increasing LCB [13, 85, 91]. Thermorheological complexity is only seen with LCB
samples and thus distinguishes the influence of LCB from the influence of short
chain comonomers [97, 101, 116].
3.3 Influence of the Catalyst and Polymerization Conditions
on LCB
Considering LCB formation via the copolymerization route, the vinyl end-group
selectivity and comonomer response of the employed catalyst are important factors
in determining the polymer structure. Besides the choice of catalyst, the choice of
polymerization conditions may also suppress or enhance formation of LCB.
3.3.1 Catalyst Structure
Differences in vinyl end-group selectivity (data in Fig. 1) and the copolymerization
ability (Table 1) suggest that metallocenes rac-Et[Ind] 2 ZrCl 2 (4) and rac-Me 2 Si
[Ind] 2 ZrCl 2 (9) would be more eager to produce LCB than metallocenes rac-Et
[H 4 Ind] 2 ZrCl 2 (6) and bis(n-BuCp) 2 ZrCl 2 (3) when activated. The rheological
properties of homopolyethylenes made with these catalysts are in line with this
thinking and with the in situ LCB mechanism.
Figure 4 shows the complex viscosity (η*) as a function of oscillation frequency
(ω) for homopolyethylenes produced with these MAO-activated catalysts under
similar polymerization conditions [46, 51]. Sample characteristics are reported in
Table 2. Polyethylenes prepared with catalysts 4/MAO and 9/MAO show low shear
rate viscosity values [at η* (ω ¼ 0.02 rad s
À1 )] that are 30- and 42- times higher
than expected for linear polymers of the same M w . In contrast, the melt viscosity is
much smaller in polymers produced with the catalysts 6/MAO and 1/MAO. For
polymers made with 3/MAO, the low shear rate viscosity value is very close to the
theoretical value, suggesting linear structure of the polymer.
Figure 5 shows dynamic modulus curves for the polyethylenes produced with
MAO-activated metallocenes 1, 3, 4, 6, and 9. All polymer samples had narrow
M w /M n (2.2–2.5) and displayed no high molecular weight tailing in SEC
[46, 51]. The modulus curves of polymers prepared with 3 and 6 are similar to
the modulus curves of linear polymers of narrow MWD. In contrast, for the
polymers based on 1, 4, and 9, the elastic modulus (G
0 ) appears very large in
comparison with the viscous modulus G
00 . Their rheological behavior suggests a
much broader MWD than the SEC-based MWD. The contrast between
SEC-measured MWD and rheological behavior is attributed to the presence of
LCB in the polymers.
198
J. Seppa ¨la ¨ et al.
samples and thus distinguishes the influence of LCB from the influence of short
chain comonomers [97, 101, 116].
3.3 Influence of the Catalyst and Polymerization Conditions
on LCB
Considering LCB formation via the copolymerization route, the vinyl end-group
selectivity and comonomer response of the employed catalyst are important factors
in determining the polymer structure. Besides the choice of catalyst, the choice of
polymerization conditions may also suppress or enhance formation of LCB.
3.3.1 Catalyst Structure
Differences in vinyl end-group selectivity (data in Fig. 1) and the copolymerization
ability (Table 1) suggest that metallocenes rac-Et[Ind] 2 ZrCl 2 (4) and rac-Me 2 Si
[Ind] 2 ZrCl 2 (9) would be more eager to produce LCB than metallocenes rac-Et
[H 4 Ind] 2 ZrCl 2 (6) and bis(n-BuCp) 2 ZrCl 2 (3) when activated. The rheological
properties of homopolyethylenes made with these catalysts are in line with this
thinking and with the in situ LCB mechanism.
Figure 4 shows the complex viscosity (η*) as a function of oscillation frequency
(ω) for homopolyethylenes produced with these MAO-activated catalysts under
similar polymerization conditions [46, 51]. Sample characteristics are reported in
Table 2. Polyethylenes prepared with catalysts 4/MAO and 9/MAO show low shear
rate viscosity values [at η* (ω ¼ 0.02 rad s
À1 )] that are 30- and 42- times higher
than expected for linear polymers of the same M w . In contrast, the melt viscosity is
much smaller in polymers produced with the catalysts 6/MAO and 1/MAO. For
polymers made with 3/MAO, the low shear rate viscosity value is very close to the
theoretical value, suggesting linear structure of the polymer.
Figure 5 shows dynamic modulus curves for the polyethylenes produced with
MAO-activated metallocenes 1, 3, 4, 6, and 9. All polymer samples had narrow
M w /M n (2.2–2.5) and displayed no high molecular weight tailing in SEC
[46, 51]. The modulus curves of polymers prepared with 3 and 6 are similar to
the modulus curves of linear polymers of narrow MWD. In contrast, for the
polymers based on 1, 4, and 9, the elastic modulus (G
0 ) appears very large in
comparison with the viscous modulus G
00 . Their rheological behavior suggests a
much broader MWD than the SEC-based MWD. The contrast between
SEC-measured MWD and rheological behavior is attributed to the presence of
LCB in the polymers.
198
J. Seppa ¨la ¨ et al.