What is the reason for this behavior?
We think that there is a rotation influence caused by the substituent at the
Cp ring. With increasing reaction temperature, the rotation volume of the substituent is enhanced and the effect is stronger for higher steric demand of the group.
In other words, with rising temperature, the catalysts substituted on the Cp ring
with 3-Me, 3-Et, and 3-iPr resemble more and more the iPr[3-tBu-Cp][Flu] ZrCl 2
catalyst and its stereospecific behavior.
We wanted to prove this mechanistic hypothesis and synthesized a catalyst
with the same basic ligand framework, but with a non-rotatable substituent at
the Cp ring. This catalyst is shown in Fig 22 below the box. Its stereospecific
polymerization behavior lies between hemiisotactic and atactic and, indeed,
scarcely changed with rising temperature. Hence, the mechanistic hypothesis of
the rotating group seems to be true. Quod erat demonstrandum!
4 Silica-Supported Metallocene/MAO Catalysts
Metallocenes immobilized on solid support materials have been successfully
introduced in industry as polymerization catalysts for the production of new,
application-oriented polymer materials (see also the contribution of Brintzinger
and Fischer [29]). Industrial polymerization processes, which are carried out either
as a slurry process in liquid propylene or as a gas-phase process, require that
catalysts are used in the form of solid grains or pellets; soluble metallocene
catalysts thus have to be supported on a solid carrier (so-called drop-in catalysts).
An additional objective of the heterogenization process was to combine the
advantages of homogeneous metallocenes with those of supported catalysts. On
the one hand, it was intended to preserve the advantages of homogeneous metallocenes, such as the high activity, narrow molecular weight distributions, stereospecificity, and uniform comonomer incorporation. On the other hand, it was intended
to combine these features with the properties of supported catalyst technologies, such
as controlled particle growth and formation of morphologically uniform polymer
particles of the desired size and shape, which mirror the starting catalyst particles
but are at least 20 times their size and show high bulk density and no reactor fouling.
4.1 Polymerization Kinetics and Polymer Particle Growth
An important aspect of olefin polymerization with solid catalysts concerns the
characteristics of particle growth during the course of polymerization, taking into
account the aspects of mass and heat transfer. Ineffective monomer transfer
can limit catalyst productivity, while ineffective removal of polymerization
heat from the growing particle in the early stages of polymerization can cause the
formation of hot spots, which may in turn lead to catalyst decay. In the absence
26
G. Fink
We think that there is a rotation influence caused by the substituent at the
Cp ring. With increasing reaction temperature, the rotation volume of the substituent is enhanced and the effect is stronger for higher steric demand of the group.
In other words, with rising temperature, the catalysts substituted on the Cp ring
with 3-Me, 3-Et, and 3-iPr resemble more and more the iPr[3-tBu-Cp][Flu] ZrCl 2
catalyst and its stereospecific behavior.
We wanted to prove this mechanistic hypothesis and synthesized a catalyst
with the same basic ligand framework, but with a non-rotatable substituent at
the Cp ring. This catalyst is shown in Fig 22 below the box. Its stereospecific
polymerization behavior lies between hemiisotactic and atactic and, indeed,
scarcely changed with rising temperature. Hence, the mechanistic hypothesis of
the rotating group seems to be true. Quod erat demonstrandum!
4 Silica-Supported Metallocene/MAO Catalysts
Metallocenes immobilized on solid support materials have been successfully
introduced in industry as polymerization catalysts for the production of new,
application-oriented polymer materials (see also the contribution of Brintzinger
and Fischer [29]). Industrial polymerization processes, which are carried out either
as a slurry process in liquid propylene or as a gas-phase process, require that
catalysts are used in the form of solid grains or pellets; soluble metallocene
catalysts thus have to be supported on a solid carrier (so-called drop-in catalysts).
An additional objective of the heterogenization process was to combine the
advantages of homogeneous metallocenes with those of supported catalysts. On
the one hand, it was intended to preserve the advantages of homogeneous metallocenes, such as the high activity, narrow molecular weight distributions, stereospecificity, and uniform comonomer incorporation. On the other hand, it was intended
to combine these features with the properties of supported catalyst technologies, such
as controlled particle growth and formation of morphologically uniform polymer
particles of the desired size and shape, which mirror the starting catalyst particles
but are at least 20 times their size and show high bulk density and no reactor fouling.
4.1 Polymerization Kinetics and Polymer Particle Growth
An important aspect of olefin polymerization with solid catalysts concerns the
characteristics of particle growth during the course of polymerization, taking into
account the aspects of mass and heat transfer. Ineffective monomer transfer
can limit catalyst productivity, while ineffective removal of polymerization
heat from the growing particle in the early stages of polymerization can cause the
formation of hot spots, which may in turn lead to catalyst decay. In the absence
26
G. Fink