(517g) Kinetic Modeling and Parameter Estimation of the Metallocene Catalyzed Slurry Polymerization of Propylene: Effect of Mao/Zr Ratio

Abstract

Homogeneous single site
catalysts have dominated the research interest in olefin polymerization over
the past 25 years. Sinn and Kaminski ¹ started this trend with the discovery that
methylaluminoxane (MAO) activates group IV transition metal catalysts. The
ability to control regio- and stereochemistries, and comonomer content and
distribution are some of the important advantages that metallocene catalysts
afford in the polymerization process. A current goal is the reduction of the
necessary amount of cocatalyst, typically methylaluminoxane (MAO), since its
high cost and the large amounts needed at industrial scale are a barrier to
making metallocene systems cost effective with respect to Ziegler-Natta
catalysts ². In this paper, we study the effect of MAO on
reaction rate, molecular weight and polymer microstructure in the slurry
polymerization of isotactic polypropylene.

The active catalyst is a
cationic species generated in an equilibrium reaction between a zirconocene
dichloride catalyst and MAO. Increasing concentrations of MAO shift the
equilibrium to the formation of the activated complex. Thus, catalyst activity
increases with increasing MAO/Zr ratio ^3,
4. In addition, MAO in solution contains residual
trimethylaluminum (TMA) in a typical concentration of 5% wt. It is known that
TMA can act as a chain transfer agent leading to the formation of an
isobutyl-terminated chain and liberation of the activated complex ⁵. In conditions where chain transfer to TMA becomes a
competitive process with respect to the other chain transfer reactions,
increasing amounts of MAO reduce the molecular weight.

Slurry polymerizations of
propylene were carried out using rac-ethylene-bis(1-indenyl)zirconium
dichloride/MAO as the catalyst system. Polymerizations were performed at 40ºC
in a 500 mL state-of-the-art semi-batch reactor. The effect of MAO/Zr ratio on
the rate of reaction, molecular weight and polymer microstructure was explored
in order to allow the chain transfer reaction to TMA to be included in our
previously developed kinetic model ⁶. An exhaustive analysis of end-groups of the polymer
chains by ¹H and ¹³C NMR was performed to gain
understanding of the chain transfer mechanisms.

Our kinetic model for the
homopolymerization of propylene is developed to predict rate of reaction,
molecular weight distribution and percentage of unsaturated end-groups of the
polymer ⁶. It is characterized by: (1) non-instantaneous
initiation by insertion of the first monomer molecule; (2) propagation by 2,1
insertion producing a regioerror that lowers the molecular weight through
competing chain transfer reactions; (3) chain transfer to TMA; and (4) catalyst
deactivation to account for decreasing catalyst activity with time. The
estimation of kinetic rate constants was performed using a systematic
optimization approach involving on-line measurements of the reaction rate and
end-of-batch measurements of molecular weight distribution and polymer
end-groups. A least-square objective function was defined to minimize the
difference between the experimental measurements and the model predictions. The
minimization was performed subject to constraints imposed by the kinetic model
equations. These constraints were posed as a set of nonlinear algebraic
equations generated by applying temporal discretization techniques to the
dynamic model. This presentation will focus on the experimental and modeling
results obtained with the rac-ethylene-bis(1-indenyl)zirconium dichloride/MAO
catalyst system.

The ultimate goal of this research is to
develop a chemically based kinetic model that can be extensible to a number of
single-site metallocene catalyst systems. To accomplish this goal, research
work with the catalysts rac-dimethylsilylbis(2-methyl-4-phenyl-indenyl)
ZrCl₂ and rac-ethylene-bis (4,7-dimethyl-1-indenyl) ZrCl₂
is underway. The former species is highly active and stereoselective, while the
later species produces isotactic polypropylene with a high frequency of
regioirregular 2,1 insertions. Our current work with these catalyst systems
will be described in the presentation.

References

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6.      Gonzalez-Ruiz RA, B.
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Submitted to AIChE Journal 2005.