Ethylene copolymerization process

Details Ethylene copolymerization process Ethylene copolymerization process

2002-10-28

2004-03-30

Choi, Ling-Siu (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymerizing in tubular or loop reactor

C526S348200, C526S348000, C526S125100, C526S159000, C526S904000, C526S123100, C502S103000

Reexamination Certificate

active

06713572

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the polymerization of ethylene. In one aspect, the present invention relates to a process for preparing an ethylene copolymer having a density of about 0.93 g/mL or less. In another aspect, the present invention relates to a slurry type copolymerization of ethylene which produces a low density copolymer. Still another aspect of the present invention relates to ethylene copolymerization using a continuous loop-type reactor.
BACKGROUND OF THE INVENTION
Various techniques for polymerizing ethylene are known. Examples include high pressure free radical polymerization, solution polymerization, gas phase polymerization, and slurry polymerization, sometimes referred to also as particle form polymerization. It is known that the density of the polyethylene can be varied by incorporating certain amounts of higher alpha-olefins during the polymerization. While the high pressure free radical type polymerization is capable of producing copolymers having densities of 0.93 g/mL and below, ethylene copolymers, referred to as linear low density polyethylene, can also be produced by copolymerizing ethylene in the presence of suitable catalysts. One technique of forming linear low density polyethylenes involves gas phase polymerization. Such processes have been found to be particularly useful for producing low density narrow molecular weight polymer that is particularly desirable for producing films. Using the particle form process in conjunction with a chromium-containing catalyst, it is also possible to make a low density polymer that is useful for the production of films; however, those polymers generally have a broader molecular weight and are not quite as clear as those produced catalytically using gas phase. Attempts have been made in the past to produce low density polyethylene in a particle form process using a titanium-containing catalyst; however, it was observed that the comonomer incorporation was generally not adequate to reduce the density to the level often desired. Generally, in order to produce lower density copolymer, it has been necessary to use much higher comonomer levels with the titanium catalysts. This results in more comonomers that must be flashed or recycled. For 1-hexene, the higher levels are also found to result in fluff swelling and reactor fouling in particle form polymerizations. To counteract that, generally lower polymerization temperatures have been employed. However, lowering the polymerization temperature lowers the rate at which the comonomer is incorporated. There is, therefore, a need for a method which allows for the copolymerization to be carried out at higher temperatures.
An object of the present invention is to provide a method for the particle form polymerization of ethylene to produce a low density polyethylene using a titanium based catalyst.
Still another object of the present invention is to provide a particle form polymerization of ethylene that is capable of yielding a low density polyethylene while using a minimum of organoaluminum cocatalyst.
Still another object of the present invention is to provide a process in which particle form copolymerization can be conducted at higher temperatures with reduced reactor fouling.
Other aspects, objects, and advantages of the present invention will be apparent to those skilled in the art having the benefit of this disclosure.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a process for preparing an ethylene copolymer having a density of about 0.93 g/mL or less in a particle form polymerization process using a titanium-containing catalyst. The process comprises contacting ethylene and at least one higher alpha-olefin in a liquid diluent with a catalyst and a cocatalyst under particle form polymerization conditions wherein the molar ratio of the comonomer to ethylene is at least about 1:1 and wherein the titanium-containing catalyst is prepared by contacting a titanium alkoxide and a magnesium dihalide in a liquid to obtain a solution, contacting the solution with a precipitating agent selected from organoaluminum compounds to obtain a solid, contacting the solid with an olefin to form a prepolymerized solid, contacting the prepolymerized solid with titanium tetrachloride, contacting the resulting solid with an orgenometallic reducing agent, and washing that resulting solid with a hydrocarbon to remove soluble material and result in a washed solid which is said catalyst.
DETAILED DESCRIPTION OF THE INVENTION
The general conditions used in particle form polymerization are well known in the art. Such processes can be carried out in a batch or continuous mode. A particularly desirable method for carrying out particle form polymerizations involve the use of continuous loop-type reactors such as disclosed in U.S. Pat. No. 3,152,872 and U.S. Pat. No. 4,424,341, the disclosures of which are incorporated herein by reference. In such processes, the polymerization conditions can be varied by changing the catalyst feed rate, the temperature, the monomer feed rate, the hydrogen feed rate, and the like.
The present invention requires the employment of a specially prepared catalyst. The preparation of the catalyst is described in EPC Published Application 480,375. The catalyst is prepared by contacting reactants comprising a titanium alkoxide and a magnesium dihalide in a suitable liquid to form a solution. The resulting solution is then contacted with a suitable precipitating agent and the resulting solid is contacted with an olefin to produce a prepolymerized solid. The prepolymerized solid is then contacted with titanium tetrachloride and then the thus resulting solid is contacted with an organometallic reducing agent. The solid resulting after that step is washed with a hydrocarbon to remove soluble material.
The invention is particularly useful when applied to a titanium-containing catalyst containing olefin prepolymer of the type disclosed in U.S. Pat. No. 4,325,837, the disclosure of which is incorporated herein by reference. Such catalysts are prepared by reacting a titanium alkoxide with a magnesium dihalide in a suitable liquid to form a solution. The resulting solution is then contacted with a suitable precipitating agent and the resulting solid is contacted with titanium tetrachloride either before or after prepolymer of an olefin is added to the solid.
Examples of the titanium alkoxides include the titanium tetraalkoxides in which the alkyl groups contain 1 to about 10 carbon atoms each. Some specific examples include titanium tetramethoxide, titanium dimethoxide diethoxide, titanium tetraethoxide, titanium tetra-n-butoxide, titanium tetrahexyloxide, titanium tetradecyloxide, titanium tetraisopropoxide, and titanium cyclohexyloxide.
The magnesium halide is preferably selected from magnesium chlorides.
The titanium alkoxide and the magnesium dihalide can be combined in any suitable liquid. Examples include substantially anhydrous organic liquids such as n-pentane, n-hexane, n-heptane, methylcyclohexane, toluene, xylenes, and the like.
The molar ratio of the transition metal compound to the metal halide can be selected over a relatively broad range. Generally, the molar ratio is within the range of about 10 to 1 to about 1 to 10, preferably between about 3 to 1 to about 0.5 to 2; however, more often the molar ratios are within the range of about 2 to 1 to about 1 to 2.
Generally, it is necessary to heat the liquid mixture to obtain a solution. Generally, the components are mixed at a temperature in the range of about 15° C. to about 150° C. The mixing can be carried out at atmospheric pressure or at higher pressures.
The time required for heating the two components is any suitable time which will result in a solution. Generally, this would be a time within the range of about 5 minutes to about 10 hours. Following the heating operation, the resulting solution can be filtered to remove any undissolved material or extraneous solid, if desired.
The precipitating agent is selected from organoaluminum compounds having the

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