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Description:
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Several ansa C1 -symmetric cyclopentadienyl -fluorenyl metallocenes based on
zirconium have been prepared with different substituents at position 3 on the
cyclopentadienyl ring . Isotactic polypropylene production from these systems depends
highly on the size of these substituents . Therefore , large groups such as 1 -methyl -4 -tbutylcyclohexyl
(metallocene 6 ) , 1 -methyl -cyclohexyl (metallocene 7 ) , 1 ,3 ,3 ,5 -
tetramethylcyclohexyl (metallocene 8 ) , and 2 ,3 ,4 -trimethyl -3 -pentyl (metallocene 9 )
have been investigated . In combination with methylaluminoxane (MAO ) , they showed
good activity and produced high molecular weight of isotactic polypropylene . In terms
of the tacticity of the polymers , metallocene 6 made the best isotactic polypropylene
with ~88 % mmmm pentad content . Also , it has been found that if the size of this
substituent is large as in 2 ,3 ,4 -trimethyl -3 -pentyl (metallocene 9 ) , then it will block the
polymerization active site which will deactivate the metallocene .
New synthetic pathways for the synthesis of cyclopentadienyl -fluorenyl
metallocenes based on titanium have been achieved . Anchoring these types of ligands
onto titanium by following the conventional method of using TiCl4 in the metallation
step has failed for the production of Me2C (3 - (diphenylmethyl ) -C5H3 ) (C13H8 )TiCl2
(metallocene 12 ) , Ph2C (C5H4 ) (C13H8 )TiCl2 (metallocene 14 ) , and
Ph2C (C5H4 ) (C13H8 )TiMe2 (metallocene 15 ) ; this is possibly due to the high reactivity of
TiCl4 . Therefore , TiCl4·2THF has been prepared and used in that step to produce these
new titanocenes with good yields .
A new ansa -C2 -symmetric substituted bis -indenyl metallocene for isotactic
polypropylene production has been successfully prepared . It is known that ansa -C2 -
symmetric metallocenes are good catalysts for isotactic polypropylene production , but in
general , their synthesis suffers from the production of the meso Cs -stereoisomer of these
catalysts , which generally produces only atactic polypropylene . Therefore , the meso
stereoisomers must be removed and this is considered a loss of the material that
increases the cost of the catalysts . Addition of bulky substituents on the indenyl groups
as in Me2Si (5 ,5 ,8 ,8 -tetramethyl -5 ,6 ,7 ,8 -tetrahydrobenz (f )indenyl )2ZrCl2 (metallocene 5 )
has prevented the meso stereoisomer production . 5 /MAO produces isotactic
polypropylene with up to ~80 % mmmm pentad content . |