METHOD OF ETHYLENE POLYMERIZATION

Abstract: The present invention relates to a method of ethylene polymerization, which comprises pre-activating a typical Ziegler-based catalyst showing high activity by reacting the same with a trialkyboron compound having reduction capability at a certain temperature, and then carrying out polymerization by adding said catalyst and a saturated hydrocarbon compound containing halogen. According to the method of ethylene polymerization of the present invention, the polymers having broad molecular weight distribution and expression of a high tail at the portion of high molecular weights can be produced. Consequently, even with a sole reactor, polymers having a bimodal molecular-weight distribution structure can be synthesized.

Description: METHOD OF ETHYLENE POLYMERIZATION Technical Field The present invention relates to a method of ethylene polymerization, or more particularly, to the method of ethylene polymerization for producing polymers having broad molecular weight distribution, which comprises pre-activating a Ziegler-based catalyst for ethylene polymerization by reacting the same with an organometallic compound, and carrying out polymerization by using said activated catalyst in a slurry process. Background Art Generally, polymers having broad molecular weight distribution are used industrially in the extrusion-blow molding process for producing products such as pipes, films, and sheets. On the other hand, polymers having narrow molecular weight distribution are generally used in extrusion-molding, etc. These two types of polymers are distinguished by the differences in the melt flow indexes and the molecular weight distributions. The molecular weight distribution is the ratio between the weight-average molecular weight and the number-average molecular weight. The polymers polymerized with a Ziegler-type catalyst in a slurry or liquid-phase sole reactor commonly have narrow molecular weight distribution. The polymers having characteristics of narrow molecular weight distribution as such suffer from the limitation of tensile strength and consequently undergo changes in shape and contraction during processing. Moreover, they are difficult to apply to a process requiring high mechanical resistance in the molten state. There has been much research into the methods for producing polymers having broad molecular weight distribution by means of using Ziegler-based catalysts, e.g., Zucchini, Ceccin et al, Adv. in Polymer Science 51, pg. 101-153 (1983). There, Zucchini reported a method of polymerization in series or in multi-step by using Ziegler-Natta based catalysts in two or more reactors to obtain polymers of broad molecular weight distribution. However, the method was complicated in process and had a disadvantage of significantly reduced productivity. With respect to the catalyst for producing polymers having broad molecular weight distribution, Altemore et al. in US Patent No. 3,899,477 disclosed a catalyst, which is used with titanium halide, vanadium halide and organo-aluminum compounds. This catalyst can produce polymers having broad molecular weight distribution if it is processed into the main catalyst prior to polymerizing alkylaluminum ,yeΛ7wtethoxide and trialkylaluminum. However, the technique has a disadvantage in that it is difficult to control the polymerization process due to the following factors: a complicated catalyst production process; difficulties in controlling operational condition for the differences in the relativities of titanium and vanadium with hydrogen, monomers and co-monomers. Consequently, it has been very difficult to industrially produce polymers of high tensile strength with broad molecular weight distribution in a sole reactor by means of using a Ziegler-type catalyst. Summary of the Invention The objective of the present invention is to provide a method of ethylene polymerization by means of using a Ziegler-based catalyst, producing ethylene polymers at a high yield, which have a molecular-weight distribution structure of broad distribution with expression of a high tail. After much research, the inventors of the present invention discovered that the aforementioned objective could be achieved by pre-activating a typical Ziegler-based catalyst and appropriately controlling the polymerization condition therein. Accordingly, the present invention was completed in this manner. In other words, the method of ethylene polymerization of the present invention comprises a catalyst pre-activation step of reacting a Ziegler-based catalyst with a trialkylboron compound at a certain temperature for a fixed period of time, and then a second step of injecting said pre-activated Ziegler-based catalyst and a saturated hydrocarbon containing halogen during polymerization. Accordingly, the method of the present invention, while showing high activity, leads to broadening of molecular weight distribution of the produced polymers with expression of a high tail with respect to the molecular weight distribution (i.e., it is not a normal distribution with respect to the molecular-weight distribution curve, and it forms a tail towards the direction of larger molecular weights, which is called a bimodal structure). The ethylene polymerization as mentioned in the present invention refers not only to homo-polymerization of ethylene but also to co-polymerization of ethylene and α- olefin (e.g., 1-butene, 1-hexene, etc.). The present invention is described in detail as below. As for the Ziegler-based catalyst used in the present invention, it could be a typical Ziegler-type catalyst, having a substrate of dichloromagnesium, dibromomagnesium, or difluoromagnesium. Its core metal is of an element of IN, N or NI Group of the Periodical Table. In the case of IV Group as a core metal, it is preferable to use dichloromagnesium as a substrate. Here, the term "typical" implies that the effects of the present invention are not limited by the catalyst components therein. In the present invention, a typical Ziegler-based catalyst is used in the polymerization process after its pre-activation by reacting the same with a trialkylboron compound in a non-polar organic solvent. As for the trialkylboron compound used during the catalyst pre-activation step of the present invention, triethylboran, tributylboron, trioctylboron, etc. can be used. Among them, the triethylboron compound shows the best results. The mole ratio of the core metal within the catalyst to the trialkylboron compound should be in the range of 1:1-10. It shows good results with the ratio of 1:3-7, or preferably, 1:5. If the mole ratio exceeds 1:10, it destroys the form of catalyst particles and induces a rapid reduction of activity. As for the appropriate non-polar organic solvent used during the catalyst pre- activation step, it includes an alkane compound of hexane, N-heptane, octane, nonane, or decane; or a cycloalkane or aromatic compound. Among these compounds, hexane seems to be the best. Preferably, the solvent is used after purification so that it does not affect the catalytic activity. The temperature of- pre-activation is 0~50°C, and the duration of 3-24 hours is preferable. If the temperature exceeds the above range, it destroys the form of catalyst particles and induces a rapid reduction of catalytic activity. Moreover, if the duration for pre-activation is less than three hours, the effect of expression of a tail at the high end of molecular weight is rather weak. If the duration exceeds 24 hours, it induces a gradual reduction of catalytic activity. Preferably, the pre-activation step is carried out at 20°C for about five hours. The method of ethylene polymerization of the present invention is carried out by injecting a Ziegler-based catalyst pre-activated by the above method along with a saturated hydrocarbon compound containing halogen into a polymerization reactor. Preferably, a co-catalyst is added thereto during the polymerization process. As for the saturated hydrocarbon compound containing halogen, which is used during polymerization, a saturated hydrocarbon containing chlorine, bromine, etc., such as chloroform, ethylchloride, tertiary butylchloride, or tetrachloromethane can be used. Among these compounds, chloroform shows the best results. Preferably, the mole ratio of the core metal within the catalyst to the saturated hydrocarbon containing halogen should be 1:1-20. In particular, when they are used with the mole ratio of approximately 1 :7, it brings about the best results. If the above ratio is less than 1 : 1, it is difficult to obtain the anticipated effects. If the ratio exceeds 1 :20, it leads to rapid reductions of catalytic activity and hydrogen reactivity. As for the co-catalyst which can be used in the method of ethylene polymerization of the present invention, it includes a typical co-catalyst used during ethylene polymerization, such as an organic aluminum compound, e.g., trialkylaluminum, dimethylalummum chloride, diethylaluminum chloride, triisobutylaluminum, trimethylaluminum, dimethlylaluminum chloride, etc. As for the amount used, it is appropriate to use a mole ratio of 20 - 100 with respect to the core metal within the catalyst. As for the molecular weight of the polymers produced by the present invention, a well-known method of control through the amount of hydrogen injected can be used. By using the method of ethylene polymerization according to the present invention, the molecular weight can be easily controlled by way of relatively small differences in the amount of hydrogen as injected therein. The above method can be confirmed by measuring the melt index of the obtained polymers. By means of the method of polymerization of the present invention as above, the polymers of broad molecular weight distribution with expression of a high tail can be obtained. On the other hand, if the polymerization is carried out without the saturated hydrocarbon containing halogen, it produces polymers of narrow molecular weight distribution with a reduced ratio of polymers of relatively high molecular weights. Detailed Description of the Preferred Embodiment The present invention is explained in detail through examples as below. Nevertheless, these examples are for illustrative purposes only and should not be construed whatsoever to limit the present invention. The characteristics of the polymers as synthesized in the examples as below are measured by the following test method: 1. The melt index was measured at 190°C with a 2.16kg weight. The weight of the molten polymers coming out during the period of 10 minutes was expressed in grams (g/lOmiuntes). 2. The molecular weight distribution (MFRR) was obtained by dividing the melt index obtained with a 21.6kg weight by the melt index obtained with a 2.16kg weight. The larger the value, the broader was the molecular weight distribution. 3. The molecular weight was measured with a gel permeation chromatography (GPC). There, the weight-average molecular weight, and the Z-average molecular weight were measured. The degree of high-tail formation was expressed as a ratio (Mz Mw) of the Z-average molecular weight measured as such to the weight-average molecular weight. The larger the value of Mz/Mw, the higher was the degree of high-tail formation. 4. The polymers obtained after polymerization were dried completely in a vacuum oven, after which their weights were measured. The polymerization activity of the catalyst was obtained by dividing the weight of the polymers as measured by the weight of the core metal (M) in the catalyst times 100 and by the duration of polymerization reaction (Kg-PE/g-M.hr). Example 1 Pre-activation of catalyst The main catalyst was prepared by means of the method introduced in Korean Patent No. 99-19193, which was as follows: First, a magnesium solution was prepared by way of contacting a halogenated magnesium compound with alcohol. Then, it was synthesized by reacting a mixture of an ester compound containing a hyndroxy group, a titanium compound, and a silicone compound in that order. There, the catalyst had the core metal of 5.0wt% of titanium. In a glove box, l.Og of the catalyst as produced above were subdivided into a 250ml flask having 3 -directional cork and a magnetic bar. While stirring with a magnetic stirrer, 150ml of dry hexane was poured into the flask in the nitrogen atmosphere. The temperature in the flask was set to 20°C, and while stirring in the nitrogen atmosphere, 5.3 ml of 1.OM of triethylboran was added thereto. The stirring was stopped after five hours. Polymerization Reaction By using a pre-activated catalyst as above, the polymerization was carried out in a 2L slurry polymerization reactor. In the nitrogen atmosphere, 1L of dry hexane was added to the polymerization reactor. In the nitrogen atmosphere, after pouring in 3ml of a triisobutylaluminum compound (1.0M) as a co-catalyst, 2ml of ethylcholride (0.1M) was poured therein. Thereafter, 4.4ml of the catalyst slurry as pre-activated above was poured therein. After venting the nitrogen from the reactor, 500ml of hydrogen was injected. After raising the temperature to 80°C, which is the polymerization temperature, ethylene was injected at 80 psi. After carrying out the polymerization reaction for one hour, the injection of ethylene was stopped. After lowering the temperature to room temperature, the gas remaining in the reactor was vented out, and then the reactor was opened. After filtering the polymers, they were dried in a vacuum oven. The results of the polymerization test are shown in Table 1. Comparative Example 1 In order to understand the effect of adding a pre-activated catalyst and a saturated hydrocarbon containing halogen during polymerization, the polymerization without undergoing the step of adding ethylchloride was carried out by using a catalyst which had not undergone the catalyst pre-activation step of Example 1. The results of the polymerization test are shown in Table 1. Comparative Example 2 Except for carrying out the polymerization without the step of adding ethylchloride, it was carried out in the same manner as Example 1 in order to understand the effect of the saturated hydrocarbon containing halogen. The results of the polymerization test are shown in Table 1. Comparative Example 3 By using a catalyst, which had not undergone the catalyst pre-activation step of Example 1, the polymerization was carried out in the same manner as Example 1. The results of the polymerization test are shown in Table 1. Table 1 As shown in the results of Table 1 , according to the method of polymerization of the present invention, it was possible to obtain polymers having a bimodal molecular- weight distribution structure by way of having broad molecular weight distribution and expression of a high tail, while maintaining a high level of polymerization activity of the catalyst. The polymers having a molecular-weight distribution structure of broad distribution and expression of a high tail tend to show high melt strength and mechanical resistance. As such, they show excellent surface characteristics, bubble stability, and other physical properties during processing. Further, since the production of thin films and lightweight products are made possible thereby, the polymers are expected to bring about the effects of reducing the amount of materials in use. In particular, when these polymers are applied to the production of pipes, it enhances the mechanical properties of extrusion processability and toughness during the molding process. As such, the creep characteristics are enhanced, which in turn is expected to enhance durability. According to the method of polymerization of the present invention as above, it leads to the effects of producing polymers in a sole polymerization reactor, while maintaining a high level of polymerization activity, wherein said polymers have a bimodal molecular-weight distribution structure by way of having broad molecular weight distribution and expression of a high tail.

Claims: CLAIMS What is claimed is: 1. A method of ethylene polymerization, which comprises pre-activating a Ziegler- based catalyst for ethylene polymerization by reacting the same with a trialkylboron compound in the presence of a non-polar organic solvent, and then carrying out polymerization by adding said pre-activated catalyst and a saturated hydrocarbon compound containing halogen. 2. The method of ethylene polymerization according of Claim 1, which comprises pre-activating said catalyst at 0~50°C for 3-24 hours. 3. The method of ethylene polymerization according of Claim 1, wherein said trialkylboron compound is triethylboron, tributylboron, or trioctylboron. 4. The method of ethylene polymerization according of Claim 1, comprising said Ziegler-based catalyst, wherein the mole ratio of the core metal thereof to the trialkylboron compound is 1 :1-10. 5. The method of ethylene polymerization according of Claim 1, wherein said saturated hydrocarbon compound containing halogen is chloroform, ethylchloride, tertiary butylchloride, or tetrachloromethane. 6. The method of ethylene polymerization according of Claim 1, comprising said Ziegler-based catalyst, wherein the mole ratio of the core metal thereof to the saturated hydrocarbon compound containing halogen is 1 : 1-20. 7. The method of ethylene polymerization according of Claim 1, which comprises adding an organo-aluminum compound as a co-catalyst during polymerization. The method of ethylene polymerization according of Claim 7, wherein said organo-aluminum compound is trialkylaluminum, dimethylaluminum chloride, diethylaluminum chloride, triisobutylaluminum, trimethylaluminum, or dimethylaluminum chloride.

Inventors: KIM SANG-YULL, LEE WEON, PARK JI-YONG, YANG CHUN-BYUNG, KIM, SANG-YULL, LEE, WEON, PARK, JI-YONG, YANG, CHUN-BYUNG
Applicants: KIM SANG YULL, LEE WEON, PARK JI YONG, SAMSUNG GENERAL CHEMICALS CO, YANG CHUN BYUNG, KIM, SANG-YULL, LEE, WEON, PARK, JI-YONG, SAMSUNG GENERAL CHEMICALS CO., LTD., YANG, CHUN-BYUNG
Application year: 2001
Main classification: C08F10
IPC codes: C08F4, C08F10, C08F110