spectroscopy [99], LA-MS, and LDI-MS [75], which were frequently involved in
recent studies of the Phillips catalyst. The characterization under close to actual
commercial conditions is also a challenge as well as an opportunity to cast some
light on the related mechanisms. At the same time, the combination of modern
spectroscopic methods with other methodologies such as polymerization kinetic,
model catalyst, and molecular modeling techniques as well as with analysis of the
microstructures of polymer chains will play more and more important roles in the
future and will be partially outlined in the following sections.
3 Approaches Using Polymerization Kinetics
Kinetic investigation through either polymerization experiments or mathematic
modeling both for slurry and gas phase polymerization is one of the most important
ways to investigate catalytic mechanisms and to provide basic data for polymerization reactor and process design. Mathematic modeling of ethylene polymerization
kinetics over Phillips catalysts has been demonstrated as a powerful tool for the
precise evaluation of the basic kinetic parameters and to establish equations for
structure–property regulation through control of process parameters
[100–103]. The polymerization kinetics of Phillips catalysts could be significantly
affected by the reductive activation process for ethylene polymerization using
different activators such as ethylene, CO, Al-alkyl cocatalysts (e.g., TEA), or
even other reducing agents. The polymerization kinetics of Phillips catalysts
using ethylene monomer itself as activator for ethylene polymerization has been
systematically investigated [2]. Typically, a linearly built-up type of kinetic curve
would be presented, with an induction period dependent on the polymerization
temperature and ethylene pressure. Reductive activation by CO only diminishes the
induction period without changing the character of the built-up type of kinetic
curve. In recent years, activation of the Phillips catalyst by Al-alkyl cocatalysts is
becoming one of the most important ways to improve the catalyst performance and
the microstructure and properties of the polyethylene (PE) products. As is well
known, Al-alkyl cocatalyst is an indispensable component for most of the olefin
polymerization catalysts such as Ziegler–Natta and metallocene catalysts. The
Al-alkyl cocatalyst could act as reducing agent, alkylation agent, poison scavenger,
and have a marked impact on the polymer microstructure by control of the chain
transfer and stereospecificity. Also, excess amount of Al-alkyl cocatalyst could
deactivate the catalyst through over-reduction of the active Cr species. Ethylene
polymerization with Phillips catalyst without using any organometal cocatalyst is
taken as the most important evidence to support the monometallic active site
mechanism. Therefore, Al-alkyl cocatalyst could be excluded as the active site
former for Phillips catalysts.
During the last few decades, experimental reports about the combination of
Al-alkyl cocatalyst with the Phillips catalyst have been very limited. Spitz et al.
[104] reported the significant effect of TEA on Phillips catalyst for the activity,
Phillips Cr/Silica Catalyst for Ethylene Polymerization
155
recent studies of the Phillips catalyst. The characterization under close to actual
commercial conditions is also a challenge as well as an opportunity to cast some
light on the related mechanisms. At the same time, the combination of modern
spectroscopic methods with other methodologies such as polymerization kinetic,
model catalyst, and molecular modeling techniques as well as with analysis of the
microstructures of polymer chains will play more and more important roles in the
future and will be partially outlined in the following sections.
3 Approaches Using Polymerization Kinetics
Kinetic investigation through either polymerization experiments or mathematic
modeling both for slurry and gas phase polymerization is one of the most important
ways to investigate catalytic mechanisms and to provide basic data for polymerization reactor and process design. Mathematic modeling of ethylene polymerization
kinetics over Phillips catalysts has been demonstrated as a powerful tool for the
precise evaluation of the basic kinetic parameters and to establish equations for
structure–property regulation through control of process parameters
[100–103]. The polymerization kinetics of Phillips catalysts could be significantly
affected by the reductive activation process for ethylene polymerization using
different activators such as ethylene, CO, Al-alkyl cocatalysts (e.g., TEA), or
even other reducing agents. The polymerization kinetics of Phillips catalysts
using ethylene monomer itself as activator for ethylene polymerization has been
systematically investigated [2]. Typically, a linearly built-up type of kinetic curve
would be presented, with an induction period dependent on the polymerization
temperature and ethylene pressure. Reductive activation by CO only diminishes the
induction period without changing the character of the built-up type of kinetic
curve. In recent years, activation of the Phillips catalyst by Al-alkyl cocatalysts is
becoming one of the most important ways to improve the catalyst performance and
the microstructure and properties of the polyethylene (PE) products. As is well
known, Al-alkyl cocatalyst is an indispensable component for most of the olefin
polymerization catalysts such as Ziegler–Natta and metallocene catalysts. The
Al-alkyl cocatalyst could act as reducing agent, alkylation agent, poison scavenger,
and have a marked impact on the polymer microstructure by control of the chain
transfer and stereospecificity. Also, excess amount of Al-alkyl cocatalyst could
deactivate the catalyst through over-reduction of the active Cr species. Ethylene
polymerization with Phillips catalyst without using any organometal cocatalyst is
taken as the most important evidence to support the monometallic active site
mechanism. Therefore, Al-alkyl cocatalyst could be excluded as the active site
former for Phillips catalysts.
During the last few decades, experimental reports about the combination of
Al-alkyl cocatalyst with the Phillips catalyst have been very limited. Spitz et al.
[104] reported the significant effect of TEA on Phillips catalyst for the activity,
Phillips Cr/Silica Catalyst for Ethylene Polymerization
155