probably due to steric crowding, whereas the siloxy-substituted catalyst exhibits
excellent copolymerization properties [69]. It is also noteworthy that such shielded
siloxy-substituted complexes revealed highest polymerization activity at low
Al(MAO)/Zr ratios. Reducing the amount of MAO in metallocene activation has
been an important research objective due to the high cost of MAO [70]. Ewen et al.
[71] also reported heteroatom-substituted aromatic ligands with a polymerization
performance rivaling that of the corresponding unsubstituted complexes. Generally,
the tuning of ligand properties with heteroatoms has been demonstrated to bear a
high potential for extending the concept of rational ligand design.
2.2 Copolymerization Behavior of Siloxy-Substituted
Bis(indenyl) Zirconium Dichlorides
Having found that siloxy-substituted complexes bear a high potential as highly
active olefin polymerization catalysts, intensive research was directed towards
optimizing the ligand design to improve the copolymerization behavior. It was
soon found that the polymerization behavior of siloxy-substituted complexes
depended markedly on the position of the siloxy substituent at the ligand [66].
Siloxy substitution at the 3-position of the indenyl ligand (17) was found to
remarkably improve the 1-olefin copolymerization ability, whereas substitution at
the 2-position (15) slightly reduced the copolymerization ability as compared to the
unsubstituted 5. The reason for this was suggested to be mainly the increased
coordination gap aperture of the 3-siloxy-substituted complexes. Table 1
summarizes the ethylene reactivity ratio data obtained for the siloxy-substituted
complexes 15, 16, and 17 The large difference in the ethylene and comonomer
reactivity ratio values, the product of which is much below unity, emphasizes the
prevailing tendency of the catalysts to produce copolymers with isolated comonomer units. The reason for the 15–40% lower incorporation of 1-hexadecene than
1-hexene was explained by the higher steric bulk and lower rate of diffusion of the
longer α-olefin.
Furthermore, decreasing the temperature from 80
C to 40
C clearly improved
the copolymerization ability of the hydrogenated 16/MAO, while the influence of
temperature on the comonomer response of the corresponding bis(indenyl) system
17/MAO was less pronounced. This was also suggested to originate from contribution of steric effects. Fluctuation between the indenyl-backward and indenylforward conformations [72] in combination with the fluctuation of the
hydrogenated six-rings of complex 16 is reduced at lower temperatures, resulting
in less interference with the comonomer coordination. The copolymerization
behavior of complex 17, which has essentially planar indenyl moieties, could be
understood to be considerably less influenced by the polymerization temperature.
Replacing the methyl groups at the silicon atom of 15 and 17 with bulkier
iso-propyl groups was found to increase the ability of comonomer incorporation
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
189
excellent copolymerization properties [69]. It is also noteworthy that such shielded
siloxy-substituted complexes revealed highest polymerization activity at low
Al(MAO)/Zr ratios. Reducing the amount of MAO in metallocene activation has
been an important research objective due to the high cost of MAO [70]. Ewen et al.
[71] also reported heteroatom-substituted aromatic ligands with a polymerization
performance rivaling that of the corresponding unsubstituted complexes. Generally,
the tuning of ligand properties with heteroatoms has been demonstrated to bear a
high potential for extending the concept of rational ligand design.
2.2 Copolymerization Behavior of Siloxy-Substituted
Bis(indenyl) Zirconium Dichlorides
Having found that siloxy-substituted complexes bear a high potential as highly
active olefin polymerization catalysts, intensive research was directed towards
optimizing the ligand design to improve the copolymerization behavior. It was
soon found that the polymerization behavior of siloxy-substituted complexes
depended markedly on the position of the siloxy substituent at the ligand [66].
Siloxy substitution at the 3-position of the indenyl ligand (17) was found to
remarkably improve the 1-olefin copolymerization ability, whereas substitution at
the 2-position (15) slightly reduced the copolymerization ability as compared to the
unsubstituted 5. The reason for this was suggested to be mainly the increased
coordination gap aperture of the 3-siloxy-substituted complexes. Table 1
summarizes the ethylene reactivity ratio data obtained for the siloxy-substituted
complexes 15, 16, and 17 The large difference in the ethylene and comonomer
reactivity ratio values, the product of which is much below unity, emphasizes the
prevailing tendency of the catalysts to produce copolymers with isolated comonomer units. The reason for the 15–40% lower incorporation of 1-hexadecene than
1-hexene was explained by the higher steric bulk and lower rate of diffusion of the
longer α-olefin.
Furthermore, decreasing the temperature from 80
C to 40
C clearly improved
the copolymerization ability of the hydrogenated 16/MAO, while the influence of
temperature on the comonomer response of the corresponding bis(indenyl) system
17/MAO was less pronounced. This was also suggested to originate from contribution of steric effects. Fluctuation between the indenyl-backward and indenylforward conformations [72] in combination with the fluctuation of the
hydrogenated six-rings of complex 16 is reduced at lower temperatures, resulting
in less interference with the comonomer coordination. The copolymerization
behavior of complex 17, which has essentially planar indenyl moieties, could be
understood to be considerably less influenced by the polymerization temperature.
Replacing the methyl groups at the silicon atom of 15 and 17 with bulkier
iso-propyl groups was found to increase the ability of comonomer incorporation
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
189