In industrial use, ansa-metallocene catalyst systems compete with the wellestablished traditional Ziegler–Natta catalysts. Because their precursors, in particular the racemic ansa-metallocenes and suitable activators like MAO, are more
costly than the relatively simple raw materials for Ziegler–Natta catalysts, the
industrial success of ansa-metallocene catalysts is contingent on their ability to
produce polymer materials with superior properties, which can be priced so as to
compensate for the higher costs of synthesizing, activating and handling these
advanced organometallic catalysts.
For isotactic polypropylene (i-PP), currently produced at a world-wide scale of
about 50 million tons per year with classical Ziegler–Natta catalysts, the situation
can be viewed as follows [66]: For many applications, the rather wide molar-mass
distribution of i-PP generated by solid-state Ziegler–Natta catalysts is an advantage
compared to the narrow molar-mass distribution of i-PP produced with supported
ansa-metallocene catalysts, since a wide molar-mass distribution leads to a more
pronounced shear-induced viscosity decrease of the polymer melt and, hence, to
higher through-put rates in typical extrusion processes. It also provides higher
polymer-melt strengths, which is desirable in thermoforming and biaxally oriented
polypropylene (BOPP) film production, both very large application segments for
i-PP. Other i-PP applications (e.g. melt-blown or spun-bonded non-woven fabrics),
however, benefit from metallocene-produced i-PP because they run more stably at
higher speeds in these spinning processes, thus providing processing advantages.
The uniform polymer chain lengths also cause a higher orientation in the spinning
process, thus providing higher tenacities of the final non-woven fabric [67].
Besides its narrow molar-mass distribution, metallocene-produced i-PP benefits
from a much more efficient control of its molar mass by hydrogen, eliminating the
need for post-reactor chain degradation using peroxides. Metallocene-produced
i-PP is thus normally free of oligomeric chain fragments and low molecular weight
residues from peroxides, which both cause spinning fumes and deposits on the
spinning dies. This reduces emissions during the spinning process and the frequency of shutdowns necessary to clean the spinning dies.
Although the wider molar-mass distribution of conventional i-PP is generally
preferred in thick-wall injection molding, where the shear-induced drop of the melt
viscosity helps to fill the mold faster, ansa-metallocene-catalyzed i-PP offers
advantages in thin-wall injection molding, where i-PP with higher melt flow rate
(lower viscosity) is generally used: The uniform polymer chain length distribution
brings a higher number of tie-molecules capable of connecting different crystallites.
This translates into higher toughness, whereas the absence of excessively long
polymer chains (with very long relaxation times) reduces warpage and gives
injection-molded articles higher dimensional stability and superior mechanical
strength; consequently, thinner walls allow material savings.
Other advantages of these materials result from their crystallization
characteristics: Due to its compositional homogeneity, polypropylene obtained
with ansa-metallocene catalysts gives rise to a more controlled initial crystallite
formation in a narrow temperature window. The ensuing crystallization process can
thus be guided so as to yield smaller crystallites than in polymers obtained with
Development of ansa-Metallocene Catalysts for Isotactic Olefin Polymerization
37
costly than the relatively simple raw materials for Ziegler–Natta catalysts, the
industrial success of ansa-metallocene catalysts is contingent on their ability to
produce polymer materials with superior properties, which can be priced so as to
compensate for the higher costs of synthesizing, activating and handling these
advanced organometallic catalysts.
For isotactic polypropylene (i-PP), currently produced at a world-wide scale of
about 50 million tons per year with classical Ziegler–Natta catalysts, the situation
can be viewed as follows [66]: For many applications, the rather wide molar-mass
distribution of i-PP generated by solid-state Ziegler–Natta catalysts is an advantage
compared to the narrow molar-mass distribution of i-PP produced with supported
ansa-metallocene catalysts, since a wide molar-mass distribution leads to a more
pronounced shear-induced viscosity decrease of the polymer melt and, hence, to
higher through-put rates in typical extrusion processes. It also provides higher
polymer-melt strengths, which is desirable in thermoforming and biaxally oriented
polypropylene (BOPP) film production, both very large application segments for
i-PP. Other i-PP applications (e.g. melt-blown or spun-bonded non-woven fabrics),
however, benefit from metallocene-produced i-PP because they run more stably at
higher speeds in these spinning processes, thus providing processing advantages.
The uniform polymer chain lengths also cause a higher orientation in the spinning
process, thus providing higher tenacities of the final non-woven fabric [67].
Besides its narrow molar-mass distribution, metallocene-produced i-PP benefits
from a much more efficient control of its molar mass by hydrogen, eliminating the
need for post-reactor chain degradation using peroxides. Metallocene-produced
i-PP is thus normally free of oligomeric chain fragments and low molecular weight
residues from peroxides, which both cause spinning fumes and deposits on the
spinning dies. This reduces emissions during the spinning process and the frequency of shutdowns necessary to clean the spinning dies.
Although the wider molar-mass distribution of conventional i-PP is generally
preferred in thick-wall injection molding, where the shear-induced drop of the melt
viscosity helps to fill the mold faster, ansa-metallocene-catalyzed i-PP offers
advantages in thin-wall injection molding, where i-PP with higher melt flow rate
(lower viscosity) is generally used: The uniform polymer chain length distribution
brings a higher number of tie-molecules capable of connecting different crystallites.
This translates into higher toughness, whereas the absence of excessively long
polymer chains (with very long relaxation times) reduces warpage and gives
injection-molded articles higher dimensional stability and superior mechanical
strength; consequently, thinner walls allow material savings.
Other advantages of these materials result from their crystallization
characteristics: Due to its compositional homogeneity, polypropylene obtained
with ansa-metallocene catalysts gives rise to a more controlled initial crystallite
formation in a narrow temperature window. The ensuing crystallization process can
thus be guided so as to yield smaller crystallites than in polymers obtained with
Development of ansa-Metallocene Catalysts for Isotactic Olefin Polymerization
37