plausible at a time when Ziegler’s “Aufbau” reaction [3] was the only known process
of ethene oligomerization under comparatively mild conditions, but much less so once
the foundations of transition metal chemistry were in place. On the other hand, ZN
catalysis is the most effective man-made reaction for making polymers, and therefore
any ZN scientist should know about polymer chemistry and physics. In particular, a
proper assessment of chain microstructure is key to understanding the properties of
polyolefin-based materials and also in investigating the behavior of organometallic
catalytic species, which remain elusive even now after 60 years of application.
The subject has been treated exhaustively in several books and reviews [4–6].
One last introductory remark concerns the bibliography. Many key achievements
of early ZN catalysis for iPP were only disclosed in patents, or appeared first in
articles published in Italian journals and language. For the sake of simplicity, I will
mainly refer to (more) easily accessible books and reviews in English, wherein
interested readers can find detailed citations of the original literature.
2 ZN Catalysts for Polypropylene: Definition
and Genealogy
According to Boor, who authored one of the first comprehensive books on the subject
[7], a ZN catalyst is a combination of a transition metal compound and a main group
metal-alkyl compound. Although the definition may look unrealistically broad (and in
fact covers a huge number of combinations that are not active as olefin polymerization
catalysts), it is a sensible one because active combinations have been reported for
transition metals in almost all groups of the periodic table and a generous number of
main group metals. In a sense it is instead too narrow, because it does not include the
so-called main group metal-alkyl-free (MAF) catalysts, which are well-documented
[8] albeit thus far irrelevant for application (with the very notable exception of
heterogeneous Cr-based systems for polyethylene, known as Phillips catalysts [9]
and starring in another fascinating story). One may also wonder if molecular
(metallocene and post-metallocene) catalysts can or should be included in the definition; my personal view is that they can but they should not, because although the basic
catalysis is the same, the activation chemistry and the high electrophilicity of the
cationic active species introduce clear aspects of specificity compared with “classical”
heterogeneous Ti-based systems [10]. Last but not least is the question of whether or
not “Ziegler–Natta” is the correct designation for the latter systems, looking back at
history and the patent litigations that went on for decades [11]. On this of course I have
an opinion, like most others in the field, but it is a personal one and as such of very
limited importance. The reason why I will refer to both catalysts and catalysis as
“Ziegler–Natta” is for the inclusive character of this choice. This book celebrates the
50th anniversary of the Nobel Prize to two outstanding scientists named Karl Ziegler
and Giulio Natta, and when I write about the marvelous chemistry that they started it
seems natural to me to merge the two names into one.
Much less pregnant of implications but of some practical relevance is the classification of ZN systems for polypropylene [11]. I have always found it confusing to
Giulio Natta and the Development of Stereoselective Propene Polymerization
39
of ethene oligomerization under comparatively mild conditions, but much less so once
the foundations of transition metal chemistry were in place. On the other hand, ZN
catalysis is the most effective man-made reaction for making polymers, and therefore
any ZN scientist should know about polymer chemistry and physics. In particular, a
proper assessment of chain microstructure is key to understanding the properties of
polyolefin-based materials and also in investigating the behavior of organometallic
catalytic species, which remain elusive even now after 60 years of application.
The subject has been treated exhaustively in several books and reviews [4–6].
One last introductory remark concerns the bibliography. Many key achievements
of early ZN catalysis for iPP were only disclosed in patents, or appeared first in
articles published in Italian journals and language. For the sake of simplicity, I will
mainly refer to (more) easily accessible books and reviews in English, wherein
interested readers can find detailed citations of the original literature.
2 ZN Catalysts for Polypropylene: Definition
and Genealogy
According to Boor, who authored one of the first comprehensive books on the subject
[7], a ZN catalyst is a combination of a transition metal compound and a main group
metal-alkyl compound. Although the definition may look unrealistically broad (and in
fact covers a huge number of combinations that are not active as olefin polymerization
catalysts), it is a sensible one because active combinations have been reported for
transition metals in almost all groups of the periodic table and a generous number of
main group metals. In a sense it is instead too narrow, because it does not include the
so-called main group metal-alkyl-free (MAF) catalysts, which are well-documented
[8] albeit thus far irrelevant for application (with the very notable exception of
heterogeneous Cr-based systems for polyethylene, known as Phillips catalysts [9]
and starring in another fascinating story). One may also wonder if molecular
(metallocene and post-metallocene) catalysts can or should be included in the definition; my personal view is that they can but they should not, because although the basic
catalysis is the same, the activation chemistry and the high electrophilicity of the
cationic active species introduce clear aspects of specificity compared with “classical”
heterogeneous Ti-based systems [10]. Last but not least is the question of whether or
not “Ziegler–Natta” is the correct designation for the latter systems, looking back at
history and the patent litigations that went on for decades [11]. On this of course I have
an opinion, like most others in the field, but it is a personal one and as such of very
limited importance. The reason why I will refer to both catalysts and catalysis as
“Ziegler–Natta” is for the inclusive character of this choice. This book celebrates the
50th anniversary of the Nobel Prize to two outstanding scientists named Karl Ziegler
and Giulio Natta, and when I write about the marvelous chemistry that they started it
seems natural to me to merge the two names into one.
Much less pregnant of implications but of some practical relevance is the classification of ZN systems for polypropylene [11]. I have always found it confusing to
Giulio Natta and the Development of Stereoselective Propene Polymerization
39