The rigid structure of 6 prevents regio- and stereo-irregular insertions to a large
extent. This is very true at low temperatures. At higher temperatures, ligand
movement is allowed and the stereocontrol effect is removed [42]. The indenyl
ligand forms a plane and improves the rigidity of the ligand compared to its
hydrogenated tetrahydroindenyl analogue. rac-Et[Ind] 2 ZrCl 2 (4) produces more
isotactic polypropylene at low monomer concentration [43] and at a higher temperature than 6 [42]. Also, 4 exhibits better copolymerization ability than 6. The
gap-opening aperture of 4 and 6 has been reported by Busico et al. [44] to be 95
and 86
, respectively, which can be understood to reduce the probability of comonomer insertion for the latter [45, 46]. Electronic effects may also play a role in the
better 1-olefin response. The electronic density around the central metal is lowered
[47] when the alkyl ligand is substituted with an aromatic ligand.
Synthetic routes of C 2 -symmetric metallocenes usually produce significant
amounts of undesired meso isomer 5 in addition to the targeted racemic isomer
4. The meso isomer does not provide stereocontrol in polypropylene polymerization
due to the very open steric environment around the active site.
The changes in ligand symmetry alter the microstructure of the prepared polymer. Catalysts with C S -symmetry enable the production of highly syndiotactic
polypropylene [48]. An example of this is iPr[Cp][Flu]ZrCl 2 (7). Based on the
published reactivity ratios, this catalyst has better copolymerization ability than
rac-Me 2 Si[Ind] 2 ZrCl 2 (9) [5, 35, 49].
Selection of the transition metal also has a significant effect on the copolymerization ability. Hf complexes have in general higher comonomer response than their
Zr analogues; e.g., rac-Et[Ind] 2 HfCl 2 (8) has almost an order of magnitude higher
comonomer response than 5. Not too many studies regarding the copolymerization
behavior of Ti-metallocenes in comparison to their Zr analogues have been
published. Based on the existing data, comonomer response appears to be at a
similar level [41].
Other means of improving the comonomer reactivity is variation of the
interannular bridge. In propylene polymerization, replacing the –CH 2 CH 2 – bridge
with a shorter –Si(Me 2 )À bridge increases the molecular weight and isotacticity
[50]. rac-Me 2 Si[Ind] 2 ZrCl 2 (9) also has a higher comonomer response than
4 [51] or 6 [33].
Early development also included some half-metallocenes like CpZrCl 3 (10),
which have been reported to give similar polymerization behavior to
1 [52]. Among this class of complexes belongs one of the highest comonomer
response catalysts: SiMe 2 [CpMe 4 ][tert-BuN]TiCl 2 (11), which has an extremely
high comonomer response [53]. Even at 140
C, an r E value as low as 8.8 has been
determined [54]. The high comonomer response has been attributed to the low
steric hindrance at the active site. However, electronic factors lower the activity of
this half-metallocene catalyst [55].
Metallocene development has been driven largely by the search for high
isotacticity for polypropylene. For example, the introduction of a 2-methyl substituent in the indenyl ligand provided (12). This metallocene has similar comonomer
response to 9 but somewhat higher isotacticity [56]. Further development of this
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
187
extent. This is very true at low temperatures. At higher temperatures, ligand
movement is allowed and the stereocontrol effect is removed [42]. The indenyl
ligand forms a plane and improves the rigidity of the ligand compared to its
hydrogenated tetrahydroindenyl analogue. rac-Et[Ind] 2 ZrCl 2 (4) produces more
isotactic polypropylene at low monomer concentration [43] and at a higher temperature than 6 [42]. Also, 4 exhibits better copolymerization ability than 6. The
gap-opening aperture of 4 and 6 has been reported by Busico et al. [44] to be 95
and 86
, respectively, which can be understood to reduce the probability of comonomer insertion for the latter [45, 46]. Electronic effects may also play a role in the
better 1-olefin response. The electronic density around the central metal is lowered
[47] when the alkyl ligand is substituted with an aromatic ligand.
Synthetic routes of C 2 -symmetric metallocenes usually produce significant
amounts of undesired meso isomer 5 in addition to the targeted racemic isomer
4. The meso isomer does not provide stereocontrol in polypropylene polymerization
due to the very open steric environment around the active site.
The changes in ligand symmetry alter the microstructure of the prepared polymer. Catalysts with C S -symmetry enable the production of highly syndiotactic
polypropylene [48]. An example of this is iPr[Cp][Flu]ZrCl 2 (7). Based on the
published reactivity ratios, this catalyst has better copolymerization ability than
rac-Me 2 Si[Ind] 2 ZrCl 2 (9) [5, 35, 49].
Selection of the transition metal also has a significant effect on the copolymerization ability. Hf complexes have in general higher comonomer response than their
Zr analogues; e.g., rac-Et[Ind] 2 HfCl 2 (8) has almost an order of magnitude higher
comonomer response than 5. Not too many studies regarding the copolymerization
behavior of Ti-metallocenes in comparison to their Zr analogues have been
published. Based on the existing data, comonomer response appears to be at a
similar level [41].
Other means of improving the comonomer reactivity is variation of the
interannular bridge. In propylene polymerization, replacing the –CH 2 CH 2 – bridge
with a shorter –Si(Me 2 )À bridge increases the molecular weight and isotacticity
[50]. rac-Me 2 Si[Ind] 2 ZrCl 2 (9) also has a higher comonomer response than
4 [51] or 6 [33].
Early development also included some half-metallocenes like CpZrCl 3 (10),
which have been reported to give similar polymerization behavior to
1 [52]. Among this class of complexes belongs one of the highest comonomer
response catalysts: SiMe 2 [CpMe 4 ][tert-BuN]TiCl 2 (11), which has an extremely
high comonomer response [53]. Even at 140
C, an r E value as low as 8.8 has been
determined [54]. The high comonomer response has been attributed to the low
steric hindrance at the active site. However, electronic factors lower the activity of
this half-metallocene catalyst [55].
Metallocene development has been driven largely by the search for high
isotacticity for polypropylene. For example, the introduction of a 2-methyl substituent in the indenyl ligand provided (12). This metallocene has similar comonomer
response to 9 but somewhat higher isotacticity [56]. Further development of this
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
187