In the polymerization of C.sub.2 to C.sub.8 .alpha.-olefins such as ethylene, propylene, butene-1, etc. and mixtures thereof, the conventional catalyst system is a titanium chloride catalyst activated with an organo aluminum compound, e.g. a cocrystallized titanium chloride-aluminum chloride catalyst of the general formula n.multidot.TiCl.sub.3 .multidot.AlCl.sub.3 activated with diethyl aluminum chloride or triethyl aluminum. It is a well known fact, that any measures taken to increase the efficiency (productivity) of the catalyst system, e.g. by raising polymerization temperature, is generally accompanied by an undesired increase in amorphous content of the polymer, as evidenced by a decrease in polymer heptane insolubles content, hereinafter called HI content.
There are limits however to the temperatures that can be used in polymerization reactions carried out in the liquid phase. These, by necessity, include process steps where the polymer product is separated from the liquid diluent, be it either an inert solvent or liquefied monomer. In these separation steps are used various pieces of equipment including conduits, filters, etc., which will plug if this polymer becomes tacky due to high amorphous polymer content. It has been found that in order to assure trouble-free operations the HI content of the polymer product from the reactor should roughly be about 80% or more, however, the actual minimum value most probably varies from one installation to another.
It is evident from the above that for any specific polymerization system there is an optimum reaction temperature or temperature range that will result in a product of acceptable quality and yield. Lower than optimum conditions would result in less than acceptable productivity, and at higher temperatures than the optimum, the increased productivity would be more than offset by the accompanying deterioration of product quality. It should be understood, that often there are many other considerations in addition to operability of this equipment that are taken into account when determining the optimum conditions, e.g. costs of raw materials and utilities costs of disposal of amorphous byproducts, desired physical properties of the finished product, etc. In other words, the optimum conditions are not necessarily those where the reaction system has reached its operational limit.
In the commercial production of highly crystalline polymers, such as propylene homopolymer, using the conventional catalyst, the optimum temperature range is typically from about 140.degree. to 160.degree. F, generally resulting in HI values between about 89 to about 93 measured on undeashed polymer product. However in the production of many other polymers of less crystallinity much lower temperatures, e.g. 140.degree. F and below, must be used or severe operational problems are experienced. In fact, attempts to commercialize processes for the production of some polymers, such as polybutene-1, various random copolymers of propylene and another comonomer such as ethylene, have either been very disappointing or completely unsuccessful, since due to the inherent lesser crystallinity of these polymers the polymerization temperatures must be maintained at such low levels that the productivities are unacceptable.
Recently several new olefin polymerization catalyst systems of increased efficiency and/or increased stereo specificity have been reported in the art. U.S. Pat. No. 3,644,320 discloses one such catalyst system, which consists of a trithiophosphate or a trithiophosphite added to a conventional titanium chloride-organo aluminum catalyst composition in amounts to provide a mole ratio of the additive to the organo aluminum component of at least 0.005. In small scale batch polymerizations of propylene it was shown that products could be obtained in higher yields and having higher heptane-insoluble contents when using the three component catalyst system instead of the conventional two component system.
During the preliminary small scale batch experimentation leading up to the present invention, the beneficial effects of including trithiophosphite in the catalyst system on polymer quality and catalyst efficiency were indeed verified. However, under continuous polymerization conditions, i.e. where monomer feed and catalyst components are fed continuously to the reactor and product is withdrawn in a continuous or "pseudo" continuous fashion, it was quite surprisingly found that the expected increase in catalyst efficiency did not materialize, in fact, the efficiency was generally somewhat lower than that of a conventional catalyst and did not improve by an increase in the trithiophosphite content. The following Table 1 sets are typical values of relative catalyst efficiencies obtained in various comparative, continuous propylene polymerization tests in the presence of a three-component catalyst system containing 3.multidot.TiCl.sub.3 .multidot.AlCl.sub.3, diethylaluminum chloride and trilauryl trithiophosphite. The catalyst efficiency of the conventional catalyst has been arbitrarily set at 100%.
TABLE 1 ______________________________________ Effect of Trialaurylphosphite on Catalyst Efficiency Thiophosphite Thiophosphite.sub.(1) Organoaluminum Efficiency Rate-ppm Mole Ratio % ______________________________________ 0 0 100 50-100 0.01-0.02 .about.85-95 280 0.06 .about.75-85 ______________________________________ .sup.(1) Based on propylene monomer feed rate
Although not wishing to be bound by any theory for an explanation of the surprising absence of any improvement in catalyst efficiency, when using the three-component catalyst system of the aforementioned patent in a continuous polymerization process, the fact still remains that the catalyst is not suitable in many commercial applications, where increase in polymer quality as well as catalyst efficiency is a requirement.
Another newly developed olefin polymerization catalyst composition having increased efficiency and stereo-specificity is disclosed in Belgian Pat. No. 818,474. Briefly described, the catalyst composition is a two-component system wherein one of the components is a titanium trichloride, preferably as cocrystallized 3.multidot.TiCl.sub.3 .multidot.AlCl.sub.3, which has been modified by treatment with phosphorus oxytrichloride in the presence of a aromatic hydrocarbon. The other component is a conventional organo aluminum compound.
In small scale batch propylene polymerization tests, the reported improvements in yield and polymer quality (i.e. heptane insolubles content) obtained with this new catalyst were verified. However, further evaluation in relatively large scale continuous polymerization tests showed that although the polymer heptane insoluble contents did improve, the efficiency of the catalyst at reactor temperatures of about 140.degree. F and below was at best no better, and in most cases lower, than that of conventional unmodified catalyst. Although, an additional advantage of the modified catalyst composition is the retention of higher polymer heptane insoluble contents when conducting the polymerization at higher temperatures to increase polymer yield, the commercial use of this catalyst composition is still somewhat limited, in that many polymers, e.g. random copolymers of ethylene and propylene containing about 3 weight percent or more polymerized ethylene, cannot be produced therewith at the required levels of polymer heptane insolubles to assure trouble-free operations.
It is therefore an object of the present invention to provide an improved olefin polymerization catalyst composition.
Another object of the invention is to provide a catalyst composition for the continuous production of .alpha.-olefin polymers of high heptane insolubles content at improved rates.
Other objects will become apparent from a reading of the specification and appended claims.