of actual growing chains during quenching. The quenching reaction is fast and is
expected to compete effectively with deactivation processes during the polymerization of ethylene. Homogeneous BI
Me2 P FeCl 2 activated with triisobutyl aluminum (TIBA), a catalyst system with a fast activation and decay of the catalytic
activity (minutes at 35
C), was evaluated in this way. The rate constant was
evaluated by assuming a rate law v ¼ k p [C 2 H 4 ][Fe*], where [Fe*] is the concentration of active sites. It was found that near the maximum rate (reached after about
2 min), 41% of the iron centers are catalytically active. This number decreases to
16% after 8 min. The rate constant for propagation near the maximum was
26,000 L mol
À1 s
À1 . BI
Me2 P FeCl 2 activated with MAO has k p values in the same
range [46]. The number of iron centers carrying an alkyl group is in this case less
than 10% in the first 1.5 min and then decreases to 4% after 9 min. In contrast, it was
found for BI
Me2 P FeCl 2 supported on Al 2 O 3 and activated with TIBA that only
about 1% of the iron centers carry an alkyl chain [61]. The rate constant k p was
determined to be 7,000 L mol
À1 s
À1 . This number increases to 18,000 L mol
À1 s
À1
when 5 vol% of dihydrogen is in the feed. The increase is interpreted as a reactivation of dormant species after 2,1-insertion of an in situ formed 1-olefin. A similar
rate constant of 12,000–45,000 L mol
À1 s
À1 was measured in supported iron
catalysts [64] (see chapter 2.4).
2.4 Ethylene Polymerization with Supported BIP FeCl 2
The catalytic properties of bis(imino)pyridyl iron catalysts (i.e., in the sense of
activity, productivity, and thermal stability) can be enhanced by immobilizing the
complex on inorganic materials. Thermal stability is of high relevance because
many systems lose their initial activity within a few minutes at temperatures over
50
C [65, 66]. Support is also essential for preventing reactor fouling [16]. The BIP
FeCl 2 catalysts are particularly easily heterogenized. Alumina, silica, and magnesium dichloride have been considered as standard supporting materials, and aluminum alkyls can be used to activate the catalyst and to influence the molecular
weight and molecular weight distribution.
Supporting BI
Me2 P FeCl 2 on silica or alumina yields active polymerization
catalysts after activation with TIBA or MAO. DRIFT analysis of the surfaces of
the supported catalyst gives an indication that the iron complexes directly interact
with hydroxide entities at the surface; interaction with the basic alumina is particularly prominent [67]. The productivity of the system after the addition of TIBA is in
the range of 200–760 kg PE g Fe
À1 in the supported catalyst at 5 bar of ethylene
pressure. The activity of 41 kg PE g Fe
À1 bar
À1 h
À1 is reached more slowly, as in the
homogenous case, but remains constant over 60 min (Fig. 4). The addition of
hydrogen at a pressure of 0.5 bar increases the activity and productivity by a factor
of about 2. The stability of the catalytic system towards deactivation is favorably
improved. Thus, higher polymerization temperatures of 80
C, with almost constant
activity over at least 1 h are now feasible.
Iron Catalyst in the Preparation of Polyolefin Composites
351
expected to compete effectively with deactivation processes during the polymerization of ethylene. Homogeneous BI
Me2 P FeCl 2 activated with triisobutyl aluminum (TIBA), a catalyst system with a fast activation and decay of the catalytic
activity (minutes at 35
C), was evaluated in this way. The rate constant was
evaluated by assuming a rate law v ¼ k p [C 2 H 4 ][Fe*], where [Fe*] is the concentration of active sites. It was found that near the maximum rate (reached after about
2 min), 41% of the iron centers are catalytically active. This number decreases to
16% after 8 min. The rate constant for propagation near the maximum was
26,000 L mol
À1 s
À1 . BI
Me2 P FeCl 2 activated with MAO has k p values in the same
range [46]. The number of iron centers carrying an alkyl group is in this case less
than 10% in the first 1.5 min and then decreases to 4% after 9 min. In contrast, it was
found for BI
Me2 P FeCl 2 supported on Al 2 O 3 and activated with TIBA that only
about 1% of the iron centers carry an alkyl chain [61]. The rate constant k p was
determined to be 7,000 L mol
À1 s
À1 . This number increases to 18,000 L mol
À1 s
À1
when 5 vol% of dihydrogen is in the feed. The increase is interpreted as a reactivation of dormant species after 2,1-insertion of an in situ formed 1-olefin. A similar
rate constant of 12,000–45,000 L mol
À1 s
À1 was measured in supported iron
catalysts [64] (see chapter 2.4).
2.4 Ethylene Polymerization with Supported BIP FeCl 2
The catalytic properties of bis(imino)pyridyl iron catalysts (i.e., in the sense of
activity, productivity, and thermal stability) can be enhanced by immobilizing the
complex on inorganic materials. Thermal stability is of high relevance because
many systems lose their initial activity within a few minutes at temperatures over
50
C [65, 66]. Support is also essential for preventing reactor fouling [16]. The BIP
FeCl 2 catalysts are particularly easily heterogenized. Alumina, silica, and magnesium dichloride have been considered as standard supporting materials, and aluminum alkyls can be used to activate the catalyst and to influence the molecular
weight and molecular weight distribution.
Supporting BI
Me2 P FeCl 2 on silica or alumina yields active polymerization
catalysts after activation with TIBA or MAO. DRIFT analysis of the surfaces of
the supported catalyst gives an indication that the iron complexes directly interact
with hydroxide entities at the surface; interaction with the basic alumina is particularly prominent [67]. The productivity of the system after the addition of TIBA is in
the range of 200–760 kg PE g Fe
À1 in the supported catalyst at 5 bar of ethylene
pressure. The activity of 41 kg PE g Fe
À1 bar
À1 h
À1 is reached more slowly, as in the
homogenous case, but remains constant over 60 min (Fig. 4). The addition of
hydrogen at a pressure of 0.5 bar increases the activity and productivity by a factor
of about 2. The stability of the catalytic system towards deactivation is favorably
improved. Thus, higher polymerization temperatures of 80
C, with almost constant
activity over at least 1 h are now feasible.
Iron Catalyst in the Preparation of Polyolefin Composites
351