least the major species of such a working catalyst, e.g. by NMR spectroscopy. In
this manner, inner-sphere ion pairs of type A
0 (Fig. 3), with a polymeryl chain and a
MeB(C 6 F 5 ) 3
À anion attached to the cationic metal center, have indeed been
identified in toluene solution at –40
C as the catalyst resting state in an ansazirconocene catalyst system activated with the Lewis-acidic borane B(C 6 F 5 ) 3 [30].
Under conditions more closely approaching practically useful polymerization
catalysis, i.e. in working catalyst systems that contain less “sticky” counter-anions
and a substantial olefin excess and that operate at ambient or higher temperatures, it
is not yet clear which fractions of the total catalyst content in a given catalyst
system correspond to ion pairs of type A
0 or to several other resting-state candidates
with regio-irregular [44], allylic [45] or agostically bound [46] chain ends.
During polymerization of 1-hexene by an MAO-activated ansa-zirconocene
catalyst system, species of types A, B and B’ were detected by NMR spetroscopy
but found to account for less than half of the initial metallocene concentration [31].
Major parts of the metallocene content of this catalyst system thus remain
unaccounted for.
Further efforts to identify the resting state(s) and the major modes of deactivation [47] of typical ansa-metallocene catalysts would appear worthwhile, since – to
quote a recent review [26] – “in the absence of this information, a rational analysis
of the effects of structural variations on catalyst activities . . . must remain
fragmentary.”
4 Industrial Use of ansa-Metallocene Catalysts
The first isospecific propene polymerization with an ethanediyl-bridged bis
(indenyl)-titanium complex demonstrated in 1983 the capability of ansametallocenes to control the tacticity of a growing polypropylene chain [4, 48],
thus creating a strong interest in exploring the potential use of ansa-metallocenes
for industrial polypropylene production. A first step towards industrially suitable
systems was the use of much more stable zirconium complexes, which when
activated by the methylaluminoxane co-catalyst [3] allowed higher polymerization
temperatures and thus higher activities [5].
However, these first ansa-zirconocenes delivered at temperatures of 70
C and
above, as typical for industrial PP processes, only oligomeric or low molecular
weight polypropylene and moderate isotacticities, both still insufficient for industrial usage. Further increases in activity and better stereospecifities were obtained
using the more rigid dimethylsilylene bridge, which was patented by Fina Technology in 1987 [15, 49] and has since become the most widely used bridging unit.
However, only the introduction of methyl substituents in the 2-position, by
the former Hoechst AG in 1990 [20, 50], led to polypropylenes with sufficiently
high molar mass under industrially relevant polymerization conditions. In 1990,
rac-dimethylsilylene-bis(2-methylindenyl)zirconium dichloride thus became the
first ansa-metallocene to combine all these improvements and be suitable for
industrial use [50, 51]. Since then, the basic concept of dimethylsilylene-bridged,
Development of ansa-Metallocene Catalysts for Isotactic Olefin Polymerization
35
this manner, inner-sphere ion pairs of type A
0 (Fig. 3), with a polymeryl chain and a
MeB(C 6 F 5 ) 3
À anion attached to the cationic metal center, have indeed been
identified in toluene solution at –40
C as the catalyst resting state in an ansazirconocene catalyst system activated with the Lewis-acidic borane B(C 6 F 5 ) 3 [30].
Under conditions more closely approaching practically useful polymerization
catalysis, i.e. in working catalyst systems that contain less “sticky” counter-anions
and a substantial olefin excess and that operate at ambient or higher temperatures, it
is not yet clear which fractions of the total catalyst content in a given catalyst
system correspond to ion pairs of type A
0 or to several other resting-state candidates
with regio-irregular [44], allylic [45] or agostically bound [46] chain ends.
During polymerization of 1-hexene by an MAO-activated ansa-zirconocene
catalyst system, species of types A, B and B’ were detected by NMR spetroscopy
but found to account for less than half of the initial metallocene concentration [31].
Major parts of the metallocene content of this catalyst system thus remain
unaccounted for.
Further efforts to identify the resting state(s) and the major modes of deactivation [47] of typical ansa-metallocene catalysts would appear worthwhile, since – to
quote a recent review [26] – “in the absence of this information, a rational analysis
of the effects of structural variations on catalyst activities . . . must remain
fragmentary.”
4 Industrial Use of ansa-Metallocene Catalysts
The first isospecific propene polymerization with an ethanediyl-bridged bis
(indenyl)-titanium complex demonstrated in 1983 the capability of ansametallocenes to control the tacticity of a growing polypropylene chain [4, 48],
thus creating a strong interest in exploring the potential use of ansa-metallocenes
for industrial polypropylene production. A first step towards industrially suitable
systems was the use of much more stable zirconium complexes, which when
activated by the methylaluminoxane co-catalyst [3] allowed higher polymerization
temperatures and thus higher activities [5].
However, these first ansa-zirconocenes delivered at temperatures of 70
C and
above, as typical for industrial PP processes, only oligomeric or low molecular
weight polypropylene and moderate isotacticities, both still insufficient for industrial usage. Further increases in activity and better stereospecifities were obtained
using the more rigid dimethylsilylene bridge, which was patented by Fina Technology in 1987 [15, 49] and has since become the most widely used bridging unit.
However, only the introduction of methyl substituents in the 2-position, by
the former Hoechst AG in 1990 [20, 50], led to polypropylenes with sufficiently
high molar mass under industrially relevant polymerization conditions. In 1990,
rac-dimethylsilylene-bis(2-methylindenyl)zirconium dichloride thus became the
first ansa-metallocene to combine all these improvements and be suitable for
industrial use [50, 51]. Since then, the basic concept of dimethylsilylene-bridged,
Development of ansa-Metallocene Catalysts for Isotactic Olefin Polymerization
35