[73]. This observation suggested that better shielding of the electron-rich oxygen in the
siloxy substituent reduces the possibility of unfavorable interactions with the MAO
cocatalyst. The enhancement of copolymerization ability due to improved shielding of
the heteroatom is supported by the finding that the copolymerization ability of the less
shielded 17 is negatively affected by increasing the Al(MAO)/Zr ratio.
In addition to enhancing copolymerization ability, 3-siloxy substitution was
found to increase the sensitivity towards excess MAO, which was observed as a
decreased comonomer incorporation and an increased induction time with an
increasing Al(MAO) concentration. Increasing the temperature from 40
C to
80
C was found to shift the induction period to higher Al/Zr ratios. Using triisobutylaluminium (TIBA) had a similar effect to increasing the temperature. These
observations indicate the presence of equilibrium reactions, which are shifted
towards more rapid formation of the active sites by increasing temperature and
decreasing MAO content. The open coordination sphere of the 3-siloxy-substituted
complexes could be more subjected towards coordination of MAO to the donor
substituents than it is for the 2-substituted complexes. Shortening of the induction
period could be achieved by addition of TIBA, probably as a result of more efficient
alkylation of the catalyst, since the alkylation by trimethylaluminum (TMA) has
been reported to be extremely slow for 2-amino-substituted catalysts [47].
Siloxy substitution at the 2-position of the indenyl ligand resulted in immediate
polymerization and extremely high polymerization activities at very low Al(MAO)
concentrations [66]. At least two reasons were suggested for the enhanced
polymerization activity of 15/MAO at low MAO concentrations:
• Preservation of the electron-donating nature of the siloxy substituent in the
presence of MAO is suggested to facilitate the formation of the active site and
to stabilize the cationic metal center
• The weaker binding of the counterion to the cationic metallocene alkyl may
increase the rate of monomer insertion into the metal–alkyl bond and thus
increase the chain propagation rate
Finally, further manifestation of the good comonomer response of meso isomers
was shown with siloxy-substituted 18 and 19, which both provide excellent comonomer response although they decompose at higher temperatures [74].
In conclusion, siloxy substitution has been demonstrated to provide a family of
highly active metallocene catalysts that reveal optimum polymerization properties
at low Al(MAO) concentrations. Good comonomer response and the reduction of
the required MAO amount are highly desirable features from an industrial point
of view.
2.3 Chain Termination
In addition to the copolymerization ability, it is of special interest to understand the
structure of end groups when defining a catalyst’s potential to promote long-chain
branch formation or to facilitate end-group functionalization. Moreover, analysis of
190
J. Seppa ¨la ¨ et al.
siloxy substituent reduces the possibility of unfavorable interactions with the MAO
cocatalyst. The enhancement of copolymerization ability due to improved shielding of
the heteroatom is supported by the finding that the copolymerization ability of the less
shielded 17 is negatively affected by increasing the Al(MAO)/Zr ratio.
In addition to enhancing copolymerization ability, 3-siloxy substitution was
found to increase the sensitivity towards excess MAO, which was observed as a
decreased comonomer incorporation and an increased induction time with an
increasing Al(MAO) concentration. Increasing the temperature from 40
C to
80
C was found to shift the induction period to higher Al/Zr ratios. Using triisobutylaluminium (TIBA) had a similar effect to increasing the temperature. These
observations indicate the presence of equilibrium reactions, which are shifted
towards more rapid formation of the active sites by increasing temperature and
decreasing MAO content. The open coordination sphere of the 3-siloxy-substituted
complexes could be more subjected towards coordination of MAO to the donor
substituents than it is for the 2-substituted complexes. Shortening of the induction
period could be achieved by addition of TIBA, probably as a result of more efficient
alkylation of the catalyst, since the alkylation by trimethylaluminum (TMA) has
been reported to be extremely slow for 2-amino-substituted catalysts [47].
Siloxy substitution at the 2-position of the indenyl ligand resulted in immediate
polymerization and extremely high polymerization activities at very low Al(MAO)
concentrations [66]. At least two reasons were suggested for the enhanced
polymerization activity of 15/MAO at low MAO concentrations:
• Preservation of the electron-donating nature of the siloxy substituent in the
presence of MAO is suggested to facilitate the formation of the active site and
to stabilize the cationic metal center
• The weaker binding of the counterion to the cationic metallocene alkyl may
increase the rate of monomer insertion into the metal–alkyl bond and thus
increase the chain propagation rate
Finally, further manifestation of the good comonomer response of meso isomers
was shown with siloxy-substituted 18 and 19, which both provide excellent comonomer response although they decompose at higher temperatures [74].
In conclusion, siloxy substitution has been demonstrated to provide a family of
highly active metallocene catalysts that reveal optimum polymerization properties
at low Al(MAO) concentrations. Good comonomer response and the reduction of
the required MAO amount are highly desirable features from an industrial point
of view.
2.3 Chain Termination
In addition to the copolymerization ability, it is of special interest to understand the
structure of end groups when defining a catalyst’s potential to promote long-chain
branch formation or to facilitate end-group functionalization. Moreover, analysis of
190
J. Seppa ¨la ¨ et al.