Since the discovery of the Ziegler-Natta catalyst, many researchers have devoted much effort to improve the catalyst performance. This work reports a simple method to improve the Ziegler-Natta catalyst performance, including catalytic activity, molecular weight, etc. The highest catalyst activity of modified catalyst is more than 3 times higher than that of the corresponding commercial Ziegler-Natta catalyst.
The polyolefin industries began in the 1950s because of the discovery of Ziegler-Natta catalyst. All of the common polyolefins led to birth in the industries by their discoveries.1 Over the years, these catalysts have evolved from simple titanium chlorides into the nowadays used MgCl2-supported TiCl4 catalyst systems.2 Till now, the research on high-activity Ziegler-Natta catalyst is still a hot issue both in academy and industry. The research targets include achieving better product properties at lower production costs and the substitution of other materials such as glass, wood, steel, and expensive engineering polymers in certain fields because polyolefins deliver better resource-saving and facile recycling solutions.3 Thus, we report, for the first time, a simple and highly efficient approach in raising the catalyst activity of MgCl2-supported TiCl4 catalyst and in improving the mechanical properties of the obtained polymer.
The electron donor in Ziegler-Natta catalyst plays an important role in regulating molecular weight and molecular weight distribution, and extensive explorations have been focused on exploring the intrinsic effects of various internal donors on polymerization behaviors.4 However, the introduction of electron donor may inevitably result in some negative impacts (e.g., decrease in catalyst activity), although the product properties were improved (e.g., molecular weight and isotactic index).5 According to published reports,6 the catalyst activity of the Ziegler-Natta catalyst was increased with the introduction of “improver” during the catalyst preparation. Unfortunately, the melt index (MI) of the obtained polyethylene (PE) drastically increased (∼3 times). The MI is inversely related to the molecular weight of the polymer, whereas the physical properties such as impact resistance always increased with the increase of molecular weight (same molecular weight distribution).
Herein, we prepared a highly active ethylene polymerization catalyst through simple modification of commercial Ziegler-Natta catalyst (MgCl2-supported TiCl4 catalyst) with sulfonate ligands. The use of sulfonate ligands has not only raised the catalyst activity of the commercial Ziegler-Natta catalyst toward ethylene polymerization but also increased the molecular weight of the product.
RESULTS AND DISCUSSION
Polymerizations of ethylene catalyzed by modified and original Ziegler-Natta catalyst with triethylaluminum as cocatalyst were performed in hexane at 70 °C. O-CAT, CH-CAT, PH-CAT, and CF-CAT were original Ziegler-Natta catalysts, whereas CH3SO3C2H5, PhSO3C2H5, and CF3SO3C2H5 were modified Ziegler-Natta catalysts. The catalyst activity significantly increased with the introduction of sulfonate ligand to the catalyst, especially for PhSO3C2H5 (Table 1, entry 3) and CF3SO3C2H5 (Table 1, entry 4). The CF-CAT catalyst showed highest activity for the current results.
Table 1. Results of Ethylene Polymerization with Different Catalystsa
The polymerization mechanism of Ziegler-Natta catalyst is poorly understood because it takes place on the surface of an insoluble particle, a difficult situation to probe experimentally. Over the years, many mechanisms have been proposed for olefin polymerization with Ziegler-Natta catalysts. The most broadly accepted is the so-called monometallic mechanism proposed by Cossee and Arlman [Scheme 1(a)].7 Polymerization occurs via two steps. First, coordination of the monomer to the active center occurs, followed by migratory insertion of the coordinated monomer into the titanium–carbon bond. In migratory insertion step, a vacant coordination site is regenerated, which enables further chain propagation. A possible mechanism of the polymerization of ethylene catalyzed by the modified Ziegler-Natta catalysts is shown in Scheme 1(b). The reaction of the TiCl4 with sulfonate ligands would form a [Cl3Ti]+[O3SR]− cation–anion ion pair. When compared with traditional Ziegler-Natta catalyst, the transition metal center of modified Ziegler-Natta catalyst has a very strong negative electron affinity. Then, the coordination and insertion rate of ethylene monomer increased. (Ethylene monomer is a very weak Lewis base.) Additionally, the increase in the electron-attractive ability of the substituent led to a decrease in the charge density of the substitute and an increase in the negative electron affinity of the transition metal. It means that the increase in the electron-attractive ability of the substituent will enhance the catalyst activity. Therefore, the catalyst activity of the catalyst modified with different electron-withdrawing substituents was in the following order: CF-CAT > PH-CAT > CH-CAT > O-CAT.
In contrast to O-CAT, the PE resulting from modified catalysts show higher fusion of heat, which implies much higher degree of crystallinity in polymer made by modified catalysts than that made by O-CAT.
The molecular weight is one of the most important parameters that determines the properties of polymers. Many mechanical properties of polymer increase with increasing molecular weight (although polymer processing also becomes more difficult with molecular weight).8 In the cases where the polymerization was carried out with Ziegler-Natta catalyst, the ultrahigh-molecular-weight PE (UHMWPE, Mv > 1 × 106) was obtained (most of the UHMWPEs produced based on the market needs were manufactured using Ziegler-Natta catalysts).9 Additionally, the molecular weight of PE obtained from sulfonate ligand-modified catalyst is higher than that of the PE obtained from O-CAT catalyst (entry 1 vs. entries 2–4). The results indicate that the mechanical properties of PE can be improved through a simple modification of O-CAT catalyst. The increased molecular weight of obtained UHMWPE could be ascribed to the faster chain propagation rate and/or lower chain transfer rate of the modified catalyst. It is well known that the molecular weight of UHMWPE made by Ziegler-Natta catalyst in the presence of internal donor is higher than that made by Ziegler-Natta catalyst in the absence of internal donor. On the other hand, the catalyst activity always decreased with the introduction of internal donor toward ethylene polymerization.10 Thus, the modification not only enhanced the catalyst activity but also increased the molecular weight of the obtained UHMWPE.
To study the applicability of the modified Ziegler-Natta catalyst, the copolymerization of ethylene and styrene were also performed by using PhSO3C2H5-modified catalyst (PH-CAT). As is seen in Table 2, the catalyst activity of the modified catalyst in the copolymerization of ethylene and styrene is higher than that of the original Ziegler-Natta catalyst (O-CAT). The obtained copolymer was extracted successively with boiling methyl ethyl ketone (MEK) to remove atactic polystyrene. It is found that the MEK-insoluble fractions of both the original and modified catalysts are lower than 60 wt %. This indicates that atactic polymerization of styrene has occurred during ethylene–styrene copolymerization. The styrene content of the copolymer was decreased with the introduction of sulfonate ligand, suggesting that the incorporation of styrene was affected by the steric hindrance of the modifiers. Additionally, it was found that the signal for Sαα, Tββ, Sαγ, and Tβδ at 43.6, 41.3, 36.6, and 43.5 ppm, respectively, are absent from the 13C NMR spectra (Supporting Information) of the copolymer that was obtained with PH-CAT catalyst. However, the content of styrene (1.2 mol %) is too low to confirm the randomness of the copolymer made by modified Ziegler-Natta catalyst, for example, pseudo-random ethylene–styrene copolymer.11 The insoluble fraction was compression molded at 300 °C with ∼1000 psi. As shown in Figure 1, the sample produced by PH-CAT catalyst is more transparent than that of the sample prepared by O-CAT catalyst. It is also worth noting that the O-CAT catalyst produced sample containing white particles. From these observations, it is considered that the product obtained by polymerization with O-CAT catalyst contains high-molecular-weight styrene homopolymers, which cannot be removed by boiling MEK. The polymer produced by O-CAT catalyst is a mixture of poly(ethylene-co-styrene), atactic polystyrene, and high-molecular-weight polystyrene.
Table 2. Results of Ethylene–Styrene Copolymerization with Different Catalystsa
All reagents and solvents were purchased from commercial sources. The commercial Ziegler-Natta catalyst was provided by Yingkou Xiangyang Chemicals Group. All manipulations were carried out under an inert atmosphere of nitrogen gas. Hexane was dried with Na/K alloy with benzophenone. The general procedure for the modification of traditional Ziegler-Natta catalyst is given. To a stirred suspension of commercial Ziegler-Natta catalyst in dried hexane, sulfonate ligand is added dropwise at 0 °C. The mixture is stirred for an additional 0.5 h, and the resultant mixture is allowed to warm up to 60 °C and stirred for an additional 6 h. After 6 h, the solid obtained is separated and washed with hot hexane (60 °C, 100 mL × 5 times) and dried in vacuum. More details are given in the Supporting Information.
In summary, the results reported herein strongly indicate that the sulfonate ligand can make the Ziegler-Natta catalyst more economically effective in the production of PE. In addition, it is particularly interesting that the mechanical properties of PE obtained by sulfonate ligand-modified Ziegler-Natta catalyst were found to be better when compared with the PE produced by original catalyst. The effort to characterize the catalytic species is still ongoing, and studies are now in progress to explore the merits of the modified catalyst.
Authors appreciate financial supports from National Science and Technology Infrastructure Program (2007BAE14B01-06) and The Fund for Creative Research Groups (50621302).