Introduction

Early transition-metal Ziegler–Natta systems and chromium based catalysts, both exploited commercially over many years, have been joined recently by several families of highly active late-transition metal catalysts based, for example, on iron, cobalt and nickel [1–3]. Amongst these new systems, the bis(imino)pyridine iron and cobalt systems, discovered independently by Brookhart, Bennett and Gibson [4–7].

In 1995 Brookhart and coworkers reported new Ni(II)- and Pd(II)-based catalysts bearing bulky diimine ligands for ethylene polymerization [8, 9]. These catalysts are highly active and produce high molecular weight polymers. The bulky substituents on the aryl groups of the ligands block an associative olefin exchange, thus effectively retarding chain transfer pro-cesses. Furthermore, the microstructure of the polymers produced is a function of temperature, monomer pressure, steric bulkiness of the auxiliary ligands, and transition metal center. Variation of these factors allows access to a range of linear and branched polyethylene. Although many researches have been devoted to the modification of BIMP catalysts, mostly attention has been paid to the groups attached to the amine nitrogen donors [10–12]. For instance, Bennett and co-workers reported an imidazolyl substituted derivative that showed similar activity to its ketimine analogue [13]. Gibson and co-workers studied systematic variations at the imine carbon position [14]. Ionkin and co-workers investigated ethylene polymerization using a series of nitro-substituted bis(imino)pyridine complexes based on iron [15].

However, less attention has been directed towards introducing functional groups to the pyridine ring of the ligand. In the present study, the synthesis, structure, and precise ethylene polymerization behavior of new BIMP catalysts based on cobalt containing electron-withdrawing and electron-donor substitutions attached to the pyridine ring were reported. Although the replacement of para substituents of the pyridine ring doesn’t provide any significant change in steric bulk, the electronic effects caused remarkable variations in catalyst activity as well as polymer characteristics. Furthermore, with the aid of theoretical study, the charges on the central metal of the catalysts were calculated and discussed.

Experimental

General methods

All manipulations were carried out under an atmosphere of nitrogen using standard Schlenk and cannula techniques or in a conventional nitrogen-filled glovebox. Solvents were refluxed over an appropriate drying agent and distilled and degassed prior to use. The viscosity average molecular weight (Mv) of some polymer samples were determined according to the previous report [16, 17]. Intrinsic viscosity [η] was measured in decaline at 135 °C using an Ubbelohde viscometer. Differential scanning calorimetry (DSC) (Universal V4IDTA) with a rate of 10 °C/min instrument was used for polymer characterization. Mv values and degree of crystallinity of a polyethylene sample were calculated according to our previous report [17]. Elemental analysis for CHN was carried out by the CHNO type from the Helaus Co. 1H NMR spectra were recorded on a Bruker spectrometer (500 MHz) at 293 K. 2,6-Lutidine, MAO (10%), 2,4,6-trimethylaniline and other chemicals were purchased from Sigma-Aldrich Chemical Co. The general procedure of polymerization was to dissolve of precatalyst in toluene (300 mL) and adding of MAO (10 wt% in toluene) to produce an orange solution. The reactor was purged with ethylene, and the contents were stirred and maintained under ethylene for the duration of the polymerization. The polymerization was terminated by the addition of aqueous hydrogen chloride. The solid PE was collected by filtration, washed with methanol (50 mL), and dried.

Ligand and catalyst preparation

4-Nitro-2,6-diacethylpyridine and 4-Methoxy-2,6-diacethylpyridine were synthesized according to our recent report [18].

2,6-Diacethyl pyridine bis(2,6-diisopropylphenylimine) (a): To a solution of 2,6-diacethylpyridine (0.98 g, 6.1 mmol) in methanol (30 ml), 2,6-diisopropylaniline (12.4 mmol, 2.38 ml) and a trace amount of 4-toluenesulfonic acid were added. The resulting yellow solution was stirred at room temperature for 2 days, a white precipitate had formed. The precipitated was filtered, washed with cold methanol, and dried. A pale yellow solid was obtained. 1H NMR study revealed that both the monoamine and diimine products were present. Therefore, the resulting solid was dissolved in boiling mixture of methanol and chloroform. Addition of chloroform was continued until the solid was dissolved following to excess addition of 2,6-diisoropylaniline (6.1 mmol, 1.19 ml). The solution was refluxed for 5 days. The solvent was removed, a yellow solid was obtained that washed with n-hexane and dried (70% yield). 1H NMR (CDCl3): δ 1.2 (d, 24H), 2.3 (s, 6H), 2.8 (m, 4H), 7–7.2 (m, 6H), 7.88 (t, 1H), 8.4 (d, 2H). Anal. (C33H43N3) calcd: C, 82.32; H, 8.94; N, 8.73. Found: C = 82.33%, H = 9.11%, N = 8.68%.

4-Nitro-2,6-diacethyl pyridine bis(2,6-diisopropylphenylimine) (b): By using the same procedure described above, the compound b was obtained in 76% yield as an orange solid. 1H NMR (CDCl3): d, 8.70 (s, 2H), 7.25–6.85 (m, 6H), 3.02 (m, 4H), 2.25 (s, 6H), 1.2 (d, 24H). Anal. (C33H42N4O2) calcd: C, 75.25; H, 8.04; N, 10.64. Found: C, 74.86; H, 8.11; N, 10.44.

4-Methoxy-2,6-diacethyl pyridine bis(2,6-diisopropylphenylimine) (c): By using the same procedure described above, the compound c was obtained in 74% yield as a pale brown solid. 1H NMR (CDCl3): d, 7.42 (s, 2H), 7.20–6.80 (m, 6H), 3.65 (s, 3H), 3.10 (m, 4H), 2.25 (s, 6H), 1.25 (d, 24H). Anal. (C33H45N3O) calcd: C, 79.80; H, 8.86; N, 8.21. Found: C, 79.27; H, 8.93; N, 8.27.

Catalyst synthesis

[2,6-diacethylpyridinebis(2,6-diisopropylphenylimine)]cobalt(ΙΙ) dichloride (A)

In a dry, oxygen free atmosphere, CoCl2 (anhydrous, 1.078 mmol) was dissolved in dry THF. [2,6-Diacethylpyridinebis(2,6-diisopropylphenylimine)], the ligand, (1.156 mmol) was added. The mixture was stirred for 3 days at room temperature and a dark blue precipitate was formed. The solid product was filtered, washed with dry n-hexane and dried under N2. The solid catalyst of [2,6-diacethylpyridinebis(2,6-diisopropylphenylimine)]cobalt(ΙΙ) dichloride (yield 75%) was obtained. 1H NMR (CD2Cl2, broad singlets are observed in each case): δ 114.91 (2H, Py-Hm), 49.85 (1H, Py-Hp), 10.01 (4H, Ar-Hm), 4.41 (6H, \NCCH3), -8.88 (2H, Ar-Hp), -16.74 (12H, iPr-Me), -18.22 (12H, iPr-Me), -81.92 (4H, iPr-CH). Anal. (C33H43Cl2CoN3) calcd: C, 64.81; H, 7.09; N, 6.87%. Found: C = 64.66%, H = 6.78%, N = 6.90%.

[4-Nitro-2,6-diacethylpyridinebis(2,6-diisopropylphenylimine)]cobalt(ΙΙ) dichloride (B): By using the same procedure described for the synthesis of catalyst A, complex B was obtained in 74% yield as an orange solid.

1H NMR (CD2Cl2, broad singlets are observed in each case): δ 96.55 (2H, Py-Hm), 10.15 (4H, Ar-Hm), 4.75 (6H, NdCCH3), -7.22 (2H, Ar-Hp), -14.62 (12H, iPr-Me), -16.15 (12H, iPr-Me), -80.44 (4H, iPr-CH). Anal. (C33H42Cl2CoN4O2) calcd: C, 60.37; H, 6.45; N, 8.53%. Found: C = 60.24, H = 6.54, N = 8.48%.

[4-Methoxy-2,6-diacethylpyridinebis(2,6-diisopropylphenylimine)]cobalt(ΙΙ) dichloride (C)

By using the same procedure described for the catalyst A, complex C was obtained in 70% yield as a brown solid. 1H NMR (CD2Cl2, broad singlets are observed in each case): δ 112.25 (2H, Py-Hm), 11.02 (4H, Ar-Hm), 4.52 (6H, NCCH3), -6.75 (2H, Ar-Hp), -17.31 (12H, iPr-Me), -16.70 (12H, iPr-Me), -22.5 (3H, OMe), -80.44 (4H, iPr-CH). Anal. C34H45Cl2CoN3O) calcd: C, 63.65; H, 7.07; N, 6.55%. Found: C = 63.69, H = 7.13, N = 6.49%.

Scheme 1. Synthetic procedure for the catalysts A-C

Scheme 1
scheme 1

Synthetic route to substituted BIMP-Co catalysts

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Results and discussion

Ethylene polymerization reactions using the prepared catalysts A-C were carried out at the different polymerization conditions. All three catalysts exhibited high activity for ethylene polymerization at low and high pressure runs. The catalysts gave the activities in the order of: catalyst B > catalyst A > catalyst C.

For all the catalysts, the effect of reaction temperature, reaction time, ethylene pressure and hydrogen concentration on catalyst activity were studied. The influence of polymerization temperature on activity was investigated at the reaction temperatures between 0 and 60 °C, while the [Al]/[Co] molar ratio was kept constant at [Al]/[Co] = 3000:1. As it can be seen in Fig. 1, the highest productivity of the catalysts A, B and C were occurred at about 30, 20 and 45 °C respectively. In the case of catalyst A, the activity increased with polymerization temperature reaching a maximum at 30 °C, but at higher temperature, activity was reduced due to the increase in catalyst deactivation rate as known for most olefin polymerization catalysts. Activity of the catalyst B bearing p-NO2 substitution reaches to a maximum value at 20 °C and further increase of temperature led to a dramatic decrease in its activity. It can be concluded that introducing the p-OMe substitution at the para-position of the pyridine ring could increase the thermal stability of the catalyst C. The catalyst C bearing p-OMe substitution exhibited more stable trend toward increase of polymerization temperature, the activity was almost stable at the temperatures between 20 and 50 °C. Higher activity of the catalyst B was offset by lower thermal stability and life time.

Fig. 1
figure 1

Effect of temperature on the average rate of polymerization. Polymerization conditions: Polymerization time = 30 min, monomer pressure = 5 bar, [Al]:[Co] = 3000:1, [Co] = 0.35 × 10-3 mmol, toluene = 300 mL

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As catalyst lifetime is a substantial factor in this scope, ethylene polymerization was carried out with prolonged reaction time in order to ascertain the lifetime of the catalysts. According to the Fig. 2, as reaction time increases, the activity increases and after reaching a certain maximum time, it decreases. At the monomer pressure of 5 bar and ambient temperature, these maximums for the catalysts A–C are around 30, 10 and 45 min, respectively.

Fig. 2
figure 2

Rp versus time. Polymerization conditions: monomer pressure = 5 bar, [Al]:[Co] = 3000:1, [Co] = 0.35 × 10-3 mmol, toluene = 300 mL

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The influence of monomer pressure between 2 and 7 bars on the catalyst behavior was studied. The polymerization behavior is shown in Fig. 3. Catalyst activities increased with ethylene pressure with the similar trend which can be explained since catalyst active sites were exposed to higher ethylene concentration at higher pressure.

Fig. 3
figure 3

Effect of monomer pressure on the polymerization behavior. Polymerization conditions: temperature = 30 °C, polymerization time = 30 min, [Al]:[Co] = 3000:1, [Co] = 0.35 × 10-3 mmol, toluene = 300 mL

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Ethylene polymerization was carried out using different amount of hydrogen as a chain transfer agent. As it can be seen in Fig. 4, higher amount of hydrogen could increase the activity of the catalysts B and C to some extent. A reasonable explanation for this enhancement effect might be the more homogeneous reaction conditns [19]. However, 2,1-reinsertion of short olefin branches, terminated by β-H elimination which can still coordinate to the active centers, is probable [20]. Activity of the catalyst A was not sensitive to the amount of hydrogen.

Fig. 4
figure 4

Effect of hydrogen on the average activity. Polymerization conditions: Polymerization time = 30 min, monomer pressure = 2 bar, [Al]:[Co] = 3000:1, [Co] = 0.35 × 10-3 mmol, toluene = 300 mL

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With the aid of Gaussian software [21] the most stable conformations and QEq charges of catalysts A–C were calculated. As expected, attachment of the nitro group led to a more electrophilic metal center (Table 1).

The presence of electron-withdrawing nitro substituent on the pyridine ring of the catalyst is considered to be highly beneficial for increasing the catalyst activity since the introduction of electron-withdrawing group on the ligand structure results in an increase in electrophilicity of the catalyst central metal. Not only the catalyst activities and catalyst behavior during the polymerization were remarkably affected by different substitution at the para-position of the pyridine ring, but the polymer characteristics such as molecular weight, degree of crystallinity and melting point were influenced (Table 2). Mv of the obtained polymer follows A > B > C. Catalyst A produced PE with the highest Mv. Furthermore, it can be concluded that the molecular weight and polydispersity of the polyethylene obtained from the synthesized catalysts were affected by the catalysts structure. The catalyst C bearing electron-donor OMe substitution at the para-position of the pyridine ring, produced PE with narrower PDI relative to polymer resulted using the other prepared catalysts.

Table 1 Correlation between catalyst activities and catalyst charges
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Table 2 Some characterization of the resulted polyethylene
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Conclusion

Attaching the NO2 and OMe substitution at the para-position of the pyridine ring caused the activity of the catalyst to vary. Polymerization reactions using the prepared catalysts were carried out at the different conditions of temperature, monomer pressure, polymerization time, and H2 concentration. Although the catalyst B showed the highest activity, lower thermal stability and lifetime was observed for the catalyst. The activity of the catalyst C toward increasing the temperature was surprisingly stable. Hydrogen could increase the activity of the catalysts B and C to some extent, while the activity of the catalyst A was less sensitive to the hydrogen concentration. Theoretical investigations revealed a more positive charge on the central metal of the catalyst B led to an increase in activity of the catalyst through increasing the metal–carbon reactivity. PE with broader PDI was obtained with the catalyst B.