7 Properties of Syndiotactic Polypropylene Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.1 Polymorphism of Syndiotactic Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.2 Thermal Properties and the Origin of Multiple Melting Behavior of s-PP . . . . . . . . . 102
7.3 Syndiotactic Polypropylene: Physical and Mechanical Properties . . . . . . . . . . . . . . . . . . 103
7.4 Syndiotactic Polypropylene: Processing and Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.5 Market Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
8 Experimental Details [187, 188] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . 107
8.1 Synthesis of Bridged, Cyclopentadienyl-Fluorenyl Zirconocenes. . . . . . . . . . . . . . . . . . 107
9 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
1 Introduction
Syndiotactic polypropylene was first isolated by Natta and coworkers as a minor
by-product of an isotactic polypropylene (i-PP) produced with a TiCl 3 -based
Ziegler–Natta (ZN) catalyst, TiCl 3 /Et 2 AlCl [1–9]. The nature of the active sites
and the mechanism of formation of this polymer (discovered more than half a
century ago) are not very well known and still a matter of much debate. It is,
however, believed that they are formed on catalytic sites with C 2 symmetry and of
low chlorine coordination (Fig. 1) via a chain-end controlled mechanism. Later,
Zambelli and coworkers produced syndiotactic polypropylene (s-PP) directly, at
subzero polymerization temperatures, using a vanadium-based catalyst. In this case,
more is known about the nature of the active site (Fig. 1) and the mechanism of the
polymerization has been elucidated satisfactorily [10–17]. It is assumed that the
polymer chains are formed at low temperatures at the homogeneous active sites
according to a mechanism that is controlled by the chirality of the last inserted
monomer unit located at the metal-end of the growing polymer chain. No singlecrystal X-ray structure of the catalyst precursor is available due to the very
temperature-sensitive nature of the catalytic species and its precursor.
After discovery of the bridged cyclopentadienyl-fluorenyl metallocene-based
syndiotactic-specific catalyst systems and the resulting s-PP polymers [18–27] it
was possible for the first time to make ever more accurate statements about the nature
of the syndiospecific active site and the mechanism of the polymerization of these
fascinating yet very complex systems. By studying the available X-ray structural data
of the metallocene molecules and their stabilized alkylmetallocenium cation [19–21]
(as the immediate active site precursor), it has become possible to make reasonable
deductions on the nature of the active sites and their mode of functioning during the
different stages of the polymerization, i.e., monomer coordination, activation, insertion, and propagation. On the other hand, the facile availability of large syndiotactic
polymer samples, prepared at different and precise polymerization temperatures/
conditions, and statistical analysis of the data extracted from their high-resolution
13
C NMR spectra, provided the means for accurate statements on the mechanism of
the polymerization and the relationship between catalyst structure and polymer
microstructure. The bridged cyclopentadienyl-fluorenyl ligand-based metallocene
structure proved to be a very versatile precatalyst system. Depending on the substitutional modifications performed on different parts of the organic ligand, the bridge,
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
45
7.1 Polymorphism of Syndiotactic Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.2 Thermal Properties and the Origin of Multiple Melting Behavior of s-PP . . . . . . . . . 102
7.3 Syndiotactic Polypropylene: Physical and Mechanical Properties . . . . . . . . . . . . . . . . . . 103
7.4 Syndiotactic Polypropylene: Processing and Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.5 Market Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
8 Experimental Details [187, 188] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . 107
8.1 Synthesis of Bridged, Cyclopentadienyl-Fluorenyl Zirconocenes. . . . . . . . . . . . . . . . . . 107
9 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
1 Introduction
Syndiotactic polypropylene was first isolated by Natta and coworkers as a minor
by-product of an isotactic polypropylene (i-PP) produced with a TiCl 3 -based
Ziegler–Natta (ZN) catalyst, TiCl 3 /Et 2 AlCl [1–9]. The nature of the active sites
and the mechanism of formation of this polymer (discovered more than half a
century ago) are not very well known and still a matter of much debate. It is,
however, believed that they are formed on catalytic sites with C 2 symmetry and of
low chlorine coordination (Fig. 1) via a chain-end controlled mechanism. Later,
Zambelli and coworkers produced syndiotactic polypropylene (s-PP) directly, at
subzero polymerization temperatures, using a vanadium-based catalyst. In this case,
more is known about the nature of the active site (Fig. 1) and the mechanism of the
polymerization has been elucidated satisfactorily [10–17]. It is assumed that the
polymer chains are formed at low temperatures at the homogeneous active sites
according to a mechanism that is controlled by the chirality of the last inserted
monomer unit located at the metal-end of the growing polymer chain. No singlecrystal X-ray structure of the catalyst precursor is available due to the very
temperature-sensitive nature of the catalytic species and its precursor.
After discovery of the bridged cyclopentadienyl-fluorenyl metallocene-based
syndiotactic-specific catalyst systems and the resulting s-PP polymers [18–27] it
was possible for the first time to make ever more accurate statements about the nature
of the syndiospecific active site and the mechanism of the polymerization of these
fascinating yet very complex systems. By studying the available X-ray structural data
of the metallocene molecules and their stabilized alkylmetallocenium cation [19–21]
(as the immediate active site precursor), it has become possible to make reasonable
deductions on the nature of the active sites and their mode of functioning during the
different stages of the polymerization, i.e., monomer coordination, activation, insertion, and propagation. On the other hand, the facile availability of large syndiotactic
polymer samples, prepared at different and precise polymerization temperatures/
conditions, and statistical analysis of the data extracted from their high-resolution
13
C NMR spectra, provided the means for accurate statements on the mechanism of
the polymerization and the relationship between catalyst structure and polymer
microstructure. The bridged cyclopentadienyl-fluorenyl ligand-based metallocene
structure proved to be a very versatile precatalyst system. Depending on the substitutional modifications performed on different parts of the organic ligand, the bridge,
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
45