1 History
Polypropylene (PP) is one of the most widely used plastics and features a wide
range of advantages such as low cost, light weight, high melting temperature, good
processability, balanced mechanical properties in terms of stiffness and impact
resistance, etc. Moreover, PP is regarded as a clean material with respect to urgent
environmental requirements, not only due to the halogen- and benzene-free structure but also due to the ease of reuse and recycle. The world production of PP in
2012 reached approximately 60 million tons per year, and is forecasted to stably
grow in the future (Fig. 1). The diverse properties of PP enable its application in a
variety of fields from commodity to specialty.
The immense growth of the polypropylene industry has been greatly driven by
the continuous developments in catalyst technology (Table 1) [1, 2 and references
therein]. The history of propylene polymerization started with the landmark discovery of a solid TiCl 3 pro-catalyst combined with diethylaluminum chloride
(DEAC) by Natta in 1954 [3, 4]. This so-called first generation catalyst enabled
the first catalytic isoselective propylene polymerization, but its poor activity and
isospecificity necessitated additional processes to extract poorly isotactic products
and violet catalyst residues from the obtained polymer. Significant efforts were then
devoted to improving the activity and isospecificity of the catalyst. For the catalyst
activity, there were two main directions of study in order to enhance the utilization
efficiency of the Ti species: preparation of TiCl 3 with larger surface area and the
search for an efficient support material for Ti halide species. The Solvay corporation invented the so-called Solvay-type TiCl 3 in the early 1970s, which was
prepared by the reduction of TiCl 4 with DEAC followed by the removal of
Al residues with the aid of ether [5]. The resultant catalyst, regarded as a second
generation catalyst, achieved improved activity and isospecificity over the first
generation of catalysts, but the level of the improvements was still insufficient to
eliminate the above-mentioned purification processes for the obtained polymer.
Regarding a support material, metal oxide (SiO 2 , Al 2 O 3 ) or hydroxide materials
[Mg(OH) 2 ] were initially considered due to the ease of the immobilization of Ti
species through covalent bonds. However, successful improvement in activity was
not achieved until Montedison and Mitsui discovered MgCl 2 support, almost at the
same time in 1968 [6, 7]. The catalysts, consisting of TiCl 4 active site precursor,
MgCl 2 support, and triethylaluminum (TEA) activator, exhibited much higher
activities than the former generation of catalysts, but their use was limited to
ethylene polymerization due to poor isospecificity. That is the story before the
appearance of “donors,” i.e., the main topic of this chapter.
The first donors in Ziegler–Natta catalysis appeared as a result of collaborative
efforts between Montedison and Mitsui to improve the isospecificity of the abovementioned MgCl 2 -supported catalyst [8, 9]. The developed catalyst, termed third
generation, achieved not only high activity but also high isospecificity by adding
benzoate to the TiCl 4 /MgCl 2 catalyst. The term “donor” originates from the
fact that additives to improve the catalyst isospecificity are Lewis bases with
82
T. Taniike and M. Terano
Polypropylene (PP) is one of the most widely used plastics and features a wide
range of advantages such as low cost, light weight, high melting temperature, good
processability, balanced mechanical properties in terms of stiffness and impact
resistance, etc. Moreover, PP is regarded as a clean material with respect to urgent
environmental requirements, not only due to the halogen- and benzene-free structure but also due to the ease of reuse and recycle. The world production of PP in
2012 reached approximately 60 million tons per year, and is forecasted to stably
grow in the future (Fig. 1). The diverse properties of PP enable its application in a
variety of fields from commodity to specialty.
The immense growth of the polypropylene industry has been greatly driven by
the continuous developments in catalyst technology (Table 1) [1, 2 and references
therein]. The history of propylene polymerization started with the landmark discovery of a solid TiCl 3 pro-catalyst combined with diethylaluminum chloride
(DEAC) by Natta in 1954 [3, 4]. This so-called first generation catalyst enabled
the first catalytic isoselective propylene polymerization, but its poor activity and
isospecificity necessitated additional processes to extract poorly isotactic products
and violet catalyst residues from the obtained polymer. Significant efforts were then
devoted to improving the activity and isospecificity of the catalyst. For the catalyst
activity, there were two main directions of study in order to enhance the utilization
efficiency of the Ti species: preparation of TiCl 3 with larger surface area and the
search for an efficient support material for Ti halide species. The Solvay corporation invented the so-called Solvay-type TiCl 3 in the early 1970s, which was
prepared by the reduction of TiCl 4 with DEAC followed by the removal of
Al residues with the aid of ether [5]. The resultant catalyst, regarded as a second
generation catalyst, achieved improved activity and isospecificity over the first
generation of catalysts, but the level of the improvements was still insufficient to
eliminate the above-mentioned purification processes for the obtained polymer.
Regarding a support material, metal oxide (SiO 2 , Al 2 O 3 ) or hydroxide materials
[Mg(OH) 2 ] were initially considered due to the ease of the immobilization of Ti
species through covalent bonds. However, successful improvement in activity was
not achieved until Montedison and Mitsui discovered MgCl 2 support, almost at the
same time in 1968 [6, 7]. The catalysts, consisting of TiCl 4 active site precursor,
MgCl 2 support, and triethylaluminum (TEA) activator, exhibited much higher
activities than the former generation of catalysts, but their use was limited to
ethylene polymerization due to poor isospecificity. That is the story before the
appearance of “donors,” i.e., the main topic of this chapter.
The first donors in Ziegler–Natta catalysis appeared as a result of collaborative
efforts between Montedison and Mitsui to improve the isospecificity of the abovementioned MgCl 2 -supported catalyst [8, 9]. The developed catalyst, termed third
generation, achieved not only high activity but also high isospecificity by adding
benzoate to the TiCl 4 /MgCl 2 catalyst. The term “donor” originates from the
fact that additives to improve the catalyst isospecificity are Lewis bases with
82
T. Taniike and M. Terano