catalyst family resulted in more sophisticated catalyst structures with improved
copolymerization abilities. rac-Me 2 Si[Me-Benz[e]Ind] 2 ZrCl 2 (13) and rac-Me 2 Si
[2-Me-4-Ph-Ind] 2 ZrCl 2 (14) are good examples of rational ligand design
[56, 57]. Mu ¨lhaupt et al. applied these systems in ethylene-1-butene [36],
ethylene-1-octene [58], and propylene-1-octene [59] copolymerization studies. In
accordance with previous propylene homopolymerization studies they reported a
beneficial influence of a 2-methyl substituent on the molar mass of the copolymers
accompanied, however, by a simultaneous decrease in polymerization activity.
Benzannelation of the indenyl ligand gave a beneficial influence on copolymerization activity and, furthermore, on the randomness and level of comonomer
incorporation. The general character of the synergistic 2-methyl-4-aryl substitution
was further demonstrated by Xu et al. [60], who synthesized a constrained geometry
catalyst (CGC) having a 2-methylbenzindenyl ligand and applied it in ethylene-1octene copolymerizations. Compared to the basic CGC catalyst with a Me 4 Cp
ligand, the activity and copolymerization ability of this rationally designed catalyst
were slightly increased and the molar mass of the resulting copolymers was
markedly increased.
2.1 Effect of Heteroatoms in the Ligand Framework
For tuning the electronic properties at the active site, the natural route for further
development of metallocenes was the use of heteroatoms (N, O, S, P) in ligand
substituents, which broadened the variety of metallocene complex families significantly. To study the electronic effects of ligand substituents, Piccolrovazzi et al.
[61] and Collins et al. [62] in the early 1990s reported introduction of methoxy
groups in the 4,7- and 5,6-positions of 4, respectively. Their approach led to a
drastic decrease in polymerization activity, which was explained as being due to
interaction between the donor substituents and MAO, resulting in inductive electron
withdrawal instead of electron donation. This also explains the low activity reported
for 2-methoxy substituted bis(indenyl)zirconocenes [63]. Introduction of amino
groups in the 2-position of indenyl ligands was found by Brintzinger et al. [64]
and Luttikhedde et al. [65] to give modest activity after an induction time of several
hours. The induction time was explained to result from inhibition of the reaction
generating the active species, probably again as a consequence of an unfavorable
interaction of the donor substituents with the MAO cocatalyst. Early successful
developments in this area included various siloxy-substituted bis(indenyl) catalysts,
e.g., 15, 16, and 17. Sterically shielded 2-siloxy-substituted bis(indenyl) complexes
were found by Leino et al. [66–68] not to suffer from induction periods and to give a
considerable increase in ethylene polymerization activity compared to their
unsubstituted analogues. An example of successful heteroatom substitution is
highlighted by comparing the copolymerization behavior of two catalysts with
different ligands, [1-tert-BuSiMe 2 Oind] versus [1-tert-BuSiMe 2 Ind]. The results
have shown that the latter silyl-substituted catalyst has a poor comonomer response,
188
J. Seppa ¨la ¨ et al.
copolymerization abilities. rac-Me 2 Si[Me-Benz[e]Ind] 2 ZrCl 2 (13) and rac-Me 2 Si
[2-Me-4-Ph-Ind] 2 ZrCl 2 (14) are good examples of rational ligand design
[56, 57]. Mu ¨lhaupt et al. applied these systems in ethylene-1-butene [36],
ethylene-1-octene [58], and propylene-1-octene [59] copolymerization studies. In
accordance with previous propylene homopolymerization studies they reported a
beneficial influence of a 2-methyl substituent on the molar mass of the copolymers
accompanied, however, by a simultaneous decrease in polymerization activity.
Benzannelation of the indenyl ligand gave a beneficial influence on copolymerization activity and, furthermore, on the randomness and level of comonomer
incorporation. The general character of the synergistic 2-methyl-4-aryl substitution
was further demonstrated by Xu et al. [60], who synthesized a constrained geometry
catalyst (CGC) having a 2-methylbenzindenyl ligand and applied it in ethylene-1octene copolymerizations. Compared to the basic CGC catalyst with a Me 4 Cp
ligand, the activity and copolymerization ability of this rationally designed catalyst
were slightly increased and the molar mass of the resulting copolymers was
markedly increased.
2.1 Effect of Heteroatoms in the Ligand Framework
For tuning the electronic properties at the active site, the natural route for further
development of metallocenes was the use of heteroatoms (N, O, S, P) in ligand
substituents, which broadened the variety of metallocene complex families significantly. To study the electronic effects of ligand substituents, Piccolrovazzi et al.
[61] and Collins et al. [62] in the early 1990s reported introduction of methoxy
groups in the 4,7- and 5,6-positions of 4, respectively. Their approach led to a
drastic decrease in polymerization activity, which was explained as being due to
interaction between the donor substituents and MAO, resulting in inductive electron
withdrawal instead of electron donation. This also explains the low activity reported
for 2-methoxy substituted bis(indenyl)zirconocenes [63]. Introduction of amino
groups in the 2-position of indenyl ligands was found by Brintzinger et al. [64]
and Luttikhedde et al. [65] to give modest activity after an induction time of several
hours. The induction time was explained to result from inhibition of the reaction
generating the active species, probably again as a consequence of an unfavorable
interaction of the donor substituents with the MAO cocatalyst. Early successful
developments in this area included various siloxy-substituted bis(indenyl) catalysts,
e.g., 15, 16, and 17. Sterically shielded 2-siloxy-substituted bis(indenyl) complexes
were found by Leino et al. [66–68] not to suffer from induction periods and to give a
considerable increase in ethylene polymerization activity compared to their
unsubstituted analogues. An example of successful heteroatom substitution is
highlighted by comparing the copolymerization behavior of two catalysts with
different ligands, [1-tert-BuSiMe 2 Oind] versus [1-tert-BuSiMe 2 Ind]. The results
have shown that the latter silyl-substituted catalyst has a poor comonomer response,
188
J. Seppa ¨la ¨ et al.