9 Outlook
The development of syndiospecific metallocene catalyst systems and s-PP from
laboratory to commercial production has been one of the most successful projects
both from scientific and technological points of view; it was driven mainly by the
discovery of a new metallocene structure. During the 5 years of research and
development there were many scientific, technical, and technological challenges,
for example, regarding optimization of metallocene structure, its large scale
production, supported catalyst development, process compatibility issues, polymer
processing, and development of new product markets. Although the industrial
production of s-PP has been a successful undertaking from both the technical and
economic points of view, the production of s-PP has remained limited in volume
and s-PP has been introduced to the market as rather a specialty product. There still
exist afew handicaps that have hampered its large market penetration despite the
attractiveness of its properties, particularly its excellent optical properties and its
unusually high radiation resistance. The stumbling block on this road was and still
is the slow crystallization rate of industrially manufactured s-PP. This originates
from the low crystallinity, which in turn emanates from the presence of high
numbers of the site epimerization-dependent meso dyad stereodefects in s-PP
samples produced with supported catalyst systems.
Despite our good understanding of cation/anion dynamics in solution, our
current understanding of the changes that the anionic MAO species undergo upon
heterogenization on the silica is still limited and therefore our capability to control
and or prevent the site epimerization processes is restricted.
We suspect either that the nature of the anionic species of MAO changes
dramatically upon heterogenization to favor site epimerization-related pathway(s)
or that, in heterogeneous polymerization, limitations in monomer-to-active site
diffusion lower the effective monomer concentration at the active site and change
the balance between the propagation rate and site epimerization rate in favor of the
latter.
7
7 Incidentally, a very similar phenomenon is observed with C 2 symmetric bis-indenyl-based
isospecific zirconocene dichloride catalysts. For example, the metallocene dimethylsilyl-bridged
2-methyl-4-phenyl-bisindenylzirconocenedichloride, once activated with MAO, produces an isotactic polypropylene with a melting point close to 160
C in liquid propylene at 60
C. However, the
same catalyst, once supported on a silica carrier, produces i-PP polymers that have melting points
ranging between 150 and 152
C, at the same polymerization conditions. In this case, the formation
of larger amounts of region-irregular units in the heterogeneously produced i-PP is the melting
point lowering factor. Interestingly, the bridged cyclopentadienyl-fluorenyl ligand-based C 1 symmetric isospecific catalyst systems are immune to this flaw. The homogeneously and heterogeneously produced isotactic polypropylenes with these catalysts show no or very few regio-irregular
errors and their isotactic polypropylene polymer pairs have similar melting points that reach, for
polymers produced with highly selective C 1 symmetric catalyst systems, values close to or even
higher than 160
C.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
109
The development of syndiospecific metallocene catalyst systems and s-PP from
laboratory to commercial production has been one of the most successful projects
both from scientific and technological points of view; it was driven mainly by the
discovery of a new metallocene structure. During the 5 years of research and
development there were many scientific, technical, and technological challenges,
for example, regarding optimization of metallocene structure, its large scale
production, supported catalyst development, process compatibility issues, polymer
processing, and development of new product markets. Although the industrial
production of s-PP has been a successful undertaking from both the technical and
economic points of view, the production of s-PP has remained limited in volume
and s-PP has been introduced to the market as rather a specialty product. There still
exist afew handicaps that have hampered its large market penetration despite the
attractiveness of its properties, particularly its excellent optical properties and its
unusually high radiation resistance. The stumbling block on this road was and still
is the slow crystallization rate of industrially manufactured s-PP. This originates
from the low crystallinity, which in turn emanates from the presence of high
numbers of the site epimerization-dependent meso dyad stereodefects in s-PP
samples produced with supported catalyst systems.
Despite our good understanding of cation/anion dynamics in solution, our
current understanding of the changes that the anionic MAO species undergo upon
heterogenization on the silica is still limited and therefore our capability to control
and or prevent the site epimerization processes is restricted.
We suspect either that the nature of the anionic species of MAO changes
dramatically upon heterogenization to favor site epimerization-related pathway(s)
or that, in heterogeneous polymerization, limitations in monomer-to-active site
diffusion lower the effective monomer concentration at the active site and change
the balance between the propagation rate and site epimerization rate in favor of the
latter.
7
7 Incidentally, a very similar phenomenon is observed with C 2 symmetric bis-indenyl-based
isospecific zirconocene dichloride catalysts. For example, the metallocene dimethylsilyl-bridged
2-methyl-4-phenyl-bisindenylzirconocenedichloride, once activated with MAO, produces an isotactic polypropylene with a melting point close to 160
C in liquid propylene at 60
C. However, the
same catalyst, once supported on a silica carrier, produces i-PP polymers that have melting points
ranging between 150 and 152
C, at the same polymerization conditions. In this case, the formation
of larger amounts of region-irregular units in the heterogeneously produced i-PP is the melting
point lowering factor. Interestingly, the bridged cyclopentadienyl-fluorenyl ligand-based C 1 symmetric isospecific catalyst systems are immune to this flaw. The homogeneously and heterogeneously produced isotactic polypropylenes with these catalysts show no or very few regio-irregular
errors and their isotactic polypropylene polymer pairs have similar melting points that reach, for
polymers produced with highly selective C 1 symmetric catalyst systems, values close to or even
higher than 160
C.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
109