enabling the latter, inter alia, to call on bright young chemists and chemical engineers
selected from all over Italy for academic or industrial positions (depending on need).
The quality of those young scientists, incidentally, is demonstrated by the fact that by
the 1960s most of them had become chair-holders and had founded schools of
excellence at top Institutes in Italy and abroad, whose legacy has not entirely dispersed
after half a century. All that could happen because the company trusted the professor,
who fully deserved the trust, and a lean legislation and bureaucracy assisted in the
endeavor. In Italy, and not only in Italy, this simple recipe was lost quite a while ago,
which may be one reason for the present stagnation. Of course, that Ziegler was also a
consultant for Montecatini, which was therefore aware about the “Nickel effect” and
passed the information over to Natta almost in real time, was crucial for the following
developments [11].
The original Ziegler’s catalyst system, i.e., a mixture of TiCl 4 and AlEt 3 in heptane,
was a rather poor catalyst for iPP; typically, less than 40% by weight of the produced
polymer had a degree of stereoregularity resulting in high crystallinity and high
melting temperature [7, 11]. Understanding that alkylated TiCl 3 was the real catalyst,
discovering the polymorphism of TiCl 3 , and finding new highly stereoselective catalyst systems for iPP based on the combination of the right polymorph(s) and Al-alkyl
(s) were the achievements of Natta’s group on the inorganic and organometallic
chemistry side. Elucidating the stereostructure of the unprecedented new polymer
(and many more deriving from the stereoselective polymerization of higher 1-alkenes,
styrene, and conjugated dienes), on the other hand, was their monumental contribution
to polymer science. By the end of the 1950s, the picture was practically complete.
In retrospect, I honestly find that even a Nobel Prize is not an adequate reward for such
an extraordinary enterprise.
4 The Structure of TiCl 3 -Based ZN Catalysts for iPP
Under standard conditions, TiCl 4 is a readily hydrolyzable molecular liquid miscible
with aliphatic and aromatic hydrocarbons, in which its tetrahedral molecules have no
tendency to aggregate. When TiCl 4 and AlEt 3 are in contact in an aliphatic hydrocarbon (e.g., heptane) a reaction occurs, as is indicated by the immediate change of the
liquid phase from colorless to yellow, and the progressive separation of a brown solid
(rather slow below room temperature; faster above, in which case the color of the
precipitate tends to darken and ultimately becomes black) [7, 11, 18, 19]. The process
is believed to entail a first step of metathesis, leading to the formation of TiCl 3 Et and
AlEt 3–x Cl x mixtures. TiCl 3 Et is metastable and decomposes homolytically into TiCl 3
and Et radicals; the latter mostly disproportionate to ethane and ethene, whereas the
former aggregate into highly defective and partly ethylated TiCl 3 crystallites,
precipitating from the nonpolar liquid in view of the predominantly ionic character
of the bonding [7, 11, 18, 19]. Natta and coworkers discovered that this heterogeneous
system was an active catalyst for the polymerization of propene, and unexpectedly
found by X-ray diffraction that the polymer was partly crystalline, which evidently
42
V. Busico
selected from all over Italy for academic or industrial positions (depending on need).
The quality of those young scientists, incidentally, is demonstrated by the fact that by
the 1960s most of them had become chair-holders and had founded schools of
excellence at top Institutes in Italy and abroad, whose legacy has not entirely dispersed
after half a century. All that could happen because the company trusted the professor,
who fully deserved the trust, and a lean legislation and bureaucracy assisted in the
endeavor. In Italy, and not only in Italy, this simple recipe was lost quite a while ago,
which may be one reason for the present stagnation. Of course, that Ziegler was also a
consultant for Montecatini, which was therefore aware about the “Nickel effect” and
passed the information over to Natta almost in real time, was crucial for the following
developments [11].
The original Ziegler’s catalyst system, i.e., a mixture of TiCl 4 and AlEt 3 in heptane,
was a rather poor catalyst for iPP; typically, less than 40% by weight of the produced
polymer had a degree of stereoregularity resulting in high crystallinity and high
melting temperature [7, 11]. Understanding that alkylated TiCl 3 was the real catalyst,
discovering the polymorphism of TiCl 3 , and finding new highly stereoselective catalyst systems for iPP based on the combination of the right polymorph(s) and Al-alkyl
(s) were the achievements of Natta’s group on the inorganic and organometallic
chemistry side. Elucidating the stereostructure of the unprecedented new polymer
(and many more deriving from the stereoselective polymerization of higher 1-alkenes,
styrene, and conjugated dienes), on the other hand, was their monumental contribution
to polymer science. By the end of the 1950s, the picture was practically complete.
In retrospect, I honestly find that even a Nobel Prize is not an adequate reward for such
an extraordinary enterprise.
4 The Structure of TiCl 3 -Based ZN Catalysts for iPP
Under standard conditions, TiCl 4 is a readily hydrolyzable molecular liquid miscible
with aliphatic and aromatic hydrocarbons, in which its tetrahedral molecules have no
tendency to aggregate. When TiCl 4 and AlEt 3 are in contact in an aliphatic hydrocarbon (e.g., heptane) a reaction occurs, as is indicated by the immediate change of the
liquid phase from colorless to yellow, and the progressive separation of a brown solid
(rather slow below room temperature; faster above, in which case the color of the
precipitate tends to darken and ultimately becomes black) [7, 11, 18, 19]. The process
is believed to entail a first step of metathesis, leading to the formation of TiCl 3 Et and
AlEt 3–x Cl x mixtures. TiCl 3 Et is metastable and decomposes homolytically into TiCl 3
and Et radicals; the latter mostly disproportionate to ethane and ethene, whereas the
former aggregate into highly defective and partly ethylated TiCl 3 crystallites,
precipitating from the nonpolar liquid in view of the predominantly ionic character
of the bonding [7, 11, 18, 19]. Natta and coworkers discovered that this heterogeneous
system was an active catalyst for the polymerization of propene, and unexpectedly
found by X-ray diffraction that the polymer was partly crystalline, which evidently
42
V. Busico