Low density polyethylene

A process for copolymerizing ethylene with another monoolefin is described wherein the other monoolefin is incorporated into the polymer at very high efficiencies and in several instances the copolymer produced contains a higher concentration of the comonomer than the gas phase in the polymerization zone. A novel ethylene/1-hexene copolymer having very high relative comonomer dispersities (99% or higher, and even above 100%) is also described.

This invention relates to a process to produce copolymers of ethylene and a 
minor amount of other 1-olefins. Another aspect of this invention relates 
to novel ethylene/1-hexene copolymers. 
BACKGROUND OF THE INVENTION 
Polyethylene is commercially produced in a variety of ways. Catalysts based 
on chromium and related polymerization processes are described in U.S. 
Pat. No. 2,825,721. The chromium catalysts and the corresponding 
polymerization processes have wide spread acceptance in the polymerization 
and copolymerization of ethylene. Slurry processes in which the catalyst, 
the monomer or monomers and a diluent are subjected to polymerization 
conditions for the monomers are known in the art. 
Many homopolymers of ethylene and copolymers of ethylene and other olefins 
have been described, produced and sold. In recent years, so called linear 
low density polyethylenes have been marketed. While the original low 
density polyethylenes were characterized by long chain branching as well 
as short chain branches and thus were not linear, the modern low density 
polyethylenes are linear polymers, i.e. they have essentially no long 
chain branching but contain short chain branching introduced into the 
molecule by a comonomer such as 1-butene which produces ethyl branches. 
The quantity and kind of such short side chains have influence on the 
physical properties of the polymers produced. 
One process for the production of linear low density polyethylene is 
through copolymerization of ethylene and other 1-olefins such as 1-butene 
in a gas phase polymerization utilizing a catalyst which comprises a 
silica support containing chromium, titanium and fluorine deposited 
thereon as described in U.S. Pat. No. 4,011,382. 1-hexene is mentioned in 
this patent as a possible comonomer. Efficient production of linear low 
density polyethylene and improving the respective processes are remaining 
goals in the industry. 
THE INVENTION 
It is one object of this invention to provide a new process for the 
production of polyethylene. 
Another object of this invention is to provide a process which allows a 
high efficiency of comonomer incorporation into the chain for the 
polymerization of ethylene. 
A still further object of this invention is to provide a highly efficient 
process for ethylene polymerization. 
Yet another object of this invention is to provide a process for ethylene 
polymerization in which a significant amount of the polymerization 
reaction cooling is achieved by using a cold feedstream. 
A yet further object of this invention is to provide a new 
ethylene/1-hexene copolymer. 
These and other objects, details, features, embodiments and advantages of 
this invention will be become apparent to those skilled in the art from 
the following detailed description and appended claims. 
In accordance with this invention it has been found that a gas phase 
polymerization of ethylene in contact with a catalyst based on 
coprecipitated silica and titania and which also contains chromium 
constitutes an unusual process with unique and unexpected features. 
Surprisingly, it has been found that the process described herein allows 
the incorporation of comonomer in the polymer chain in a "super-random" 
fashion, in which the comonomer units are very well isolated by ethylene 
units in the polymer chain. It has also been found that it is possible to 
incorporate higher concentrations of 1-olefin comonomers into the polymer 
chain of ethylene units in even higher concentrations than that of the 
comonomer in the gas phase of the polymerization zone. This latter 
observation is a particularly surprising feature and is of significance 
for commercial application of the process because 1-olefin comonomers 
normally require a higher feed temperature to prevent their condensation. 
On the other hand, a low feed temperature is desirable to provide cooling 
of the polymerization reaction. 
The process of this invention allows one the use of low concentrations of 
the conomoner in the gas phase and thus to use cooler feedstreams, while 
at the same time a relatively high concentration of the comonomer in the 
copolymer is achieved. 
Furthermore it has been discovered that the process of this invention 
produces copolymers of ethylene and other 1-olefins which contain the 
comonomer units separated from one another; the comonomer units are not to 
any significant extent present as clusters or blocks. In some instances 
the dispersity of the comonomer units in the polymer chain has been found 
to be higher than the ideally random distribution. This result is an 
entirely unexpected result. 
Thus in accordance with a further embodiment of this invention a novel 
ethylene/1-hexene copolymer is provided. This copolymer is characterized 
by having a relative monomer dispersity of above 99%. Most preferably the 
copolymer has a relative comonomer dispersity of 100% or more. 
THE POLYMERIZATION PROCESS 
The process of this invention comprises as its main step a gas phase 
polymerization of ethylene. Preferably this step is a copolymerization of 
ethylene and one or more 1-olefin comonomers. The polymerization is 
carried out in contact with a catalyst containing cogelled silica and 
titania and further containing chromium. Such catalysts are described for 
instance in U.S. Pat. No. 3,887,494; the disclosure of this patent is 
herewith incorporated by reference. The catalyst used in this invention is 
either a silica/titania cogel containing chromium oxide or it is a 
silica/titania/chromium oxide tergel obtained by simultaneously gelling 
silica, titania and chromium oxide. 
The catalyst used in the process of this invention can be characterized by 
the following ranges of properties or respective ingredients: 
______________________________________ 
Weight Percent 
Generally 
Preferred 
______________________________________ 
Silica.sup.1 80 to 99.8 
90 to 98 
Titanium as titania 1 to 10 2 to 5 
(coprecipitated with silica).sup.1 
Chromium as chromium oxide (when 
0.1 to 10 0.2 to 3 
deposited on the 
silica/titania cogel).sup.1 
Chromium as chromium oxide (when 
0.1 to 10 0.2 to 3 
coprecipitated with 
silica and titania).sup.1 
Pore Volume.sup.2 (cc/g) 
1.8 to 3.5 2.0 to 3.0 
Surface Area.sup.3 (square meters per g) 
200 to 500 350 to 450 
Particle Size.sup.4 (microns) 
10 to 300 50 to 150 
______________________________________ 
.sup.1 Based on total weight of catalyst. 
.sup.2 Determined by nitrogen absorption. 
.sup.3 Determined by BET. 
.sup.4 Determined by screening. 
The catalyst used in accordance with this invention is activated generally 
in the regular way chromium oxide catalysts are activated. The activation 
includes contacting the catalyst with free oxygen at high temperatures. 
Specifically activation temperatures range from 177.degree. to 
1093.degree. C. Catalyst poisons or deactivators such as water or other 
hydroxyl containing compounds should be kept away from the activated 
catalyst. For this procedure, too, standard techniques such as a 
protection of the catalyst with nitrogen gas can be utilized. In the 
process of this invention the other 1-olefins used together with ethylene 
are also valuable in activating the catalyst and increasing the reaction 
rate. Thus the process of this invention actually involves two activation 
steps. The first activation step is the regular activation with high 
temperature and free oxygen. The second activation step is the initial 
contacting of the catalyst with the olefins which cause the activation of 
the catalyst and increase in the reaction rate. This step is then followed 
by polymerization. 
It is within the scope of this invention to use a promoter or adjuvant. 
Examples of such adjuvants are trialkylaluminum, e.g. triethylaluminum, 
trialkylboranes, e.g., triethylborane, magnesium alkyls, e.g. dibutyl 
magnesium or mixtures thereof. Many of these adjuvants are known in the 
art as adjuvants for slurry and solution polymerizations of olefins. The 
adjuvant may be added by impregnating the catalyst with a solution of the 
adjuvant and evaporating the solvent. The impregnation can also be done in 
a fluidized bed by spraying the adjuvant solution onto the fluidized 
catalyst at a temperature preferably above the boiling point of the 
solvent. The adjuvant when used is added to the catalyst after the high 
temperature activation, generally at a temperature in the range of 
50.degree. to 200.degree. C. The adjuvant is generally used in a 
concentration of 1 to 5 weight % based on total catalyst. 
The monomer feedstreams used in the process of this invention are in gas 
phase and contain ethylene and the comonomer or comonomers. The comonomers 
are 1-olefins having 4 to 10 carbon atoms. The comonomers are employed in 
a ratio of ethylene:comonomer in a range of about 1000:1 to 11:1. In 
accordance with this invention it is possible to operate with relatively 
low concentrations of the comonomer in the gas phase surrounding the 
catalyst. Specifically, it is possible and presently preferred to use a 
molar concentration of comonomer which is in contact with the catalyst in 
the gas phase as defined by the following relationship: 
EQU CG=k.times.CP 
wherein 
CG is the concentration of comonomer in the gas phase in the catalyst zone 
or reactor expressed as mole percent based on the total moles of olefins 
in that same zone as 100% 
CP is the concentration of the comonomer units in the polymer chain based 
on total molar units of ethylene and comonomer units in the polymer as 
100%. 
k is a factor in the range of 1/8 to 3/2. 
In several circumstances it has been found in accordance with this 
invention that a higher comonomer concentration was achieved in the 
polymer than was present in the gas phase of the catalytic polymerization 
zone. This effect appears to be more pronounced with increasing molecular 
weight of the comonomer. Therefore, the lower end of the range for the 
factor k above is associated with higher molecular weight comonomers such 
as octenes, while the higher range for the factor k is associated with 
comonomers having a lower molecular weight, i.e. butenes and pentenes. 
The concentration of the comonomer (mole % of all olefins) in the preferred 
process of this invention can be described for the individual comonomers 
in the gas phase of the polymerization zone relative to the concentration 
of this comonomer in the copolymer by the formulae: 
##EQU1## 
CGB=mole concentration 1-butene in gas phase. CPB=mole concentration 
1-butene in copolymer. 
##EQU2## 
CGH=mole concentration 1-hexene in gas phase. CPH=mole concentration 
1-hexene in copolymer. 
##EQU3## 
CGO=mole concentration 1-octene in gas phase. CPO=mole concentration 
1-octene in copolymer. 
##EQU4## 
CGF=mole concentration of 4-methyl-1-pentene in gas phase. CPF=mole 
concentration of 4-methyl-1-pentene in copolymer. 
and by the ranges for the respective factors K as follows: 
______________________________________ 
Comonomer K Range for the K-factors 
______________________________________ 
1-butene K4 0.6 to 1.2 
1-hexene K6 1.4 to 2.5 
1-octene K8 4 to 7 
4-methyl-1-pentene 
K51 1 to 1.4 
______________________________________ 
Among the 1-olefin comonomers having 4 to about 10 carbon atoms the most 
preferred comonomers are presently 1-butene, 1-hexene, 4-methyl-1-pentene 
and 1-octene. Combinations of two or more of these comonomers can be used 
in the process of this invention. 
The gas phase used in the polymerization step can contain other components. 
Diluent gases such as nitrogen or noble gases can be utilized. It is 
presently preferred, however, to carry out the polymerization step in the 
catalytic polymerization zone using a gas phase consisting essentially of 
the 1-olefins defined above, i.e. ethylene and the comonomer or 
comonomers, and hydrogen when employed. For melt index and molecular 
weight control it is presently preferred to carry out the polymerization 
step in the presence of hydrogen. In addition to increasing the melt index 
of the polymer added hydrogen also functions to increase the reaction rate 
of polymerization in the process of this invention. 
The gas phase polymerization step of this invention is usually conducted at 
significant ethylene pressures which are above atmospheric pressure. 
Usually the pressure in the catalytic polymerization zone is in the range 
of 50 to 1000 psig, preferably in the range of 100 to 500 psig. Ethylene 
partial pressure is generally in the range of 50 to 1000 psig and 
preferably in the range of 100 to 500 psig. If hydrogen is used for 
molecular weight control the partial pressure of the hydrogen will be in 
the range of 10 to 150 psig. 
The polymerization is carried out at a temperature which can vary widely. 
An upper limit for the temperature in the polymerization zone is set only 
by the temperature at which the polymer begins to fuse, causing 
agglomeration of the polymer particles and loss of catalyst fluidization. 
The upper temperature limit varies from about 120.degree. C. for ethylene 
homopolymer to about 90.degree. C. for copolymers of 0.920 g/cc density. 
Copolymers have lower temperatures of fusion than homopolymers. 
The cooling of the catalytic reaction zone can be accomplished by various 
means including indirect heat exchange or evaporation of a hydrocarbon 
spray. It is presently preferred to use the ambient feedstream temperature 
created as the gas stream flows into the catalytic polymerization zone. In 
view of the fact that the concentration of the comonomer in accordance 
with this invention can be low, it is possible to operate with relatively 
low feedstream temperatures. Generally the feed temperature will be in a 
range of about 25.degree. to 60.degree. C. and the outlet temperature, 
i.e., the temperature of the gas leaving the catalytic polymerization 
zone, will be in a range of about 70.degree. to 120.degree. C. For 
copolymers in particular, the outlet temperatures will be generally below 
100.degree. C. It is presently preferred that the temperature difference 
between the inlet and the outlet temperature of the catalytic reaction 
zone be in a range of 20.degree. to 60.degree. C. 
The gas phase polymerization step of this invention is preferably carried 
out in a fluidized bed reactor. The catalyst can be fluidized by the gas 
passing upwardly through a distribution plate and into the catalytic 
polymerization zone containing the catalyst and the polymer formed. The 
actual linear velocity of the gas can range from about 0.5 to 5 ft/sec as 
measured at the reaction conditions employed. It is, however, equally 
effective to achieve the fluidization by mechanical means. Thus powdered 
catalyst and finely divided polymer can be fluidized by a stirring or 
agitating mechanism such as a marine type propeller or an anchor type 
mixer. When ethylene gas flow is used for the fluidization, the conversion 
per pass is usually kept low and the gas carries most of the heat of 
reaction from the reaction zone. This gas is cooled before recycle. It is 
also possible in accordance with this invention to use mechanical 
agitation in an autoclave and to actually internally circulate ethylene, 
and to remove the heat of reaction by transferring this heat to internal 
walls or cooling plates or coils. 
If required the polymer can be separated from the catalyst particles by 
standard techniques such as by dissolution in a hot solvent such as 
cyclohexane and filtration or centrifugation of the solution to remove 
catalyst. The polymer is then recovered by removal of the solvent such as 
by evaporation or steam stripping as known in the art. If the productivity 
of the catalyst utilized is sufficiently high, e.g., at least 2000 g 
polymer per g catalyst, it is also within the scope of this invention to 
recover and utilize the polymer without separating catalyst residues. 
ETHYLENE/1-HEXENE COPOLYMER 
Another embodiment of this invention resides in a new ethylene/1-hexene 
copolymer. This copolymer is characterized by a relative dispersity of the 
hexene units of 99% or more. The most preferred copolymers of ethylene and 
1-hexene have a relative comonomer dispersity of over 100%. These relative 
dispersities are based on the ideally random distribution as a reference 
point. Some of the novel copolymers have a dispersion of the comonomers 
which is better (less clustering) than that of an ideally random dispersed 
copolymer. 
The most preferred copolymer of this invention is characterized in addition 
to the relative dispersity of the comonomer by the following properties. 
______________________________________ 
Property General Range 
Preferred Range 
______________________________________ 
Density.sup.1 (g/cc) 
0.910-0.935 0.915-0.930 
Melt Index.sup.2 (grams/10 minutes) 
0.6-15 1-10 
1-Hexene concentration in the 
0.5-7 1-6 
polymer (mole percent) 
Molecular weight.sup.3 
50-190 90-180 
Heterogeneity Index.sup.4 
above 6 7.5-10 
(Mw/Mn) 
______________________________________ 
.sup.1 ASTM D1505, g/cc. 
.sup.2 ASTM D1238, Condition E, g/10 minutes. 
.sup.3 Size exclusion chromatography (SEC); weight average in thousands. 
.sup.4 Weight average molecular weight divided by number average molecula 
weight, M.sub.n also determined by SEC. 
RELATIVE COMONOMER DISPERSITY 
The relative dispersity of the comonomer in the polymer chain, RMD, is 
defined by the following formula and determined as described below: 
##EQU5## 
wherein AMD represents the absolute comonomer dispersity and BMD 
represents the perfectly random comonomer dispersity or Bernoullian 
dispersity. 
The absolute monomer dispersity is determined by the following procedure. 
The absolute monomer dispersity is defined as the ratio of the number (N) 
of clusters of comonomers per average molecule divided by the number (X) 
of comonomer units per average polymer chain. If n.sub.1 represents the 
number of isolated comonomer units, n.sub.2 represents an adjacant pair 
cluster of comonomer units up to a . . . n.sub.x cluster of x contiguous 
comonomer units present in the copolymer, X and N are defined as follows: 
##EQU6## 
the absolute monomer dispersity is defined by the following relationship: 
##EQU7## 
Thus, if only isolated comonomer units are present in the polymer 
molecule, AMD would be 100. Conversely, if all comonomer units were 
concentrated in one block, AMD would be approximately 0. 
The ideally random or Bernoullian distribution, BMD, is determined by the 
following formula: 
##EQU8## 
wherein MC is the concentration in mole percent of the comonomer in the 
polymer. Thus if the polymer consists of 95% ethylene and 5% 1-hexene BMD 
is 95. 
The absolute monomer dispersity AMD is determined by NMR methods as 
follows: 
An NMR spectrum is taken of the polymer. The peaks in accordance with 
standard NMR practice can be determined and characterized by their 
position (in ppm) relative to tetramethylsilane. In view of higher 
operating temperatures the actual "calibration" is done relative to 
hexamethyldisiloxane having its peak at 2.03 ppm relative to 
tetramethylsilane. The peaks listed in the following table for the polymer 
of this invention are given in ppm relative to tetramethylsilane. 
The spectrum of the ethylene 1-hexene copolymer will show peaks which have 
the following assignments: 
______________________________________ 
Chemical Shift 
Carbon Sequence 
PPM, TMS Assignment 
Assignment 
______________________________________ 
41.40 .alpha..alpha. 
HHHH 
40.86 .alpha..alpha. 
HHHE 
40.18 .alpha..alpha. 
EHHE 
38.13 Methine EHE 
35.85 Methine EHH 
35.37 4B.sub.4 HHH 
.alpha..gamma. 
HHEH 
35.00 .alpha..gamma. 
EHEH 
.alpha..delta.+ 
HHEE 
34.90 4B.sub.4 HHE 
34.54 .alpha..delta.+ 
EHEE 
34.13 4B.sub.4 EHE 
33.57 Methine HHH 
30.94 .gamma..gamma. 
HEEH 
30.47 .gamma..delta.+ 
HEEE 
29.98 .delta. + .delta.+ 
(EEE).sub.n 
29.51 3B.sub.4 EHE 
29.34 3B.sub.4 EHH 
29.18 3B.sub.4 HHH 
27.28 .beta..delta.+ 
EHEE 
27.09 .beta..delta.+ 
HHEE 
24.53 .beta..beta. 
EHEHE 
24.39 .beta..beta. 
EHEHH 
24.25 .beta..beta. 
HHEHH 
23.37 2B.sub.4 EHE + EHH + HHH 
14.12 1B.sub.4 EHE + EHH + HHH 
______________________________________ 
In this table the abbreviations .alpha., .beta., .gamma., .delta.+, 
1B.sub.4, 2B.sub.4, 3B.sub.4, 4B.sub.4, and methine are used in the usual 
way well known in NMR technology for the characterization of the relative 
position of carbon atoms in the polymer chain. The Greek letters refer to 
a distance in carbon atoms of 1 (for .alpha.), 2 (for .beta.) . . . 4 or 
more (for .delta.+) from the respective methylene carbon atom from a 
branch site. The terms 2B.sub.4 etc. refer to the position of a carbon 
atom in a side chain, the subscript of B characterizing the length of the 
side chain which in the case of butyl is always 4 while the prescript 
characterizes the number of the carbon atom investigated starting with the 
methyl carbon as "1"; thus the methyl carbon is 1B.sub.4. "Methine" 
characterizes the carbon atom to which the branch is attached and can only 
be one of three types, EXE, EXX and XXX. 
The triad distribution is determined. Although this represents only one of 
several possibilities, it is presently preferred to use the triad 
distribution to determine the absolute monomer dispersity. Other methods 
developed analogously to the triad distribution would be to use either a 
dyad or tetrad distribution. These other methods do not have as many of 
the advantages with respect to accuracy or ease of calculation as does the 
triad distribution. The triad distribution in essence determines the 
relative concentration of EXE, EXX, XXX contiguous sequences in the 
polymer molecule where E stands for ethylene and X stands for the 
comonomer unit, here in particular 1-hexene. For more details reference is 
specifically made to Eric T. Hsieh and James C. Randall, Ethylene-1-Butene 
Copolymers 1. Comonomer Sequence Distribution, Macromolecules, 15, (2), 
353 (1982). Reference is also made to standard NMR techniques for 
measuring both the peak height and the peak areas, although the latter 
measurement is preferred. 
Since every cluster of two or more X units will contribute to two EXX 
units, the following relationships exist: 
EQU EXE=n.sub.1 
EQU EXX=2(n.sub.2 +n.sub.3 +. . . +n.sub.i +. . . ) 
or combining these equations 
EQU EXE+1/2.multidot.EXX=n.sub.1 +n.sub.2 +n.sub.3 +. . . +n.sub.i +. . . =N 
Similarly, since the triad XXX is found once in XXX, twice in XXXX, three 
times in XXXXX, etc. the relationship 
EQU XXX=n.sub.3 +2n.sub.4 +3n.sub.5 +. . . (i-2)n.sub.i +. . . 
Combining the last three questions one finds readily 
EQU EXE+EXX+XXX=n.sub.1 +2n.sub.2 +3n.sub.3 +. . . +i.multidot.n.sub.1. . . =X 
Thus the absolute monomer dispersity is determined by this NMR evaluation 
as 
##EQU9## 
The individual concentrations of EXE, EXX and XXX being determined from the 
peak heights or peak areas. In this instance H represents 1-hexene 
replacing X of the above generic description. 
From the so determined value (AMD) for the absolute monomer dispersity, the 
relative monomer dispersity is determined in accordance with the above 
formula. In the ensuing discussions the relative and absolute monomer 
dispersities shown have been determined as described above. 
The following examples are intended to further illustrate preferred 
embodiment of this invention without undo limitation of its scope.