Dual reactor ethylene polymerization process

Details Dual reactor ethylene polymerization process Dual reactor ethylene polymerization process

1999-08-30

2001-08-21

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

C526S116000, C526S119000, C526S133000, C526S161000, C526S352000, C502S155000

Reexamination Certificate

active

06277931

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the solution polymerization of ethylene in two reactors using two different catalyst systems.
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. The “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 broader molecular weight distribution and a broader 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 impact strength and optical properties in comparison to “conventional” LLDPE. However, one advantage of the “conventional” LLDPE is that it is usually easier to “process” in its existing mixers and extruders. Accordingly, it would be highly desirable to prepare LLDPE products which possess the improved physical properties offered by single site catalysts and retain the broad molecular weight distribution (for improved processability) which is associated with conventional LLDPE.
One approach which has been used to achieve this object is the use of mixed catalyst systems. For example, U.S. Pat. No. (USP) 4,530,914 (Ewen et al, to Exxon) teaches the use of two different metallocenes 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 may be quickly ascertained by reviewing the patent literature.
However, the use of “mixed” catalyst systems is often 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, 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. Accordingly, a “mixed catalyst” process which mitigates some of these difficulties 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 polymerization reactor at a temperature of from 80 to 200° C. and a pressure of from 500 to 8,000 pounds per square inch gauge (“psi”) in the presence of (a) a first catalyst which is an organometallic complex of a group 4 or 5 metal that is characterized by having at least one phosphinimine ligand; and (b) a first cocatalyst; and
B) passing said first polymer solution into a second polymerization reactor and polymerizing ethylene, optionally with one or more C
3-12
alpha olefins, in said second stirred polymerization reactor at a higher polymerization temperature than that of said first reactor in the presence of a Ziegler Natta catalyst, wherein said Ziegler Natta catalyst comprises a transition metal compound of a transition metal selected from groups 3, 4 or 5 of the Periodic Table (using IUPAC nomenclature) and an organoaluminum component which is defined by the formula:
Al(X′)
a
(OR)
b
(R)
c
wherein: X′ is a halide (preferably chlorine); OR is an alkoxy or aryloxy group; R is a hydrocarbyl (preferably an alkyl having from 1 to 10 carbon atoms); and a, b, or c are each 0, 1, 2 or 3 with the provisos that a+b+c=3 and b+c≧1.
Thus, the process of the present invention requires two solution polymerization reactors and two distinct catalyst systems. The first catalyst must have a phosphinimine ligand (and, hence, is sometimes referred to herein as a “phosphinimine catalyst” or “PIC”).
The first reactor uses the “phosphinimine catalyst”. Conventional process control techniques may be used to operate the first reactor as there is only one catalyst to deal with.
Preferred phosphinimine catalysts for use in the first reactor are titanium species which contain one cyclopentadienyl ligand, one phosphinimine ligand and two chloride ligands.
It is particularly preferred that the concentration of titanium in the first reactor be less that 1 part per million (ppm) especially less that 0.5 ppm (based on the weight of titanium divided by the weight of the reactor contents).
Exemplary cocatalysts for the phosphinimine catalyst are alumoxanes and/or ionic activators. Preferred cocatalysts for the phosphinimine catalyst are a combination of:
1) an alumoxane (in which the Al/Ti molar ratio, based on the alumoxane and the titanium in the phosphinimine catalyst is between 10/1 and 200/1, most preferably from 40/1 to 120/1); and
2) a boron-containing ionic activator (in which the B/Ti ratio, based on the boron in the ionic activator to the titanium in the phosphinimine catalyst is between 0.5/1 and 1.5/1).
The polymer solution from the first reactor is transferred to the second solution polymerization reactor. A Ziegler Natta catalyst is used in the second reactor. It is preferred that the Ziegler Natta catalyst contains at least one transition metal selected from titanium and vanadium, and that the molar concentration of titanium/vanadium which is added to the second reactor is at least 10 times greater than the titanium concentration in the first reactor.
Thus, the second polymerization reactor must use a Ziegler Natta catalyst. Additionally, the second polymerization reactor must be operated at a higher temperature from the first—most preferably, at least 30° C. higher than the first.
While not wishing to be bound by any particular theory, it is believed that the reactor conditions in the second reactor “overwhelm” the catalyst from the first reactor (i.e. for process control purposes, any residual catalyst from the first reactor is not a concern in the second reactor). This is desirable from a process operability perspective as it reduces the number of variables which need to be considered when controlling the polymerization reaction in the second reactor.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Part 1. Description of First Catalysts
The catalyst used in the first reactor of the process of this invention (“first catalyst”) is an organometallic complex of a group 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 first 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); Pl 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 catal

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