Process for the polymerization of olefins in the gas phase

Details Process for the polymerization of olefins in the gas phase Process for the polymerization of olefins in the gas phase

1999-12-23

2002-08-20

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...

C526S075000, C526S126000, C526S127000, C526S129000, C526S130000, C526S134000, C526S160000, C526S161000, C526S172000, C526S901000, C526S904000

Reexamination Certificate

active

06437060

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for the polymerisation of olefins and in particular to a process for the homopolymerisation of ethylene or copolymerisation of ethylene and alpha-olefins in the gas phase by use of a prepolymerised transition metal complex catalyst.
Traditional olefin polymerisation catalysts have been based on transition metal salts of Group IV to VIII metals in combination with base metal alkyls of Group I to III metals. Such catalysts known as Ziegler-Natta catalysts have been used to polymerise olefins in solution, slurry and gas phase processes. Another catalyst system used for polymerisation of olefins is based on chromium oxide and is often referred to as Phillips-type catalyst system.
A problem encountered when such catalyst systems have been used in the gas phase has been the control of the morphology of the polymer produced. The morphology of polymers produced in the gas phase has been improved by use of prepolymerisation processes in which typically in a first stage the contact between one or more olefins with the Ziegler-Natta catalyst results in the formation of a prepolymer in the form of solid particles. In a second stage the prepolymer is contacted with one or more olefins under polymerisation conditions in the gas phase to produce polymers directly in the form of powders. In this way the morphology of the final polymer may be improved. A typical prepolymerisation process is described in EP 99774.
Catalysts based on cyclopentadienyl metal complexes have also been widely used for the polymerisation of olefins. These complexes may be used in catalyst systems which comprise a bis(cyclopentadienyl) transition metal complex and a cocatalyst. Such bis (Cp) transition metal complexes have been referred to as metallocenes and are typically based on titanium or zirconium metals and may be cocatalysed with aluminum compounds such as aluminoxanes. When used in gas phase processes such bis (Cp) metallocene systems may be supported on silica.
More recently another type of transition metal complex has been used to prepare olefin polymers. Such complexes have a single cyclopentadienyl ring ligand and a hetero atom bonded to the metal atom and may also be used in conjunction with aluminoxanes. Such ‘constrained geometry’ catalysts are described in EP 420436 and EP 416815.
Similar catalyst systems are taught in EP 418044 and WO 92/00333. In these systems the catalyst is prepared as the product of a mono(cyclopentadienyl) heteroatom metal complex and an ionic activator compound and such systems have been referred to as ionic mono(cyclopentadienyl) catalysts. Typical ionic activators for such systems may be exemplified by borates.
The complexes described above may be optionally prepolymerised. For example WO 93/23439 describes supported bis (Cp) metallocene catalyst systems, activated with alumoxanes which may be optionally prepolymerised in order to impart improved catalyst particle strength. In this reference the prepolymerisation is performed in the slurry phase at a temperature in the range −15° C. to 30° C. preferably at less than 25° C.
Further examples of the use of prepolymerisation with such bis (Cp) metallocene complexes may be found in EP 452920, EP 516458, EP 582480 and EP 605952.
WO 94/03506 describes supported ionic catalysts based on mono (cyclopentadienyl) complexes and ionic activators which may also be optionally prepolymerised in order to achieve improved particle strength and size and reduced reactor fouling during polymerisation.
WO 94/28034 describes supported bridged bis (Cp) metallocene catalysts in which prepolymerisation reduces the reactor fouling tendencies of the catalyst and enhances the particle morphology control of the final polymer formed.
WO 96/00243 describes chiral metallocenes for the production of highly isotactic polypropylene copolymners in which prepolymerisation is found to improve particle morphology.
In all these systems the valency of the transition metal in the metallocene complex is in either the +3 or more usually in the highest oxidation state of +4.
WO 95/00526 describes titanium or zirconium complexes in which the transition metal is in the +2 formal oxidation state. The complex also comprises a neutral, conjugated or non-conjugated diene ligand which forms a &pgr;-complex with the metal. Such complexes are rendered catalysts by combination with an activating cocatalyst for example aluminoxanes, boranes or borates. When used in a gas phase process these catalysts are suitably supported on silica. However there is no mention of prepolymerisation as an option when using such catalyst systems in the gas phase.
Accordingly in the above complexes when prepolymerisation has been suggested it is in order to either reduce reactor fouling or to improve the morphology of the final polymer both advantages typically claimed with the earlier Ziegler-Natta or chromium systems.
SUMMARY OF THE INVENTION
We have now found that prepolymerisation in the presence of transition metal complexes may be used to improve reactivity in particular when performed in the gas phase, for example in an agitated dry phase reactor.
In particular we have now found that the catalytic activity of certain transition metal complex catalysts in the gas phase may be improved by use of an initial prepolymerisation step performed at low temperature (with respect to the final polymerisation temperature) either in a separate stage or in-situ prior to the final polymerisation stage.
Thus according to the present invention there is provided a process for polymerising ethylene or copolymerising ethylene and one or more alpha-olefins in the gas phase comprising:
(1) in a first stage prepolymerising ethylene or ethylene and one or more alpha-olefins in the gas phase at a temperature in the range 20 to 70° C. in the presence of a catalyst system comprising (a) a supported transition metal complex and (b) an activator,
(2) optionally, recovering the prepolymerised catalyst, and
(3) in a second stage polymerising ethylene or ethylene and one or more alpha-olefins in the gas phase at a temperature in the range 65 to 100° C. in the presence of said prepolymerised catalyst.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is particularly suitable for use with ‘constrained geometry complexes’.
The term ‘constrained geometry complex’ will be readily understood by one skilled in the art to mean complexes in which the metal atom is forced into greater exposure of the active metal site because of one or more substituents on the delocalised &pgr; bonded moiety. Such complexes are described in detail in EP 416815 incorporated herein by reference.
The process of the present invention may be performed in a single gas phase reactor in which both stages are performed or the prepolymerised catalyst from the first stage may be recovered before use in the final polymerisation.
The prepolymerised catalyst may be recovered by conventional means.
The prepolymerisation stage is most preferably carried out at a temperature in the range 20 to 65° C. most preferably in the range 25-40° C. and the final polymerisation stage at a preferred temperature in the range just above 65° C. to 100° C., most preferably in the range 70 to 85° C.
During the prepolymerisation stage the pressure is typically in the range 0.1 to 10 bar. In the final polymerisation stage the pressure is increased and is typically in the range 5 to 20 bar.
Titanium (II) or zirconium (II) complexes are particularly suitable for use as the constrained geometry complex in the process of the present invention. Such complexes are disclosed in the aforementioned WO 95/00526 which is incorporated herein by reference. The complexes have the general formula:
wherein
R′ each occurrence is independently selected from hydrogen, hydrocarbyl, silyl, germyl, halo, cyano, and combinations thereof, said R′ having up to 20 non hydrogen atoms, and optionally, two R′ groups (where R′ is not hydrogen, halo or cyano) together form a divalent derivative

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