(prochiral!) molecule, after activation with MAO, could provide an enantiotopically
chiral catalyst that could produce a crystalline polypropylene with syndiotactic chain
architecture according to a new mechanism that follows, in a rhythmic alternation,
the si/re prochiral face-selective coordination and insertion of propylene molecules at
its two available enantiotopic coordination positions.
To understand the functioning mode of this catalyst system and the mechanistic
aspects of the stereochemical events involved in the formation of s-PP chains, it is
very important to first examine more closely the molecular structure and structural
characteristics of the complex 1 (and/or 2). The structural data can be then
correlated to the information gleaned from different polymer analyses in order to
determine the polymerization mechanism and the elementary steps involved in the
formation of s-PP chains on the basis of the well-established principle of catalyst
structure–polypropylene chain microstructure interrelationship.
2.1 Molecular Structure of Isopropylidene(cyclopentadienylfluorenyl)MCl 2 (M ¼ Zr, Hf): Bonding and Symmetry
Two different views of the molecular structures of the metallocene complexes
isopropylidene(cyclopentadienyl-fluorenyl)MCl 2 , where M ¼ Zr (1) or Hf (2),
are depicted in Fig. 2. As evident from the molecular views presented in Fig. 2,
the stereorigid metallocene molecule 1 (and 2) possess, in the solid state, a bilateral
symmetry (or a plane of symmetry, σ v ). Consequently, the left and right halves of
the molecule, if bisected by the imaginary symmetry plane, would be mirror image
related. Apart from this global observation, detailed examination of the structural
data, particularly those concerning the Zr–C and C–C bond distances, reveals the
following facts. Despite the presence of a relatively short bridge (basically a carbon
atom) as the inter-annular tie between the two aromatic ring systems (the cyclopentadienyl and fluorenyl centroids), the bonding relationship of these rings to the
zirconium metal is very strong and of a η
5 nature. The thermal stability of 1 (and 2)
also favors the notion of a doubly η
5 -bonded centroid–M–centroid molecular
structure. The observed variation in the bond distances with a progressive increase
in the Zr–C flu bond distances from the bridge-head carbon to the proximal carbons,
extending to the distal carbons, is believed to be caused by the nonbonding
repulsive steric interaction between the two σ-bonded chloride ligands and the
distal C–H groups of the two six-membered rings of the fluorenyl section [19].
Similar reasoning applies to the Zr–C cp bond distance variation due to different
carbon atoms in the cyclopentadienyl moiety. The Zr–C bond distance lengthening,
on average about 0.2 A ˚ , due to nonbonded contact and repulsive interaction
between the σ-ligands and the distal C–H groups of both aromatic systems is of
prime importance for the stereoselectivity and the degree of enantioselectivity
of the resulting catalysts (Sect. 2.4). The distal parts of cyclopentadienyl and
fluorenyl moieties of the ligand are positioned exactly above and below the two
48
A. Razavi
chiral catalyst that could produce a crystalline polypropylene with syndiotactic chain
architecture according to a new mechanism that follows, in a rhythmic alternation,
the si/re prochiral face-selective coordination and insertion of propylene molecules at
its two available enantiotopic coordination positions.
To understand the functioning mode of this catalyst system and the mechanistic
aspects of the stereochemical events involved in the formation of s-PP chains, it is
very important to first examine more closely the molecular structure and structural
characteristics of the complex 1 (and/or 2). The structural data can be then
correlated to the information gleaned from different polymer analyses in order to
determine the polymerization mechanism and the elementary steps involved in the
formation of s-PP chains on the basis of the well-established principle of catalyst
structure–polypropylene chain microstructure interrelationship.
2.1 Molecular Structure of Isopropylidene(cyclopentadienylfluorenyl)MCl 2 (M ¼ Zr, Hf): Bonding and Symmetry
Two different views of the molecular structures of the metallocene complexes
isopropylidene(cyclopentadienyl-fluorenyl)MCl 2 , where M ¼ Zr (1) or Hf (2),
are depicted in Fig. 2. As evident from the molecular views presented in Fig. 2,
the stereorigid metallocene molecule 1 (and 2) possess, in the solid state, a bilateral
symmetry (or a plane of symmetry, σ v ). Consequently, the left and right halves of
the molecule, if bisected by the imaginary symmetry plane, would be mirror image
related. Apart from this global observation, detailed examination of the structural
data, particularly those concerning the Zr–C and C–C bond distances, reveals the
following facts. Despite the presence of a relatively short bridge (basically a carbon
atom) as the inter-annular tie between the two aromatic ring systems (the cyclopentadienyl and fluorenyl centroids), the bonding relationship of these rings to the
zirconium metal is very strong and of a η
5 nature. The thermal stability of 1 (and 2)
also favors the notion of a doubly η
5 -bonded centroid–M–centroid molecular
structure. The observed variation in the bond distances with a progressive increase
in the Zr–C flu bond distances from the bridge-head carbon to the proximal carbons,
extending to the distal carbons, is believed to be caused by the nonbonding
repulsive steric interaction between the two σ-bonded chloride ligands and the
distal C–H groups of the two six-membered rings of the fluorenyl section [19].
Similar reasoning applies to the Zr–C cp bond distance variation due to different
carbon atoms in the cyclopentadienyl moiety. The Zr–C bond distance lengthening,
on average about 0.2 A ˚ , due to nonbonded contact and repulsive interaction
between the σ-ligands and the distal C–H groups of both aromatic systems is of
prime importance for the stereoselectivity and the degree of enantioselectivity
of the resulting catalysts (Sect. 2.4). The distal parts of cyclopentadienyl and
fluorenyl moieties of the ligand are positioned exactly above and below the two
48
A. Razavi