1 Introduction
In this article, we try to give a brief account of the development of ansametallocene catalysts for isotactic olefin polymerizations, especially with regard
to some points of particular interest to the authors of this article. For this, we will
refer to appropriate previous reviews and then try to assess the state of the art.
2 Structural Variants of ansa-Metallocene Catalysts
In the three decades that have passed since the first syntheses of chiral ethanediylbridged bis(indenyl)titanium and bis(indenyl)zirconium complexes [1, 2] and the
first reports on their use – together with methylalumoxane as activator [3] – as
catalysts for isotactic olefin polymerization [4, 5], this field of catalysis research has
seen many structural variations on the ansa-metallocene theme.
Among ansa-metallocene catalysts with different group IV transition metals,
ansa-titanocene-based catalysts (with rare exceptions [6]) lose most of their activity
above 0
C, whereas ansa-hafnocene catalysts usually give lower activities than the
analogous ansa-zirconocene catalysts [7, 8], which have thus received most
research interest.
Variations in catalyst structures mostly involved changes in the substitution
patterns of the ansa-metallocene ring ligands, since catalyst properties are clearly
influenced most directly by the steric environment of the metallocene coordination
sites [7–9]. Much interest has been directed at the possibility of obtaining
polyolefins with tacticities different from those of the isotactic polymers obtained
with C 2 -symmetric ansa-metallocene catalysts. The observation that zirconocene
catalysts with C S -symmetric geometry, i.e. with enantiotopic metallocene coordination sites, generate syndiotactic polypropylene [10] thus helped to establish
explanations for the enantioselectivity of the chain-growth process; these
explanations are universally accepted today [11, 12].
Especially interesting in this regard are ansa-metallocene catalysts with a
C 1 -symmetric structure, in which the steric environments of the metallocene coordination sites are unrelated to each other. Depending on the steric environment of
each coordination site, polypropylene tacticities can range here from almost atactic
to highly isotactic [7, 13]. Due to the possibility of finely adjusting stereoerror
frequencies, and thereby tuning the flexibility of polymer chains and the ensuing
properties of polymer materials, this type of catalysts continues to be of practical
interest [14].
With regard to variations in bridging units, interanular bridging via a (CH 3 ) 2 Si
unit proved to endow ansa-metallocene catalysts with higher degrees of stereoselectivity and with higher activities than bridges with two-atom or longer chains
[15], probably due to a greater stereorigidity of the ligand framework and a wider
opening of the interanular wedge angle [7, 8, 16]. Bridging units with spatially more
demanding, asymmetric or chiral substituents have likewise been shown to influence the catalytic properties of ansa-metallocene catalysts [17], especially those
with C 1 symmetry [7], presumably via their effects on ring-ligand conformation.
30
H.H. Brintzinger and D. Fischer
In this article, we try to give a brief account of the development of ansametallocene catalysts for isotactic olefin polymerizations, especially with regard
to some points of particular interest to the authors of this article. For this, we will
refer to appropriate previous reviews and then try to assess the state of the art.
2 Structural Variants of ansa-Metallocene Catalysts
In the three decades that have passed since the first syntheses of chiral ethanediylbridged bis(indenyl)titanium and bis(indenyl)zirconium complexes [1, 2] and the
first reports on their use – together with methylalumoxane as activator [3] – as
catalysts for isotactic olefin polymerization [4, 5], this field of catalysis research has
seen many structural variations on the ansa-metallocene theme.
Among ansa-metallocene catalysts with different group IV transition metals,
ansa-titanocene-based catalysts (with rare exceptions [6]) lose most of their activity
above 0
C, whereas ansa-hafnocene catalysts usually give lower activities than the
analogous ansa-zirconocene catalysts [7, 8], which have thus received most
research interest.
Variations in catalyst structures mostly involved changes in the substitution
patterns of the ansa-metallocene ring ligands, since catalyst properties are clearly
influenced most directly by the steric environment of the metallocene coordination
sites [7–9]. Much interest has been directed at the possibility of obtaining
polyolefins with tacticities different from those of the isotactic polymers obtained
with C 2 -symmetric ansa-metallocene catalysts. The observation that zirconocene
catalysts with C S -symmetric geometry, i.e. with enantiotopic metallocene coordination sites, generate syndiotactic polypropylene [10] thus helped to establish
explanations for the enantioselectivity of the chain-growth process; these
explanations are universally accepted today [11, 12].
Especially interesting in this regard are ansa-metallocene catalysts with a
C 1 -symmetric structure, in which the steric environments of the metallocene coordination sites are unrelated to each other. Depending on the steric environment of
each coordination site, polypropylene tacticities can range here from almost atactic
to highly isotactic [7, 13]. Due to the possibility of finely adjusting stereoerror
frequencies, and thereby tuning the flexibility of polymer chains and the ensuing
properties of polymer materials, this type of catalysts continues to be of practical
interest [14].
With regard to variations in bridging units, interanular bridging via a (CH 3 ) 2 Si
unit proved to endow ansa-metallocene catalysts with higher degrees of stereoselectivity and with higher activities than bridges with two-atom or longer chains
[15], probably due to a greater stereorigidity of the ligand framework and a wider
opening of the interanular wedge angle [7, 8, 16]. Bridging units with spatially more
demanding, asymmetric or chiral substituents have likewise been shown to influence the catalytic properties of ansa-metallocene catalysts [17], especially those
with C 1 symmetry [7], presumably via their effects on ring-ligand conformation.
30
H.H. Brintzinger and D. Fischer