Polymerization process

Details Polymerization process Polymerization process

1997-07-31

2002-06-25

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...

C526S113000, C526S118000, C526S119000, C526S160000, C526S901000, C526S943000

Reexamination Certificate

active

06410659

ABSTRACT:

The present invention relates to a process for preparing copolymers, in particular to a process for preparing copolymers of ethylene with alpha-olefins having a bimodal molecular weight distribution.
Bimodal or multimodal polyolefins with broad molecular weight distributions are obtained commercially using Ziegler catalysts in slurry or gas phase polymerisation processes in which different operating conditions, are employed. Such processes are known as cascade processes. Polymers obtained in such processes often selectively incorporate comonomers in one part of the molecular weight distribution. Other parts of the polymer often contain little or no comonomer incorporation.
These polymers have been found to offer advantages in processability and tear, impact, stress crack and fracture properties depending upon the polymer application envisaged.
The cascade process relies upon different operating conditions, often using two separate reactors. It would be advantageous to be able to produce such polymers in a single reactor preferably in the gas phase under steady state conditions.
Bimodal polyolefins may be prepared by using combinations of polymerisation catalysts as components, for example a metallocene and a Ziegler catalyst or alternatively two different metallocene catalysts may be used. Such catalyst systems may be referred to as multisite catalysts. In such systems the different catalyst components must be able to produce polyolefins of different molecular weights under a single set of reactor process operating conditions, so that a bimodal molecular weight distribution is formed. Typically the low molecular weight portion of such bimodal polymers are derived from the metallocene component(s) of the catalyst.
It would also be desirable to use multisite catalysts to prepare polyolefins with specific comononer distributions across their bimodal molecular weight distributions. Unfortunately metallocene components conventionally used in multisite catalysts are known for the ability to incorporate high quantities of comonomer relative to Ziegler catalysts, resulting in bimodal MWD polymers in which comonomer is concentrated in the low MW portion.
It has now been found that certain metallocene components have a low propensity for incorporating comonomer into bimodal polymer even in the presence of high concentrations of comonomer and may be utilized to control the comonomer distribution.
The comonomer distribution is dependent upon the comonomer incorporation properties of the individual components of the multisite catalyst. Hence by using such metallocene components having a low propensity for incorporating comonomer, polymers may be obtained which typically exhibit a bimodal comonomer distribution in which the comonomer is more evenly distributed over the MWD or is even concentrated in the high molecular weight component.
Thus according to the present invention there is provided a process for preparing bimodal molecular weight distribution-copolymers of ethylene with alpha-olefins having 3 to 20 carbon atoms, said copolymers having:
(a) a comonomer distribution wherein the comonomer level at the mid-position of the low molecular weight component is <3 times the level at the mid position of the high molecular weight component, and
(b) a total average comonomer content in the range 0.5-20 short chain branches (SCB)/1000 C atoms
and characterised in that said process is carried out in the presence of a supported multisite catalyst.
The “comonomer level” defined in (a) represents the comonomer content, measured in short chain brafiches per thousand backbone carbon atoms (SCB/1000 C), of the polymer at the specified molecular weight which is independent of the proportion of the total polymer represented by the polymer at that molecular weight.
The “total average comonomer content” defined in (b) is the average comonomer content, in SCB/1000 C, of all polymer over the entire molecular weight range.
The multisite catalyst is defined as comprising two active components for example a Ziegler catalyst component producing a high molecular weight polymer component and a metallocene component producing a low molecular weight polymer component. The metallocene component may also be comprised of two or more different metallocene species provided that together they provide the required low molecular weight polymer component.
Examples of metallocenes suitable for use in the present invention are represented by the following Formulae:
(C
5
R
5
)(C
5
R
1
5
)MY
2
  (I)
(C
5
R
2
H
3
)(C
5
R
1
2
H
3
)MY
2
  (II)
(C
5
R
4
)Z(C
5
R
1
4
)MY
2
  (III)
(C
5
R
m
H
5−m
)(C
5
R
1
n
H
5−n
)MY
2
  (IV)
wherein,
C
5
R
5
and C
5
R
1
5
etc represent a cyclopentadienyl ligand,
R and R
1
alkyl, aryl, alkylaryl, alkenyl, or haloalkyl and may be the same or different,
z=bridging group comprising CX
2
, SiX
2
, GeX
2
etc,
X=hydrogen or as defined by R and R
1
above,
M=Zr, Ti or Hf,
Y=univalent anionic ligand for example halide, alkyl, alkoxy, etc.
and wherein in Formula (II) at least one of R and R
1
has ≧3 carbon atoms and in Formula (IV) m=3 or 4.
Examples of suitable metallocenes as represented by Formula (I) and (II) are bis(pentamethylcyclopentadienyl) zirconium dichloride and bis(1-propenyl-2-methylcyclopentadienyl) zirconium dichloride respectively.
These metallocenes are represented by the Formula:
By using the multisite catalysts of the present invention, polymer compositions containing a lower absolute comonomer incorporation level may be obtained for a given set of reaction conditions.
The metallocenes may be prepared in accordance with literature methods eg J E Bercaw et al JACS 100, 10, 3078, Canadian Journal of Chemistry 69, 1991, 661-672 and E Samuel et al J. Organometallic Chem. 1976, 113, 331-339.
Bimodal distribution is defined as relating to copolymers which show a substantially different molecular weight distribution between the low and the high molecular weight components.
Typically the low molecular weight component has a mid-position in the range 1000-300,000 preferably in the range 5000-50,000 and the high molecular weight component has a mid-position in the range 100,000-10,000,000 preferably in the range 150,000-750,000.
The total average comonomer content is preferably in the range 1 to 20 SCB/1000 C atoms.
The multisite catalyst for use in the present invention may be used in the presence of suitable co-catalysts. Suitable co-catalysts are organometallic compounds having a metal of Group IA, IIA, IIB or IIIB of the periodic table. Preferably, the metals are selected from the group including lithium, aluminium, magnesium, zinc and boron. Such co-catalysts are known for their use in polymerisation reactions, especially the polymerisation of olefins, and include organo aluminium compounds such as trialkyl, alkyl hydrido, alkyl halo, alkyl alkoxy aluminium compounds and alkyl aluminoxanes. Suitably each alkyl or alkoxy group contains 1 to 6 carbons. Examples of such compounds include trimethyl aluminium, triethyl aluminium, diethyl aluminium hydride, triisobutyl aluminium, tridecyl aluminium, tridodecyl aluminium, diethyl aluminium methoxide diethyl aluminium ethoxide, diethyl aluminium phenoxide, diethyl aluminium chloride, ethyl aluminium dichloride, methyl diethyoxy aluminium and methyl aluminoxane.
The preferred compounds are alkyl aluminoxanes, the alkyl group having 1 to 10 carbon atoms, especially methyl aluminoxane (MAO) and trialkyl aluminium compounds eg trimethylaluminium. Other suitable co-catalysts also include Bronsted or Lewis acids.
The co-catalyst may be mixed with the supported multisite catalyst. For example the metallocene component and co-catalyst (eg MAO) may be added to a supported Ziegler catalyst. During the subsequent polymerisation process a second cocatalyst (eg trimethylaluminium) may be added to the reaction medium.
Catalyst supports used with the multisite catalyst may comprise a single oxide or a combination of oxides or metal halides. They may also be physical

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