684
syndiotactic chains are desirable. Typically, the isotactic and syndiotactic character
is defined by the amount of m and r-diads present in the polymer. In an isotactic
polypropylene, the extent of m-diad can be more than 99%, whereas the amount of
r-diad in a syndiotactic polypropylene can extend up to 85% [21].
Collette et al. developed a series of metal oxide-supported group IVB metalbased R 4 M (M = Ti, Zr, and Hf; R = benzyl, neophyl, and neopentyl) type catalyst
systems [23]. These R 4 M catalysts supported on Al 2 O 3 exhibited high efficiency
with 600 g of polymer/nM R 4 M, and the resulting polymers were of high molecular
weight. The isotactic content in the polypropylene reached up to 70%. The stereoregulation was strongly dependent on the M:Al 2 O 3 ratio and the amount of M on the
surface. Presence of H 2 further improved the yield but compromised the crystallinity. The Table 4 below summarizes the yield, molecular weight, and thermal properties of the polypropylene obtained based on different catalyst. The T m (147 °C) of
the polypropylene obtained using Al 2 O 3 -supported catalyst was observed closed to
that of the TiCl 4 /MgCl 2 /AlEt 3 -based system (T m = 159 °C). However, the polydispersity indices (M w /M n ) of the former (8.0) were compared to that of the TiCl 4 /
MgCl 2 -based system (5.6). Presence of H 2 marginally increased the T m of the samples to 151.8 °C, though the M w /M n further increased to 10.0. The major concern in
these solid supported systems was the yield. The value (47.7 g/mmol of metal) was
much inferior compared to that of the TiCl 4 /MgCl 2 -based system (2680 g/mmol of
metal). Importantly, the molecular weight (M n = 75,500 g/mol) of the polypropylene
based on solid-supported catalyst was much higher compared to that of the TiCl 4 /
MgCl 2 -based system (22,400 g/mol) suggesting the polymerization is faster in case
of the former.
Ewen et al. showed that catalyst systems may be developed by selecting suitable
ligands to predominantly synthesize either isotactic or syndiotactic polypropylene.
A C 2 -symmetric metallocene catalyst based on indene produced isotactic polypropylene (Fig. 6a). A stereorigid C 5 -symmetric prochiral catalyst based on cyclopentene and fluorene produced syndiotactic polymer predominantly (Fig. 6b). The
mechanism of syndiotactic polymer formation was attributed to the chain migratory
insertion of monomer as shown in Scheme 5. The two accessible sites on the catalytic sites are for the growing chain and the incoming monomer. If the growing
chain shifts between these two sites at each monomer addition, then a syndiotactic
polypropylene is the preferred product. However, several authors also hypothesized
that the steric repulsion between the methyl group of the complexed monomer and
last unit of the growing chain guided the above stereoselectivity of the polymer [24].
Table 3 Properties of isotactic, syndiotactic, and atactic polypropylene (PP) [22]
Properties
PP
Isotactic PP
Syndiotactic PP
UTS (MPa)
40
600
330
Elongation at break (%)
100
130
230
Young’s modulus (GPa)
1.9
20.0
3.0
Density (g/cm
3
)
0.89–0.92
0.91–0.94
–
Melting point (°C)
–
171
130
A. S. K. Sinha and U. Ojha
syndiotactic chains are desirable. Typically, the isotactic and syndiotactic character
is defined by the amount of m and r-diads present in the polymer. In an isotactic
polypropylene, the extent of m-diad can be more than 99%, whereas the amount of
r-diad in a syndiotactic polypropylene can extend up to 85% [21].
Collette et al. developed a series of metal oxide-supported group IVB metalbased R 4 M (M = Ti, Zr, and Hf; R = benzyl, neophyl, and neopentyl) type catalyst
systems [23]. These R 4 M catalysts supported on Al 2 O 3 exhibited high efficiency
with 600 g of polymer/nM R 4 M, and the resulting polymers were of high molecular
weight. The isotactic content in the polypropylene reached up to 70%. The stereoregulation was strongly dependent on the M:Al 2 O 3 ratio and the amount of M on the
surface. Presence of H 2 further improved the yield but compromised the crystallinity. The Table 4 below summarizes the yield, molecular weight, and thermal properties of the polypropylene obtained based on different catalyst. The T m (147 °C) of
the polypropylene obtained using Al 2 O 3 -supported catalyst was observed closed to
that of the TiCl 4 /MgCl 2 /AlEt 3 -based system (T m = 159 °C). However, the polydispersity indices (M w /M n ) of the former (8.0) were compared to that of the TiCl 4 /
MgCl 2 -based system (5.6). Presence of H 2 marginally increased the T m of the samples to 151.8 °C, though the M w /M n further increased to 10.0. The major concern in
these solid supported systems was the yield. The value (47.7 g/mmol of metal) was
much inferior compared to that of the TiCl 4 /MgCl 2 -based system (2680 g/mmol of
metal). Importantly, the molecular weight (M n = 75,500 g/mol) of the polypropylene
based on solid-supported catalyst was much higher compared to that of the TiCl 4 /
MgCl 2 -based system (22,400 g/mol) suggesting the polymerization is faster in case
of the former.
Ewen et al. showed that catalyst systems may be developed by selecting suitable
ligands to predominantly synthesize either isotactic or syndiotactic polypropylene.
A C 2 -symmetric metallocene catalyst based on indene produced isotactic polypropylene (Fig. 6a). A stereorigid C 5 -symmetric prochiral catalyst based on cyclopentene and fluorene produced syndiotactic polymer predominantly (Fig. 6b). The
mechanism of syndiotactic polymer formation was attributed to the chain migratory
insertion of monomer as shown in Scheme 5. The two accessible sites on the catalytic sites are for the growing chain and the incoming monomer. If the growing
chain shifts between these two sites at each monomer addition, then a syndiotactic
polypropylene is the preferred product. However, several authors also hypothesized
that the steric repulsion between the methyl group of the complexed monomer and
last unit of the growing chain guided the above stereoselectivity of the polymer [24].
Table 3 Properties of isotactic, syndiotactic, and atactic polypropylene (PP) [22]
Properties
PP
Isotactic PP
Syndiotactic PP
UTS (MPa)
40
600
330
Elongation at break (%)
100
130
230
Young’s modulus (GPa)
1.9
20.0
3.0
Density (g/cm
3
)
0.89–0.92
0.91–0.94
–
Melting point (°C)
–
171
130
A. S. K. Sinha and U. Ojha