Other explanations are side reactions such as deactivation of MAO with the -
metallocene-like α-hydrogen transfer reactions or that only a part of MAO with
a special structure is suitable for the activation. Knowing that the bulky structure
of MAO could be necessary for the activation of metallocene catalysts, other
bulky and weakly coordinating cocatalysts such as tris(pentafluorophenyl)borane
or organic salts of the non-coordinating tetrakis(pentafluorophenyl)borate
[(C 6 F 5 ) 4 B]¯, and aluminum fluorides were introduced by Marks [48, 49] and others
[50–53]. With these cocatalysts, a metallocene/cocatalyst ratio of 1:1 is used, but
only if a high excess of an aluminum alkyl as scavenger is present. Details of the
polymerization using other cocatalysts are described by Shiono [54].
Meanwhile, the polyolefin industries used MAO-containing catalysts on a large
scale. Companies such as Albermale, Akzo, Chemtura, and Mitsui produce
hundreds of tonnes of MAO by reaction of water or ice with TMA and in some
cases add other aluminum alkyls to increase the solubility.
3 New Polyolefins
3.1 Homopolymers
Metallocenes, especially zirconocenes but also titanocenes, hafnocenes, and other
transition metal complexes treated with MAO are highly active for the polymerization of olefins, diolefins, and styrene. The polymerization activity, which is up to
100 times higher than for classical Ziegler catalysts, as well as the possibility to
easily tailor the microstructure of the polymer chain and to obtain polymers with
special properties have motivated research groups worldwide to produce thousands
of patents and publications in the last 20 years. An overview can be found in
selected review articles and books [55–68]. A metallocene/MAO catalyst
containing 1 g zirconium produced 40 Â 10
6 g polyethylene in 1 h at 95
C and
8 bar ethene pressure (Table 1).
Nearly every zirconium atom forms an active center as shown by Tait [69] and
Chien [70] and produces about 46,000 polymer chains per hour. The insertion time
of one ethene unit is only 3 Â 10
À5 s. For the first time, it could be shown that a
soluble catalyst such as Cp 2 ZrCl 2 /MAO is able to produce polyethylene with high
molecular weights and a narrow molecular weight distribution of approximately
two. All active sites are similar and form polymers with the same average chain
length (single-site catalysts). Only traces of low molecular weight oligomers are
formed.
Zirconocenes with different symmetries and substitutions are shown in Fig. 6.
There are a great variety of structures of metallocenes that can be used for the
polymerization. The cyclopentadienyl, indenyl, and fluorenyl ligands can be
hydrated or substituted by alkyl, aryl, methoxy, siloxy, or other groups. Ethanediyl
(C 2 H 4 ), dimethylsilandiyl [(CH 3 ) 2 Si], or isopropandiyl [(CH 3 ) 2 C] are mainly used
as interannular bridges between the rings. Central metals could be Ti, Zr, or
Methylaluminoxane: Key Component for New Polymerization Catalysts
9
metallocene-like α-hydrogen transfer reactions or that only a part of MAO with
a special structure is suitable for the activation. Knowing that the bulky structure
of MAO could be necessary for the activation of metallocene catalysts, other
bulky and weakly coordinating cocatalysts such as tris(pentafluorophenyl)borane
or organic salts of the non-coordinating tetrakis(pentafluorophenyl)borate
[(C 6 F 5 ) 4 B]¯, and aluminum fluorides were introduced by Marks [48, 49] and others
[50–53]. With these cocatalysts, a metallocene/cocatalyst ratio of 1:1 is used, but
only if a high excess of an aluminum alkyl as scavenger is present. Details of the
polymerization using other cocatalysts are described by Shiono [54].
Meanwhile, the polyolefin industries used MAO-containing catalysts on a large
scale. Companies such as Albermale, Akzo, Chemtura, and Mitsui produce
hundreds of tonnes of MAO by reaction of water or ice with TMA and in some
cases add other aluminum alkyls to increase the solubility.
3 New Polyolefins
3.1 Homopolymers
Metallocenes, especially zirconocenes but also titanocenes, hafnocenes, and other
transition metal complexes treated with MAO are highly active for the polymerization of olefins, diolefins, and styrene. The polymerization activity, which is up to
100 times higher than for classical Ziegler catalysts, as well as the possibility to
easily tailor the microstructure of the polymer chain and to obtain polymers with
special properties have motivated research groups worldwide to produce thousands
of patents and publications in the last 20 years. An overview can be found in
selected review articles and books [55–68]. A metallocene/MAO catalyst
containing 1 g zirconium produced 40 Â 10
6 g polyethylene in 1 h at 95
C and
8 bar ethene pressure (Table 1).
Nearly every zirconium atom forms an active center as shown by Tait [69] and
Chien [70] and produces about 46,000 polymer chains per hour. The insertion time
of one ethene unit is only 3 Â 10
À5 s. For the first time, it could be shown that a
soluble catalyst such as Cp 2 ZrCl 2 /MAO is able to produce polyethylene with high
molecular weights and a narrow molecular weight distribution of approximately
two. All active sites are similar and form polymers with the same average chain
length (single-site catalysts). Only traces of low molecular weight oligomers are
formed.
Zirconocenes with different symmetries and substitutions are shown in Fig. 6.
There are a great variety of structures of metallocenes that can be used for the
polymerization. The cyclopentadienyl, indenyl, and fluorenyl ligands can be
hydrated or substituted by alkyl, aryl, methoxy, siloxy, or other groups. Ethanediyl
(C 2 H 4 ), dimethylsilandiyl [(CH 3 ) 2 Si], or isopropandiyl [(CH 3 ) 2 C] are mainly used
as interannular bridges between the rings. Central metals could be Ti, Zr, or
Methylaluminoxane: Key Component for New Polymerization Catalysts
9