Data on the number of active centers and propagation rate constants for olefin
polymerization on traditional ZN catalysts and highly active supported ZN catalysts,
obtained by use of SF and QR methods, will be presented and discussed below.
2.3 Number of Active Centers in Ethylene and Propylene
Polymerization on Traditional ZN Catalysts and
Supported Titanium–Magnesium Catalysts
It is known that the activity of catalysts changes very much depending on their
composition, preparation and activation methods, and polymerization conditions.
The most plausible reason for this is a change in the number of active centers.
From the data obtained using a QR RO
3 H method [155, 157, 161, 162] or
using labeled cocatalyst Al(
14 C 2 H 5 ) 3 [15, 146, 154] for ethylene and propylene
polymerization on TiCl 3 /AlR x Cl y catalysts, the number of active centers is
(1–10) Â 10
À2 mol/mol Ti. However, in work [156] it has been shown that the rate
of transfer reaction of polymer chain with AlR 3 decreases sharply with increasing
polymerization time (polymer yield). Taking into account this effect, it was found
[156] that the C p values for propylene polymerization on TiCl 3 /AlEt 2 Cl (AlEt 3 )
catalyst are lower and only (0.1–0.3) Â 10
À2 mol/mol Ti. This value is close
to the number of active centers, (0.1–0.5) Â 10
À2 mol/mol Ti, found later with
use of the QR
14
СО method for ethylene and propylene polymerization on similar
catalysts, TiCl 3 /AlEt 2 Cl (AlEt 3 , AliBu) 3 [21, 165, 166, 174, 175]. Thus, the number
of active centers for traditional ZN catalysts based on TiCl 3 is very low and,
according to works [21, 59, 166, 175] (QR methods), makes up no more than
0.8 mol% of the total content of Ti in catalysts. By estimation [174], this value
can correspond to the number of titanium chlorides located on the lateral sides
of crystals, making microparticles of δ-TiCl 3 with a surface of 70–100 m
2
/g.
A great increase in activity of ZN catalysts is reached by supporting TiCl 4
on “activated” magnesium dichloride, having a high surface and disordered
crystal structure (supported TMC catalysts) [176, 177]. The activity of these
catalysts in ethylene and propylene polymerization is usually 30–150 kg/g Ti h atm,
which exceeds the activity of traditional ZN catalysts based on δ-TiCl 3 by approximately two orders of magnitude. According to works [74, 165, 173, 178–180]
(QR
14
СО method) and works [181, 182] (QR RO
3 H method), such an increase
in activity is connected with a great increase in the number of active centers,
which for these catalysts is 1–7 mol% of the total Ti content. Further, in works
[153, 183–192] with use of the SF method in the case of propylene polymerization
on ТМC, C p values of the same order (0.5–11 mol%) have been obtained. It is
necessary to notice that the number of active centers changes considerably with
variation in the composition and conditions of catalyst preparation. For example, in
work [183] for three types of ТМC with different activities, C p values from 0.8 to
9.9 mol% were obtained by the SF method.
Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler. . .
121
polymerization on traditional ZN catalysts and highly active supported ZN catalysts,
obtained by use of SF and QR methods, will be presented and discussed below.
2.3 Number of Active Centers in Ethylene and Propylene
Polymerization on Traditional ZN Catalysts and
Supported Titanium–Magnesium Catalysts
It is known that the activity of catalysts changes very much depending on their
composition, preparation and activation methods, and polymerization conditions.
The most plausible reason for this is a change in the number of active centers.
From the data obtained using a QR RO
3 H method [155, 157, 161, 162] or
using labeled cocatalyst Al(
14 C 2 H 5 ) 3 [15, 146, 154] for ethylene and propylene
polymerization on TiCl 3 /AlR x Cl y catalysts, the number of active centers is
(1–10) Â 10
À2 mol/mol Ti. However, in work [156] it has been shown that the rate
of transfer reaction of polymer chain with AlR 3 decreases sharply with increasing
polymerization time (polymer yield). Taking into account this effect, it was found
[156] that the C p values for propylene polymerization on TiCl 3 /AlEt 2 Cl (AlEt 3 )
catalyst are lower and only (0.1–0.3) Â 10
À2 mol/mol Ti. This value is close
to the number of active centers, (0.1–0.5) Â 10
À2 mol/mol Ti, found later with
use of the QR
14
СО method for ethylene and propylene polymerization on similar
catalysts, TiCl 3 /AlEt 2 Cl (AlEt 3 , AliBu) 3 [21, 165, 166, 174, 175]. Thus, the number
of active centers for traditional ZN catalysts based on TiCl 3 is very low and,
according to works [21, 59, 166, 175] (QR methods), makes up no more than
0.8 mol% of the total content of Ti in catalysts. By estimation [174], this value
can correspond to the number of titanium chlorides located on the lateral sides
of crystals, making microparticles of δ-TiCl 3 with a surface of 70–100 m
2
/g.
A great increase in activity of ZN catalysts is reached by supporting TiCl 4
on “activated” magnesium dichloride, having a high surface and disordered
crystal structure (supported TMC catalysts) [176, 177]. The activity of these
catalysts in ethylene and propylene polymerization is usually 30–150 kg/g Ti h atm,
which exceeds the activity of traditional ZN catalysts based on δ-TiCl 3 by approximately two orders of magnitude. According to works [74, 165, 173, 178–180]
(QR
14
СО method) and works [181, 182] (QR RO
3 H method), such an increase
in activity is connected with a great increase in the number of active centers,
which for these catalysts is 1–7 mol% of the total Ti content. Further, in works
[153, 183–192] with use of the SF method in the case of propylene polymerization
on ТМC, C p values of the same order (0.5–11 mol%) have been obtained. It is
necessary to notice that the number of active centers changes considerably with
variation in the composition and conditions of catalyst preparation. For example, in
work [183] for three types of ТМC with different activities, C p values from 0.8 to
9.9 mol% were obtained by the SF method.
Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler. . .
121