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Description:
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The sterically expanded octamethyloctahydrodibenzofluorene moiety , C29H38
(Oct ) , has been incorporated into ansa -metallocenes and constrained geometry catalysts
(CGC's ) . Utilization of this sterically expanded version of fluorene has resulted in solidstate
anomalies for both systems .
The ansa -metallocenes Me2C (n5 -C5H4 ) (n5 -C29H36 )MCl2 , M = Zr , Hf and
Me2C (n5 -C5H4 ) (n5 -C29H36 )ZrBn2 demonstrate crystal motifs expected for ansametallocenes
while Me2C (n5 -C5H4 ) (n5 -C29H36 )MMe2 , M=Zr , Hf exhibit diffuse
diffraction , a phenomenon that is extremely unusual for organometallic complexes . This
crystalline anomaly is the result of a disorder restricted to two dimensions caused by the
rare pillared motif of the system . The best solution for this system consists of parallel
and anti -parallel pillars present in a 60 :40 ratio .
The solid state anomaly observed for the Oct -CGC's occurs on a molecular level .
The parent Oct -CGC , Me2Si (1 -C29H36 ) (1 -N -tBu )ZrCl2·OEt2 , demonstrates an
unprecedented n1 ligation to the fluorenyl -based ring . Systematic derivatization of this
system via halide substitution , alkylation , and exchange of Zr for Hf has revealed that the n1 ligation persists for systems with small substituents on the metal center capable of
retaining a coordinated ether .
It is hypothesized that the unusual structure of this new Oct -CGC results in
Me2Si (1 -C29H36 ) (1 -N -tBu )ZrCl2·OEt2 /MAO (MAO = methylaluminoxane ) being six
times more active in the homopolymerization of 1 -octene than ethylene . When
compared to the prototypical Ti -CGC Me2Si (n5 -C5Me4 ) (n1 -N -tBu )TiCl2 /MAO , the Oct -CGC is 85 times more reactive in the homopolymerization of 1 -octene and 52 times
more active in the copolymerization of 1 -octene and 4 -methyl -1 -pentene . The high
reactivity of the Oct -CGC towards alpha -olefins results in the observation of an unyielding
comonomer effect in the copolymerization of these olefins with ethylene .
In addition , the Oct -CGC is perhaps the most syndioselective catalyst known .
With an enantiofacial selectivity of 99 .7 % and a remarkably high activity towards alpha -olefins , the Oct -CGC is capable of producing the highest melting syndiotactic
polypropylene (Tm = 165oC , annealed = 174oC ) reported thus far . The high activity and
syndioselectivity of the Oct -CGC can be extended to the production of syndiotactic
poly (4 -methyl -1 -pentene ) with the highest melting point thus far reported (Tm = 215oC ) . |