Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Reexamination Certificate
2000-10-31
2002-04-16
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in two or more physically distinct zones
C526S127000, C526S160000, C526S161000, C526S943000, C502S152000, C502S155000
Reexamination Certificate
active
06372864
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the solution polymerization of ethylene in two reactors using a catalyst having a phosphinimine ligand.
BACKGROUND OF THE INVENTION
The use of so-called “single-site” catalysts such as metallocene catalysts to prepare polyethylene having a narrow molecular weight distribution is well known. In addition, “linear low density polyethylene” (or “LLDPE”, a copolymer of ethylene and a higher alpha olefin) prepared with such catalysts typically exhibits a very uniform composition distribution (i.e. the comonomer is very uniformly distributed within the polymer chains). The combination of narrow molecular weight distribution and uniform composition distribution distinguishes these polymers from “conventional” LLDPE which is commercially manufactured with a Ziegler Natta catalyst or a chromium catalyst. In particular, the conventional LLDPE products have a broad molecular weight distribution and a broad composition distribution. These compositional differences are manifested in the form of differences in the physical properties of the two types of LLDPE polymers. Most notably, LLDPE prepared with a single site catalyst has improved dart impact strength and optical properties in comparison to “conventional” LLDPE. However, the “conventional” LLDPE is usually easier to “process” in its existing mixing and extrusion equipment. Accordingly, it would be highly desirable to prepare LLDPE products which possess the improved physical properties offered by single site catalysts and also exhibit processability characteristics which are similar to those of conventional LLDPE.
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) teaches the use of two different metallocenes and U.S. Pat. No. 4,701,432 (Welborm, 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. Pats. No. 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 Welborm 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 as 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 metallocene catalysts in two different polymerization reactors. However, process control problems relating to interactions between the two different catalysts might also be anticipated in such a process. Accordingly, a “dual reactor” process which mitigates some of these problems would be a useful addition to the art.
SUMMARY OF THE INVENTION
The present invention provides a medium pressure solution polymerization process characterized by:
A) polymerizing ethylene, optionally with one or more C
3-12
alpha olefins, in solvent in a first stirred polymerization reactor at a temperature of from 80 to 200° C. and a pressure of from 1500 to 5000 pounds per square inch gauge (psi) in the presence of (a) a catalyst which is an organometallic complex of a group 3, 4 or 5 metal, characterized by having at least one phosphinimine ligand; and (b) a cocatalyst which contains an alumoxane; then
B) passing said first polymer solution into a second stirred polymerization reactor and polymerizing ethylene, optionally with one or more C
3-12
alpha olefins, in said second stirred polymerization reactor in the presence of (a) a catalyst which is an organometallic complex of a group 3, 4 or 5 metal, characterized by having at least one phosphinimine ligand; and (b) a cocatalyst which contains an ionic activator.
Thus, the process of the present invention requires two solution polymerization reactors and a catalyst having a phosphinimine ligand (“phosphinimine catalyst”).
Preferred catalysts are titanium complexes which contain one cyclopentadienyl ligand, one phosphinimine ligand and two chloride ligands. The same phosphinimine catalyst may be used in both reactors or, alternatively, a different type of phosphinimine catalyst may be used in the two reactors. It is preferred to use the same catalyst in both reactors. Preferred co-catalysts are selected from a boron-containing “ionic activators” and alumoxanes.
Thus, in the present process, the cocatalyst system used in the first reactor must be different from the cocatalyst system used in the second reactor. In addition, it is preferred that polymerization temperature in the second reactor is different than the polymerization temperature of the first reactor.
Most preferably, the second polymerization reactor is operated at a higher temperature than the first (ideally at least 25° C. higher than the first).
Certain LLDPE polymers produced according to the preferred process of this invention exhibit an outstanding balance of physical properties, optical properties and “processability”. As will be recognized by those skilled in the art, this balance of characteristics is highly desirable for the production of LLDPE film. Thus, the present invention also provides a LLDPE film having a dart impact strength as determined by ASTM D-1709 of greater than 700 grams/mil, a haze as determined by ASTM D-1003 of less that 6%, a 45° gloss as determined by ASTM D-2457 of greater that 65% and a machine direction tear resistance as determined by ASTM D-1922 of greater than 300 grams/mil.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Part 1. Description of Catalysts
The catalyst used in the process of this invention is an organometallic complex of a group 3, 4 or 5 metal which is characterized by having at least one phosphinimine ligand (where the term phosphinimine is defined in section 1.2 below).
Any such organometallic having a phosphinimine ligand which displays catalytic activity for ethylene polymerization may be employed. Preferred catalysts are defined by the formula:
wherein M is a transition metal selected from Ti, Hf and Zr (as described in section 1.1 below); PI is a phosphinimine ligand (as described in section 1.2 below); L is a monanionic ligand which is a cyclopentadienyl-type ligand or a bulky heteroatom ligand (as described in section 1.3 below); X is an activatable ligand which is most preferably a simple monanionic ligand such as alkyl or a halide (as described in section 1.4 below); M is 1 or 2, n is 0 or 1, and p is fixed by the valence of the metal M.
The most preferred first catalysts are group 4 metal complexes in the highest oxidation state. For example, a preferred catalyst may be a bis(phosphinimine) dichloride complex of titanium, zirconium or hafnium. However, it is preferred that the first catalyst contain one phosphinimine ligand, one “L” ligand (which is most preferably a cyclopentadienyl-type ligand) and two “X” ligands (which are preferably both chloride).
1.1 Metals
The catalyst is an organometallic complex of a group 3, 4 or 5 metal (where the numbers refer to columns in the Periodic Table of the Elements using IUPAC nomenclature). The preferred metals are from group 4, (especially titanium, hafnium or zirconium) with titanium being most preferred.
1.2 Phosphinimine Ligand
The first catalyst must contain a phosphinimine ligand which is covalently bonded to the metal. This ligand is 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 a
Harlan R.
Johnson Kenneth H.
NOVA Chemicals (International ) S.A.
Wu David W.
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