polymerization procedure. Then, the reactor was charged with quantitative chlorobenzene (30 mL) and dMMAO solution under nitrogen, and polymerization was initiated by introducing the catalyst solution at the desired temperature and desired pressure. The polymerization was conducted for a specified time and terminated with acidic alcohol. The polymers obtained were washed by alcohol to remove dMMAO and ligand residue, and dried under vacuum at 80 • C for 6 h to a constant weight.

Analytical Procedure
A single crystal each from complexes 1 and 2 was mounted under nitrogen atmosphere at low temperature. The data collection was made on a Bruker APEX2 diffractometer (Karlsruhe, Germany) using graphite monochromated with Mo Ka radiation (λ = 0.71073 Å). The SMART program package was used to determine the unit cell parameters. The absorption correction was applied using the SADABS program [36]. All structures were solved by direct methods and refined on F 2 by full-matrix least-squares techniques with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were placed at calculated positions and were included in the structure calculation. Calculations were carried out using the SHELXS-97, SHELXL-2014, or Olex2 program [37][38][39][40][41][42]. Crystallographic data are summarized in Table 1, and CIF files are provided in the Supplementary Materials. However, disorder modeling of the butyl group for complex 1 was observed, and the ratio of the occupancies was fixed at 60:40; it was not refined by means of free variable for any reason. Molecular weight and molecular weight distribution of the polymers were determined by a polymer laboratory PL GPC-220 (Santa Clara, CA, USA) with one guard column (PL# 1110-1120) and two 30 cm columns (PLgel 10 µm MIXED-B 7.5 × 300 mm). Polymer characterization was carried out at 150 • C using 1,2,4-trichlorobenzene as the eluent and calibrated by polystyrene standards. All 13 C NMR spectra were recorded on a Bruker-600 spectrometer (Karlsruhe, Germany) at ambient temperature unless otherwise indicated. The chemical shifts of the 13 C HMR spectra are referenced to the carbon resonance of 1,1,2,2-tetrachloroethane-d2 (δ: 74.47). DSC analyses were performed on a TA differential scanning calorimeter Q2000 (New Castle, DE, USA) and the DSC curves of the samples were recorded under nitrogen atmosphere at a heating rate of 10 • C/min from −30 to 220 • C.

Molecular Structure of Complexes
The (methylphenyl)silylene-bridged complex 1 and (diphenyl)silylene-bridged complex 2 were synthesized using a one-pot reaction of the corresponding ligand with a 2-fold excess of MeLi and TiCl 4 in hexane. As compared to the 1 H NMR spectrum of complex F reported previously, complexes 1 and 2 showed that the methyl groups bonded to Si and Ti atoms are non-equivalent, indicating that the phenyl substituent on the silylene bridge should affect the rotation of the naphthyl amido ligand to keep the C 1 -symmetric nature in solution. The molecular structures of 1 and 2, as determined by single crystal X-ray analysis, are shown in Figure 1, and the selected bond lengths and angles of complexes are shown in Table 2.

Molecular Structure of Complexes
The (methylphenyl)silylene-bridged complex 1 and (diphenyl)silylene-bridged complex 2 were synthesized using a one-pot reaction of the corresponding ligand with a 2-fold excess of MeLi and TiCl4 in hexane. As compared to the 1 H NMR spectrum of complex F reported previously, complexes 1 and 2 showed that the methyl groups bonded to Si and Ti atoms are non-equivalent, indicating that the phenyl substituent on the silylene bridge should affect the rotation of the naphthyl amido ligand to keep the C1-symmetric nature in solution. The molecular structures of 1 and 2, as determined by single crystal X-ray analysis, are shown in Figure 1, and the selected bond lengths and angles of complexes are shown in Table 2.   93.88 (13) 93.32 (7) 94.27 (11) In complexes 1 and 2, the bond lengths between the titanium and fluorenyl carbons C(1), C(2), and C (5) (7) 100.10(10) N(1)-Si(1)-C (1) 93.88 (13) 93.32 (7) 94.27 (11) In complexes 1 and 2, the bond lengths between the titanium and fluorenyl carbons C(1), C(2), and C (5)  coordinates to the titanium metal with an η 5 -form [43]. Longer bond lengths (2.525-2.638) between the titanium and the fluorenyl carbons C(3) and C(4) were observed. The results indicate that the fluorenyl ligand is coordinating to the titanium metal with an η 3 -to η 1 -form irrespective of the structure of the substituent. In each molecular structure, the lack of a symmetric plane or axis also confirmed the C 1 -symmetric nature of 1 and 2 in the solid state.

Propylene Polymerization
Propylene polymerization was conducted using complexes 1 and 2 activated by dMMAO using a semi-batch method under different propylene pressures at 20 • C. The results are summarized in Table 3. The activity strongly depended on the silylene-bridged substituent and propylene pressure. In contrast to the results obtained with complex F, which contains a dimethyl silylene bridge, under an atmosphere pressure of propylene, complexes 1 and 2 were inactive for propylene polymerization, because of the steric hindrance effect of the phenyl group on the silylene bridge (entries 1 and 2, Table 3). The increase of the propylene pressure significantly improved the catalytic activities of complexes (up to 600 kg mol −1 h −1 ), affording high molecular weight polypropylene (M n value up to 500 kg mol −1 ). Diphenyl silylene-bridged complex 2 exhibited much lower activity than that of (methylphenyl)silylene-bridged complex 1, also indicating that the sterically bulkier phenyl substituent prevents the rotation of the naphthyl amido ligand, resulting in steric hindrance on the metal center.  To investigate the structure-stereospecificity relationships of propylene polymerization using complexes 1 and 2, the microstructures of polypropylene were measured by 13 C NMR analysis. The 13 C NMR spectra of the methyl region of polypropylene obtained with F, 1, and 2 under a propylene pressure of 8.0 atm are shown in Figure 2, with the steric triad distributions in the main chain. As compared to the statistically atactic PP obtained with F, unexpected results were observed: 1 and 2 produced syndiotactic-rich polypropylene rather than isotactic PP, with rr values of 0.55 and 0.51, respectively, although complexes 1 and 2 display a C 1 -symmetric nature, as described by 1 H NMR and X-ray crystal analyses. Consequently, all polymers obtained were amorphous, with the similar T g value of −4 • C, regardless of the structure of the complex used.  Table 3.

Mechanical Properties of Polymers
To investigate the effect of the microstructure on the mechanical properties of the polypropylenes, tensile strength was measured for the polypropylene samples obtained with F, 1, and 2 under a propylene pressure of 8.0 atm. The strain-stress curves are shown in Figure 3. The polymers obtained with 1 and 2 exhibited tensile strength values of 2.4 MPa and 1.6 MPa and elongation at break values of 1500-2800%, respectively. The elastic properties of the polymer obtained with 1, which showed the highest tensile strength value, was then investigated. The sample was cyclically loaded and unloaded 10 times to 300% strain ( Figure 4). The sample displayed better elastomeric properties after the first stress-strain cycle, with a certain amount of unrecovered strain due to a permanent structural change. For comparison, the polymer generated by complex F showed a very low tensile strength value (0.86 MPa) without elastic properties (elongation at break value of 5000%, instrument detection limit). The results indicate that the mechanical properties of the polypropylene samples are affected by their microstructures, since they showed almost the same molecular weight.  Table 3.  Table 3.

Mechanical Properties of Polymers
To investigate the effect of the microstructure on the mechanical properties of the polypropylenes, tensile strength was measured for the polypropylene samples obtained with F, 1, and 2 under a propylene pressure of 8.0 atm. The strain-stress curves are shown in Figure 3. The polymers obtained with 1 and 2 exhibited tensile strength values of 2.4 MPa and 1.6 MPa and elongation at break values of 1500-2800%, respectively. The elastic properties of the polymer obtained with 1, which showed the highest tensile strength value, was then investigated. The sample was cyclically loaded and unloaded 10 times to 300% strain ( Figure 4). The sample displayed better elastomeric properties after the first stress-strain cycle, with a certain amount of unrecovered strain due to a permanent structural change. For comparison, the polymer generated by complex F showed a very low tensile strength value (0.86 MPa) without elastic properties (elongation at break value of 5000%, instrument detection limit). The results indicate that the mechanical properties of the polypropylene samples are affected by their microstructures, since they showed almost the same molecular weight.  Table 3.

Mechanical Properties of Polymers
To investigate the effect of the microstructure on the mechanical properties of the polypropylenes, tensile strength was measured for the polypropylene samples obtained with F, 1, and 2 under a propylene pressure of 8.0 atm. The strain-stress curves are shown in Figure 3. The polymers obtained with 1 and 2 exhibited tensile strength values of 2.4 MPa and 1.6 MPa and elongation at break values of 1500-2800%, respectively. The elastic properties of the polymer obtained with 1, which showed the highest tensile strength value, was then investigated. The sample was cyclically loaded and unloaded 10 times to 300% strain ( Figure 4). The sample displayed better elastomeric properties after the first stress-strain cycle, with a certain amount of unrecovered strain due to a permanent structural change. For comparison, the polymer generated by complex F showed a very low tensile strength value (0.86 MPa) without elastic properties (elongation at break value of 5000%, instrument detection limit). The results indicate that the mechanical properties of the polypropylene samples are affected by their microstructures, since they showed almost the same molecular weight.  Table 3.  Table 3.   Table 3).

Conclusions
In conclusion, two novel ansa-silylene(fluorenyl)(amido) dimethyl titanium complexes 1 and 2 with a phenyl substituent on the silylene bridge were synthesized and characterized by 1 H NMR, elemental analysis, and X-ray crystal analysis. The complexes displayed a C1-symmetric nature in both solution and solid state, as can be determined by the 1 H NMR and X-ray analyses, respectively. Complex 1 containing the (methylphenyl)silylene bridge showed higher activity than (diphenyl)silylene-bridged complex 2 in propylene polymerization. The increase of propylene pressure was effective for enhancing polymerization activity (up to 600 kg mol −1 h −1 ) to produce high molecular weight polypropylene. The modification of the silylene bridge was not efficient for the isospecificity of a CGC catalyst, affording syndiotactic-rich polypropylene with an rr value of 0.50. The microstructure of polypropylene influenced the mechanical properties of the resulting polymers.
Author Contributions: All authors tried their best to contribute effectively to perform and analyze this experimental work. They all participated to the writing of the present manuscript. H.W. performed the overall experimental work. Y.L. participated in analysis of structural data. The settings up of the experimental protocols as well as the interpretation of the obtained results were performed under the supervision of Z.C.