Highly Linear Polyethylenes Achieved Using Thermo-Stable and Efficient Cobalt Precatalysts Bearing Carbocyclic-Fused NNN-Pincer Ligand

Six examples of 2-(1-arylimino)ethyl-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridine-cobalt(II) chloride complexes, [2-(1-ArN)C2H3-9-ArN-5,6,7,8-C5H8C5H3N]CoCl2, (Ar = 2-(C5H9)-6-MeC6H3 Co1, 2-(C6H11)-6-MeC6H3 Co2, 2-(C8H15)-6-MeC6H3 Co3, 2-(C5H9)-4,6-Me2C6H2 Co4, 2-(C6H11)-4,6-Me2C6H2 Co5, and 2-(C8H15)-4,6-Me2C6H2 Co6), were synthesized by the direct reaction of the corresponding ortho-cycloalkyl substituted carbocyclic-fused bis(arylimino)pyridines (L1–L6) and cobalt(II) chloride in ethanol with good yields. All the synthesized ligands (L1–L6) and their corresponding cobalt complexes (Co1–Co6) were fully characterized by FT-IR, 1H/13C-NMR spectroscopy and elemental analysis. The crystal structure of Co2 and Co3 revealed that the ring puckering of both the ortho-cyclohexyl/cyclooctyl substituents and the one pyridine-fused seven-membered ring; a square-based pyramidal geometry is conferred around the metal center. On treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the six complexes showed high activities (up to 4.09 × 106 g of PE mol−1 (Co) h−1) toward ethylene polymerization at temperatures between 20 °C and 70 °C with the catalytic activities correlating with the type of ortho-cycloalkyl substituent: Cyclopentyl (Co1 and Co4) > cyclohexyl (Co2 and Co5) > cyclooctyl (Co3 and Co6) for either R = H or Me and afforded strictly linear polyethylene (Tm > 130 °C). The narrow unimodal distributions of the resulting polymers are consistent with single-site active species for the precatalyst. Furthermore, compared to the previously reported cobalt analogues, the titled precatalysts exhibited good thermo-stability (up to 70 °C) and possessed longer lifetime along with a higher molecular weight of PE (Mw: 9.2~25.3 kg mol−1).

To enhance the thermal stability, one efficient method was introduced to the bulky substituent on the ortho-position of N-aryl group [6][7][8][9][10][11][12][13][14][15] (Chart 1). Wu's group have reported that bis(imino)pyridine iron complexes (A1) bearing a bulky 2-methyl-6-sec-phenethyl group, can produce linear high molecular weight polyethylene with higher activity at elevated temperature [31]. In the recent five years or so, our group have developed series of dibenzhydryl substituted bis(imino)pyridyl iron and cobalt complexes (A2) that displayed good catalytic activity at high temperature and explored a very good thermal stability [34]. There were also reports about incorporating the cycloalkyl substituents in the 2,6-positions of the N-aryl groups of the bis(imino)pyridine (A3) and it has been found to efficiently increase the temperature stability of the iron catalysts over their alkyl analogues [35]. Later Russia's group documented the 2,6-bis(arylimino)pyridy liron(II) chloride complexes by the introduction of cycloaliphatic substituents into the ortho position of the aryl groups substantially enlarges the temperature interval of efficient use of the related catalysts for ethylene polymerization reactions [36]. Consequently, they have prepared the multifunctional bis(imino)pyridine iron chloride complexes incorporating the cycloalkyl substituent that could display more efficient ethylene polymerization at elevated temperatures (60-90 °C) while retaining high molecular weights of the resulting PE, as distinct from the analogous monofunctional systems (A3) [37]. On the other hand, the modification of bis(imino)pyridine backbone to enhance the catalytic properties at a higher temperature. Our group has been interested in introducing cycloalkyl unit to bis(imino)pyridine ligand sets by adjusting the flexibility of the exterior imine donors to modify the donor properties of the NNN-pincer ligand set and in-turn the performance of the catalyst [6]. With regards to cobalt precatalysts (Chart 1), one [38][39][40][41][42] or two [43][44][45][46][47][48] cycloalkyl unit fused derivatives having ring sizes between five and eight are accessible (see for example B [39], C [40], D [41], E [43,44], F [45], and G [47,48], Chart 1). In terms of catalytic performance and the polymeric products On the other hand, the modification of bis(imino)pyridine backbone to enhance the catalytic properties at a higher temperature. Our group has been interested in introducing cycloalkyl unit to bis(imino)pyridine ligand sets by adjusting the flexibility of the exterior imine donors to modify the donor properties of the NNN-pincer ligand set and in-turn the performance of the catalyst [6]. With regards to cobalt precatalysts (Chart 1), one [38][39][40][41][42] or two [43][44][45][46][47][48] cycloalkyl unit fused derivatives having ring sizes between five and eight are accessible (see for example B [39], C [40], D [41], E [43,44], F [45], and G [47,48], Chart 1). In terms of catalytic performance and the polymeric products properties, we have observed significant differences between cobalt catalysts (B-G). For example, five-membered ring fused bis(imino)pyridine cobalt precatalyst [38], showed lower activities (2.89 × 10 4 g of PE mol −1 (Co) h −1 ) than their analogues of A and B-G [6]; while using B that containing slightly larger six-membered ring (Chart 1) [39], results in much higher activities (up to 1.08 × 10 7 g of PE mol −1 (Co) h −1 ) and better thermal stability than A and C [4,6,39,40], and produces polyethylene wax with narrower molecular weight distribution. Expanding the ring size to seven (D and E, Chart 1) [41,43,44], the more flexible structure, leads to increased molecular weight (Mw up to 54.0 kg mol −1 ) which is most apparent with doubly-fused E containing ortho-cycloalkyl substituent, but E display slightly lower activity their counterparts D. Beyond this, it was found that cyclohexyl fused pyridine cobalt complexes (B and C) produced the polyethylene with very low molecular weight (0.82 kg mol −1 ) and linear saturated structure. The seven-membered ring fused pyridine(imino)cobalt precatalyst (D and E) produced the higher molecular weight polyethylene (3.2 kg mol −1 ) that contained the vinyl group, which has a significant potential application in the polywax in the package field [6]. The other interesting is the one cycloalkyl fused pyridine (imino) cobalt (B and D) showed better thermal stability (optimum temperature: 50~60 °C) and higher activity (8.65-10.09 × 10 6 g of PE mol −1 (Co) h −1 ) than that by double cyclcoalkyl fused bis(imino)pyridine cobalt complexes (C, E, F, and G) (activity: 2.89-5.04 ×10 6 g of PE mol −1 (Co) h −1 and optimum temperature at 30-40 °C). Very recently, we have reported a series of E derivatives that incorporate the cycloalkyl substitution on the ortho position of N-aryl group, which exhibited good activity (2.0 × 10 6 g of PE mol −1 (Co) h −1 ) with the optimum temperature of 30 °C [43]. Considering the potential positive effect by the cycloalkyl substituent on the ortho position of N-aryl group and the backbone of monocycloheptyl fused pyridine(imino) on the thermal stability and polymerization activity toward ethylene polymerization, therefore, in this work we focused on the mono cycloheptyl-fused bis(imino)pyridine cobalt complexes that incorporated the different ring size of cycloalkyl on the ortho position of N-aryl group and their catalytic behavior toward ethylene polymerization in detail.

Single-Crystal X-ray Diffraction Study
The single-crystals of Co2 and Co3 suitable for the X-ray determinations were obtained by slow diffusion of n-hexane into their solution in dichloromethane at room temperature. Their molecular structures are shown in Figure 1a,b, and selected bond lengths and angles in Table 1. which is similar to our previous work [49,50]. The isomers with N-H groups are confirmed with the observation range from 3372 to 3379 cm −1 in their IR spectra, while the isomers containing the C=N groups were observed characteristic band around 1644 cm −1 . Then using the literature procedure [40,49], the corresponding NNN-pincer ligands (L1/L1'-L6/L6) reacted with cobalt(II) dichloride in ethanol to give the corresponding 2-(1-arylimino)ethyl-9-arylimino-5,6,7,8-tetrahydrocyclohepta The cobalt complexes Co1-Co6 have been characterized by IR and elemental analysis and the structure of Co2 and Co3 was confirmed by single-crystal X-ray diffraction. Comparing the IR spectra of free ligands with their corresponding Co complexes, there is no absorption related to the N-H groups; moreover, the adsorption around 1644 cm −1 for νC=N in ligand compounds is shifted to lower wavenumber around 1609 cm −1 within the cobalt complexes indicating effective coordination of Co-Nsp2 [40,41].

Ethylene Polymerization using Co2/MAO
With MAO as co-catalyst, Co2 was employed as precatalyst to investigate the ethylene polymerization systematically at various parameters, such as different Al/Co molar ratios, reaction temperatures and run times and polymerization results are collected in Table 2.
First of all, with the ethylene pressure set at 10 atm and the Al/Co ratio fixed at 1000, ethylene polymerizations were conducted at a different temperature from 30 to 70 • C (runs 1-5, Table 2). The maximum activity of 2.89 × 10 6 g of PE mol −1 (Co) h −1 was found at 50 • C affording a polymeric product; no trace of oligomers could be detected. Further, higher temperature led to the dramatically decreasing of the polymerization activity ( Figure 2a). Meanwhile, the molecular weight (M w ) of the polymer was found to decrease from 22.97 kg mol −1 to 6.56 kg mol −1 when increasing the temperature from 30 • C to 70 • C, which can be ascribed to either the more facile chain transfer or chain termination with respect to the chain propagation yielding lower molecular weight PE [4,40,41,45,[51][52][53]. Moreover, their GPC curves showed the unimodal distribution and the resultant polyethylene possessed narrow polydispersity (PDI = 2.0~3.8), which is consistent with the characters of single-site active species [6,[39][40][41][42][43][44][45].
Fixing the temperature at 50 • C, ethylene polymerizations by Co2/MAO were conducted at a different Al/Co molar ratio from 500 to 3500 (runs 3 and 6-11, Table 2). The highest activity was observed as 3.57 × 10 6 g of PE mol −1 (Co) h −1 with a molar ratio of 2500 (run 9, Table 2), a lower or higher ratio of co-catalyst led to lower activities. The molecular weight of the polymer (M w ) gradually decreased from 14.2 kg mol −1 to 9.39 kg mol −1 , and the molecular weight distribution kept very narrow (PDI = 2.4 − 2.7). This result can be ascribed to increased chain transfer to aluminum occurring as a consequence of the larger amounts of alkyl aluminum reagent employed [4,40,41,44,51,52]. To our surprise with the Al/Co molar ratios changing between 3000 and 3500, a dramatic increase of molecular weight was observed and up to 13.96 kg mol −1 with 3500 equivalents of MAO (Figure 2b). It would seem plausible that the formation of new active species has been influenced within this molar ratio window, a similar observation has been noted elsewhere [38,44]. On the other hand, with regards to the molecular weight distribution, the PDI of polymers are falling in the range from 2.4 to 3.0, slightly broader than their counterparts D containing symmetric para-R 1 groups (PDI = 2.2) [41], this observation was similar to complex E incorporating α, α -bis(arylimino)-2,3:5,6-bis(hexamethylene)pyridine containing cycloalkyl ortho-substituents [43,44].

Run
Precat. Al/Co  [4,40,41,44,51,52]. To our surprise with the Al/Co molar ratios changing between 3000 and 3500, a dramatic increase of molecular weight was observed and up to 13.96 kg mol −1 with 3500 equivalents of MAO (Figure 2 (b)). It would seem plausible that the formation of new active species has been influenced within this molar ratio window, a similar observation has been noted elsewhere [38,44].
To the end, the lifetime of the Co2/MAO catalyst was probed by conducting the polymerization over 5, 15, 30, 45 and 60 min (runs 9 and 12-15, Table 2). Polymerization activity gradually decreased from 8.12 × 10 6 g of PE mol −1 (Co) h −1 to 2.48 × 10 6 g of PE mol −1 (Co) h −1 with prolonging the reaction time from, 5 to 60 min, which can be explained by the rapid formation of active species after the addition of MAO and gradual deactivation over extended reaction time [34,[40][41][42][43][44][45]. It is indicated that a short time frame was required to generate all active species, and then the onset of partial deactivation of active species occurred over the course of the reaction [34]. Decreasing ethylene pressure to 5 atm, the polymerization activity (2.85 × 10 6 g of PE mol −1 (Co) h −1 ) is much lower than that (3.57 × 10 6 g of PE mol −1 (Co) h −1 ) at 10 atm (runs 9 and 16, Table 2), but the resultant PE polymer possessed much higher molecular weight (13.04 vs. 9.39 kg mol −1 ).
In order to explore the effect imparted by the cycloalkyl ortho-substitution pattern on performance and polymer properties, the other complexes were additionally screened using the optimized reaction conditions established independently for Co2/MAO and the results were collected in Table 2 (runs 17-21). On activation with MAO, all the cobalt complexes Co1-Co6 displayed good activities in the range of 3.21-4.09 × 10 6 g of PE mol −1 (Co) h −1 , which fall in the order: (Figure 3a). These findings indicate that the size of cycloalkyl group on the ortho position of N-aryl group affects the catalytic activity, in which less bulky cyclopentyl systems gave the higher activities (Co1, Co4) and the bulkiest group of cyclooctyl showed the lowest (Co3, Co6) activities. All the resultant PE by different cobalt complexes Co1-Co6 possessed narrow distribution and PDI value ranged from 2.4 to 3.9, a slightly broader than that found in the D/MAO system. The GPC traces clearly showed unimodal distribution (Figure 3). But the polymer by Co3 and Co6 that bear the bulkiest ortho-substituent of cyclooctyl possessed the highest molecular weight among their analogues, the similar trends were also observed for E/MAO system [44]. The para-methyl substituted complexes Co4-Co6 showed slightly lower activity than unsubstituted ones Co1-Co3, which was demonstrated by the activity order: . However, Figure 3 shows that molecular weights by Co1-Co3 are lower than that by Co4-Co6 respectively. These trends are similar to their analog complex of B [39]. To the end, the lifetime of the Co2/MAO catalyst was probed by conducting the polymerization over 5, 15, 30, 45 and 60 min (runs 9 and 12-15, Table 2). Polymerization activity gradually decreased from 8.12 × 10 6 g of PE mol −1 (Co) h −1 to 2.48 × 10 6 g of PE mol -1 (Co) h -1 with prolonging the reaction time from, 5 to 60 min, which can be explained by the rapid formation of active species after the addition of MAO and gradual deactivation over extended reaction time [34,[40][41][42][43][44][45]. It is indicated that a short time frame was required to generate all active species, and then the onset of partial deactivation of active species occurred over the course of the reaction [34]. Decreasing ethylene pressure to 5 atm, the polymerization activity (2.85 × 10 6 g of PE mol −1 (Co) h −1 ) is much lower than that (3.57 × 10 6 g of PE mol −1 (Co) h −1 ) at 10 atm (runs 9 and 16, Table 2), but the resultant PE polymer possessed much higher molecular weight (13.04 vs. 9.39 kg mol −1 ).
In order to explore the effect imparted by the cycloalkyl ortho-substitution pattern on performance and polymer properties, the other complexes were additionally screened using the optimized reaction conditions established independently for Co2/MAO and the results were collected in Table 2 (runs 17-21). On activation with MAO, all the cobalt complexes Co1-Co6 displayed good activities in the range of 3.21-4.09 × 10 6 g of PE mol −1 (Co) h −1 , which fall in the order: (Figure 3 (a)). These findings indicate that the size of cycloalkyl group on the ortho position of N-aryl group affects the catalytic activity, in which less bulky cyclopentyl systems gave the higher activities (Co1, Co4) and the bulkiest group of cyclooctyl showed the lowest (Co3, Co6) activities. All the resultant PE by different cobalt complexes Co1-Co6 possessed narrow distribution and PDI value ranged from 2.4 to 3.9, a slightly broader than that found in the D/MAO system. The GPC traces clearly showed unimodal distribution ( Figure 3). But the polymer by Co3 and Co6 that bear the bulkiest ortho-substituent of cyclooctyl possessed the highest molecular weight among their analogues, the similar trends were also observed for E/MAO system [44]. The para-methyl substituted complexes Co4-Co6 showed slightly lower activity than unsubstituted ones Co1-Co3, which was demonstrated by the activity order: Co1 (R 2 = H) > Co4 (R 2 = Me), Co2 (R 2 = H) > Co5 (R 2 = Me) and Co3 (R 2 = H) > Co6 (R 2 = Me). However, Figure 3 shows that molecular weights by Co1-Co3 are lower than that by Co4-Co6 respectively. These trends are similar to their analog complex of B [39].  Table 2).
In comparison with previously reported cobalt precatalysts, such as A, B and D (Chart 2) [4,[39][40][41]44], the current systems, Co1-Co6, under comparable polymerization conditions (namely MAO as co-catalyst, 10 atm C 2 H 4 , 30 min) exhibited relatively lower catalytic activity (3.21-4.09 × 10 6 g of PE mol −1 (Co) h −1 ), but are higher than that by precatalyst E (Chart 2). On the other hand, the resulting polymers by Co1-Co5/MAO possessed much higher molecular weight than that found in D/MAO. These findings heights cycloalkyl ortho-substituents in the N-aryl group favored the high molecular weight polymer formation.

Ethylene Polymerization Using Co2/MMAO
Under the optimized molar ratio for Co2/MAO (Al/Co = 2500), ethylene polymerization by Co2/MMAO were conducted at various temperatures (from 30 °C to 60 °C) at 10 atm ethylene pressure (runs 1-4, Table 2). The results showed that the highest activity of 2.95 × 10 6 g of PE mol -1 (Co) h -1 was also observed at 50 °C (run 3, Table 3), similar to the results by Co2/MAO (2.89 × 10 6 g of PE mol -1 (Co) h -1 ). Further, higher temperature led to the decrease of activity, indicating the partial decomposition of the active species [4,40,41,45,[51][52][53]. The molecular weight of PE decreased from 12.98 kg mol −1 to 6.41kg mol −1 with the increasing the temperature from 30 °C to 60 °C, which can be ascribed to either increased chain transfer to aluminum or chain termination by β-H elimination at the higher temperature [47][48][49][50][51][52][53][54]. However, all the resultant PE possessed quite narrow distribution (PDI = 2.5) and GPC traces showed unimodal distribution, which is a typical character of a single site catalyst system. A similar observation was also reported for their counterparts B, D and E (Chart 2).
With the temperature fixed at 50 °C, the influence of the Al/Co molar ratio was investigated in the range of 1500 to 3000 (runs 3, 5-7, Table 3). A topmost of activity 2.95 × 10 6 g of PE mol −1 (Co)h −1 was achieved at the molar ratio of 2500, which is similar to the results with MAO. As with MMAO, the resultant polyethylenes possessed high molecular weights (8.19-10.22 kg mol −1 ) and narrow polydispersities (Mw/Mn ≈ 2.5), which is consistent with single-site characteristics for the active species [40][41][42][43][44][45].
In order to investigate the lifetime of the active species, the polymerization tests were conducted over different reaction time from 5, 15, 30 and 60 min (runs 3 and 8-10, Table 3). The results showed that polymerization activity decreased from 7.04 to 2.07 × 10 6 g of PE mol −1 (Co) h −1 when increasing the time from 5 to 60 min. As with MMAO system, the molecular weights of the resultant polymer are falling in the range from 8.19 to 10.49 kg mol −1 , and displaying narrow molecular weight distribution (Mw/Mn = 2.4-3.0). With the ethylene pressure reduced to 5 atm, the activity (1.90 × 10 6 g of PE mol −1 (Co) h −1 ) was much lower than that at 10 atm (run 8, Table 3).
With MMAO as co-catalyst, all the cobalt complexes Co1-Co6 were evaluated for their ethylene polymerization and they also displayed good activity (2.24-3.34 × 10 6 g of PE mol −1 (Co) h −1 ). The similar trends in activity and molecular weight as MAO are observed, in which the cyclopentyl substituted Co1 and Co4 displayed the highest activity and cyclooctyl substituted cobalt complex Chart 2. Comparison of the M w , polydispersity (PDI) and polymerization activity of previously reported cobalt precatalysts (A [4], B [39], D [40], E [44] and H [38]) with MAO as activator under related condition.

Ethylene Polymerization Using Co2/MMAO
Under the optimized molar ratio for Co2/MAO (Al/Co = 2500), ethylene polymerization by Co2/MMAO were conducted at various temperatures (from 30 • C to 60 • C) at 10 atm ethylene pressure (runs 1-4, Table 2). The results showed that the highest activity of 2.95 × 10 6 g of PE mol −1 (Co) h −1 was also observed at 50 • C (run 3, Table 3), similar to the results by Co2/MAO (2.89 × 10 6 g of PE mol −1 (Co) h −1 ). Further, higher temperature led to the decrease of activity, indicating the partial decomposition of the active species [4,40,41,45,[51][52][53]. The molecular weight of PE decreased from 12.98 kg mol −1 to 6.41kg mol −1 with the increasing the temperature from 30 • C to 60 • C, which can be ascribed to either increased chain transfer to aluminum or chain termination by β-H elimination at the higher temperature [47][48][49][50][51][52][53][54]. However, all the resultant PE possessed quite narrow distribution (PDI = 2.5) and GPC traces showed unimodal distribution, which is a typical character of a single site catalyst system. A similar observation was also reported for their counterparts B, D and E (Chart 2).
With the temperature fixed at 50 • C, the influence of the Al/Co molar ratio was investigated in the range of 1500 to 3000 (runs 3, 5-7, Table 3). A topmost of activity 2.95 × 10 6 g of PE mol −1 (Co)h −1 was achieved at the molar ratio of 2500, which is similar to the results with MAO. As with MMAO, the resultant polyethylenes possessed high molecular weights (8.19-10.22 kg mol −1 ) and narrow polydispersities (M w /M n ≈ 2.5), which is consistent with single-site characteristics for the active species [40][41][42][43][44][45].
In order to investigate the lifetime of the active species, the polymerization tests were conducted over different reaction time from 5, 15, 30 and 60 min (runs 3 and 8-10, Table 3). The results showed that polymerization activity decreased from 7.04 to 2.07 × 10 6 g of PE mol −1 (Co) h −1 when increasing the time from 5 to 60 min. As with MMAO system, the molecular weights of the resultant polymer are falling in the range from 8.19 to 10.49 kg mol −1 , and displaying narrow molecular weight distribution (M w /M n = 2.4-3.0). With the ethylene pressure reduced to 5 atm, the activity (1.90 × 10 6 g of PE mol −1 (Co) h −1 ) was much lower than that at 10 atm (run 8, Table 3).
With MMAO as co-catalyst, all the cobalt complexes Co1-Co6 were evaluated for their ethylene polymerization and they also displayed good activity (2.24-3.34 × 10 6 g of PE mol −1 (Co) h −1 ). The similar trends in activity and molecular weight as MAO are observed, in which the cyclopentyl substituted Co1 and Co4 displayed the highest activity and cyclooctyl substituted cobalt complex (Co3 and Co6) gave the lowest activity, but produced the highest molecular weight polyethylene (runs 3, 12-16, Table 3). In comparison with D/MMAO, the ortho-cycloalkyl systems in general exhibited more than twice higher molecular weight, although the catalytic activity of D/MMAO is nearly twice as much as that seen for Co1 and Co4, and almost three times that for Co2-Co3 and Co5-Co6. In addition, the molecular weights obtained using Co1-Co6 (range: 8.19-15.94 kg mol −1 ) are all significantly higher than that seen for D/MMAO system (2.5 kg mol −1 ) [40], which also illustrated the steric effect of the ortho cycloalkyl substituents suppressed the chain transfer and led to the high molecular weight of the polymer. As shown by their GPC curves in Figure 4, the MWD of polymeric products are falling in the range 2.4 to 4.4. (Co3 and Co6) gave the lowest activity, but produced the highest molecular weight polyethylene (runs 3, 12-16, Table 3). In comparison with D/MMAO, the ortho-cycloalkyl systems in general exhibited more than twice higher molecular weight, although the catalytic activity of D/MMAO is nearly twice as much as that seen for Co1 and Co4, and almost three times that for Co2-Co3 and Co5-Co6. In addition, the molecular weights obtained using Co1-Co6 (range: 8.19-15.94 kg mol −1 ) are all significantly higher than that seen for D/MMAO system (2.5 kg mol −1 ) [40], which also illustrated the steric effect of the ortho cycloalkyl substituents suppressed the chain transfer and led to the high molecular weight of the polymer. As shown by their GPC curves in Figure 4, the MWD of polymeric products are falling in the range 2.4 to 4.4. Table 3. Ethylene polymerization results using Co2/ MMAO a.    Table 3).

Characterization of Polyethylene
Regardless of MMAO or MAO activation, narrow unimodal distributions of the polymers are observed (M w /M n =2.2-4.5) consistent with single-site active species. All the T m values of the polymer ranging from 129 • C to 133 • C [6,[40][41][42][43][44][45]. To provide further support for the linearity of the polymers, representative PE samples using Co1/MAO at 50 • C (run 9, Table 2) were characterized by 1 H/ 13 C-NMR spectroscopy at high temperature (recorded in 1,1,2,2-tetrachloroethane-d 2 at 120 • C, Figures 5 and 6). The 1 H-NMR spectra clearly showed the weak downfield multiplets at δ 5.90 and δ 5.00, the typical vinylic signal ( Figure 5). While the integration ratio for H 1 :H 2 /H 2 ':H I is close to 1:2:3, indicating the only the unsaturated chain structure in the PE. The shift 114.38 and 139.54 in 13 C-NMR spectra was ascribed to the unsaturated chain ends (-CH=CH 2 ) [16,40,41,43,45], which agreed well with 1 H-NMR spectra. In addition, the lower intensity peaks at 32.24, 22.92 and 14.22 corresponding to an n-propyl end-group further supported the linear structure of the polyethylene [16,18,45]. These findings are further evidenced by their high melting temperature (T m = 130.0 • C, run 9, Table 2).
The resultant PE by Co2/MMAO were also characterized by 1 H/ 13 C-NMR spectra, and shown in Figures 7 and 8. The 1 H-NMR spectrum also showed the signal of a vinyl group (δ 5.90 and δ 5.00), which was also confirmed by the signal in 13 C-NMR (δ 139. 55, 114.40). However, the integration ratio for H 1 :H 2 /H 2 ':H I is close to 1:2:5 (Figure 7), dissimilar to that seen with MAO at the same temperature, which suggested both the saturated and unsaturated chain structure in polyethylene. Moreover, the signal of the lower intensity peaks at 32.24, 22.93 and 14.26 in 13 C-NMR corresponds to n-propyl end-group, indicating the linear structure of PE. These observations would suggest that, chain termination via β-H elimination in Co2/MMAO system is no longer the sole chain transfer pathway operative, with chain transfer to aluminum now competitive [16,18,44]. The resultant PE by Co2/MMAO were also characterized by 1 H/ 13 C-NMR spectra, and shown in Figures 7 and 8. The 1 H-NMR spectrum also showed the signal of a vinyl group (δ 5.90 and δ 5.00), which was also confirmed by the signal in 13 C-NMR (δ 139. 55, 114.40). However, the integration ratio for H1:H2/H2':HI is close to 1:2:5 (Figure 7), dissimilar to that seen with MAO at the same temperature, which suggested both the saturated and unsaturated chain structure in polyethylene. Moreover, the signal of the lower intensity peaks at 32.24, 22.93 and 14.26 in 13 C-NMR corresponds to n-propyl end-group, indicating the linear structure of PE. These observations would suggest that, chain termination via β-H elimination in Co2/MMAO system is no longer the sole chain transfer pathway operative, with chain transfer to aluminum now competitive [16,18,44].  Table 3); recorded in 1,1,2,2-tetrachloroethane-d2 at 120 °C.  Table  3); recorded in 1,1,2,2-tetrachloroethane-d2 at 120 °C.  Table 3); recorded in 1,1,2,2-tetrachloroethane-d 2 at 120 • C. The resultant PE by Co2/MMAO were also characterized by 1 H/ 13 C-NMR spectra, and shown in Figures 7 and 8. The 1 H-NMR spectrum also showed the signal of a vinyl group (δ 5.90 and δ 5.00), which was also confirmed by the signal in 13 C-NMR (δ 139. 55, 114.40). However, the integration ratio for H1:H2/H2':HI is close to 1:2:5 (Figure 7), dissimilar to that seen with MAO at the same temperature, which suggested both the saturated and unsaturated chain structure in polyethylene. Moreover, the signal of the lower intensity peaks at 32.24, 22.93 and 14.26 in 13 C-NMR corresponds to n-propyl end-group, indicating the linear structure of PE. These observations would suggest that, chain termination via β-H elimination in Co2/MMAO system is no longer the sole chain transfer pathway operative, with chain transfer to aluminum now competitive [16,18,44].  Table 3); recorded in 1,1,2,2-tetrachloroethane-d2 at 120 °C.  Table  3); recorded in 1,1,2,2-tetrachloroethane-d2 at 120 °C.  Table 3); recorded in 1,1,2,2-tetrachloroethane-d 2 at 120 • C.

General Considerations
All the synthetic procedures for air/moisture-sensitive compounds were performed under a nitrogen atmosphere with standard Schlenk techniques. Toluene as a solvent for polymerization was refluxed over sodium (a small amount of benzophenone) and distilled under nitrogen prior to use. Methylaluminoxane (MAO, 1.46 M solution in toluene) and modified methylaluminoxane (MMAO, 1.93 M in n-heptane) were bought from Akzo Nobel Corp (Nanjing, China). High-purity ethylene was bought from Beijing Yanshan Petrochemical company (Beijing, China) and used as received. NMR spectra were recorded using a Bruker DMX 400 MHz instrument (Beijing, China) at ambient temperature using TMS as an internal standard. FT-IR spectra were recorded using a Perkin-Elmer System 2000 FT-IR spectrometer (Shanghai, China). Elemental analysis was carried out using a Flash EA1112 microanalyzer (Beijing, China). Molecular weights and molecular weight distribution of polyethylenes were determined using a PL-GPC220 GPC/SEC High Temperature System (Beijing, China). Data collection and handling were carried out using Cirrus GPC Software (Beijing, China) and Multi Detector Software (Beijing, China). The calibrants for constructing conventional calibration is Polystyrene Calibration KitS-M-10 from PL Company (Beijing, China). The true average molecular weights of PE are transferred by inputting the M-H constants of PE. K of 0.727 and α of 40.6 are provided by PL Company (Beijing, China). Samples were dissolved at a concentration of 1.0 to 2.5 mg mL −1 , depending on the molecular weights. DSC trace and melting points of polyethylene were obtained from the second scanning run on DSCQ2000 at a heating rate of 10 • C min −1 from −20 • C to 200 • C. 1 H/ 13 C-NMR spectra of the polyethylene were recorded using a Bruker DMX 300 MHz instrument (Beijing, China) at 120 • C in deuterated 1,1,2,2-tetrachloroethane with TMS as an internal standard. The compound of 2-acetyl-5,6,7,8-tetrahydrocycloheptapyridin-9-one was synthesized according to the literature [49] and the 2-cycloalkylaniline hydrochlorides were prepared using literature methods [34].

General Procedure for Ethylene Polymerization under 5/10 Atm Pressure
A 250 mL stainless steel autoclave (Dalian Sanling Electronic Manufacture, Dalian, China), equipped with an ethylene pressure control system, a mechanical stirrer and a temperature controller, was employed for the reaction. The autoclave was evacuated and refilled with ethylene three times. When the desired reaction temperature was reached, toluene, the co-catalyst (MAO or MMAO), and a toluene solution of the catalytic precursor (the total volume was 100 mL) were injected into the autoclave by using syringes. Then, the ethylene pressure was increased to 5/10 atm, maintained at this level with constant feeding of ethylene. After the reaction was carried out for their quired period, the reactor was cooled with a water bath and the excess ethylene was vented. The reaction solution was quenched with 10% HCl/ethanol. The precipitated polymer was collected through filtration, washed with ethanol and dried under vacuum at 60 • C until constant weight.

Conclusions
A family of six cobalt (II) chloride complexes, Co1-Co6, bound by single ring-fused 2-(1-cycloalkylphenylimino)ethyl-9-cycloalkylphenylimino-5,6,7,8-tetrahydrocyclo-heptapyridines, was prepared by the reaction of the corresponding carbocyclic-fused NNN-pincer ligands (L1/L1 -L6/L6 ) and cobalt(II) chloride and fully characterized. On activation with either MMAO or MAO, all these ortho cycloalkyl substituted cobalt complexes displayed good activity for ethylene polymerization with the optimum temperature of 50 • C, affording the linear polyethylene. The molecular weight (Mw = 9.78-25.6 kg mol −1 ) is higher than that observed with their analogues D bearing alkyl substituent. The ring size of the ortho substituent greatly affected their molecular weight and polymerization activity, which was demonstrated by the highest activity achieved by the cyclopentyl substituted ones (Co1 and Co4) and the highest molecular weight by cyclooctyl substituted ones (Co3 and Co6). Notably, polyethylene using MAO as activator displayed high selectivity for vinyl end-groups (-CH=CH 2 ), while with MMAO the polyethylene possessed both the saturated and saturated linear structure.

Conflicts of Interest:
The authors declare no conflict of interest.