SUMMARY:The molecular weight (MW) and the molecular weight distribution (MWD) of polyethylene (PE) prepared with various metallocene catalysts were investigated. The weight-average molecular weight (MW) of PE prepared with dicyclopentadienyltitanium dichloride/ aluminoxane (Cp,TiCl,/MAO) was about 4,O. lo', while that of PE obtained withPE of bimodal MWD was obtained with the mixed catalyst Cp,TiCl,/Cp,ZrCl,. Narrow MWD PE was obtained with the mixed catalyst Cp2ZrC1,/Et(ind),ZrC1,, because each catalyst produced an M, of 1,0 * lo5. However, the change of temperature during the polymerization with the mixture Cp,ZrC1,/Et(ind)2ZrClz produced PE with a bimodal MWD and a value of the ratio of weight-to number-average molecular weights of a w / M n = 4.
Et(ind)zZrClz (C2H,(indenyl)zZrC12) confined inside regular pores of molecular sieves MCM-41 and VPIJ were prepared and used to polymerize propene with high activity. Stereoregularity, melting point and molecular weight of polypropene obtained were increased and the polymerization behavior was quite different from that prepared with homogeneous Et(ind)zZrC12. The small, regular and cylindrical pores of MCM-4 1 and VPI-5 suppress the formation of inactive binuclear complexes between metallocene and metallocene, or between metallocene and methylaluminoxane, resulting in stable active sites and high activity in propene polymerization.
' )Present address: Polymer Technology Lab.,
Metallocene catalysts entrapped inside the supercages of NaY zeolite were prepared by reacting NaY with methylaluminoxane (MAO) or trimethylaluminium (TMA) and then with Cp2ZrCI, (Cp: cyclopentadienyl) or Cp2TiCI2. NaY/MAO/Cp,ZrCl, and NaY/MAO/ Cp,TiCI, catalysts could polymerize ethylene. The amount of additional MA0 for the polymerization was lowered to a mole ratio of AVZr of 186. Molecular weights and melting points of polyethylene polymerized with Nay-supported catalysts were higher than those of polyethylene obtained with homogeneous metallocene catalysts. It could be confirmed by extraction experiments that the metallocene catalyst was confined securely inside the supercage of the NaY zeolite.
For a determination of the active site concentrations, [C*], the
simultaneous kinetics and
inhibition method using CO as a catalyst inhibitor were examined.
It is noticed that this evaluation
method is not quantitatively exact because MAO-activated alkyl Zr sites
exist in dynamic equilibrium
between dormant states and a chain propagating site and because some CO
molecules coordinate even
to inactive species. The number of active sites was evaluated on
the assumption that two molecules of
CO coordinate to each active center. The increase in Al/Zr ratio
and temperature increased both the
number of active sites and the value of k
p for
ethylene polymerization over Cp2ZrCl2/MAO.
Especially,
a drastic increase of activity is mainly due to a much higher value of
k
p above a Al/Zr ratio of 2000.
A
comonomer enhanced drastically the catalytic activity to more than 10
times for ethylene−propylene
copolymerization over the
rac-Me2Si(Ind)2ZrCl2/MAO
system. This is due to the drastic increase in the
propagating rate constant, k
p, rather than the
increase in the formation of new active sites by the
addition
of comonomer.
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