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Description:
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Utilization of methylaluminoxane (MAO ) activated metallocene and constrained
geometry (CGC ) olefin polymerization catalysts containing fluorenyl or
octamethyloctahydrodibenzofluorenyl (Oct ) moieties has yielded three series of
syndiotactic copolymers of propylene with higher a -olefins . The melting temperatures
of these polymers were analyzed , and found to correspond directly with the mole percent
incorporation of comonomer , as well as with the frequency of stereoerrors in the
polymers . Further analysis indicated that rmrr stereoerrors , a result of site
epimerization , occur in close proximity to the incorporated comonomers .
The MAO -activated fluorenyl /Oct -containing metallocene and CGC catalysts were
further utilized to produce syndiotactic samples of poly (1 -butene ) (s -PB ) and poly (1 -
pentene ) (s -PPe ) . The syndiotacticity of the samples was quantified by 13C NMR and
the melting temperatures determined by DSC . The samples of s -PB and s -PPe produced
by Me2Si ( h1 -C29H36 ) ( h1 -N -tBu )ZrCl2·OEt2 (Oct -CGC ) were found to melt at higher
temperatures (55 .9 and 43 .1 °C , respectively ) than any previously reported samples . The MAO -activated Oct -CGC was also used to produce polyethylene samples at a
variety of polymerization temperatures and pressures . All of the samples were found to
contain an unprecedented degree of branching (13 -65 total branches per 1000 carbon
atoms ) for an early transition metal single -site catalyst . The branches were found to be
almost exclusively of two or greater than five carbon atoms in length , and the levels of
the longer branches could be controlled by varying the polymerization conditions . The
number of ethyl branches was roughly 5 per 1000 carbon atoms for all samples .
Finally , a binary catalyst system comprising the Oct -CGC and a chromium -based
ethylene trimerization catalyst , ( (tBuSCH2CH2 )2NH )CrCl3 , was developed . This MAOactivated
catalyst system could be tuned to produce polyethylene samples with 17 -49
total branches per 1000 carbon atoms . Between 4 and 16 of these branches were found
to arise from incorporation of 1 -hexene produced by the chromium oligomerization
catalyst . Adjusting the ratios of oligomerization catalyst , polymerization catalyst , and
activator was found to allow rational control over the branch content of the polymers .
The branching levels could also be varied by altering the time between injection of the
oligomerization and polymerization catalysts into the system . |