carbon in the molecule (see Fig. 10). Other trimeric isomers were formed by double
bond migration, 2,1-, and 1,3-insertion and were evidence for the mechanism of
chain termination in the olefin polymerization by metallocene catalysts. It was
possible to obtain the propene trimer 2,4-dimethyl-1-heptene with 95.3 enantiomeric
excess at 20
C oligomerization temperature [94]. The tetrameric oligopropylene,
under the same preparation conditions, shows a higher optical rotation than the
trimer because the molecule consists of two chiral carbon atoms.
These measurements were direct proof that a chiral (pure enantiomer) soluble
zirconocene/MAO catalyst inserts the propene units in the same stereospecific
manner and produces optically active oligomers and, by a continued insertion,
isotactic polymers. Such polymer chains contain a vinyl group at the end formed
by the main termination reaction, and all methyl groups are found at the top or the
bottom. Using a racemic mixture of the zirconocene, 50% of the polymer chains
show the methyl groups at the top and the other 50% at the bottom.
3.2 Copolymers
Because metallocene/MAO catalysts were found to be highly active polymerization
catalysts not only for ethene and propene but also for longer chained 1-olefins, they
are suitable for different copolymerizations. Copolymers with new microstructures
can be obtained, such as:
Ethene–propene (EP) [25]
Ethene–propene, diene (EPDM) [95]
Ethene–1-butene, hexene (LLDPE) [96]
Ethene–1-octene (LLDPE) [97]
Ethene–1,5-hexadiene elastomer [95]
Ethene–cyclopentene (COC) [98]
Ethene–norbornene (COC) [91, 98]
Ethene–1,3-butadiene (elastomer) [95]
Ethene–styrene (elastomer) [99, 100]
EP, EPDM (ethene propene diene monomers), and LLDPE (linear low-density
polyethylene) are of high interest for the polymer industry. All copolymers produced
by metallocene catalysts are characterized by a narrow molecular weight distribution
of 2 and a uniform microstructure. Although the comonomers are distributed randomly
in the polymer chain, only low amounts are needed to decrease the density and the
melting point of ethene copolymers. The low amount of oligomers compared to
copolymers produced by Ziegler–Natta catalysts is responsible for a high tensile
strength and other mechanical properties of the obtained LLDPE. Mechanical
properties can be increased if there are some long chain branches in the polymer
chain. In particular, half-sandwich complexes (constrained geometry catalysts) are
able to incorporate higher 1-olefins into the growing chain (Fig. 11) [78]. These
complexes are activated by MAO or fluorinated phenylborates.
16
W. Kaminsky and H. Sinn
bond migration, 2,1-, and 1,3-insertion and were evidence for the mechanism of
chain termination in the olefin polymerization by metallocene catalysts. It was
possible to obtain the propene trimer 2,4-dimethyl-1-heptene with 95.3 enantiomeric
excess at 20
C oligomerization temperature [94]. The tetrameric oligopropylene,
under the same preparation conditions, shows a higher optical rotation than the
trimer because the molecule consists of two chiral carbon atoms.
These measurements were direct proof that a chiral (pure enantiomer) soluble
zirconocene/MAO catalyst inserts the propene units in the same stereospecific
manner and produces optically active oligomers and, by a continued insertion,
isotactic polymers. Such polymer chains contain a vinyl group at the end formed
by the main termination reaction, and all methyl groups are found at the top or the
bottom. Using a racemic mixture of the zirconocene, 50% of the polymer chains
show the methyl groups at the top and the other 50% at the bottom.
3.2 Copolymers
Because metallocene/MAO catalysts were found to be highly active polymerization
catalysts not only for ethene and propene but also for longer chained 1-olefins, they
are suitable for different copolymerizations. Copolymers with new microstructures
can be obtained, such as:
Ethene–propene (EP) [25]
Ethene–propene, diene (EPDM) [95]
Ethene–1-butene, hexene (LLDPE) [96]
Ethene–1-octene (LLDPE) [97]
Ethene–1,5-hexadiene elastomer [95]
Ethene–cyclopentene (COC) [98]
Ethene–norbornene (COC) [91, 98]
Ethene–1,3-butadiene (elastomer) [95]
Ethene–styrene (elastomer) [99, 100]
EP, EPDM (ethene propene diene monomers), and LLDPE (linear low-density
polyethylene) are of high interest for the polymer industry. All copolymers produced
by metallocene catalysts are characterized by a narrow molecular weight distribution
of 2 and a uniform microstructure. Although the comonomers are distributed randomly
in the polymer chain, only low amounts are needed to decrease the density and the
melting point of ethene copolymers. The low amount of oligomers compared to
copolymers produced by Ziegler–Natta catalysts is responsible for a high tensile
strength and other mechanical properties of the obtained LLDPE. Mechanical
properties can be increased if there are some long chain branches in the polymer
chain. In particular, half-sandwich complexes (constrained geometry catalysts) are
able to incorporate higher 1-olefins into the growing chain (Fig. 11) [78]. These
complexes are activated by MAO or fluorinated phenylborates.
16
W. Kaminsky and H. Sinn