Syndiotactic polymerization of styrene and copolymerization with ethylene catalyzed by chiral half-sandwich rare-earth metal dialkyl complexes

The syndiotactic polymerization of styrene (St) and the copolymerization of St with ethylene (E) were carried out by using a series of chiral half-sandwich rare-earth metal dialkyl complexes (Cpx*) as the catalysts. The complexes are Ln(CH2SiMe3)2(THF) (1−4: Ln = Sc (1), Ln = Lu (2), Ln = Y (3), Ln = Dy (4)) bearing chiral cyclopentadienyl ligand containing bulky cylcohexane derivatives in the presence of activator and AliBu3. For the St polymerization, a high activity up to 3.1 × 106 g of polymer molLn−1·h−1 and a high syndiotactic selectivity more than 99% were achieved. The resulting syndiotactic polystyrenes (sPSs) have the molecular weights (Mn) ranging from 3700 g·mol−1 to 6400 g·mol−1 and the molecular weight distributions (Mw/Mn) from 1.40 to 5.03. As for the copolymerization of St and E, the activity was up to 2.4 × 106 g of copolymer molSc−1·h−1·MPa−1, giving random St-E copolymers containing syndiotactic polystyrene sequences with different St content in the range of 15 mol%−58 mol%. These results demonstrate that the bulky cyclopentadienyl ligands of the chiral half-sandwich rare-earth metal complexes effectively inhibit the continued insertion of St monomers into the (co)polymer chain to some extent in comparison with the known half-sandwich rare-earth metal complexes.


INTRODUCTION
The exploitation of highly efficient and regio-/stereoselective homogeneous organometallic catalysts is a perennial subject of interest in the coordinative-insertive polymerization of olefins, which helps to realize effective preparation of high-performance polymers with controlled structures and desired properties [1] . Nowadays, syndiotactic polystyrene (sPS) has drawn considerable attention not only in basic scientific research but also in industrial applications due to its intrinsic properties, such as high melting temperature (ca. 270 °C), high modulus of elasticity, fast crystallization rate, good resistance to heat and chemicals, and low dielectric constant [2−4] . Since Ishihara and co-workers firstly reported the preparation of sPS via the polymerization of styrene (St) by using the half-sandwich titanium complex (Cp * TiCl 3 ) in combination with MAO to achieve high activity up to 3.6 × 10 6 g of polymer mol Ti −1 ·h −1 [5,6] , a lot of titanium complexes have been intensively investigated to afford sPSs with high content of racemic pentads [7−22] . Recently, the rare-earth metal complexes have entered people's vision, which usually exhibit extremely high activity and high syndiotactic selectivity in the polymerization of St similar to the known titanium complexes [23,24] . In 2004, the Carpentier's group and Hou's group independently reported the highly syndiospecific polymerization of St catalyzed by the metallocenelanthanideallyl complexes (Cp-CMe 2 -Flu) Ln(C 3 H 5 )(THF) (Ln = Y, La, Nd and Sm) alone [25] or the half-sandwich scandium dialkyl complex (C 5 Me 4 SiMe 3 )-Sc(CH 2 SiMe 3 ) 2 (THF) in the presence of activator [Ph 3 C][B(C 6 F 5 ) 4 ] [26] . Especially, the latter cationic catalytic system exhibited extremely high activity up to 1.4 × 10 7 g of polymer mol Sc −1 ·h −1 at room temperature, affording the sPSs with high molecular weights and narrow molecular weight distributions (M n = 9 × 10 4 g·mol −1 to 3.8 × 10 5 g/mol, M w /M n = 1.29−1.55). Inspired by these fantastic works, numerous attempts have been made to develop new rare-earth metal complexes for highly syndiospecific polymerization of St. For example, Okuda and co-workers found that the halfsandwich scandium dialkyl complex [C 5 Me 4 SiMe 2 (C 6  with moderate activities [27,28] . In 2007, several kinds of half-sandwich rare earth metal bis(aminobenzyl) complexes bearing η 5 -phospholyl-, η 5 -pyrrolyl-, or η 5 -1,2-azaborolylligand(Cp * )Sc(CH 2 C 5 -H 4 NMe 2 -o) 2 were reported by Hou and co-workers. These complexes showed high activity up to 3.1 × 10 6 g of polymer mol Sc −1 ·h −1 and high syndiospecific selectivity up to 100% for the polymerization of St in the presence of [Ph 3 C][B(C 6 F 5 ) 4 ] [29−31] . In 2009, Chen's group described that the high syndiospecific polymerization of St was catalyzed by using a half-sandwich scandium complex bearing indenyl ligand (Ind)Sc(CH 2 SiMe 3 ) 2 (THF) activated by [Ph 3 C][B(C 6 F 5 ) 4 ] with an extremely high activity of ca. 1.21 × 10 7 g of polymer mol Sc −1 ·h −1 [32] . At the same time, Visseaux and co-workers demonstrated that half-sandwich scandium borohydrido complex (Cp * )Sc(BH 4 ) 2 (THF) promoted the high syndiospecific polymerization of St with moderate activity (2.0 × 10 5 g of polymer mol Sc −1 ·h −1 ) and high syndioselectivity (> 99.9%) in combination with [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i Bu 3 [33] . In 2011, Luo and coworkers represented that a series of half-sandwich rare-earth metal bis(amide) complexes (Cp * )Sc(N(SiMe 3 ) 2 ) 2 (THF) showed high activity up to 3.12 × 10 6 g of polymer mol Sc −1 ·h −1 and high syndiotacticity (rrrr > 99%) in the polymerization of St [34,35] . In 2012, a series of CGC rare-earth metal complexes (Flu-CH 2 -Py)Sc(CH 2 SiMe 3 ) 2 (THF) were reported by Cui and coworkers, which could serve as the high efficient and syndiotactic catalyst precursors for the polymerization of St with an extremely high activity up to 1.6 × 10 7 g of polymer mol Ln −1 ·h −1 and high syndiotactic selectivity (rrrr > 99%) [36,37] . In 2013, we also discovered the syndiotactic polymerization of St catalyzed by a series of fluorenyl-ligated scandium dialkyl complexes (Flu)Sc(CH 2 SiMe 3 ) 2 (THF) in combination with borate and AlR 3 (activity up to 3.4 × 10 7 g of polymer mol Sc −1 ·h −1 , syndiotactic selectivity (rrrr) > 99%) [38] .
In comparison with the known titanium complexes, the advantage of these half-sandwich rare-earth metal complexes is that they can promote the copolymerization of St with ethylene (E), affording the random St-E copolymers containing syndiotactic polystyrene sequences [24,39] . Such St-E copolymer can overcome sPS's drawbacks such as brittleness and poor processing performance due to its high melting temperature, and broaden its industrial applications [40] . Despite these rare-earth metal complexes are available for the syndiotactic polymerization of St, only a few rare-earth metal complexes are active for the copolymerization of St with E until now [41,42] . In 2004, Hou firstly reported the random copolymerization of St and E by using the half-sandwich rare-earth metal complex (C 5 Me 4 SiMe 3 )Sc(CH 2 SiMe 3 ) 2 (THF) under the activation of [Ph 3 C][B(C 6 F 5 ) 4 ], affording the St-E copolymers containing syndiotactic polystyrene sequences with St content in the range of 13 mol%−87 mol% [26] . In 2013, we also described the random copolymerization of St and E by using the fluorenyl-ligated half-sandwich scandium complexes to give the St-E copolymers containing syndiotactic polystyrene sequences with the St content ranging from 17 mol% to 80 mol% [38] . Nevertheless, almost all of these known half-sandwich rare-earth metal complexes contain an achiral cyclopentadienyl derivative. Up to date, the synthesis of rare-earth metal complexes bearing chiral cyclopentadienyl ligands and the application of them in coordinative-insertive polymerization of olefins have rarely been described [43] . In this work, we report the syndiotactic polymerization of St and the copolymerization of St with E by using a series of chiral half-sandwich rare-earth metal complexes bearing a chiral cyclopentadienyl ligand under the activation of borate and Al i Bu 3 . The obtained sPSs show low molecular weight, and the random St-E copolymers have the St content ranging from 15 mol% to 58 mol%.

Materials
All manipulations of air and moisture-sensitive compounds were performed under a dry nitrogen atmosphere by using Schlenk techniques or in an Mbraun glove box filled with nitrogen. Nitrogen and ethylene (purchased from Beijing AP Beifen Gases Industrial Co., Ltd.) were purified by passing through a dry clean column (4Å molecular sieves, Dalian Replete Science And Technology Co., Ltd.) and a gas clean column (Dalian Replete Science And Technology Co., Ltd.). The nitrogen in the glove box was constantly circulated through a copper/molecular sieves catalyst unit. The oxygen and moisture concentrations in the glove box atmosphere were monitored by an O2/H2O Combi-Analyzer (Mbraun) to ensure that both of them were always below 0.0001‰. The Cp x* -H ligand and the chiral half-sandwich rare-earth metal complexes 1−4 were prepared according to the procedures reported in the literatures [43−48] . Anhydrous THF, hexane and toluene were purified by a solvent purification system (SPS-800, Mbraun), and dried over fresh Na chips in the glove box. St was purchased from Sigma-Aldrich, dried over CaH 2 and degassed by two freeze-pump-thaw cycles.

Measurements
Samples of half-sandwich metal complexes for nuclear magnetic resonance (NMR) spectroscopic measurements were prepared in the glove box using J. Young valve NMR tubes. The 1 H-and 13 C-NMR spectra of catalyst precursors were recorded on an AVANCE 400 spectrometer at room temperature with C 6 D 6 as the solvent. The 1 H-and 13 C-NMR spectra of polystyrene and copolymer samples were recorded on an AVANCE 400 spectrometer in 1,1,2,2-C 2 D 2 Cl 4 at 120 °C. Elemental analyses were performed on an Elementary Vario MICRO CUBE (Germany). The molecular https://doi.org/10.1007/s10118-018-2060-8 weights and the molecular weight distributions of polystyrenes were determined at 140 °C by gel permeation chromatography (GPC) on a HLC-8320 apparatus. 1,2,4-Trichloro-benzene was used as an eluent at a flow rate of 0.35 mL·min −1 . The molecular weights and the molecular weight distributions of copolymers were determined at 145 °C by GPC on a PL-GPC 220/HT apparatus (Tosoh Corp.), and o-dichlorobenzene was used as an eluent at a flow rate of 1.0 mL·min −1 . All of the calibrations were made by polystyrene standard EasiCal PS-1 (PL Ltd). Any thermal history difference in the polymers was eliminated by first heating the specimen to 350 °C, cooling to −50 °C at a rate of 10 K·min −1 , and then recording the second differential scanning calorimetry (DSC) scan.     ) were added subsequently at room temperature. A large amount of polystyrene solid was precipitated out after 10 min. The reaction mixture was stirred for 30 min. Then the flask was taken out of the glove box, quenched with methanol (150 mL containing 5% butylhydroxytoluene (BHT)), and filtered. The precipitate was dried under vacuum at 30 °C overnight to a constant weight (410 mg, yield: 75%). The resulting polymer is soluble in 1,1,2,2-tetrachloroethane at 120 °C.

Scheme 1 Synthesis of chiral half-sandwich rare-earth metal dialkyl complexes
Complexes 1-4 have good solubility in common solvents such as hexane, benzene, toluene, and tetrahydrofuran (THF), and give well resolved NMR spectra in standard solvents without the ligand redistribution (see electronic supplementary information, ESI). The single crystal of Lu complex 2 was obtained from hexane/toluene mixed solution at −30 °C. The ORTEP drawing of complex 2 is shown in Fig. 1. Its selected bond lengths and angles are summarized in Table 1. Similar to the Sc and Y complexes 1 and 3 reported by our group before, the Lu complex 2 also contained one cyclopentadienyl ligand, two trimethylsilylmethyl ligands, and one THF molecule. Among the bond distances of five carbon atoms C(1)−C(5) to the metal center, the bond distance of Lu-C(3) is apparently shorter than the bond distances of Lu-C(1) and Lu-C(5), and the bond distances   Table 2.
The metal center of these complexes significantly affected the polymerization activity. Similar to the known half-sandwich rare-earth metal complexes, activated by borate B and Al i Bu 3 , Sc complex 1 exhibited the highest activity (ca. 6.9 × 10 4 g of polymer mol Sc −1 ·h −1 ) for the https://doi.org/10.1007/s10118-018-2060-8   (Table 2, entries 1 and 7). When borane C was used as an activator, only trace amount of polystyrene was obtained even during a long polymerization time (5 h) ( Table 2, entry 6). When the reaction time was prolonged from 2 min to 30 min, the yield slightly increased from 55% to 75%. However, the catalytic activities dramatically decreased from 8.6 × 10 5 g of polymer mol Sc −1 ·h −1 to 7.8 × 10 4 g of polymer mol Sc −1 ·h −1 ( Table 2, entries 7−9), affording the polystyrene with high content of racemic pentads (rrrr > 99%) and low molecular weights (M n = 5200−5900 g·mol −1 ). The amount of Al i Bu 3 also played an important role on the polymerization activity of the complex 1 in the St polymerization ( Solvent fractionation experiment demonstrated that either atactic polystyrene or isotactic polystyrene could be produced. The microstructures of resulting polystyrenes were highly syndiotactic with the racemic pentad configuration rrrr more than 99%, as evidenced in 13 C-NMR spectra with the signals at 145.5 (ipso-C), 44.5 (Sαα), and 41.2 ppm (Tββ) (see ESI). The high melt point values around 270 °C measured by DSC also confirmed the formation of sPSs. GPC profiles indicated that the sPSs obtained by the chiral half-sandwich rare-earth metal complexes 1−4/activator/Al i Bu 3 ternary catalytic systems possessed low molecular weights (M n = 3700− 6400 g·mol −1 ), which were much lower than those obtained https://doi.org/10.1007/s10118-018-2060-8 by the known half-sandwich rare-earth metal complexes. The unimodal GPC curves (M w /M n = 1.40−5.03) also implied the generation of the single-site cationic active species by these catalytic systems in the St polymerization.

Copolymerization of Styrene with Ethylene
The chiral half-sandwich Sc complex 1/[Ph 3 C][B(C 6 F 5 ) 4 ]/ Al i Bu 3 ternary catalytic system was also active for the copolymerization of St with E under 0.1 MPa E pressure in toluene at 25 °C, affording the random St-E copolymers containing syndiotactic polystyrene sequences with different St contents in the range of 15 mol%−58 mol%. Representative results are displayed in Table 3.
Before the copolymerization, the homopolymerization of E was also carried out by using the complex 1/[Ph 3 C] [B(C 6 F 5 ) 4 ]/Al i Bu 3 catalytic system under atmospheric E pressure in toluene at 25 °C (Table 3, entry 1). A moderate activity ca. 8.5 × 10 5 g of copolymer mol Sc −1 ·h −1 ·MPa −1 was obtained, and the resulting polyethylene had moderate molecular weight and bimodal molecular weight distribution (M n = 2.8 × 10 4 g·mol −1 , M w /M n = 21.22). In the copolymerization of St with E under the same conditions, as the [St]/[complex 1] molar ratio increased from 500 to 2000, the catalytic activity gradually increased from 7.0 × 10 5 to 2.35 × 10 6 g of copolymer mol Sc −1 ·h −1 ·MPa −1 ( Table 3, entries 2−5). For the resulting St-E copolymers, the molecular weights gradually decreased from 1.2 × 10 4 g·mol −1 to 1000 g·mol −1 , and the molecular weight distributions decreased from 40.99 to 1.52. However, the St content of  Fig. 2 13 C-NMR spectra of styrene-ethylene copolymers