catalyst system, into the calculations. The relevance of the symmetry and
stereorigidity of the metallocene structure for the syndiospecificity of the final catalyst
is debated by presenting a few “non-conforming” catalyst examples.
Finally, the challenges involved in the manufacture of syndiotactic polypropylene
in commercial scale continuous production processes are highlighted as well as the
polymer’s unique structural, physical, mechanical and rheological properties.
Keywords Counter-anion effects Á C s symmetric zirconocene catalyst Á Enantioselectivity Á Enantiomeric mis-insertion Á Heterogenization Á Site epimerization Á
Syndiotactic polypropylene
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2 Bridged Cyclopentadienyl-Fluorenyl Metallocene Molecules . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 46
2.1 Molecular Structure of Isopropylidene(cyclopentadienyl-fluorenyl)MCl 2 (M ¼ Zr,
Hf): Bonding and Symmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.2 Crystal Structure of the Metallocenium–Monoalkyl Cation and the Structure of the
Putative Active Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.3 Polymerization Behavior of 1/MAO and 2/MAO Catalyst Systems . . . . . . . . . . . . . . . . . 51
2.4 Mechanism of Syndiospecific Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.5 Syndiospecific Transition State Structure and Syndio-Insertion Catalytic Cycle . . . . 55
3 Structural Modifications to Enhance the Syndiospecific Catalytic Performance . . . . . . . . . . 58
3.1 Modification of the Bridge and Syndiotactic Polymer Molecular Weight . . . . . . . . . . . 59
3.2 Bridge Size Modification: 1,2-Ethano-Bridge Versus 2,2-Propano-Bridge . . . . . . . . . . 60
3.3 Bridge Substituents and Syndiotactic Polypropylene Molecular Weight . . . . . . . . . . . . 65
3.4 Polymerization Behavior of Diphenylmethylidene(cyclopentadienyl-fluorenyl)
MCl 2 /MAO: Methyl Versus Phenyl Substituent in the Bridge . . . . . . . . . . . . . . . . . . . . . . . 65
4 Fluorenyl Substituents and Catalyst Enantioselectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.1 Stereoregularity Improvement and Frontal Substituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.2 Computational Calculations: Determination and/or Prediction of Enantioselectivity
of Syndiospecific Catalyst Systems . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. 73
4.3 Importance of the Frontal Substituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5 Stereorigidity of Bridged Metallocenes and Stereoselectivity of the Catalysts . . . . . . . . . . . . 81
5.1 Catalysts Stereorigidity and Site Epimerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2 The Origin of Site Epimerization: Computational Investigation . . . . . . . . . . . . . . . . . . . . . 84
5.3 Site Epimerization in the “Absence” of the Counter-Ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.4 Counter-Ion-Assisted Site Epimerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6 Metallocene Molecular Symmetry and the Catalyst’s Syndiotactic Specificity . . . . . . . . . . . 89
6.1
Syndio- and Nonsyndiospecific Catalyst Systems with C s Symmetric Metallocene
Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.2
Other Types of C s Symmetric Metallocene Catalysts with Syndio- and
Nonsyndiospecific Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.3
C 1 Symmetric Structures: Syndio- and Nonsyndiospecific Catalyst Systems . . . . . . 93
6.4
Other C 1 Symmetric but Syndiospecific Catalyst Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.5
Industrial Production of Syndiotactic Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.6
Optimization of the Procedure for Synthesis of the Metallocene Molecule . . . . . . . . 97
6.7
Proper Choice of the Silica and MAO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.8
Large-Scale Preparation of the Supported Metallocene Catalysts . . . . . . . . . . . . . . . . . . 98
6.9
Large-Scale Production of Syndiotactic Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.10 Processing of Syndiotactic Polypropylene Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
44
A. Razavi
stereorigidity of the metallocene structure for the syndiospecificity of the final catalyst
is debated by presenting a few “non-conforming” catalyst examples.
Finally, the challenges involved in the manufacture of syndiotactic polypropylene
in commercial scale continuous production processes are highlighted as well as the
polymer’s unique structural, physical, mechanical and rheological properties.
Keywords Counter-anion effects Á C s symmetric zirconocene catalyst Á Enantioselectivity Á Enantiomeric mis-insertion Á Heterogenization Á Site epimerization Á
Syndiotactic polypropylene
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2 Bridged Cyclopentadienyl-Fluorenyl Metallocene Molecules . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 46
2.1 Molecular Structure of Isopropylidene(cyclopentadienyl-fluorenyl)MCl 2 (M ¼ Zr,
Hf): Bonding and Symmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.2 Crystal Structure of the Metallocenium–Monoalkyl Cation and the Structure of the
Putative Active Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.3 Polymerization Behavior of 1/MAO and 2/MAO Catalyst Systems . . . . . . . . . . . . . . . . . 51
2.4 Mechanism of Syndiospecific Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.5 Syndiospecific Transition State Structure and Syndio-Insertion Catalytic Cycle . . . . 55
3 Structural Modifications to Enhance the Syndiospecific Catalytic Performance . . . . . . . . . . 58
3.1 Modification of the Bridge and Syndiotactic Polymer Molecular Weight . . . . . . . . . . . 59
3.2 Bridge Size Modification: 1,2-Ethano-Bridge Versus 2,2-Propano-Bridge . . . . . . . . . . 60
3.3 Bridge Substituents and Syndiotactic Polypropylene Molecular Weight . . . . . . . . . . . . 65
3.4 Polymerization Behavior of Diphenylmethylidene(cyclopentadienyl-fluorenyl)
MCl 2 /MAO: Methyl Versus Phenyl Substituent in the Bridge . . . . . . . . . . . . . . . . . . . . . . . 65
4 Fluorenyl Substituents and Catalyst Enantioselectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.1 Stereoregularity Improvement and Frontal Substituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.2 Computational Calculations: Determination and/or Prediction of Enantioselectivity
of Syndiospecific Catalyst Systems . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. 73
4.3 Importance of the Frontal Substituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5 Stereorigidity of Bridged Metallocenes and Stereoselectivity of the Catalysts . . . . . . . . . . . . 81
5.1 Catalysts Stereorigidity and Site Epimerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2 The Origin of Site Epimerization: Computational Investigation . . . . . . . . . . . . . . . . . . . . . 84
5.3 Site Epimerization in the “Absence” of the Counter-Ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.4 Counter-Ion-Assisted Site Epimerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6 Metallocene Molecular Symmetry and the Catalyst’s Syndiotactic Specificity . . . . . . . . . . . 89
6.1
Syndio- and Nonsyndiospecific Catalyst Systems with C s Symmetric Metallocene
Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.2
Other Types of C s Symmetric Metallocene Catalysts with Syndio- and
Nonsyndiospecific Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.3
C 1 Symmetric Structures: Syndio- and Nonsyndiospecific Catalyst Systems . . . . . . 93
6.4
Other C 1 Symmetric but Syndiospecific Catalyst Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.5
Industrial Production of Syndiotactic Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.6
Optimization of the Procedure for Synthesis of the Metallocene Molecule . . . . . . . . 97
6.7
Proper Choice of the Silica and MAO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.8
Large-Scale Preparation of the Supported Metallocene Catalysts . . . . . . . . . . . . . . . . . . 98
6.9
Large-Scale Production of Syndiotactic Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.10 Processing of Syndiotactic Polypropylene Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
44
A. Razavi