Preparation of very low molecular weight polyethylene in a fluidized bed

Ethylene copolymers having a very low molecular weight are continuously prepared in a fluidized bed in the presence of reduced concentrations of hydrogen, at improved catalyst activity and polymer productivity, by effecting polymerization in the presence of a dialkylzinc compound.

FIELD OF THE INVENTION 
This invention relates to a process for preparing very low molecular weight 
ethylene copolymers in a fluidized bed. More particularly, this invention 
relates to a fluidized bed process for preparing ethylene copolymers 
having a melt index of from about 500 g/10 minutes to about 2500 g/10 
minutes. 
BACKGROUND OF THE INVENTION 
The low molecular weight polyethylene market is spanned by products having 
molecular weights varying from just a few hundred to several thousand. 
Polymers having weight average molecular weights of from about 2,000 to 
about 4,000 are generally classified as waxes, while polymers having 
weight average molecular weights of from about 4,000 to about 14,000 are 
generally classified as wax-resins. The waxes have melt indices varying 
from about 1000 g/10 minutes to about 2500 g/10 minutes, while the 
wax-resins have melt indices varying from about 500 g/10 minutes to about 
1000 g/10 minutes. 
Polyethylene waxes and wax-resins are conventionally prepared by the high 
pressure (&gt;100,000 kPa) homopolymerization of ethylene in stirred and 
elongated tubular reactors in the absence of solvent using free radical 
initiators. However, the elevated pressure required to produce these low 
molecular weight materials by this technique necessitates a high level of 
energy consumption which materially affects the manufacturing cost of 
these products. 
Gas-phase, fluidized bed processes, such as described in U.S. Pat. Nos. 
4,302,565 and 4,302,566, are well known cost effective methods of 
producing high modulus, high molecular weight ethylene copolymers having a 
density of from 0.91 g/cm.sup.3 to 0.96 g/cm.sup.3. However, such 
processes have not heretofore been employed to produce very low molecular 
weight products such as waxes and wax-resins. The reason for this is that 
low molecular weight materials of this type have low sintering 
temperatures which causes the polymer particles to soften and stick 
together at the reactor temperatures normally employed in fluidized bed 
polymerizations. As a result of this particle agglomeration, fluidization 
soon ceases and polymerization comes to a halt due to reactor fouling. 
European patent 0 120 053 discloses that low modulus ethylene copolymers 
having a density of less than 0.91 g/cm.sup.3 can be prepared in gas phase 
in a fluidized bed provided that a large volume of a diluent gas is 
present in the reaction mixture. According to this reference, hydrogen may 
be employed as a diluent gas in a mol ratio of hydrogen to ethylene of 
from 0.01:1 to 0.5:1. In this process, the hydrogen acts not only as a 
diluent, but also as a chain transfer agent to regulate the molecular 
weight of the copolymers produced by the process. Copolymers having a melt 
index of from greater than 0 g/10 minutes to about 25 g/10 minutes are 
produced by the process. 
Attempts have been made to produce very low molecular weight waxes and 
wax-resins in a fluidized bed by modifying the procedure of European 
patent 0 120 053, e.g., by increasing the concentration of hydrogen in the 
reactor. Hydrogen, of course, is a well known chain transfer agent and is 
frequently employed in ethylene polymerizations to control molecular 
weight. However, it has been found that catalyst activity is adversely 
affected at the high concentrations of hydrogen required to produce the 
desired products (at least 60 mol percent hydrogen in the reaction 
mixture), and if the concentration of hydrogen exceeds 50 mol percent, 
catalyst activity virtually ceases and polymerization in effect comes to a 
halt. 
Copending U.S. application Ser. No. 458,343 of M. C. Hwu et al., filed Dec. 
28, 1989, discloses that ethylene can be successfully polymerized with at 
least one higher alpha-olefin in a fluidized bed in the presence of high 
concentrations of hydrogen to produce ethylene copolymers having a melt 
index of from about 500 g/10 minutes to about 2500 g/10 minutes, while 
still maintaining satisfactory catalyst activity, provided that 
polymerization is first effected in the presence of a hydrogen 
concentration of less than 50 mol percent before the hydrogen 
concentration is increased to the level required to produce the desired 
products. 
S. Lin, J. Chen and Y. Lu have disclosed (Kinetic Study Of Supported 
Catalytic Ethylene Polymerization With The Effect Of Diethyl Zinc On 
Catalyst Efficiency And Regulation Of The Molecular Weight. Gao Fen Zi 
Tong Xun (Polymer Communications), No. 5, pg. 326-331, October, 1986. 
Institute Of Polymer Science, Zhongshen University, Guang Zhou) that 
diethylzinc can be employed to regulate the molecular weight of 
polyethylene and to increase the efficiency of a TiCl.sub.4 /MgCl.sub.2 
/Al(C.sub.2 H.sub.5).sub.3 catalyst prepared by grinding. However, no 
hydrogen was employed in the reaction system and no very low molecular 
weight polymers were produced in this manner. 
SUMMARY OF THE INVENTION 
In accordance with the present invention it has now been discovered that 
ethylene can be continuously polymerized with at least one higher 
alpha-olefin in a fluidized bed in the presence of reduced concentrations 
of hydrogen to produce ethylene copolymers having a melt index of from 
about 500 g/10 minutes to about 2500 g/10 minutes, at improved levels of 
catalyst activity and polymer productivity, provided that polymerization 
is conducted in the presence of a dialkylzinc compound. 
Thus, the present invention provides a process for continuously producing 
ethylene copolymers having a melt index of from about 500 g/10 minutes to 
about 2500 g/10 minutes at high levels of catalyst activity and polymer 
productivity which comprises copolymerizing ethylene and at least one 
higher alpha-olefin, by continuously contacting, in a fluidized bed, at a 
temperature of from 10.degree. C. up to 110.degree. C. and a pressure no 
greater than 7,000 kPa, a gaseous mixture containing 
(a) ethylene and at least one higher alpha-olefin in a molar ratio of such 
higher alpha-olefin to ethylene of from 0.01:1 to 2:1, and 
(b) from 30 mol percent to 90 mol percent hydrogen, 
with 
(I) a catalyst composition prepared by (i) forming a precursor composition 
from a magnesium compound, titanium compound, and electron donor compound; 
(ii) diluting said precursor composition with an inert carrier; and (iii) 
activating the diluted precursor composition with an organoaluminum 
compound; and 
(II) a dialkylzinc compound.

DETAILED DESCRIPTION OF THE INVENTION 
Fluidized bed reactors suitable for continuously preparing ethylene 
copolymers have been previously described and are well known in the art. 
Fluidized bed reactors useful for this purpose are described, e.g., in 
U.S. Pat. Nos. 4,302,565 and 4,302,566, the disclosures of which are 
incorporated herein by reference. 
In order to produce ethylene copolymers having a melt index of from about 
500 g/10 minutes to about 2500 g/10 minutes, it has previously been found 
necessary for the gaseous reaction mixture present in the polymerization 
reactor to contain at least 60 mol percent hydrogen. Ordinarily, however, 
as the concentration of hydrogen in the reactor increases, the rate of 
polymerization declines, and finally, at the hydrogen concentration levels 
required to produce the desired polymers, polymerization ceases 
completely. This effect is due both to (i) lowered ethylene partial 
pressure in the reactor and (ii) deactivation of the catalyst by the 
hydrogen as the hydrogen partial pressure increases. Surprisingly, 
however, it has recently been reported by M. C. Hwu et al., supra, that 
the desired polymers can be produced in high yields at satisfactory 
polymerization rates, in spite of the presence of hydrogen concentrations 
in excess of 60 mol percent in the reactor, if polymerization is first 
effected in the presence of less than 50 mol percent hydrogen before the 
hydrogen concentration is increased to the desired level of in excess of 
60 mol percent. And now, in accordance with the present invention, it has 
been discovered that such two-stage polymerization can be eliminated, and 
catalyst activity and polymer productivity can be further enhanced, if 
polymerization is conducted in the presence of a dialkylzinc compound. 
Such compounds evidently act to minimize the de-activating effect which 
hydrogen exerts on the catalyst, and thereby serve to increase polymer 
productivity. Further, because dialkylzinc compounds act as chain transfer 
agents in addition to activity promoters, it is possible to produce the 
desired low molecular weight polymers in the presence of reduced 
concentrations of hydrogen. 
The hydrogen employed in the process of the invention not only serves to 
reduce the molecular weight of the polymers produced by acting as a chain 
transfer agent, but also acts as a diluent gas which helps reduce the 
tackiness of the polymer particles and their tendency to agglomerate. In 
order to produce the desired copolymers, the gaseous mixture present in 
the reactor must contain at least 30 mol percent hydrogen. This 
concentration of hydrogen is necessary not only to reduce the molecular 
weight of the copolymers to the desired level, but also to prevent 
agglomeration of polymer particles and consequent fouling of the reactor. 
However, because of the presence of dialkylzinc in the reactor, it is 
possible to produce such copolymers in the presence of less than 60 mol 
percent hydrogen, and without a two-stage polymerization. If desired, 
however, hydrogen concentrations up to 90 mol percent can be tolerated. 
Preferably, the reaction mixture contains from 40 to 75 mol percent 
hydrogen, but most preferably less than 60 mol percent hydrogen. Generally 
the reaction mixture contains ethylene in an amount sufficient to produce 
a hydrogen to ethylene mol ratio of from 2.5:1 to 18:1, preferably from 
3.5:1 to 11:1. 
The dialkylzinc compounds employed in the process of the present invention 
can be illustrated by the formula 
EQU ZnRR' 
wherein R and R' are alkyl radicals, which radicals may be the same or 
different. Generally R and R' are alkyl radicals containing from 1 to 12 
carbon atoms, usually from 1 to 6 carbon atoms. Such radicals may be 
cyclic, branched or straight chain, and may be substituted with one or 
more substituents which are nonreactive with all the components of the 
catalyst system as well as all the components of the reaction system. 
Illustrative of such radicals are methyl, ethyl, n-propyl, isopropyl, 
n-butyl, tert-butyl, n-hexyl, n-decyl, and the like. 
The dialkylzinc compounds can be used individually or in combination 
thereof, and include compounds such as diethylzinc, diisobutylzinc, and 
di-n-decylzinc. Diethylzinc is particularily preferred. 
The higher alpha-olefin employed to copolymerize with ethylene to produce 
the copolymers of the present invention plays an important role in 
determining the properties of such copolymers. Thus, for example, as the 
amount of higher alpha-olefin which copolymerizes with ethylene increases, 
copolymers having progressively lower densities are obtained at any given 
melt index. The amount of higher olefin needed to produce copolymers of a 
given density will vary from olefin to olefin, under the same conditions, 
with larger amounts of olefin required as the number of carbon atoms in 
the olefin decreases. 
The higher alpha-olefin employed to copolymerize with ethylene also affects 
the molecular weight of the copolymers produced, with higher melt indices 
being obtained as the concentration of comonomer in the polymer increases. 
Polymer production also increases in the presence of comonomer. 
The higher alpha-olefin which is copolymerized with ethylene to produce the 
low molecular weight copolymers of the present invention may be present in 
the gaseous reaction mixture in an amount sufficient to provide a molar 
ratio of higher olefin to ethylene of from 0.01:1 to 2:1, preferably from 
0.05:1 to 1:1. Such ratios will produce copolymers having a density of 
from 0.88 g/cm.sup.3 to 0.96 g/cm.sup.3. 
The higher alpha-olefins which can be polymerized with ethylene to produce 
the low molecular weight copolymers of the present invention can contain 
from 3 to 8 carbon atoms. These alpha-olefins should not contain and 
branching on any of their carbon atoms closer than two carbon atoms 
removed from the double bond. Suitable alpha-olefins include propylene, 
butene-1, pentene-1, hexene-1, 4-methylpentene-1, heptene-1 and octene-1. 
The preferred alpha-olefins are propylene, butene-1, hexene-1, 
4-methylpentene-1 and octene-1. 
In addition to hydrogen, ethylene, and at least one higher alpha-olefin, 
the reaction mixture may also contain from 10 to 40 mol percent of an 
inert gas. By an "inert" gas is meant a gas which is nonreactive under the 
conditions employed during polymerization, i.e., does not decompose and/or 
react with the polymerizable monomers or other components of the reaction 
system under the polymerization conditions employed in the reactor. In 
addition such gas should be insoluble in the polymer product produced so 
as not to contribute to polymer tackiness. Among such gases are nitrogen, 
argon, helium, methane, ethane, and the like. 
The gaseous reaction mixture should, of course, be substantially free of 
catalyst poisons, such as moisture, oxygen, carbon monoxide, carbon 
dioxide, acetylene and the like. 
In order to prevent polymer agglomeration, it is, of course, necessary to 
conduct polymerization at a temperature below the sintering temperature of 
the polymer product. Since the sintering temperature decreases along with 
the molecular weight of the product, the greater the concentration of 
hydrogen and dialkylzinc employed in the reactor, the lower the reaction 
temperature must be in order to prevent agglomeration. On the other hand, 
the temperature employed must be sufficiently elevated to prevent 
substantial condensation of the reaction mixture to the liquid state, as 
such condensation will cause the polymer particles being produced to 
cohere to each other and likewise aggravate the polymer agglomeration 
problem. This difficulty is normally associated with the use of 
alpha-olefins having 5 or more carbon atoms which have relatively high dew 
points. While some minor condensation is tolerable, anything beyond this 
will cause reactor fouling. Generally, in order to continuously produce 
the desired polymers while preventing polymer agglomeration, the 
temperature must not be permitted to rise above 110.degree. C. Usually 
temperatures of from 10.degree. C. to 100.degree. C. are employed, 
depending upon the particular comonomer employed, the concentration of 
such comonomer in the reactor, and the molecular weight of the polymer 
product. Temperatures above 100.degree. C. should be avoided when the 
hydrogen concentration in the reaction mixture exceeds 60 mol percent. 
Pressures of up to about 7000 kPa can be employed in the process. Pressures 
of from about 70 kPa to 2500 kPa are preferred. 
In order to maintain a viable fluidized bed, the superficial gas velocity 
of the gaseous reaction mixture through the bed must exceed the minimum 
flow required for fluidization, and preferably is at least 0.2 feet per 
second above the minimum flow. Ordinarily the superficial gas velocity 
does not exceed 5.0 feet per second, and most usually no more than 2.5 
feet per second is sufficient. 
The catalyst compositions employed in the process of the present invention 
are produced by (i) forming a precursor composition from a magnesium 
compound, titanium compound, and electron donor compound; (ii) diluting 
said precursor composition with an inert carrier; and (iii) activating the 
diluted precursor composition with an organoaluminum compound. 
The precursor composition is formed by dissolving at least one titanium 
compound and at least one magnesium compound in at least one electron 
donor compound at a temperature of from about 20.degree. C. up to the 
boiling point of the electron donor compound. The titanium compound(s) can 
be added to the electron donor compound(s) before or after the addition of 
the magnesium compound(s), or concurrent therewith. Dissolution of the 
titanium compound(s) and the magnesium compound(s) in the election donor 
compound(s) can be facilitated by stirring, and in some instances by 
refluxing, these two compounds in the electron donor compound(s). After 
the titanium compound(s) and the magnesium compound(s) are dissolved, the 
precursor composition may be isolated by crystallization or by 
precipitation with an aliphatic or aromatic hydrocarbon containing from 5 
to 8 carbon atoms, such as hexane, isopentane or benzene. The crystallized 
or precipitated precursor composition may be isolated in the form of fine, 
free-flowing particles having an average particle size of from about 10 
microns to about 100 microns after drying at temperatures up to 60.degree. 
C. 
About 0.5 mol to about 56 mols, preferably about 1 mol to about 10 mols, of 
the magnesium compound(s) are used per mol of the titanium compound(s) in 
preparing the precursor composition. 
The titanium compound(s) employed in preparing the precursor composition 
has the structure 
EQU Ti(OR").sub.a X.sub.b 
wherein R" is an aliphatic or aromatic hydrocarbon radical containing from 
1 to 14 carbons atoms, or COR'" where R'" is an aliphatic or aromatic 
hydrocarbon radical containing from 1 to 14 carbon atoms, 
X is selected from the group consisting of Cl, Br, I, and mixtures thereof, 
a is 0, 1 or 2, b is 1 to 4 inclusive, and a +b=3 or 4. 
R" and R'" may be substituted with one or more substituents which are inert 
under the reaction conditions employed during preparation of and 
polymerization with the precursor composition. When R" and R'" are 
aliphatic, they may be cyclic, branched or straight chain. 
Suitable titanium compounds include TiCl.sub.3, TiCl.sub.4, 
Ti(OCH.sub.3)Cl.sub.3, Ti(OC.sub.6 H.sub.5)Cl.sub.3, 
Ti(OCOCH.sub.3)Cl.sub.3 and Ti(OCOC.sub.6 H.sub.5)Cl.sub.3. TiCl.sub.3 is 
preferred because catalysts containing this material show higher activity 
at the high hydrogen concentrations employed in the process of the present 
invention. In addition, limited ethylene hydrogenation occurs in the 
presence of TiCl.sub.3 compared to other titanium materials. 
The magnesium compound(s) employed in preparing the precursor composition 
has the structure 
EQU MgX.sub.2 
wherein X is selected from the group consisting of Cl, Br, I, and mixtures 
thereof. 
Suitable magnesium compounds include MgCl.sub.2, MgBr.sub.2 and MgI.sub.2. 
Anhydrous MgCl.sub.2 is particularly preferred. 
The electron donor compound(s) employed in preparing the precursor 
composition is an organic compound which is liquid at 25.degree. C. and in 
which the titanium and magnesium compounds are soluble. The electron donor 
compounds are known as such, or as Lewis bases. 
Suitable electron donor compounds include alkyl esters of aliphatic and 
aromatic carboxylic acids, aliphatic ethers, cycloaliphatic ethers and 
aliphatic ketones. Particularly useful are alkyl esters of saturated 
aliphatic carboxylic acids containing from 1 to 4 carbon atoms; alkyl 
esters of aromatic carboxylic acids containing from 7 to 8 carbon atoms; 
aliphatic ethers containing from 2 to 8 carbon atoms, preferably from 4 to 
5 carbon atoms; cycloaliphatic ethers containing from 4 to 5 carbon atoms, 
preferably mono- or di-ethers containing 4 carbon atoms; and aliphatic 
ketones containing from 3 to 6 carbon atoms, preferably from 3 to 4 carbon 
atoms. The most preferred electron donor is tetrahydrofuran. If desired, 
these electron donor compounds may be substituted with one or more 
substituents which are inert under the reaction conditions employed during 
preparation of and polymerization with the precursor composition. 
After the precursor composition has been prepared it is diluted with an 
inert carrier material by (1) mechanically mixing or (2) impregnating such 
composition into the carrier material. 
Mechanical mixing of the inert carrier and precursor composition is 
effected by blending these materials together using conventional 
techniques. The blended mixture suitably contains from about 3 percent by 
weight to about 50 percent by weight of the precursor composition. 
Impregnation of the inert carrier material with the precursor composition 
may be accomplished by dissolving the precursor composition in the 
electron donor compound, and then admixing the support with the dissolved 
precursor composition to impregnate the support. The solvent is then 
removed by drying at temperatures up to about 85.degree. C. 
The support may also be impregnated with the precursor composition by 
adding the support to a solution of the chemical raw materials used to 
form the precursor composition in the electron donor compound, without 
isolating the precursor composition from said solution. The excess 
electron donor compound is then removed by drying at temperatures up to 
about 85.degree. C. 
The blended or impregnated precursor composition prepared as disclosed 
herein has the formula 
EQU Mg.sub.m Ti(OR").sub.n X.sub.p [ED].sub.q 
wherein R" is an aliphatic or aromatic hydrocarbon radical containing from 
1 to 14 carbon atoms, or COR'" wherein R'" is also an aliphatic or 
aromatic hydrocarbon radical containing from 1 to 14 carbon atoms, 
X is selected from the group consisting of Cl, Br, I, and mixtures thereof, 
ED is an electron donor compound, 
m is 0.5 to 56, preferably 1.5 to 5, 
n is 0, 1 or 2, 
p is 2 to 116, preferably 6 to 14, and 
q is 2 to 85, preferably 3 to 10. 
Suitably, the impregnated carrier material contains from about 3 percent by 
weight to about 50 percent by weight, preferably from about 10 percent by 
weight to about 30 percent by weight, of the precursor composition. 
The carrier materials employed to dilute the precursor composition are 
solid, particulate, porous materials which are inert to the other 
components of the catalyst composition, and to the other active components 
of the reaction system. Suitable carrier materials include inorganic 
materials such as oxides of silicon and/or aluminum. Usually these 
materials have an average particle size of from about 10 microns to about 
250 microns, preferably from about 20 microns to about 150 microns, and a 
surface area of at least 3 square meters per gram, preferably at least 50 
square meters per gram. Polymerization activity of the catalyst can be 
improved by employing a silica support having an average pore size of at 
least 80 Angstrom units, preferably at least 100 Angstrom units. The 
carrier material should be dry, that is, free of absorbed water. Drying of 
the carrier material can be effected by heating, e.g., at a temperature of 
at least 600.degree. C. when silica is employed as the support. 
Alternatively, when silica is employed, it may be dried at a temperature 
of at least 200.degree. C. and treated with about 1 weight percent to 
about 8 weight percent of one or more of the aluminum activator compounds 
described below. Modification of the support with an aluminum compound in 
this manner increases catalyst activity and improves polymer morphology of 
the resulting ethylene copolymers. Other organometallic compounds, such as 
diethylzinc, may also be used to modify the support. 
To be useful in producing ethylene copolymers, the precursor composition 
must be activated with a compound capable of transforming the titanium 
atoms in the precursor composition to a state which will cause ethylene to 
effectively copolymerize with higher alpha olefins. Such activation is 
effected by means of an organoaluminum compound having the structure 
EQU Al(R"").sub.d X'.sub.e H.sub.f 
wherein X' is Cl or OR'"", 
R"" and R'"" are saturated hydrocarbon radicals containing from 1 to 14 
carbon atoms, which radicals may be the same or different, 
e is 0 to 1.5, 
f is 0 or 1, and 
d+e+f=3. 
R"" and R'"" may be substituted with one or more substituents which are 
inert under the reaction conditions employed during polymerization. 
Preferably R"" and R'"" are alkyl radicals containing from 1 to 8 carbon 
atoms. 
Such activator compounds can be employed individually or in combination 
thereof and include compounds such as Al(C.sub.2 H.sub.5).sub.3, 
Al(C.sub.2 H.sub.5).sub.2 Cl, Al.sub.2 (C.sub.2 H.sub.5).sub.3 Cl.sub.3, 
Al(C.sub.2 H.sub.5).sub.3, Al(C.sub.2 H.sub.5).sub.2)Cl, Al(i-C.sub.4 
H.sub.9).sub.3, Al(i-C.sub.4 H.sub.9).sub.2 H, Al(C.sub.6 H.sub.13).sub.3 
and Al(C.sub.8 H.sub.17).sub.3. 
If desired, the precursor composition may be partially activated before it 
is introduced into the polymerization reactor. However, any activation 
undertaken outside of the polymerization reactor should be limited to the 
addition of an amount of activator compound which does not raise the molar 
ratio of activator compound:electron donor in the precursor composition 
beyond 0.5:1, as higher ratios have been found to substantially lower 
catalyst activity. In order to maintain maximum catalyst activity, the 
activator compound is preferably employed in an amount which will provide 
the precursor composition with an activator compound:electron donor molar 
ratio of from about 0.1:1 to about 0.3:1. Such partial activation is 
carried out in a hydrocarbon solvent slurry followed by drying of the 
resulting mixture, to remove the solvent, at temperatures of from about 
20.degree. C. to about 80.degree. C., preferably from about 50.degree. C. 
to about 70.degree. C. The resulting product is a free-flowing solid 
particulate material which can be readily fed to the polymerization 
reactor where the activation is completed with additional activator 
compound which can be the same or a different compound. 
Alternatively, when an impregnated precursor composition is employed, it 
may, if desired, be completely activated in the polymerization reactor 
without any prior activation outside of the reactor, in the manner 
described in U.S. Pat. No. 4,383,095. 
During the continuous gas phase fluidized bed polymerization disclosed 
herein, discrete portions of the partially activated or totally 
unactivated precursor composition are continually fed to the reactor, 
along with discrete portions of the activator compound necessary to 
complete the activation of the partially activated or totally unactivated 
precursor composition, in order to replace active catalyst sites that are 
expended during the course of the reaction. Discrete portions of the 
dialkylzinc compound are also continually fed to the reactor in order to 
minimize the de-activating effect which the high concentration of hydrogen 
exerts on the catalyst, thereby enhancing catalyst activity and polymer 
productivity. 
The dialkylzinc compound, the partially activated or totally unactivated 
precursor composition, and the required amount of activator compound 
necessary to complete activation of the precursor composition are 
preferably introduced into the reactor through separate feed lines. The 
dialkylzinc compound and the activator compound are conveniently and 
preferably strayed into the reactor dissolved in an inert liquid solvent, 
i.e., a solvent which is nonreactive with all the components of the 
catalyst system as well as all the components of the reaction system. 
Hydrocarbons such as isopentane, hexane, and mineral oil are preferred for 
this purpose. Generally, such solutions contain from about 2 weight 
percent to about 30 weight percent of the activator compound and/or the 
dialkylzinc compound. If desired, less concentrated or more concentrated 
solutions can be employed, or, alternatively, the activator compound and 
the dialkylzinc compound can be added in the absence of solvent, or, if 
desired, suspended in a stream of liquid monomer. 
Any solvent employed to introduce the dialkylzinc compound and the 
activator compound into the reactor is, of course, immediately vaporized 
in the reactor so that gaseous conditions are maintained in the reactor at 
all times. The amount of solvent employed should, of course, be carefully 
controlled so as to avoid the use of excessive quantities of liquid which 
would prevent the rapid vaporization thereof. 
The activator compound should be added to the reactor in such amounts as to 
provide, in the reactor, a total aluminum:titanium atomic ratio of from 
about 10:1 to about 400:1, preferably from about 25:1 to about 60:1. 
The dialkylzinc compound should be added to the reactor in such amounts as 
to provide an atomic ratio of zinc:titanium of from about 3:1 to about 
40:1, preferably from about 5:1 to about 25:1. 
By operating under the polymerization conditions described herein it is 
possible to continuously polymerize ethylene in a fluidized bed with one 
or more higher alpha olefins containing from 3 to 8 carbon atoms to 
produce low molecular weight copolymers having a melt index of from about 
500 g/10 minutes to about 2500 g/10 minutes. By "continuously polymerize" 
as used herein is meant the capability of uninterrupted polymerization for 
weeks at a time, i.e., at least in excess of 168 hours, and usually in 
excess of 1000 hours without reactor fouling due to the production of 
large agglomerations of polymer. 
The low molecular weight ethylene copolymers produced in accordance with 
the process of the present invention usually have a density of from 0.88 
g/cm.sup.3 to 0.96 g/cm.sup.3. Such copolymers contain from 54 mol percent 
to 94 mol percent of polymerized ethylene and from 6 mol percent to 46 mol 
percent of polymerized alpha-olefin containing from 3 to 8 carbon atoms. 
The low molecular weight copolymers produced in accordance with the present 
invention have a weight average molecular weight (M.sub.w) of from about 
2,000 to about 14,000 and a number average molecular weight (M.sub.n) of 
from about 1,000 to about 7,000. The molecular weight distribution 
(M.sub.w /M.sub.n) of such copolymers, defined as the ratio of weight 
average molecular weight to number average molecular weight, may vary from 
about 1.5 to about 3.0, usually from about 1.7 to about 2.5. 
The low molecular weight ethylene copolymers produced in accordance with 
the process of the present invention have a melt index of from about 500 
g/10 minutes to about 2500 g/10 minutes, preferably of from about 500 g,10 
minutes to about 2000 g/10 minutes. The melt index of a polymer varies 
inversely with its molecular weight and is a function of the 
hydrogen/monomer ratio employed in the reaction system, the amount of 
dialkylzinc present, the polymerization temperature, and the density of 
the polymer. Thus, the melt index is raised by increasing the 
hydrogen/monomer ratio, the concentration of dialkylzinc, the 
polymerization temperature, and/or the ratio of higher alpha-olefin to 
ethylene employed in the reaction system. 
The low molecular weight copolymers produced in accordance with the present 
invention have a melting point of from about 110.degree. C. to about 
145.degree. C. 
The low molecular weight copolymers produced in accordance with the present 
invention have a residual catalyst (precursor) content of less than 0.05 
weight percent. The residual titanium content of such copolymers is less 
than 4 parts per million (ppm), usually from 2 to 3 parts per million 
(ppm). When produced in the absence of dialkylzinc compound, such 
copolymers have a residual catalyst (precursor) content of at least 0.05 
weight percent up to about 0.20 weight percent. The residual titanium 
content of such copolymers is from about 4 to 15 parts per million (ppm), 
usually from about 4 to 8 parts per million (ppm). 
The low molecular weight ethylene copolymers produced in accordance with 
the present invention are granular materials having an average particle 
size of from about 0.01 to about 0.07 inches, usually of from about 0.02 
to about 0.05 inches, in diameter. Particle size is important for the 
purpose of readily fluidizing the polymer particles in the fluid bed 
reactor. 
The ethylene polymers produced in accordance with the process of the 
present invention have a bulk density of from about 16 pounds per cubic 
foot to about 25 pounds per cubic foot. 
The following Examples are designed to illustrate the present invention and 
are not intended as a limitation upon the scope thereof. 
The properties of the polymers described herein were determined by the 
following test methods: 
Density 
ASTM D-1505. A plaque is made and conditioned for one hour at 100.degree. 
C. to approach equilibrium crystallinity. Measurement for density is then 
made in a density gradient column and density values are reported as 
grams/cm.sup.3. 
MELT INDEX (MI) 
Determined at 190.degree. C. for polymers having a melt index of less than 
1000 grams per 10 minutes by applying a weight load of 2050 grams to the 
sample and counting the number of seconds required for the molten polymer 
to flow one inch through a die having a diameter of 0.041.+-.0.002 inch 
and a height of 0.157.+-.0.002 inch. Melt index was then calculated 
according to the equation: 
##EQU1## 
where T is the time in seconds and K is a constant determined from a 
polymer of known melt index. 
The melt index of polymers having a melt index of greater than 1,000 grams 
per 10 minutes is determined in a similar manner at 125.degree. C. using a 
weight load of 225 grams. 
Fines 
Weight percent of polymer particles which pass through a 120 mesh screen. 
Rubble 
Weight percent of polymer particles which are collected on a 10 mesh 
screen. 
Productivity 
A sample of the resin product is ashed, and the weight percent of ash is 
determined. The amount of Ti, Mg and halide in the ash is determined by 
elemental analysis. Productivity is expressed in terms of pounds of 
polymer produced per pound of catalyst (precursor) consumed and/or parts 
per million of Ti in the polymer. 
Average Particle Size 
Calculated from sieve analysis data according to ASTM D-1921, Method A, 
using a 500 g sample. Calculations are based on weight fractions retained 
on the screens. 
Bulk Density 
ASTM D-1895, Method B The resin is poured via 3/8" diameter funnel into a 
400 ml graduated cylinder to the 400 ml line without shaking the cylinder, 
and weighed by difference. 
EXAMPLE 1 
Impregnation of Support with Precursor 
In a 12 liter flask equipped with a mechanical stirrer were placed 41.8g 
(0.439 mol) of anhydrous MgCl.sub.2 and 2.5 liters of tetrahydrofuran 
(THF). To this mixture, 29.0g (0.146 mol) of TiCl.sub.3 .multidot.0.33 
AlCl.sub.3 were added, and the resulting slurry was heated at 60.degree. 
C. for 1/2 hour in order to completely dissolve the magnesium and titanium 
compounds. 
Five hundred grams (500 g) of silica were dehydrated by heating at a 
temperature of 600.degree. C. and slurried in 3 liters of isopentane. The 
slurry was stirred while 186 ml. of a 20 percent by weight solution of 
triethylaluminum in hexane was added thereto over a 1/4 hour period. The 
resulting mixture was then dried under a nitrogen purge at 60.degree. C. 
over a period of about 4 hours to provide a dry, free-flowing powder 
containing 5.5 percent by weight triethylaluminum. 
The treated silica was then added to the solution prepared as described 
above. The resulting slurry was stirred for 1/4 hour and then dried under 
a nitrogen purge at 60.degree. C. over a period of about 4 hours to 
provide a dry, impregnated, free-flowing powder containing about 9 percent 
THF. 
EXAMPLE 2 
Preparation of Partially Activated Precursor 
The silica-impregnated precursor composition prepared in accordance with 
Example 1 was slurried in 3 liters of anhydrous isopentane and stirred 
while a 20 percent by weight solution of tri-n-hexylaluminum in anhydrous 
hexane was added thereto over a 1/4 hour period. The tri-n-hexyl(aluminum 
solution was employed in an amount sufficient to provide 0.2 mols of this 
compound per mol of tetrahydrofuran in the precursor. The mixture was then 
dried under a nitrogen purge at a temperature of 65.+-.10.degree. C. over 
a period of about 4 hours to provide a dry, free-flowing powder. This 
material was stored under dry nitrogen until it was needed. 
Comparative Example A 
The silica-impregnated precursor composition prepared in accordance with 
Example 1 and partially activated in accordance with Example 2 was 
employed together with triethylaluminum, as cocatalyst, to copolymerize 
ethylene with hexene-1 in the presence of 4.4 mol percent hydrogen in a 
fluidized bed reactor system similar to the one described and illustrated 
in U.S. Pat. Nos. 4,302,565 and 4,302,566. The polymerization reactor had 
a lower section 10 feet high and 18 inches in diameter, and an upper 
section 12.5 feet high and 30 inches in diameter. 
The silica-impregnated precursor composition was continually fed to the 
polymerization reactor along with a 5 percent solution of 
tri-triethylaluminum in isopentane so as to provide a completely activated 
catalyst in the reactor having an aluminum:titanium atomic ratio of about 
45:1. 
Polymerization was effected at a pressure of 2068 kPa and a temperature of 
86.degree. C. using a gaseous reaction mixture containing 30.5 mol percent 
ethylene, 2.5 mol percent hexene-1, 4.4 mol percent hydrogen, and about 
55.5 mol percent nitrogen. The gaseous mixture was passed through the bed 
at a superficial gas velocity of about 1.8 ft/sec. 
Table 1 below summarizes the reaction conditions employed in this 
polymerization, the properties of the copolymer produced by such 
polymerization under steady state conditions, and the productivity of the 
catalyst system employed. 
Comparative Example B 
Ethylene was copolymerized with hexene-1 in the presence of 33.3 mol 
percent hydrogen using the same fluidized bed reactor system and catalyst 
system employed in Comparative Example A. 
Polymerization was effected at a pressure of 2068 kPa and a temperature of 
100.degree. C. using a gaseous reaction mixture containing 33.3 mol 
percent ethylene, 1.7 mol percent hexene-1, 33.3 mol percent hydrogen, and 
25.0 mol percent nitrogen. The gaseous mixture was passed through the bed 
at a superficial gas velocity of about 1.8 ft/sec. 
After steady state conditions were attained, the copolymer being produced 
had a melt index of 140 g/10 minutes compared to 0.5 g/10 minutes in 
Comparative Example A. The titanium content of the copolymer was 3.60 ppm 
compared to 3.32 ppm in Comparative Example A. The conditions attained at 
that time are set forth in Comparative Example B of Table 1. 
Comparative Example C 
A 5 percent by weight solution of diethylzinc in isopentane was continually 
added to the reaction mixture of Comparative Example B over a 20 hour 
period until the atomic ratio of zinc:titanium in the reactor reached a 
ratio of 6:1 and the conditions shown in Comparative Example C of Table 1 
were attained. At the end of the 20 hour period, the copolymer being 
produced had a melt index of 302 g/10 min. compared to 140 g/10 min. in 
Comparative Example B. The titanium content of the copolymer was 2.71 ppm 
compared to 3.60 ppm in Comparative Example B. 
EXAMPLE 3 
The composition of the gaseous reaction mixture of Comparative Example C 
was altered over a period of 4 days until the mixture contained 47.9 mol 
percent hydrogen and the atomic ratio of zinc:titanium in the reactor 
reached a ratio of 21:1. 
After steady state conditions were attained, the copolymer being produced 
had a melt index of 930 g/10 min. compared to 302 g/10 min. in Comparative 
Example C. Despite the presence of almost 12 percent more hydrogen in the 
reactor, the copolymer had a titanium content of 2.37 ppm compared to 2.71 
ppm in Comparative Example C. The conditions attained at that time are set 
forth in Example 3 of Table 1. 
TABLE 1 
______________________________________ 
Comp. Comp. Comp. 
Example Exp. A Exp. B Exp. C Exp. 3 
______________________________________ 
Polymerization 
Conditions 
Temperature, .degree.C. 
86 100 100 100 
Pressure, kPa 
2068 2068 2068 2068 
Al/Ti Atomic Ratio 
45 72 75 74 
Zn/Ti Atomic Ratio 
0 0 6 21 
Space-Time Yield 
4 5 4 7 
(lb/hr/ft.sup.3) 
Mol % Hydrogen 
4.4 33.3 36.0 47.9 
Mol % Nitrogen 
.about.55.5 
25.0 27.0 10.0 
Mol % Ethylene 
30.5 33.3 34.3 34.0 
Mol % Hexene 2.5 1.7 1.7 2.0 
Hydrogen/Ethylene 
0.14 1.00 1.05 1.41 
Mol Ratio 
Hexene/Ethylene 
0.08 0.05 0.05 0.06 
Mol Ratio 
Polymer Properties 
Melt Index, g/10 min 
0.5 140 302 930 
Density, gm/cc 
0.930 0.948 0.945 
0.946 
Bulk Density, lbs/ft.sup. 3 
27.0 23.0 21.2 19.7 
Avg. Particle Size, in 
0.030 0.013 0.011 
0.012 
Fines, % 1.2 8.3 7.9 4.0 
Rubble, % 0 0 0 0 
Productivity 
Lbs Polymer/Lb 
3010 2780 3690 4220 
Catalyst 
Ti, ppm. 3.32 3.60 2.71 2.37 
______________________________________