Unifying Molecular Weights of Highly Linear Polyethylene Waxes through Unsymmetrical 2,4-Bis(imino)pyridylchromium Chlorides

By dealing CrCl3∙3THF with the corresponding ligands (L1–L5), an array of fluoro-substituted chromium (III) chlorides (Cr1–Cr5) bearing 2-[1-(2,4-dibenzhydryl-6-fluoro- phenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine (aryl = 2,6-Me2Ph Cr1, 2,6-Et2Ph Cr2, 2,6-iPr2Ph Cr3, 2,4,6-Me3Ph Cr4, 2,6-Et2-4-MePh Cr5) was synthesized in good yield and validated via Fourier Transform Infrared (FT-IR) spectroscopy and elemental analysis. Besides the routine characterizations, the single-crystal X-ray diffraction study revealed the solid-state structures of complexes Cr2 and Cr4 as the distorted-octahedral geometry around the chromium center. Activated by either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the chromium catalysts exhibited high activities toward ethylene polymerization with the MMAO-promoted polymerizations far more productive than with MAO (20.14 × 106 g (PE) mol−1 (Cr) h−1 vs. 10.03 × 106 g (PE) mol−1 (Cr) h−1). In both cases, the resultant polyethylenes were found as highly linear polyethylene waxes with low molecular weights around 1–2 kg mol−1 and narrow molecular weight distribution (MWD range: 1.68–2.25). In general, both the catalytic performance of the ortho-fluorinated chromium complexes and polymer properties have been the subject of a detailed investigation and proved to be highly dependent on the polymerization reaction parameters (including cocatalyst type and amount, reaction temperature, ethylene pressure and run time).


X-ray Crystallographic Studies
Single-crystals of the complexes Cr2 and Cr4 suitable for the X-ray determination were individually grown by the slow diffusion of n-heptane into their respective dichloromethane solutions. The Oak Ridge Thermal Ellipsoid Plot (ORTEP) diagrams of Cr2 and Cr4 are presented in Figure 1, respectively while the selected bond lengths and angles of Cr2 and Cr4 are listed in Table 1. These two complexes have similar coordination geometry hence they will be discussed together. Complexes Cr2 and Cr4 were mononuclear species in which chromium center coordinated with three chloride atoms forming a six-coordinate geometry described as a distorted-octahedral geometry. These three chloride ligands were disposed in a mer arrangement. The Cl1, N1, N2, and N3 atoms constituted the equatorial plane while two axial bonds nearly formed a linear through the chromium center [Cl(1)-Cr(1)-Cl(3), 91.04(8) • for Cr2 and 93.74(4) • for Cr4, respectively]. The bond length of Cr-N pyridine (Cr1-N1, 2.003(5) Å for Cr2 and 1.985(3) Å for Cr4) was evidently shorter than that of corresponding Cr-N imino (Cr1-N2, 2.157(5) Å and Cr1-N3, 2.133(5) Å for Cr2; Cr1-N2, 2.132(3) Å and Cr1-N3, 2.136(3) Å for Cr4) highlighting that a stronger bond between the pyridine donor and the metal center was present than that between the imine-nitrogen and the metal center. Compared to the ortho-chloro substituted chromium (III) complexes, in this work the Cr-N pyridine bond length was longer [2.003(5) Å vs. 1.994(2) Å for Cr2 in both cases] while Cr-Cl and imine bond lengths were generally shorter due to the halogen effects [64]. Furthermore, the obvious deviation of bond lengths of the two Cr-N imino bonds (Cr1-N2 and Cr1-N3) in Cr2 and Cr4 was mainly due to the unsymmetrical framework consistent with the observations of their analogs [49][50][51][52][53][62][63][64]. The bond lengths of the two imine nitrogen atoms in Cr4 were also distinct (1.297(5) for C2-N2 and 1.2817(6) for C8-N3), potentially due to different steric properties of N-aryl groups [62,63].

Ethylene Polymerization
To identify a suitable polymerization condition that can be used to evaluate all the five chromium pre-catalysts (Cr1-Cr5) for the polymerization of ethylene, Cr4 was chosen as the test precatalyst in the first instance to allow an optimization of various catalytic parameters. Based on previous studies of structurally related N,N,N-bound chromium(III) complexes [49][50][51][52][53]64], methylaluminoxane (MAO) and modified methylaluminoxane (MMAO) have proved the most effective co-catalysts to promote ethylene polymerization. Hence, these two cocatalysts were employed to activate the chromium precatalysts and the optimum reaction conditions including run temperature, Al/Cr molar ratio, reaction time and ethylene pressure were separately ascertained for the catalytic system composed of Cr4/MAO and Cr4/MMAO.

Catalytic Evaluation of Cr4/MAO Catalytic System
With the reaction temperature fixed at 30 • C, variation of the molar ratio Al/Cr from 2000 to 4000 was investigated (entries 1-7, Table 2). As the Al/Cr molar ratio was increased, the catalytic activity reached a maximum of 5.46 × 10 6 g (PE) mol −1 (Cr) h −1 at the Al/Cr ratio of 3500 (entry 5, Table 2). The Gel Permeation Chromatography (GPC) data generally revealed a narrow and unimodal polydispersity (M w /M n range = 1.46-1.89) for the polyethylene formed ( Figure 2a); moreover, there was no clear effects shown by the amount of co-catalyst on the molecular weight with very similar values observed across the ratio range (M w = 1.04-1.22 kg mol −1 ). Interestingly, the Melting temperature (T m ) value of the resultant polymers followed a similar trend with their corresponding molecular weights (Table 2), a finding that indicates the highly linear properties of the obtained polymers [64].
On varying the polymerization temperature from 30 to 70 • C with the Al/Cr molar ratio fixed at 3500 and the reaction time for 30 min (entries 5, 8-11, Table 2), a peak in catalytic activity was achieved of 10.03 × 10 6 g (PE) mol −1 (Cr) h −1 at 60 • C. Further raising the temperature to 70 • C led to a rapid decrease in its activity from 10.03 × 10 6 g (PE) mol −1 (Cr) h −1 to 6.24 × 10 6 g (PE) mol −1 (Cr) h −1 (Figure 2b), which can be ascribed to the partial deactivation of the active species resulting from increased chain transfer to aluminum at the higher temperature [65][66][67][68][69] and the lower solubility of ethylene in toluene at elevated temperatures [70][71][72]. Although the molecular weights obtained at different temperatures have little difference, the highest molecular weight (1.61 kg mol −1 ) obtained at the optimum temperature 60 • C somehow reflects more probability of chain propagation at this temperature [57]. To investigate the lifetime of the active species in the Cr4/MAO system, the catalytic screens were conducted over time (5,15,30,45 and 60 min) with the reaction temperature maintained at 60 • C and the Al/Cr molar ratio of 3500 (entries 10 and 12-15, Table 2). The highest activity of 10.03 × 10 6 g (PE) mol −1 (Cr) h −1 was observed at the 30 min mark (entry 10, Table 2) while this activity value was similar with that obtained at 5 and 15 min indicating no obvious induction period needed to generate the active species. Then the catalytic activity gradually decreased with the reaction time extension reaching its lowest value of 5.27 × 10 6 g (PE) mol −1 (Cr) h −1 at 60 min (entry 15, Table 2) suggesting that the active species formed slowly after the addition of MAO and underwent progressive deactivation over time [49][50][51][52][53][62][63][64]. While the obtained polyethylene had increased molecular weight over time illustrating that there were sufficient active species present to maintain chain propagation despite gradual deactivation ( Figure 3a) [49][50][51][52][53]. With other reaction parameters maintained at the optimum values, on lowering the ethylene pressure from 10 to 5 atm the catalytic activity has more than halved (entry 16 vs. entry 10, Table 2). Further reducing the ethylene pressure to 1 atm, only a trace amount of polymer was obtained (entry 17, Table 2), which is in accord with the previous observations for structurally related chromium pre-catalysts [62][63][64]. These results manifest that high pressure of ethylene is necessary to achieve satisfactory activities consistent with the direct correlation between catalytic activity and ethylene concentration [54][55][56][57][58][59][60][61][62][63][64].  Table 2); (b) GPC curves of the polyethylene formed using Cr4/MAO at different temperatures with the Al/Cr molar ratio fixed at 3500 (entries 5 and 8-11, Table 2).  Table 2); (b) Comparative activity of Cr1-Cr5 and M w of the corresponding polymers (Table 3). With an aim to investigate the influence of structural variations made to the chromium precatalysts on catalyst performance and polymer properties, the remaining four pre-catalysts were investigated for ethylene polymerization under the optimal conditions (Al/Cr = 3500, run temperature = 60 • C, run time = 30 min) established by Cr4/MAO (entry 10, Table 2). The activity of these catalysts (Cr1-Cr5) and molecular weight of the obtained polymers were described in Figure 3b. High activities in the range of (6.16-10.03) × 10 6 g (PE) mol −1 (Cr) h −1 were observed with the following order: Table 3), which indicates the catalytic activity closely related with both steric and electronic effects imparted by the second N-aryl imine group. Cr3 containing bulky 2,6-diisopropyl-imine groups was found to exhibit lower catalytic activity as the crowded space around the chromium center led to lower ethylene coordination and insertion rates [57,62,63,73]. The electronic effect was reflected in the presence of an electron-donating para-methyl substituent of Cr4 and Cr5, which was beneficial to the improvement of catalytic activity when compared with para-hydrogen substituted Cr1 and Cr2, respectively [63]. By comparison with polymerization data recorded for ortho-chloro-substituted chromium complexes, all the chromium complexes in this work generally showed higher catalytic activity producing polyethylene with lower molecular weight [64].
To investigate the microstructural properties of the polyethylenes generated using Cr4/MAO, both DSC and high temperature 1 H and 13 C NMR spectroscopic measurements were employed. T m values of the resultant polymers exceeding 120 • C indicates the obtained high-density polyethylene possessed highly linear structures (Tables 2 and 3). For further confirming this speculation, a representative sample with the highest yield obtained by Cr4/MAO at 60 • C (entry 10, Table 2) was subjected to the 1 H and 13 C high-temperature spectroscopy (recorded in 1,1,2,2-tetrachloroethane-d 2 at 100 • C) ( Figure 4). The prominent singlets at δ =1.35 in the 1 H spectrum and δ = 30.00 in the 13 C spectrum corresponding to the repeating -(CH 2 ) n -repeat units again reflected the strict linearity of resultant polyethylene [49][50][51][52][53]. However, there was no evidence of double bonds present in the chain of polymer meaning that no unsatarated polymer formed along a termination pathway involving β-hydrogen elimination or transfer [51].   Table 2).

Catalytic Evaluation of Cr4/MMAO Catalytic System
To complement the study performed with MMAO as co-catalyst; the results are collected in Table 4. Once again, Cr4 was chosen as the test pre-catalyst to allow an optimization of the polymerization parameters. The polymerization was conducted at 10 atm of ethylene pressure, and the screening results are given in Table 4. Initially, on increasing Al/Cr molar ratio from 2000 to 4500 at 30 • C, a maximum activity of 20.14 × 10 6 g (PE) mol −1 (Cr) h −1 was found with an Al/Cr ratio of 4000 (entry 6, Table 4). Cr4/MMAO generates ca. 3.5-fold higher activity in comparison to Cr4/MAO, similar to observations reported elsewhere [74]. When further raising the amount of co-catalyst, the activity was sharply reduced to 11.14 × 10 6 g (PE) mol −1 (Cr) h −1 as a result of the increased chain transfer from the chromium center to aluminum [61]. In addition, the molecular weight distribution (M w /M n range = 1.62-2.14) remained particularly narrow and unimodal, as shown by the GPC curves (Figure 5a).
With the Al/Cr molar ratio kept at 4000 and the run time set at 30 min, the reaction temperature had been increased from 20 to 60 • C (entries 6 and 9-12, Table 4). The best catalytic activity was attained at 30 • C with a value of 20.14 × 10 6 g (PE) mol −1 (Cr) h −1 . The differences in optimum polymerization temperature between Cr/MAO and Cr/MMAO catalytic system may be attributed to the different energy barrier for cocatalysts to activate the chromium precatalyst [56]. At higher temperature, the activity was decreased slowly due to the partially deactivation of the active species [65][66][67][68][69] and lower solubility of ethylene [70][71][72] (Figure 5b) but nevertheless revealed a good level of 11.76 × 10 6 g (PE) mol −1 (Cr) h −1 at 60 • C (entry 12, Table 4). Similar to MAO case, the molecular weight of obtained polyethylene reached a peak up to 1.43 kg mol −1 at the optimum temperature which indicates that chain propagation served as the dominant reaction before reaching the optimum temperature (≤30 • C) but when further increasing the reaction temperature the higher rate of chain termination resulted in the lower molecular weight of polyethylene [57,[65][66][67][68][69]. In the next step, the temperature was kept at 30 • C and the Al/Cr molar ratio at 4000, the effect of time was investigated by conducting the polymerizations using Cr4/MMAO at 5, 15, 30, 45 and 60 min intervals (entries 6 and 13-16, Table 4) (Figure 6a). Similar with the Cr4/MAO system, the optimal activity of 20.14 × 10 6 g (PE) mol −1 (Cr) h −1 was again achieved within 30 min (entry 6, Table 4). Between 5 to 30 min the activity was gradually increased, while during the second 30 min it was slowly decreased with the onset of catalyst deactivation [49][50][51][52][53][62][63][64]. With regard to molecular weight of the resultant polymers, longer reactions were accompanied by an increase in the value of molecular weight from 0.67 to 1.55 kg mol −1 ; this observation can be attributed to stable presence of sufficient active species over longer reaction time during the polymerization process [57]. Reducing the ethylene pressure was also significantly affected the catalytic performance, which was demonstrated by the much lower activity at 5 atm of C 2 H 4 and only traced amounts of the polymer were gained at 1 atm of ethylene (entries 17 and 18, Table 4). Additionally, the molecular weight of the obtained polymer at 5 atm of ethylene pressure was lower than that achieved at 10 atm of ethylene pressure, which can be attributed to the slower propagation rate at lower ethylene pressure (entries 6, 17 and 18, Table 4) [54][55][56][57][58][59][60][61][62][63][64].  Table 4); (b) GPC curves of the polyethylene formed using Cr4/MMAO at different temperatures with the Al/Cr molar ratio fixed at 4000 (entries 6 and 9-12, Table 4).  Table 4); (b) Comparative activity of Cr1-Cr5 and M W of the corresponding polymers (Table 5).

Ethylene Polymerization with the Cr1-Cr5/MMAO Using Optimal Reaction Conditions
Using the favored operating conditions established using Cr4/MMAO (Al/Cr = 4000, run temperature = 30 • C, run time = 30 min), the remaining precatalysts, Cr1-Cr3 and Cr5, were all evaluated using MMAO as cocatalyst (Table 5). According to the data, all the chromium complexes (Cr1-Cr5) exhibited activities in the range of 7.59-20.14 × 10 6 g (PE) mol −1 (Cr) h −1 (Table 5) which were generally higher when compared to Cr/MAO catalytic system (6.16-10.03 × 10 6 g (PE) mol −1 (Cr) h −1 ) (Table 3) highlighting the importance of the aluminoxane activator. The overall activity decreased in the order Cr4 [2,4,6- ] as a result of the combined action of electronic and steric effects of the ligands (Figure 6b) [57,62,63,73]. By way of comparison, structurally related chromium precatalysts bearing 2,4-dibenzhydryl-6-chlorophenyl groups displayed relatively lower activity while the most hindered Cr3 showed lower activity than that in this work (Chart 1) indicating that the solubility of catalyst also affected their catalytic activity [64]. To further study the effect of cocatalyst type on the microstructures of the polymers, the sample achieved by Cr4/MMAO at 30 • C (entry 6, Table 4) was also measured by high temperature 1 H-NMR and 13 C NMR spectroscopic study. Similar with the result in MAO case, the presence of singlet resonances in both the 1 H-NMR spectrum (at δ 1.35, Figure 7a) and the 13 C-NMR spectrum (at δ 30.0, Figure 7b) is characteristic with high linearity polyethylene, corresponding to the methylene (-CH 2 -) repeat unit, again confirmed the formation of highly linear polyethylene which was further corroborated by its high melting temperature (T m > 119 • C) (Figure 7) [49][50][51][52][53]. To allow a comparison of these current chromium precatalysts (G in Chart 1) with the structurally related chromium systems, E and F (Chart 1), the optimum activity and the molecular weight as well as the polydisperisity of resultant polymers observed for each precatalyst are depicted in Figure 8. All polymerization tests were performed under their optimum condition at 10 atm C 2 H 4 over 30 min using MMAO as cocatalyst [62][63][64]. Inspection of Figure 8 shows that G exhibited the highest catalytic activity of all four classes highlighting the beneficial effect of strong electro-withdrawing group (ortho-fluoro substitution) on improving catalytic activity of precatalysts [75]. Moreover, the chromium precatalysts E containing 2,6-dibenzhydryl group delivered polyethylene with much higher molecular weight in the lowest yield when compared to F and G substituted by 2,4-dibenzhydryl group indicating that the requisite steric protection could largely inhibit chain termination as well as chain propagation leading to high molecular weight polymer inefficiently. In this work, the polyethylenes generated using G/MMAO are characteristic of polyethylene waxes displaying the lowest molecular weights (1.0-2.5 kg mol −1 ) and relatively narrower molecular weight distributions (1.92-2.27) among these four chromium catalysts in Figure 8. Therefore, G/MMAO shows great promise for potential industrial applications in the production of low molecular weight highly linear polyethylene waxes [51,75].  Table 5) with structrually related chromium-containing E NO2 [62], E tBu [63] and F [64] (Chart 1); all polymerizations were recorded at 10 atm C 2 H 4 , 30 min using MMAO as co-catalyst.

General Considerations
The air-and moisture-sensitive compounds were synthesized and handled under nitrogen atmosphere using standard Schlenk techniques. Prior to use, toluene was refluxed over sodium under nitrogen for 10h. The cocatalysts, methylaluminoxane (MAO, 1.46 M Al solution in toluene) and modified methylaluminoxane (MMAO, 2.00 M Al solution in n-heptane), were purchased from Albemarle Corp. (Baton Rouge, LA, USA). High-purity ethylene was purchased from Beijing Yanshan Petrochemical Co. (Beijing, China) and used as received. Other reagents were purchased from Aldrich (Beijing, China), Acros (Beijing, China) or Beijing Chemicals (Beijing, China). A Bruker Avance III 400 HD instrument (Bruker, Fällanden, Switzerland) was used to record the 1 H-and 13 C-NMR spectra of compounds and ligands at ambient temperature using TMS as an internal standard. The 1 H-and 13 C-NMR spectra of the resultant polyethylene were recorded on a Bruker DMX 300 MHz instrument (Bruker, Fällanden, Switzerland) at 100 • C using the deuterated 1,1,2,2-tetrachloroethane as the deuterium reagent. IR spectra were conducted on a System 2000 FT-IR spectrometer (Perkin-Elmer, Waltham, MA, USA) and elemental analysis (C, H, and N) was carried out using a Thermo Flash Smart EA microanalyzer (Thermo Fisher Scientific, Waltham, MA, USA). The molecular weight (M w ) and molecular weight distributions (M w /M n ) of resultant polyethylenes were determined by the Agilent PL-GPC220 GPC/SEC high-temperature system at 150 • C with 1,2,4-trichlorobenzene (TCB) as eluent with a flow rate of 1.0 mL min −1 . Additionally, the melting point of polyethylene was measured from the fourth scanning run by the PerkinElmer TA-Q2000 differential scanning calorimetry (DSC) analyzer (TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere.

Ethylene Polymerization at 1 Atmosphere Ethylene Pressure
The precatalyst Cr4 (1.9 mg, 2.0 µmol) was added to a Schlenk vessel which was equipped with a stirrer, followed by freshly distilled toluene (30 mL). When the chromium catalysts were completely dissolved in the toluene, the required amount of co-catalyst was then added by syringe. Under 1 atm of ethylene pressure, the reaction mixture kept stirring at the designated reaction temperature for 30 min. After reaction, the mixture was quenched with 10% hydrochloric acid in ethanol. The obtained polymer was washed with ethanol and dried under reduced pressure at 80 • C and weighed.

Ethylene Polymerization at 5/10 Atmosphere Ethylene Pressure
The high-pressure polymerization reactions were carried out in a stainless-steel autoclave (250 mL) equipped with a mechanical stirrer, a temperature controller and an ethylene pressure control system. Freshly distilled toluene (25 mL) and toluene solution with chromium complex (50 mL) were successively injected into the autoclave when the designated reaction temperature reached. Then the required amount of co-catalyst was injected and more toluene (25 mL) was introduced to complete the addition. The autoclave was immediately pressurized to the designated ethylene pressure and the stirring commenced at the same time. When the reaction time was up, stop stirring and cool the reactor. The ethylene pressure was vented and 10% hydrochloric acid in ethanol was used to quenched the reaction mixture. The obtained polymer was washed with ethanol and dried under reduced pressure at 80 • C and weighed.

Conclusions
A series of ortho-fluorinated 2-[1-(2,4-dibenzhydryl-6-fluorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridylchromium (III) chloride complexes has been synthesized in good yield and fully characterized including the molecular structure of complex Cr2 and Cr4 using-crystal X-ray diffraction. In the presence of MAO or MMAO, complexes Cr1-Cr5 showed exceptionally good performance toward ethylene polymerization and produced highly linear polyethylene waxes. In general, the MMAO-activated chromium catalysts displayed higher activities than that seen earlier with Cr/MAO highlighting the effect of cocatalyst type on catalytic performance. Moreover, the activity of Cr4/MMAO was especially outstanding as a result of combination effects of steric and electronic properties and reached at 20.14 × 10 6 g (PE) mol −1 (Cr) h −1 which was much higher than that of previously reported chromium analogs. This work further illustrates the systematic modification in the steric and electronic substituents of complexes providing a way to improve the catalyst performance and polymer microstructure.