6.10 Processing of Syndiotactic Polypropylene Polymers
Highly stereoselective metallocene catalysts, undergoing few site epimerization
errors, that produce high crystallinity s-PP with melting points close to or higher
than 160
C and fast crystallization rates are available for homogeneous polymerization [195–197]. The very same catalysts systems, however, when supported on a silica
carrier, produce s-PP polymers with much lower melting points and crystallinities.
Consequently, the crystallinities and crystallization rates decrease in heterogeneously
produced s-PP polymer samples. For example, the same MAO-activated metallocene
structure, in homogeneous and heterogeneous polymerization reactions, would
produce two s-PP polymers whose melting points could differ by 5–20
C. Strangely,
the higher the stereospecificity of the metallocene precatalyst and the higher the
melting point of the homogeneous s-PP polymer, the lower is the melting point of
the heterogeneously produced s-PP polymer counterpart, e.g., a metallocene structure
providing s-PP polymers with a melting points around 132
C in homogeneous
polymerization, produces s-PP polymers having melting points of about 127
C in
heterogeneous polymerization under the same conditions. However, if the improved
metallocene catalyst produces a s-PP polymer with a melting point of 160
C in a
homogeneous polymerization [195–197, 198–202], the same structure under the same
polymerization conditions produces a s-PP polymer with a melting point of only
140
C in heterogeneous polymerization. The analysis of the two polymers reveals
that the heterogeneously produced polymer samples contain a much larger number
of site epimerization-related errors, whereas the meso triad-related errors remain
basically unchanged and almost equal for both polymer samples.
Therefore, and inadvertently, the s-PP resins that are produced industrially with
supported metallocene catalysts have lower crystallinity and relatively low crystallization rates, which makes their processing challenging. The s-PP polymer melt
that exits the extruder does not solidify quickly enough to proceed effectively with
the pelletization process. In practice, one can mitigate the problem by addition of a
nucleating agent or by cooling the extrudate. Another side-effect of the low
crystallinity is that the commercially produced s-PP resins, with melting points
below 130
C, contain a substantial amount of absorbed propylene monomer, which
is released with time and necessitates vigorous aeration of the storage hanger. These
handicaps hamper the fast and efficient extrusion and pelletization of very large
quantities of commercially produced s-PP and have so far prevented its vast market
penetration as a commodity product compared to other olefin-based thermoplastics.
7 Properties of Syndiotactic Polypropylene Polymers
7.1 Polymorphism of Syndiotactic Polypropylene
Syndiotactic polypropylene (s-PP) presents a very complex polymorphic behavior
(the first part of this section on s-PP modifications is reported in the publication
[203–212]). s-PP chains crystallize to various polymorphic crystalline phases and,
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
99
Highly stereoselective metallocene catalysts, undergoing few site epimerization
errors, that produce high crystallinity s-PP with melting points close to or higher
than 160
C and fast crystallization rates are available for homogeneous polymerization [195–197]. The very same catalysts systems, however, when supported on a silica
carrier, produce s-PP polymers with much lower melting points and crystallinities.
Consequently, the crystallinities and crystallization rates decrease in heterogeneously
produced s-PP polymer samples. For example, the same MAO-activated metallocene
structure, in homogeneous and heterogeneous polymerization reactions, would
produce two s-PP polymers whose melting points could differ by 5–20
C. Strangely,
the higher the stereospecificity of the metallocene precatalyst and the higher the
melting point of the homogeneous s-PP polymer, the lower is the melting point of
the heterogeneously produced s-PP polymer counterpart, e.g., a metallocene structure
providing s-PP polymers with a melting points around 132
C in homogeneous
polymerization, produces s-PP polymers having melting points of about 127
C in
heterogeneous polymerization under the same conditions. However, if the improved
metallocene catalyst produces a s-PP polymer with a melting point of 160
C in a
homogeneous polymerization [195–197, 198–202], the same structure under the same
polymerization conditions produces a s-PP polymer with a melting point of only
140
C in heterogeneous polymerization. The analysis of the two polymers reveals
that the heterogeneously produced polymer samples contain a much larger number
of site epimerization-related errors, whereas the meso triad-related errors remain
basically unchanged and almost equal for both polymer samples.
Therefore, and inadvertently, the s-PP resins that are produced industrially with
supported metallocene catalysts have lower crystallinity and relatively low crystallization rates, which makes their processing challenging. The s-PP polymer melt
that exits the extruder does not solidify quickly enough to proceed effectively with
the pelletization process. In practice, one can mitigate the problem by addition of a
nucleating agent or by cooling the extrudate. Another side-effect of the low
crystallinity is that the commercially produced s-PP resins, with melting points
below 130
C, contain a substantial amount of absorbed propylene monomer, which
is released with time and necessitates vigorous aeration of the storage hanger. These
handicaps hamper the fast and efficient extrusion and pelletization of very large
quantities of commercially produced s-PP and have so far prevented its vast market
penetration as a commodity product compared to other olefin-based thermoplastics.
7 Properties of Syndiotactic Polypropylene Polymers
7.1 Polymorphism of Syndiotactic Polypropylene
Syndiotactic polypropylene (s-PP) presents a very complex polymorphic behavior
(the first part of this section on s-PP modifications is reported in the publication
[203–212]). s-PP chains crystallize to various polymorphic crystalline phases and,
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
99