Synthesis of Ethylene/1-Octene Copolymers with Ultrahigh Molecular Weights by Zr and Hf Complexes Bearing Bidentate NN Ligands with the Camphyl Linker

: Ultrahigh molecular weight polyethylene (UHMWPE) is a class of high-performance engineering plastics, exhibiting a unique set of properties and applications. Although many advances have been achieved in recent years, the synthesis of UHMWPE is still a great challenge. In this contribution, a series of zirconium and hafnium complexes, [2,6-(R 1 ) 2 -4-R 2 -C 6 H 2 -N-C(camphyl)=C(camphyl)- N-2,6-(R 1 ) 2 -4-R 2 -C 6 H 2 ]MMe 2 (THF) ( 1-Zr : R 1 = Me, R 2 = H, M = Zr; 2-Zr : R 1 = Me, R 2 = Me, M = Zr; 1-Hf : R 1 = Me, R 2 = H, M = Hf; 2-Hf : R 1 = Me, R 2 = Me, M = Hf), bearing bidentate NN ligands with the bulky camphyl backbone were synthesized by the stoichiometric reactions of α -diimine ligands with MMe 4 (M = Hf or Zr). All Zr and Hf metal complexes were analyzed using 1 H and 13 C NMR spectroscopy, and the molecular structures of complexes 1-Zr and 1-Hf were determined by single-crystal X-ray diffraction, revealing that the original α -diimine ligand was selectively reduced into the ene-diamido form and generated an 1,3-diaza-2-metallocyclopentene ring in the metal complexes. Zr complexes 1-Zr and 2-Zr showed moderate activity (up to 388 kg(PE) · mol − 1 (M) · h − 1 ), poor copolymerization ability, but unprecedented molecular weight capability toward ethylene/1-octene copolymerization. Therefore, copolymers with ultrahigh molecular weights (>600 or 337 × 10 4 g · mol − 1 ) were successfully synthesized by 1-Zr or 2-Zr , respectively, with the borate cocatalyst [Ph 3 C][B(C 6 F 5 ) 4 ]. Surprisingly, Hf complexes 1-Hf and 2-Hf showed negligible activity under otherwise identical conditions, revealing the great inﬂuence of metal centers on catalytic performances.


Introduction
The compound α-diimine has been employed as an important synthetic ligand in the fields of coordination chemistry and olefin polymerization [1][2][3][4]. In the middle 1990s, Brookhart and co-workers reported their landmark research on cationic α-diimine based Ni and Pd catalysts that builds a new era in the olefin polymerization field [5]. For example, branched polyolefins have been previously produced through copolymerization of ethylene with α-olefins by early transition metal catalysts [6][7][8][9], while Brookhart-type α-diimine based late transition metal catalysts can synthesize such branched polymers using ethylene as the only feed via the unique chain walking mechanism [10,11]. Moreover, Ni and Pd late transition metal catalysts have low oxophilicity, and thus show great potential for copolymerization of olefins with polar comonomers [12,13]. In the last two decades, this class of catalysts has received increasing research interest owing to their versatile structures, easy synthesis, and capability to tune structures of products [14]. Therefore, low molecular weight oligomers [15][16][17], ultrahigh molecular weights polyethylenes (UHMWPE) [18,19], highly branched [20] or linear [21][22][23] polymers are readily prepared by facilely tuning the substituents and backbone of ligands [24,25]. Besides, living polymerization of α-olefins has been also achieved by the delicate design of α-diimine based catalysts [26,27].
Although α-diimine compounds have been intensively investigated as supporting ligands for late transition metal catalysts in the field of olefin polymerization, they are rarely used as ligand precursors for early transition metal catalysts. In 2007, Klosin and coworkers reported imino-amido Zr and Hf tribenzyl complexes, which were synthesized by the reaction of α-diimine ligand with M(CH 2 Ph) 4 (M = Zr or Hf) and showed good activity toward ethylene/1-octene copolymerizations [28]. However, using imino-amido Zr and Hf tribenzyl complexes as catalysts for olefin polymerizations is limited, because they are usually unstable at high temperature and would go through dibenzyl elimination to form ene-diamido complexes, which exhibit relatively lower activity and form polymers with lower molecular weights and broad distributions [29]. Therefore, increasing the thermal stability of α-diimine based metal complexes is highly desired for developing new olefin polymerization catalysts. For late transition metal complexes, the modifications of N-aryl substituents of α-diimine ligands have been successfully employed to improve their thermal stability. Ionkin and coworkers reported that ortho-difurylaryl-substituted Ni α-diimine catalysts displayed unprecedently stability under 150 • C for ethylene polymerization [30]. More recently, Sun, Long, Chen, and others reported the benzhydryl-derived ligands could extremely enhance the thermal stability of α-diimine catalysts [23,[31][32][33][34][35][36]. On the other hand, the modification of the backbone of α-diimine ligand is an alternative to tuning the catalytic properties of α-diimine catalysts [37][38][39]. Gao and Wu and coworkers reported α-diimine Ni complexes with the bulky camphyl backbone, which display excellent thermal stability and produce polymers with high molecular weights [40][41][42]. For early transition metal catalysts, a series of α-diimine derivatives including imine-enamide and amide-quinoline have been used to support Zr and Hf complexes [43][44][45][46][47][48][49], but direct modifications of N-aryl substituents or backbones of α-diimine ligands are undeveloped. Inspired by these studies, we report herein the synthesis of Zr and Hf complexes from α-diimine ligands with the bulky camphyl backbone. Surprisingly, ene-diamido Zr and Hf complexes were selectively obtained as indicated by the analysis of NMR spectroscopy and X-ray diffraction. Copolymerization of ethylene and 1-octene with these metal complexes have been investigated, and the preliminary results show that Zr complexes have moderate activity (up to 388 kg(PE)·mol −1 (M)·h −1 ), poor copolymerization ability (less than 1 mol%), but unprecedented molecular weight capability (>600 × 10 4 g·mol −1 ) toward ethylene/1-octene copolymerization, while Hf complexes show negligible activity under otherwise identical conditions. Note that Ultrahigh molecular weight polyethylenes (UHMWPE) have a unique set of properties such as high chemical resistance, outstanding mechanical property, good corrosion stability, excellent abrasive wear resistance, and thus are extensively used as a class of high-performance engineering plastics [50,51].

Results and Discusssion
The α-diimine ligands 1 and 2 with the camphyl linker were prepared according to literature procedures [42]. According to previous report, imino-amido tribenzyl Zr complexes were readily prepared through the reaction of α-diimine ligand with ZrBn 4 via migratory insertion of one benzyl group into one C=N bond [48]. In this study, ZrCl 4 reacted with 4 equivalents of MeMgBr at low temperature (about −40 • C) in toluene to give ZrMe 4 , which was further used in situ to react with α-diimine ligands to form methyl Zr complexes (Scheme 1) [43,52]. In contrast to previous report [28], the desired imino-amido Zr complexes did not formed and ene-diamido complexes 1-Zr and 2-Zr were exclusively obtained. Using the similar method, the corresponding Hf complexes 1-Hf and 2-Hf were also prepared as yellow solids. The NMR spectroscopic data of the Zr and Hf complexes (Figures S1-S8) are consistent with the presence of distinct ene-diamido structure. For example, the 13 C NMR signals of the C atoms of the camphyl backbone in 1-Zr appear in the olefinic range, 119.08 and 117.44 ppm ( Figure S2), respectively.
In both 1 H and 13 C NMR spectra, each pair of substituents on the N atoms exhibits two characteristic sets of resonances, suggesting a C 1 -symmetric structure due to the unsymmetric camphyl backbone. The broad signals at 3.53 and 1.08 ppm ( Figure S1), whose integrations correspond to four protons, indicate the presence of one coordinated THF molecule [53].
(Figures S1-S8) are consistent with the presence of distinct ene-diamido structure. For example, the 13 C NMR signals of the C atoms of the camphyl backbone in 1-Zr appear in the olefinic range, 119.08 and 117.44 ppm ( Figure S2), respectively. In both 1 H and 13 C NMR spectra, each pair of substituents on the N atoms exhibits two characteristic sets of resonances, suggesting a C1-symmetric structure due to the unsymmetric camphyl backbone. The broad signals at 3.53 and 1.08 ppm ( Figure S1), whose integrations correspond to four protons, indicate the presence of one coordinated THF molecule [53].
Complexes 1-Zr, 2-Zr, 1-Hf and 2-Hf were preliminarily evaluated as catalysts for ethylene/1-octene copolymerization at 120 • C under 3 MPa of ethylene pressure, and the data are presented in Table 1. According to previous report, the ene-diamido complexes usually showed a lower activity in comparison to that of the imino-amido complexes [28].
In the presence of [Ph 3 C][B(C 6 F 5 ) 4 ] as cocatalyst, 1-Zr and 2-Zr exhibited moderate activity toward ethylene/1-octene copolymerization ( Table 1, Runs 1 and 2). It is worth noting that subtle changes in ligand resulted in 1-Zr and 2-Zr showing quite different activity and polymer molecular weight capability. 2-Zr, with one more methyl on the paraposition of the aryl group, exhibited a higher activity (388 kg(PE)·mol −1 (M)·h −1 ) than 1-Zr (236 kg(PE)·mol −1 (M)·h −1 ). In contrast to previous reports that ene-diamido complexes usually formed polymers with low molecular weights and broad distributions [28,29], 1-Zr and 2-Zr in this study displayed remarkable molecular weight capability and produced ultrahigh molecular weight polyethylene (UHMWPE). A polymer sample with molecular weight as high as 337 × 10 4 g·mol −1 was synthesized by 2-Zr/[Ph 3 C][B(C 6 F 5 ) 4 ] ( Figure S9), which is much higher than previous reports [28,29,[46][47][48][49]. The polymer prepared by 1-Zr/[Ph 3 C][B(C 6 F 5 ) 4 ] possessed an even higher molecular weight, which should be over 600 × 10 4 g·mol −1 but could not be accurately calibrated because of the limitation of GPC equipment. Molecular weights determined through universal calibration relative to polystyrene standards in our hand are only effective in the range of 10 3 g·mol −1 to 600 × 10 4 g·mol −1 . On the other hand, the copolymerization ability of 1-Zr and 2-Zr were poor in comparison to previously reported imino-amido analogues [28], and less than 1 mol% of 1-octene were incorporated in this study (Table 1, Runs 1 and 2; Figures S10 and S11). Hf complexes were also tested under otherwise identical conditions, and it is surprising that only trace polymers were obtained by 1-Hf and 2-Hf. These results suggest that the catalytic properties highly depend on both the structure of ligands and the nature of the metal centers.  Complexes 1-Zr, 2-Zr, 1-Hf and 2-Hf were preliminarily evaluated as catalysts for ethylene/1-octene copolymerization at 120 °C under 3 MPa of ethylene pressure, and the data are presented in Table 1. According to previous report, the ene-diamido complexes usually showed a lower activity in comparison to that of the imino-amido complexes [28].   Complexes 1-Zr, 2-Zr, 1-Hf and 2-Hf were preliminarily evaluated as catalysts for ethylene/1-octene copolymerization at 120 °C under 3 MPa of ethylene pressure, and the data are presented in Table 1. According to previous report, the ene-diamido complexes usually showed a lower activity in comparison to that of the imino-amido complexes [28].

Synthesis of 1-Zr
ZrCl 4 (0.256 g, 1.1 mmol) was suspended in anhydrous toluene (5 mL), and the MeMgCl solution (1.6 mL, 4.8 mmol, 3 M in THF) was added slowly using a syringe at −40 • C. The resultant mixture was stirred at −40 • C for 1 h, and then ligand 1 (0.372 g, l.0 mmol) in toluene (8 mL) was added at −40 • C. The resultant suspension was stirred under −40 • C for 2 h, and at ambient temperature for another 3 h that formed a dark-brown mixture. The volatiles were removed under reduced pressure to give an oil residue that was extracted with n-hexane for 3 times (30, 20, 10 mL). The yellow solid 1-Zr was obtained from the concentrated hexane solution with a yield of 64% (0.36 g, 0.64 mmol). 1 13

Synthesis of 1-Hf
HfCl 4 (0.352 g, 1.1 mmol) was suspended in anhydrous toluene (5 mL), the MeMgCl solution (1.6 mL, 4.8 mmol, 3 M in THF) was added slowly using a syringe at −40 • C. The resultant mixture was stirred at −40 • C for 1 h, and then ligand 1 (0.372 g, 1.0 mmol) in toluene (8 mL) was added at −40 • C. The resultant suspension was stirred at −40 • C for 2 h, and then at ambient temperature for another 3 h that gives a dark-brown mixture. The volatiles were removed under reduced pressure to give a residue that was extracted with n-hexane for 3 times (30 mL, 20 mL, 10 mL). The product 1-Hf as a yellow solid was obtained from the concentrated hexane solution with a yield of 69% (0.45 g, 0.69 mmol). 1

Conclusions
The reactions of α-diimine ligands having the bulky camphyl backbone with MMe 4 (M = Hf or Zr) led to the selective formation of ene-diamido Zr and Hf complexes. The ene-diamido structure has been characterized and determined by 1 H and 13 C NMR spectroscopy, and single-crystal X-ray diffraction. Zr complexes 1-Zr and 2-Zr showed moderate activity (236 and 388 kg (PE)·mol −1 (M)·h −1 , respectively) toward ethylene/1-octene copolymerization at high temperature (120 • C) and high pressure (3 MPa of ethylene) and produced copolymers with ultrahigh molecular weights. Under otherwise identical conditions, Hf complexes 1-Hf and 2-Hf exhibited only negligible activity. This study reveals that the catalytic properties, including activity, molecular weight capability and copolymerization ability, highly depend on the structure of α-diimine ligands and the nature of the metal centers.