Solution polymerization process

Details Solution polymerization process Solution polymerization process

2002-04-17

2004-08-17

Wu, David W. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S172000, C526S170000, C526S160000, C526S131000, C526S134000, C526S153000

Reexamination Certificate

active

06777509

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for the preparation of polyethylene having a broad molecular weight distribution.
BACKGROUND OF THE INVENTION
It is well known that linear polyethylene may be prepared by the polymerization of ethylene (optionally with one or more olefins or diolefins such as butene, hexene, octene or hexadiene) using a “Ziegler” catalyst system which comprises a transition metal compound (such as a titanium halide) or a vanadium halide and an aluminum alkyl. Polyethylene produced in this manner, particularly “linear low density polyethylene”, is in widespread commercial use. However, the conventional linear low density polyethylene (“lldpe”) made with Ziegler catalysts suffers from a number of deficiencies. Most notably, conventional lldpe is a heterogeneous product which contains a small fraction of low molecular weight wax and a comparatively large amount of very high molecular weight homopolymer. The heterogeneous nature of these polymers generally detracts from the physical properties of finished goods made from them.
Accordingly, a great deal of effort has been directed towards the preparation of “homogeneous” lldpe resins which mitigate this problem. In particular, it is now well known to those skilled in the art that so-called “metallocene” catalysts may be used to produce homogeneous lldpe resin. These homogeneous resins are, however, not without problems. Most notably, these homogeneous resins typically have a narrow molecular weight distribution and are difficult to “process” or convert into finished polyethylene products. Thus, efforts to improve the processability of homogeneous polyethylene resin by broadening the molecular weight distribution are disclosed in the art.
One approach which has been used to achieve this object is the use of mixed catalyst systems in a single reactor. For example, U.S. Pat. No. 4,530,914 (Ewen et al., to Exxon) teach the preparation of “broad” polymers through the use of two different metallocene catalysts and U.S. Pat. No. 4,701,432 (Welborn, to Exxon) teaches the use of a supported catalyst prepared with a metallocene catalyst and a Ziegler Natta catalyst. Many others have subsequently attempted to use similar mixed catalyst systems, as described in U.S. Pat. Nos. 5,767,031; 5,594,078; 5,648,428; 4,659,685; 5,145,818; 5,395,810; and 5,614,456.
However, the use of “mixed” catalyst systems is generally associated with operability problems. For example, the use of two catalysts on a single support (as taught by Welborn in U.S. Pat. No. 4,701,432) may be associated with a reduced degree of process control flexibility (e.g. if the polymerization reaction is not proceeding as desired when using such a catalyst system, then it is difficult to establish which corrective action should be taken because the corrective action will typically have a different effect on each of the two different catalyst components). Moreover, the two different catalyst/cocatalyst systems may interfere with one another—for example, the organoaluminum component which is often used in Ziegler Natta or chromium catalyst systems may “poison” a metallocene catalyst.
Another alternative is to use two different homogeneous catalysts in two different polymerization reactors. In commonly assigned U.S. Pat. No. 6,063,879 (Stephan et al.) there are disclosed certain phosphinimine catalysts which may be used to produce homogeneous polyethylene. The use of such phosphinimine catalysts in a “dual reactor” polymerization system to prepare polymers having a broad molecular weight distribution is also disclosed in a commonly assigned patent application (Brown et al., U.S. Ser. No. 09/364,703 and corresponding Canadian application 2,245,375).
Each of the approaches to produce broad MWD polymers requires the use of at least two distinct polymerization catalysts or two distinct polymerization reactors.
We have now discovered a process to prepare broad MWD ethylene polymers in a single polymerization reactor using a single phosphinimine catalyst and a specific activation system.
The present process is simpler/more elegant than the above described prior art process in the sense that the present process does not require the use of two catalysts and/or two reactors. The simplicity of the present process offers the potential to improve process control and reduce costs in comparison to the prior art processes.
SUMMARY OF THE INVENTION
A process for preparing thermoplastic ethylene alpha olefin copolymer having a polydispersity (or molecular weight distribution, Mw/Mn) of greater than 2, said process comprising polymerizing ethylene and at least one other C
3
to 10 alpha olefin under medium pressure solution polymerization conditions at a temperature of greater than 170° C. to 300° C. in the presence of a catalyst system comprising:
1) an organometallic catalyst comprising a group 4 metal, at least one phosphinimine ligand and at least one activatable ligand;
2) a four coordinate boron activator; and
3) at least one trialkyl aluminum.
DETAILED DESCRIPTION
The catalyst used in this invention is an organometallic complex of a group 4 metal having at least one phosphinimine ligand and at least one activatable ligand. Highly preferred catalysts also contain a cyclopentadienyl ligand.
The preferred phosphinimine catalysts used in this invention are defined by the formula:
(Cp)
a
M(Pl)
b
(L)
c
wherein Pl is a phosphinimine ligand (see section 1.1 below); Cp is a cyclopentadienyl-type ligand (section 1.2 below); L is an activatable ligand (section 1.3 below); M is a metal selected from Ti, Hf and Zr; and wherein a is 0 or 1; b is 1 or 2; a+b=2; c is 1 or 2; and a+b+c=the valence of the metal M.
The most preferred catalysts are those in which the metal is 4 valent. For example, a catalyst may be a cyclopentadienyl-phosphinimine complex of titanium, zirconium, or hafnium having two additional, monoanionic ligands. It is particularly preferred that each catalyst contains one phosphinimine ligand, one cyclopentadienyl ligand and two chloride or alkyl ligands.
Each catalyst must contain at least one phosphinimine ligand which is covalently bonded to the metal. Phosphinimine ligands are defined by the formula:
wherein each R
1
is independently selected from the group consisting of a hydrogen atom, a halogen atom, C
1-20
hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C
1-8
alkoxy radical, a C
6-10
aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
—Si—(R
2
)
3
wherein each R
2
is independently selected from the group consisting of hydrogen, a C
1-8
alkyl or alkoxy radical, C
6-10
aryl or aryloxy radicals, and a germanyl radical of the formula:
Ge—(R
2
)
3
wherein R
2
is as defined above.
The preferred phosphinimines are those in which each R
1
is a hydrocarbyl radical. A particularly preferred phosphinimine is tri-(tertiary butyl) phosphinimine (i.e. where each R
1
is a tertiary butyl group).
As used herein, the term cyclopentadienyl-type ligand is meant to convey its conventional meaning, namely a ligand having a five carbon ring which is bonded to the metal via eta-5 bonding. Thus, the term “cyclopentadienyl-type” includes unsubstituted cyclopentadienyl, substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted fluorenyl and substituted fluorenyl. An exemplary list of substituents for a cyclopentadienyl ligand includes the group consisting of C
1-10
hydrocarbyl radical (which hydrocarbyl substituents are unsubstituted or further substituted); a halogen atom, C
1-8
alkoxy radical, a C
6-10
aryl or aryloxy radical; an amido radical which is unsubstituted or substituted by up to two C
1-8
alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C
1-8
alkyl radicals; silyl radicals of the formula —Si—(R)
3
wherein each R is independently selected from the group consisting of hydrogen, a C
1-8
alkyl or alkoxy radical C
6-10
aryl or aryloxy radicals; germanyl radicals of the formula Ge—(R)

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