identify a ZN catalyst in terms of “generations” and in the following I refrain from
using a scheme that is not univocal. I prefer to set just one major divide between
unsupported (or self-supported) TiCl 3 -based catalysts [7] and (MgCl 2 -)supported ones
[11], and within the former to classify TiCl 3 polymorphs according to their crystal
structure, i.e., as fibrillar (β) or layered (α, γ and δ) [12].
The emphasis of this chapter will be on the basic principles of ZN catalysis for iPP,
rather than on iPP properties and applications. In particular, the objective is to illustrate
the mechanism of asymmetric induction in the insertion of a monomer that has no
functional groups other than a C¼C bond, and yet reacts yielding a much larger
enantiomeric excess than most highly functional substrates of enantioselective catalysis.
That this occurs at the surface of simple and inexpensive inorganic solids is another
amazing aspect that contributes to make ZN catalysts “unique and marvelous” [7].
3 From Ziegler’s Metallorganische Mischkatalysator
to ZN Catalysts
It is a fact that the impact of transition metals on olefin polymerization was discovered
by accident. The details of how the accident occurred may vary somewhat depending
on the literature source, but what is certain is that the fortuitous presence of traces of Ni
in a reactor where ethene oligomerization at Al centers was being carried out changed
the process into a selective dimerization [13]. I find this an example of how Fate
challenges humans. Of all transition metals, Ni is one of the least suited to mediate
polyolefin chain growth; the strong propensity of Ni-alkyl bonds to undergo β-H
elimination make Ni-based catalysts mainly suited to oligomerization [14]. Indeed,
Ni contamination in Ziegler’s autoclave led to 1-butene, which is the shortest oligomer
that can form from ethene polyinsertion under fast β-H elimination. Decades afterwards, elegant work by Brookhart and coworkers demonstrated that high molecular
weight polyethylene can actually form in the presence of Ni-based catalysts bearing a
proper ancillary ligand framework [15], but that’s yet another story. For the one of
interest here, Fate’s verdict was: “Ni is no good for the ‘Aufbau’ reaction.” In Ziegler’s
group, on the other hand, they knew about Sybilline oracles and re-phrased the verdict
into a more general “[Transition] Metals can change the course of the ‘Aufbau’
reaction,” and realized its vast implications. The systematic screening that followed
was serendipitous and fortunate; nowadays, even a freshman student of organometallics would privilege early transition metals, but in 1953 Zr and Ti were just
metals other than Ni. In the modern jargon of high-throughput experimentation [16],
Cr was a “hit” and Zr a “lead.” The real breakthrough followed, i.e., the combination of
AlEt 3 and TiCl 4 [13]. What that meant for polyethylene can be read in a previous
chapter of this book.
Why not polypropylene too? This embarrassing question may be given many
different answers, but the simplest probably is that whoever finds a treasure tends to
enjoy it for a while before searching for another. On the other hand, once it is known
that somebody has found a treasure in a certain place, it is natural for others to
40
V. Busico
using a scheme that is not univocal. I prefer to set just one major divide between
unsupported (or self-supported) TiCl 3 -based catalysts [7] and (MgCl 2 -)supported ones
[11], and within the former to classify TiCl 3 polymorphs according to their crystal
structure, i.e., as fibrillar (β) or layered (α, γ and δ) [12].
The emphasis of this chapter will be on the basic principles of ZN catalysis for iPP,
rather than on iPP properties and applications. In particular, the objective is to illustrate
the mechanism of asymmetric induction in the insertion of a monomer that has no
functional groups other than a C¼C bond, and yet reacts yielding a much larger
enantiomeric excess than most highly functional substrates of enantioselective catalysis.
That this occurs at the surface of simple and inexpensive inorganic solids is another
amazing aspect that contributes to make ZN catalysts “unique and marvelous” [7].
3 From Ziegler’s Metallorganische Mischkatalysator
to ZN Catalysts
It is a fact that the impact of transition metals on olefin polymerization was discovered
by accident. The details of how the accident occurred may vary somewhat depending
on the literature source, but what is certain is that the fortuitous presence of traces of Ni
in a reactor where ethene oligomerization at Al centers was being carried out changed
the process into a selective dimerization [13]. I find this an example of how Fate
challenges humans. Of all transition metals, Ni is one of the least suited to mediate
polyolefin chain growth; the strong propensity of Ni-alkyl bonds to undergo β-H
elimination make Ni-based catalysts mainly suited to oligomerization [14]. Indeed,
Ni contamination in Ziegler’s autoclave led to 1-butene, which is the shortest oligomer
that can form from ethene polyinsertion under fast β-H elimination. Decades afterwards, elegant work by Brookhart and coworkers demonstrated that high molecular
weight polyethylene can actually form in the presence of Ni-based catalysts bearing a
proper ancillary ligand framework [15], but that’s yet another story. For the one of
interest here, Fate’s verdict was: “Ni is no good for the ‘Aufbau’ reaction.” In Ziegler’s
group, on the other hand, they knew about Sybilline oracles and re-phrased the verdict
into a more general “[Transition] Metals can change the course of the ‘Aufbau’
reaction,” and realized its vast implications. The systematic screening that followed
was serendipitous and fortunate; nowadays, even a freshman student of organometallics would privilege early transition metals, but in 1953 Zr and Ti were just
metals other than Ni. In the modern jargon of high-throughput experimentation [16],
Cr was a “hit” and Zr a “lead.” The real breakthrough followed, i.e., the combination of
AlEt 3 and TiCl 4 [13]. What that meant for polyethylene can be read in a previous
chapter of this book.
Why not polypropylene too? This embarrassing question may be given many
different answers, but the simplest probably is that whoever finds a treasure tends to
enjoy it for a while before searching for another. On the other hand, once it is known
that somebody has found a treasure in a certain place, it is natural for others to
40
V. Busico