In only a few polymerization processes are metallocene catalysts used in a soluble form. Supported metallocene catalysts are preferred for the pro- duction of polyethylene or isotactic polypropylene on an industrial scale, especially in the slurry and gas-phase processes. To use them in existing technological processes (drop-in technology) as replacements for the con- ventional Ziegler–Natta catalysts, the metallocenes have to be anchored to an insoluble powder support, including silica, alumina, and magnesium dichloride (208–217). Various methods of anchoring catalysts to supports are possible (Fig. 25):
1. Initial absorption of MAO on the support with subsequent addition to metallocenes in a second step; this is the procedure most commonly used.
The washed catalysts are used in combination with additional MAO or other aluminum alkyls in polymerization (208–210).
2. A mixture of the metallocene and MAO is adsorbed on the support. In this case, the prereaction time plays an important role (213).
3. Absorption and immobilization of the metallocene on the support sur- face first; then, after addition of MAO, this catalytic is used in the polymer- ization process (213, 214).
4. The metallocene can be bonded directly to the support by a spacer and an anchor group, resulting in effective immobilization (211).
Each of these procedures affords a different catalyst, and each in turn produces polyolefins with different properties. As a result of supporting a
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 137
FIG. 25. Methods for supporting metallocenes (221).
metallocene catalysts, the energy of the transition state is increased, leading in most cases to a decrease in the catalytic activity. On the other hand, the steric hindrance and the rigidity increase, which can change the tacticity of the product polypropylene.
The polymers obtained with catalysts made by method 1 are very similar to those obtained with the homogeneous system. Each metallocene on the support forms an active center and thus the starting point of the growth of a polymer chain. Because the active sites on the surface of each catalyst grain are virtually identical, all chains grow uniformly, resulting in polymers with the same microstructures and very narrow molecular weight distributions.
Molecular weight and isotacticity are similar to those of the homogeneous system, but the activity is about two times lower because one side of the active site is blocked by the support. On the other hand, some deactivation steps are also hindered by this blocking, which means that the large excess of MAO that is necessary to achieve good activity in solution can be decreased from the range of 3000–10,000 molar ratio AlMAO: Zr (homogeneous) to 100–500 Al : Zr (heterogeneous). For ethylene polymerization, Finket al.(218, 219) showed that the method of supporting the metallocene on porous silica has a substantial influence on the progress of polymerization. Fragmentation of catalyst particles by the growing polymer can be circumvented so that the catalytic activity can be maintained.
If the metallocene is linked to the support first, bonding occurs on various adsorption sites, giving nonuniform supported species and much broader molecular weight distributions in the polymer product than those obtained in the corresponding homogeneous catalysis. Furthermore, a large part of the metallocene may be destroyed by acidic centers of the support; then the activity of the supported catalyst is much lower than in the case of the homogeneous system.
Table XIV shows the results of the polymerization of propylene with var- ious supported metallocenes and a comparison with the homogeneous sys- tem; silica grade 952 was used as the support (214). The molecular weights of the polypropylene obtained with the supported catalysts are 15–30 times higher than those obtained with the homogeneous ones; the isotacticity, de- termined by the concentration of mmmm pentads, is also greater. On the other hand, the activities are much lower. The molar ratio Al : Zr for an ac- tive system was decreased to 150 : 1 from 5000 : 1. Surprisingly, the nature of the tacticity changes when a catalyst is supported; when a metallocene that gave syndiotactic polymer was anchored to a support, the mmmm pentads, measured by13C NMR spectroscopy increased from 0.4 to 96%, represent- ing an isotactic polymer. The melting points of polypropylene produced by supported metallocenes can reach 163◦C, whereas the homogeneous catalysts under the same conditions produce only low-melting polymers (with melting points of about 122◦C).
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 139
TABLE XIV
Comparison of the Polymerization of Propene with Homogeneous and Silica-Supported [End(Ind)2]Zr Cl2(I) or [Me2C(FluoCp)]ZrCl2(II)a
Al : Zr molar Activity in kg Mn Melting Tacticity Catalyst ratio [PP/(mol Zrãh)] (g/mol) point (◦C) mmmm (%)
I/MAO-homo 4000 1690 20,000 122 74
Silica/I/MAO 150 26 520,000 157 94
Silica/I/MAO 200 18 640,000 158 96
Silica/II/MAO 200 6 335,000 163 98
II/MAO-homo 5900 1500 47,000 131 0.4
Silica/II/MAO 180 20 350,000 158 96
aCocatalyst, 300–400 mg MAO in 100 ml toluene; temperature, 50◦C; [Zr]SiO2 = 3–5×10−5 mol/liter, [Zr]homo=1×10−6 mol/liter. Adapted from Kaminsky and Remmer (214).
The most common industrial method of producing a supported metal- locene catalyst is to treat the support with MAO first and then adsorb the metallocene on it. Many procedures for such preparations have been re- ported. The properties of the support provide some control of the morphol- ogy of the polymer produced; the polymer particles are often replicates of the catalyst particles (220).
The adsorbed metallocene can be prereacted with additional MAO, and the prereaction time plays a role. Scavengers can be used, which react with poisons such as oxygen, alkynes, and sulfur compounds. During the prereaction, the active site is formed faster by higher concentrations of the compounds. The supported catalyst is washed to remove the excess MAD.
To demonstrate changes in the tacticity of the product polypropylene as a consequence of anchoring the catalyst to a carrier, Me2C(Cp Flu)Zr Cl2was supported on MAO–SiO2 (221). Triisobutylaluminum (TIBA) was added to the reaction mixture as scavenger. The zirconocene is not very strongly adsorbed on the silica surface, and it can be washed out, especially if MAO is used as an additional cocatalyst and scavenger in the solvent (Table XV).
When the supported catalyst is washed with TIBA (ratio TIBA to Zr=500), the filtrate shows no polymerization activity, even when additional TIBA is added. However, if some MAO is added to the same filtrate, a weak polymer- ization activity is observed (0.21 g of polymer), showing that small amounts of Me2C(CpFlu)ZrCl2are extracted into solution, and these are activated not by TIBA but by MAO. The extracted catalyst is evidently not destroyed—
it produces syndiotactic polypropylene. On the other hand, polymerization activity of the filtrate is high if the supported catalyst is washed with MAO, which has a stronger Lewis acidity than TIBA. The amount of catalyst ex- tracted by MAO is about 20 times greater than that extracted with TIBA.
TABLE XV
Removable Amount of Zirconocene by Washing of Me2C(CpFlu)ZrCl2/MAO-SiO2Measured by Polymerization Activity
of Propene
Filtrate is Yield Leaching by Scavenger used polymerization active (g of PP)
TIBA (1 : 500) TIBA (1 : 500) No —
TIB (1 : 500) MAO (1 : 3000) Yes, sPP 0.21
MAO (1 : 3500) TIBA (1 : 500) Yes, sPP 4.26
This difference shows that when additional MAO is used as scavenger, a sup- ported metallocene catalyst in a slurry process gives a mixture of polymers, some produced on the supported catalyst and some in the homogeneous phase.
The tacticities are also different, depending on whether the metallocene is supported. The homogeneous Me2C(CpFlu)ZrCl2/MAO catalyst gives a higher yield of syndiotactic polypropylene than the supported catalyst (Fig. 26).
The dependence of the catalyst performance on temperature is nearly the
FIG. 26. 13C-NMR measured rrrr pentads in dependence of the polymerization temperature (221).
TABLEXVI InfluenceofPropeneConcentrationontheMicrostructure(13 C-NMRMeasuredPentades)ofPolypropenesbySupported andUnsupportedCatalysts Propene (moles/liter)mmmm(%)mmmr(%)rmmr(%)mmrr(%)rmrr/mrmm(%)mrmr(%)rrrr(%)rrrm(%)mrrm(%) Me2CCpFluZrCl2/MAO-SiO2 0.060.00.02.14.010.40.069.713.70.0 0.290.00.02.24.38.30.074.310.90.0 1.290.00.01.73.85.20.081.87.50.0 3.270.00.01.83.93.20.085.15.90.0 6.20.00.01.93.42.90.086.85.10.0 Me2CCpFluZrCl2 0.060.00.00.52.18.60.875.712.20.0 0.290.00.00.91.83.51.287.55.20.0 1.290.00.01.22.41.80.091.72.90.0 3.290.00.01.11.50.80.095.41.30.0 aAdaptedfromKaminskyandWinkelbach(221).
141
same for the soluble and supported catalysts. Similar values observed for the rmmr pentads indicate that there is no loss of the enantiomorphic site control resulting from anchoring of the catalyst.
The dependence the polymer microstructure on the propylene concentra- tion in the reactor is shown in Table XVI. By varying the monomer concen- tration in operation with the supported catalyst, one can obtain polypropy- lenes with relatively low concentration of rrrr pentads. At low propylene concentrations, it is possible that m insertions are formed by epimerization.
The yield of these isolated m diads is significantly increased when the cata- lyst is supported. Similar effects were shown by catalysts producing isotactic polymer.