Exploring cocatalyst type effect on the Ziegler-Natta catalyzed ethylene polymerizations: experimental and DFT studies

Due to the important role of cocatalyst in the polymerization process employing industrially favored Ziegler-Natta catalysts, its effect on kinetic behavior, catalyst activity, and polymer properties is discussed. In this paper triethyl aluminum (TEA) and triisobutyl aluminum (TIBA) have been used as the main cocatalyst ingredient with 10-20 mole percent of diethyl aluminum chloride (DEAC) and ethyl aluminum dichloride (EADC) cocatalysts. Among studied systems, neat TEA demonstrated the highest activity. Moreover, TEA-DEAC and TEA-EADC cocatalysts revealed a built-up kinetic profile, while TIBA-DEAC and TIBA-EADC show a decay-type kinetic curve. According to melt flow index results, no considerable change in flowability was detected in the synthesized polyethylenes (PE). On the other hand, the ethylene insertion and 2 chain termination mechanisms were scrutinized by means of density functional calculations using Ti active center located in (110) and (104) facets of the MgCl2 surface. Results revealed that TiCl4 supported (110) termination favors the PE chain production, being the ethylene insertion, the rate-determining step. To shed light on the bulkiness level of employed cocatalysts, buried volume (VBur) together with the two-dimensional map of cocatalyst molecules were considered. Higher VBur of TIBA can explain its lower activity and decay type kinetic profile obtained in the experimental section.


Introduction
Nowadays, it can be boldly said that the production of polymers using Ziegler-Natta catalysts is among the most important processes and has the largest portion in the production of plastics [1,2]. Metallocene catalysts are nearly four decades old. Although advanced companies such as Basell, Dow, Sabic, Mitsui, Sommito, Borealis, Exxon Mobile, and Inoe use metallocene catalysts to produce some polyolefin grades, still more than 70 % of polyolefins are prepared by Ziegler-Natta type catalysts [3,4]. It is well stablished that the first and most important parameter controlling the final polymer properties is the catalyst [5,6]. On the other hand, it is noticed that the nature of this important chemical is practically undeniable without the presence of an activator known as cocatalyst [7,8]. Many advances have been made in the catalysts structural studies and polymerization condition, although the effect of important factors such as electron donor, cocatalyst, and comonomer in the catalyst performance and polymerization process is not very well explored and understood [9]. Among these factors, cocatalyst, due to its primary role in switching the polymerization reaction, has relevant importance. In the past, researchers believed that the cocatalyst is a passive component that only has the function of "removing impurity" and "alkylating" the active catalytic centers at the beginning of the polymerization, and then, does not play a specific role in the ongoing process. It has now been shown that the type of cocatalyst not only affects catalyst productivity but also alters the microstructure and arrangement of monomers in polymer chains during the polymerization reaction [10,11]. In fact, after the catalyst, the selection of the appropriate cocatalyst is the most important and effective parameter in olefin polymerization processes [12]. Despite many studies on the catalyst structure, activity, polymerization, and properties of the polymer [13,14], only a few investigations have been carried out on the reactions and interactions of cocatalyst [15,16].
The process of cocatalyst interaction with Ziegler-Natta catalysts is well understood, but the performance of different types of cocatalyst in these interactions is still unclear [17,18]. It is well-known that, in a polymerization process, the cocatalyst i) activates pre-catalyst particles, ii) participates in chain transfer reactions, iii) changes the isotacticity and molecular weight of the polymer [19], and iv) alters the kinetic stability and resistance to the decomposition of catalyst [20]. Notably, these functions by controlling catalyst performance, greatly alter the final properties of the produced polymer. Among the most common cocatalysts used in the commercial production of polyolefins are triethyl aluminum (TEA), triisobutyl aluminum (TIBA), diethyl aluminum chloride (DEAC), and ethyl aluminum dichloride (EADC). It is pointed out that such compounds, due to their various chemical structure, exhibit different behaviors in the olefin polymerizations. In fact, cocatalysts can form mono-and dimeric moieties, each one indicates different capabilities toward pre-catalyst reduction/activation [21].
In addition to the type of cocatalyst, their concentration plays a relevant role. Indeed, at lower cocatalyst concentrations, a large percentage of the primary catalyst remains inactive, and impurities and toxins are not removed appropriately. As a result, the consumption of the catalyst increases dramatically, which is not economically affordable due to the high price of catalysts.
Moreover, the high amount of cocatalyst (too optimal) causes excessive reduction of active catalytic centers and conversion of Ti 3+ to Ti 2+ , with the subsequent inactivation and deactivation in ethylene and propylene polymerizations, respectively. Owing to the fact that some cocatalysts have a positive effect on catalyst activity and others have a better ability to improve the product's properties, it has encouraged researchers to use a combination of cocatalysts [22]. Due to the industrial and academic importance of the topic, in this paper we thoroughly investigated the kinetics of catalyst behavior, and properties of the final polyethylene produced via a commercial

Materials
Hydrogen gas (purity >99%) as an active chain transfer agent was supplied from Roham Gas Company. An industrial TiCl4/MgCl2 based Ziegler-Natta catalyst, hexane as main medium in polymerization reactions, the polymerization grade of ethylene gas, were all acquired from Jam Company, Iran. Hexane was stored over a 4 Å activated molecular sieve. Triethylaluminium

Ethylene polymerization
In this study, all the operations were carried out in dry and oxygen free media under nitrogen flow. Polymerizations were carried out in stainless-steel setup equipped with a mechanical stirrer, high-pressure injection system, a pressure, and a temperature sensor. In order to prepare the reactor for polymerization, the reactor was exposed to nitrogen for 1 h at 115 °C to remove impurities and moisture. After that, 650 mL of n-hexane was fed to the reactor at room temperature under nitrogen flow. After 3 min of degassing, the reactor was heated up to 80 °C, the 3 bar of hydrogen gas, 5 bar of ethylene gas, cocatalyst (with a different molar ratio of cocatalyst combination) and catalyst were injected into the reactor. After 2 h, the gases were completely vented, and the polymerization medium was cooled to room temperature. Finally, the polymer powder was dried at 85 °C.

Polymer characterization
The melt flow index (MFI) of the synthesized polymers was measured using a Gottfert (MI-4) instrument. The polymer powders were mixed with 400 and 800 ppm of antioxidants (IRGANOX 1010, 168, respectively) before adding to the barrel. The MFI of powders was determined using ASTM D-1238 test method. The measurements were performed at 190 °C using 21.6, 5, and 2.16 kg loads, respectively.

Computational details
DFT calculations have been carried out with the Gausian16 [23] code using the BP86 functional of Becke and Perdew [24,25], including corrections due to dispersion through the Grimme's method (GD3 keyword in Gaussian) [26]. The electronic configuration of the atoms has been described with the triple-ζ basis set with polarization of Ahlrichs for main-group atoms (def2-TZVP keyword in Gaussian) [27,28]. The geometry optimizations were performed without symmetry constraints, and analytical frequency calculations confirmed the character of the located stationary points (only one negative frequency). These frequencies were used to calculate unscaled zero-point energies (ZPEs) as well.

Characterization of catalyst
In the study of catalyst and polymerization process, the most basic task is to identify the structure, components, configuration, size, and size distribution of the catalyst. With a basic knowledge of the structure and chemistry of a catalyst, a good prediction of the behavior of the catalyst in the polymerization process can be provided. Table 1 shows the results of the structure and properties of the catalyst .   The XRD spectrum of the catalyst in 2θ angles between 5 and 65° is illustrated in Figure 2. In this pattern, the peaks at 2θ angles of 14° (stacking of Cl-Mg-Cl triple layers along the crystallographic direction), 33° (corresponded to (104) plane) and 50° (corresponded to (110) plane) were detected [29]. According to the literature, highly disordered catalyst is usually obtained by the rotational disorder of Cl-Mg-Cl triple layers, which subsequently lead to the formation of classically considered surfaces, i.e. (104) and (110) facets [6]. Therefore, in building molecular structure of a Ziegler-Natta catalyst, these surfaces should be considered.

Ethylene Polymerization in the presence of various cocatalysts
As mentioned, cocatalysts generally contain aluminum center, together with alkyl groups, chlorine atoms, etc., which based on their structure tune the performance of catalyst. Selecting the appropriate alkylaluminum can play a key role in the behavior of the catalyst, including polymerization activity, stereoselectivity, amount and distribution of active catalytic centers, as well as final characteristic of the polymer produced [4][5][6][7][8]. According to Table 2 results, catalyst activity was higher when TEA used as cocatalyst, in compare with TIBA. It can be attributed to the bulkiness of iso-buthyl groups which hinders its approaching to the Ti centers located in the interior parts of catalyst. By employing Cl containing Al complexes, EADC and DEAC, in the cocatalyst composition, the activity suppressed, however, no clear trend was found between the amount of Cl-containing cocatalyst and its type with the catalyst activity. In addition, the results of the Table 2 showed that the type of cocatalyst did not have a significant effect on the melt flow index. Although the polymers obtained in the presence of TIBA catalyst and TIBA/DEAC, TIBA/EADC compared to TEA and TEA/DEAC and TEA/EADC compounds had lower melt flow index and narrower FRR, this difference was not large enough to be considered. Therefore, it can be said that the type and amount of cocatalyst has not affected on the melt flow index.   compositions, decay type polymerization behavior was identified. This can be attributed to the less bulkiness of TEA (will discussed as VBur term in the simulation section) which facilitates its effusion into the small cavities of catalysts where Ti centers were located. Therefore, it can be said that in an ethylene polymerization under the same condition, TEA compared to TIBA, due to the small size of ethyl groups, will have a higher alkylation ability and consequently a higher polymerization rate.

Molecular modelling results
To elucidate the role of MgCl2 surface on chain termination mechanism, DFT simulation has been performed. In particular, the (104)  On (110) surface, the ethylene coordination was 2.3 kcal/mol favored than (104), although the energy barrier for the insertion into the Ti-C bond for (104) termination was 0.6 kcal/mol lower than for (110). The second energy barrier to consider is the H transfer from molecular H2. As it is shown in Figure 5, the hydrogen transfer was not the rate determining step (rds) of the reaction on (110) surface, since the energy barrier was 9.9 kcal/mol, thus, 1.0 kcal/mol lower than the previous ethylene insertion. The transition state structure depicted on Figure 6a shows that butyl molecule moves away from the Ti atom while one of the H atoms is bonded to Ti and the other forms the C-H bond with butyl. Similar mechanism was observed for (104) termination (see Figure 6b), although an energy barrier over than 45 kcal/mol was found, discarding a priori the H transfer reaction. Both transition state structures implied the bonding of both hydrogens on Ti as a previous step to eject and hydrogenate the butyl moiety. It has been observed that Ti atom supported on (104) move upwards during the proton transfer reaction, elongating the Ti-Clsurface bonds (~0.2 Å) and shortening the Ti-Clcluster bonds (~0.1 Å) to accommodate protons and throw out the butyl moiety (see Figure 6b). Instead, the Ti-Cl bond elongation was not observed for (110) since the Ti atom did not change the initial position. This may explain the large energy difference found by DFT simulations. According to literature, polymerization rate has a direct relationship with the formation of active centers through alkylation of TiCl4 by the cocatalysts. As a result, each cocatalyst can play a major role in the rate of polymerization due to its unique chemical structure [19]. Considering that TiCl4 catalysts supported on (110) termination is more catalytically active than on (104) cut, the effect of Al complexes in the transalkylation process to insert the ethyl on TiCl4 clusters has been investigated. The structures have been optimized adsorbing the Al complex on chloride surface atom near to Ti site. The computed reaction energies suggested that the transalkylation is thermodynamically favored using DEAC, by -5.8 kcal/mol Although the process using TEAL and EADC was endothermic, by 6.3 and 3.9 kcal/mol, respectively, the unfavorable thermodynamic energy values were relatively low, and thus, the reaction was still possible at room temperature. The use of the %VBur index, developed by Cavallo et al, [30] gives values of 78.7, 80,6, 74.6, and 76.2, for the TEA, TIBA, DEAC, and EADC, Al based systems, respectively [31]. Thus, the thermodynamic results are directly correlated with the sterical hindrance of the cocatalysts on the surface, and the steric maps in Figure 7 confirmed this hypothesis [32][33][34]. Indeed, when the steric hindrance increases, [35] the performance is worse [36,37]. This fact could explain the lower activity of TIBA containing cocatalysts in comparison to TEA and the decay type of kinetic curves in the experimental section. Finally, the reaction mechanism was computed considering the most favorable cocatalysts according to experimental results (TEA) on (110) surface of MgCl2. With the goal to explore the role of Clcontaining cocatalysts, the reaction mechanism with DEAC and EADC was investigated as well.
To carry out these simulations, the Al complex was placed close to the Ti clusters and in top of chloride surface atom. Table 3 report the energy barriers for butyl formation (Eb 1) and proton transfer (Eb 2). In general, no huge variations on the rate limiting step of the reaction were found in comparison to the energy barriers calculated for the TiCl4 cluster supported on MgCl2 (110) surface. As it can be observed, TEA show the same energy value for the energy barriers, 11 kcal/mol. The DEAC cocatalysts show slightly lower energy barriers (1.4 kcal/mol) with respect to the use of TEA and the calculations without cocatalysts. Nevertheless, experimental results did not show an improvement of the catalytic activity combining DEAC (>20% molar concentration) with TEA. These results can be related with the low concentration of DEAC, which reduce the probability to be close to Ti sites. Therefore, our simulations suggest that the use DEAC as main cocatalyst would slightly improve the catalytic activity of the system.
With respect to the EADC, the cocatalysts that contain more chloride ligands, presents the larger barriers.
On the other hand, one can hypothesize that cocatalyst may play a most relevant role on MgCl2 (104) termination, where the energy barrier is larger than 45 kcal/mol. The presence of cocatalyst may stabilize the Ti cluster, reducing the energy value of the rate limiting step. The %VBur show the higher free volume around TEA in compare with TIBA, confirming that steric hindrance problems of cocatalysts affects the catalytic activity. Finally, the reaction mechanism computed with TEA, DEAC, and EADC as cocatalyst, reveal that EADC is prejudicial due to the increase of the energy barriers, and DEAC slightly improve the catalytic activity.