Polymerization process

Details Polymerization process Polymerization process

1997-07-31

2001-10-09

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...

C526S123100, C526S129000, C526S127000, C526S130000, C526S901000, C526S943000

Reexamination Certificate

active

06300437

ABSTRACT:

The present invention relates to a process for preparing polyolefins, in particular to a process for preparing polyolefins having a degree of long chain branching and which show improved processability.
Polyethylenes produced from conventional Ziegler catalysts have a narrower molecular weight distribution than those produced using for example conventional chromium catalysts and generally show only average processability. For example conventional linear low density polyethylene (LLDPE) prepared using titanium based catalysts show a poorer processability than low density polyethylenes (LDPE).
Polyethylenes produced using conventional metallocene catalysts eg bis(cyclopentadienyl) zirconium systems have an even narrower molecular weight distribution and also show processability problems eg melt fracture, low melt tension etc.
In order to improve processability the breadth of molecular weight distribution may be increased or the degree of long chain branching in the polymer may be increased. Products exhibiting higher shear thinning whilst maintaining impact properties etc. are desirable. It is advantageous however that any such improvement in processability can be achieved by polymerisation the gas phase.
Several attempts have been made to improve processability. WO 93/08221 discloses the use of specific constrained geometry catalysts to introduce long chain branching to give increased processability at narrow molecular weight distributions.
EP 452920 discloses the use of a series of prepolymerised bridged metallocene catalysts which result in a narrow composition distribution and an increase in melt tension for improved bubble stability.
Another solution to improve processability is to blend a LLDPE (prepared using a conventional Ziegler catalyst) with a LDPE (prepared using a high pressure free radical process) to obtain the benefits of the LLDPE mechanical properties with the LDPE processability. However blending may introduce problems eg degradation, colour, antioxidants and properties such as heat seal may be reduced due to poor mixing or incompatibility of multi-component systems.
We have now found that by using specific metallocene complexes as catalysts polymers may be produced in the gas phase in a single step which show properties consistent with a blend of LDPE and LLDPE.
Thus according to the present invention there is provided a process for producing polyolefins which have at least 0.01 long chain branches/1000 carbon atoms along the polymer backbone (as measured by flow activation) and a breadth of molecular weight distribution (M
w
/M
n
) greater than 2.5 said process comprising polymerising an olefin monomer or monomers in the presence of a catalyst comprising a metallocene complex having the general formula:
wherein
Cp
1
, Cp
2
are independently a substituted or unsubstituted indenyl or hydrogenated indenyl group,
Y is a univalent anionic ligand,
M is zirconium, titanium or hafnium, and
Z is a bridging group comprising an alkylene group having 1 to 20 carbon atoms or a dialkyl silyl- or germanyl-group, or alkyl phophine or amine radical.
Preferred complexes are those in which M is zirconium.
The univalent anionic ligands are suitably hydrogen, halide, hydrocarbyl, alkoxide, amide or phosphide and are preferably halide.
When Z is an alkylene group it has preferably 2 carbon atoms.
A particularly preferred metallocene complex is C
2
-bridged bis(indenyl) zirconium dichloride represented by the formula:
The metallocene for use in the present invention may be used in the presence of a suitable co-catalyst. Suitably the co-catalyst is an organometallic compound having a metal of Group 1A, 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 hydride, alkyl halo and alkyl alkoxy aluminium compounds. 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 (MAO), 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. Suitable co-catalysts also include Bronsted or Lewis acids.
The in-situ co-catalyst may be mixed with the metallocene, optionally on an inorganic support. Alternatively, the co-catalyst may be added to the polymerisation medium along with the metallocene complex. Suitably, the amount of co-catalyst mixed with the metallocene complex may be such as to provide an atom ratio (M) from the metallocene to the metal in the co-catalyst of 1-10,000:10,000-1 for aluminoxanes and 1-100:100-1 otherwise.
Catalyst supports may comprise a single oxide or a combination of oxides. They may also be physical mixtures of oxides. The supports may have a high surface area (250-1000M
2
/g) and a low pore volume (0-1 ml/g) or a low surface area (0-250M
2
/g) and high pore volume (1-5 ml/g) or preferably high surface area (250-1000M
2
/g) and high pore volume (1-5 ml/g) (mesoporous). Preferred support materials are silica, alumina, titania, boria and anhydrous magnesium chloride or mixtures thereof, although any support used in heterogeneous catalysis/polymer catalysis may be employed.
The support may undergo a pretreatment to modify its surface eg thermal or chemical dehydroxylation or any combination of these, using agents such as hexamethyldisilazane and trimethylaluminium. Other reagents that can be used are triethyaluminium, methylaluminoxane and other aluminium containing alkyls, magnesium alkyls especially dibutyl magnesium and alkyl magnesium halides, zinc alkyls and lithium alkyls. Different impregnation regimes may be used to add the surface treatment and subsequent metallocene impregnation. Metallocene or metallocene/cocatalyst may be added to the support or other supported polymerisation catalyst before, during or after surface treatment to modify the support/catalyst surface or any combination of these. Impregnation may take place sequentially or in a number of separate steps or in a single step using any method known in the prior art including vapour phase treatment/impregnation techniques.
The olefin polymerisation catalyst used in the process according to the present invention may be used to produce both homopolymers or copolymers using solution polymerisation, slurry polymerisation or gas phase polymerisation techniques. Suitably alpha olefins used in copolymerisation may be those having up to 20 carbon atoms in particular butene-1, hexene-1, 4-methyl pentene-1 or octene-1. Methods and apparatus for effecting such polymerisation reactions are well known and described in, for example, named
Encycopaedia of Polymer Science and Engineering
published by John Wiley and Sons, 1987, Volume 7, pages 480 to 488 and 1988, Volume 12, pages 504 to 541. The catalyst according to the process of the invention can be used in similar amounts and under similar conditions to known olefin polymerisation catalysts.
The polymerisation may optionally be carried out in the presence of hydrogen. Hydrogen or other suitable chain transfer agents may be employed in the polymerisation to control the molecular weight of the produced polyolefin. The amount of hydrogen may be such that the percentage of the partial pressure of hydrogen to that of olefin(s) is from 0.01-200%, preferably from 0.01-10%.
Typically, the temperature is from 30 to 110° C. for the slurry or “particle form” process or for the gas phase process. For the solution process the temperature is typically from 30 to 250° C. The pressure used can be selected from a relatively wide range of suitable pressure, eg from subatmospheric to

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