A majority of all polyethylene grades are ethene/α-olefin copolymers (LLDPE,
MDPE), which benefit directly from the use of single-site catalysts like ansametallocenes. Most polypropylenes, on the other hand, are homo-polymers, with
one exception: biphasic i-PP reactor blends contain elastomeric ethene/propene
copolymers (EP rubbers) dispersed in a matrix of crystalline i-PP homo-polymer.
Such reactor blends, used e.g. in the manufacture of automobile parts, retain the high
mechanical strength of isotactic polypropylene while being endowed with increased
toughness, also at lower temperatures, from the dispersed EP rubber particles. However, these ethene/propene copolymers, for which similar considerations should hold
as for m-LLDPE, were not accessible by first-generation 2-methyl-substitued
bisindenyl ansa-metallocenes because of a drastic reduction in polymer chain length
in the presence of ethene. Therefore, the development of catalysts bearing a ligand
framework with suitably substituted α-positions [25, 55, 70], as discussed in Sect. 2,
was a prerequisite for the production of i-PP reactor blends with metallocene catalysts.
Such a specially adapted ansa-zirconocene catalyst is used by LyondellBasell to
produce EP/i-PP “impact” polymer blends, which belong to their family of “Clyrell”
grades [71].
The unique properties of i-PP produced with ansa-metallocene catalysts, including their low content of easily migrating short-chain polymer, desirable in particular for food and medical packaging and related purposes [72], has driven the
industrial use of i-PP produced with ansa-metallocene catalysts in some demanding
market sectors. However, higher costs, the dominance of homo-polypropylene,
improved Ziegler–Natta catalysts and, last but not least, a complicated patent
situation have as of today restricted their use in the i-PP field to specialized niche
applications.
The possibility of controlling the interanular wedge angle of ansa-metallocene
catalysts and their resulting capability to also enchain olefins, which are otherwise
not readily amenable to insertion polymerization, allows the production of
copolymers of ethene with cyclic olefins, in particular with norbornene or its
substituted derivatives. Based on first observations in this regard [73, 74], ethene/
norbornene copolymers, e.g. with alternating microstructures [75–77] and with
excellent optical and barrier properties, are now commercially produced with
ansa-metallocene catalysts by Topas Advanced Polymers (TOPAS
® ) [78] and by
Mitsui Chemical (Apel) [79].
5 Concluding Remarks
Since the first reports on their usage for isospecific propene polymerization [4, 5, 48],
ansa-metallocenes have inspired industrial and academic research for almost 30 years.
Initially introduced for isospecific olefin polymerization, the concept of bridging their
two ring ligands has substantially enhanced the variability and design options of
metallocene catalysts in general. In addition to paving the way for industrial i-PP
Development of ansa-Metallocene Catalysts for Isotactic Olefin Polymerization
39
MDPE), which benefit directly from the use of single-site catalysts like ansametallocenes. Most polypropylenes, on the other hand, are homo-polymers, with
one exception: biphasic i-PP reactor blends contain elastomeric ethene/propene
copolymers (EP rubbers) dispersed in a matrix of crystalline i-PP homo-polymer.
Such reactor blends, used e.g. in the manufacture of automobile parts, retain the high
mechanical strength of isotactic polypropylene while being endowed with increased
toughness, also at lower temperatures, from the dispersed EP rubber particles. However, these ethene/propene copolymers, for which similar considerations should hold
as for m-LLDPE, were not accessible by first-generation 2-methyl-substitued
bisindenyl ansa-metallocenes because of a drastic reduction in polymer chain length
in the presence of ethene. Therefore, the development of catalysts bearing a ligand
framework with suitably substituted α-positions [25, 55, 70], as discussed in Sect. 2,
was a prerequisite for the production of i-PP reactor blends with metallocene catalysts.
Such a specially adapted ansa-zirconocene catalyst is used by LyondellBasell to
produce EP/i-PP “impact” polymer blends, which belong to their family of “Clyrell”
grades [71].
The unique properties of i-PP produced with ansa-metallocene catalysts, including their low content of easily migrating short-chain polymer, desirable in particular for food and medical packaging and related purposes [72], has driven the
industrial use of i-PP produced with ansa-metallocene catalysts in some demanding
market sectors. However, higher costs, the dominance of homo-polypropylene,
improved Ziegler–Natta catalysts and, last but not least, a complicated patent
situation have as of today restricted their use in the i-PP field to specialized niche
applications.
The possibility of controlling the interanular wedge angle of ansa-metallocene
catalysts and their resulting capability to also enchain olefins, which are otherwise
not readily amenable to insertion polymerization, allows the production of
copolymers of ethene with cyclic olefins, in particular with norbornene or its
substituted derivatives. Based on first observations in this regard [73, 74], ethene/
norbornene copolymers, e.g. with alternating microstructures [75–77] and with
excellent optical and barrier properties, are now commercially produced with
ansa-metallocene catalysts by Topas Advanced Polymers (TOPAS
® ) [78] and by
Mitsui Chemical (Apel) [79].
5 Concluding Remarks
Since the first reports on their usage for isospecific propene polymerization [4, 5, 48],
ansa-metallocenes have inspired industrial and academic research for almost 30 years.
Initially introduced for isospecific olefin polymerization, the concept of bridging their
two ring ligands has substantially enhanced the variability and design options of
metallocene catalysts in general. In addition to paving the way for industrial i-PP
Development of ansa-Metallocene Catalysts for Isotactic Olefin Polymerization
39