In order to increase the melting point of the semicrystalline s-PP polymers, it is
necessary to increase the stereoregularity of the individual polymer chains. This in
turn requires the suppression of the concentration of both types of stereodefects
(m and mm) in the backbone of the polymer chains, whose combined numbers
determine the crystallinity, melting point, and most of the physical properties, such
as crystallization rate, optics, etc.
In the following sections, a few examples are presented as case studies to show
how these substitutional modifications are realized and in what way they bring
about the desired improvements. The discussion starts with the topic of modification of the bridge and bridge substituents to increase the syndiotactic polymer’s
molecular weight. We continues with the subject of structural modifications for
improving the stereoselectivity of the catalyst and the stereoregularity of the
resulting s-PP.
Before entering the discussions, it should be noted from the onset that there exist
only limited options for substitutional modifications with the aim of increasing
molecular weight and/or improving the enantioselectivity of the 1/MAO catalyst
system without tampering with its salient syndiospecific characteristics. Therefore,
for these investigations, only limited substitution modification patterns have been
selected that can bring about the desired improvement but otherwise leave the
syndiotactic selective behavior of the system intact.
3.1 Modification of the Bridge and Syndiotactic Polymer
Molecular Weight
The importance of the bridge in ansa-metallocene catalysts was recognized early on
during the development of chiral metallocene catalysts for olefin polymerization
[43–45]. Its existence is the prerequisite for stereorigidity of the metallocene
structure, designed to prevent the free rotation of the aromatic ring(s) and to
maintain the appropriate chirality of the corresponding cationic active species
during the initial stages and the entire process of the polymerization. Later, it was
recognized that the size of the bridge connecting the two aromatic rings could be
additionally used as a regulating tool for modifying the active site’s bite angle. For
example, by replacing a one-carbon bridge such as ¼CR 2 with a two-carbon bridge
like –RC–CR– or a silicon bridge like ¼SiR 2 it is possible to decrease the opening
around, and control the monomer accessibility to, the transition metal center and
thus influence the activity of the catalyst [107, 108, 109–112].
Based on a series of molecular mechanic calculations and polymerization
experiments performed with a number of ansa-bis-(fluorenyl)zirconium dichloride
complexes, Alt et al. [109–112] demonstrated that the size of the bridging unit plays
an important role in the final catalytic activity of these complexes towards ethylene
polymerization. A bigger bite angle, i.e., bigger opening around the active site, is
supposed to increase the catalytic activity because of a less-hindered monomer
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
59
necessary to increase the stereoregularity of the individual polymer chains. This in
turn requires the suppression of the concentration of both types of stereodefects
(m and mm) in the backbone of the polymer chains, whose combined numbers
determine the crystallinity, melting point, and most of the physical properties, such
as crystallization rate, optics, etc.
In the following sections, a few examples are presented as case studies to show
how these substitutional modifications are realized and in what way they bring
about the desired improvements. The discussion starts with the topic of modification of the bridge and bridge substituents to increase the syndiotactic polymer’s
molecular weight. We continues with the subject of structural modifications for
improving the stereoselectivity of the catalyst and the stereoregularity of the
resulting s-PP.
Before entering the discussions, it should be noted from the onset that there exist
only limited options for substitutional modifications with the aim of increasing
molecular weight and/or improving the enantioselectivity of the 1/MAO catalyst
system without tampering with its salient syndiospecific characteristics. Therefore,
for these investigations, only limited substitution modification patterns have been
selected that can bring about the desired improvement but otherwise leave the
syndiotactic selective behavior of the system intact.
3.1 Modification of the Bridge and Syndiotactic Polymer
Molecular Weight
The importance of the bridge in ansa-metallocene catalysts was recognized early on
during the development of chiral metallocene catalysts for olefin polymerization
[43–45]. Its existence is the prerequisite for stereorigidity of the metallocene
structure, designed to prevent the free rotation of the aromatic ring(s) and to
maintain the appropriate chirality of the corresponding cationic active species
during the initial stages and the entire process of the polymerization. Later, it was
recognized that the size of the bridge connecting the two aromatic rings could be
additionally used as a regulating tool for modifying the active site’s bite angle. For
example, by replacing a one-carbon bridge such as ¼CR 2 with a two-carbon bridge
like –RC–CR– or a silicon bridge like ¼SiR 2 it is possible to decrease the opening
around, and control the monomer accessibility to, the transition metal center and
thus influence the activity of the catalyst [107, 108, 109–112].
Based on a series of molecular mechanic calculations and polymerization
experiments performed with a number of ansa-bis-(fluorenyl)zirconium dichloride
complexes, Alt et al. [109–112] demonstrated that the size of the bridging unit plays
an important role in the final catalytic activity of these complexes towards ethylene
polymerization. A bigger bite angle, i.e., bigger opening around the active site, is
supposed to increase the catalytic activity because of a less-hindered monomer
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
59