Thus, the results of definition of the number of active centers for propylene
polymerization on ТМC, obtained by independent methods (SF and QR methods),
show the following:
1. The great increase in activity of ТМC in comparison with traditional ZN
catalysts is defined, mainly, by an increase in the number of active centers
2. The number of active centers is less than 10 mol% of the total titanium
content in the catalysts (usually in limits from 1% to 10%) and depends on the
composition and procedure of TMC preparation; most of the titanium is in the
inactive form and does not participate in polymerization
It has been found [80, 193] that a great increase in activity of ТМC in
ethylene polymerization (from 50–100 kg/g Ti h atm to 400–600 kg/g Ti h atm)
is observed with a decrease in titanium content in the catalyst from 1–4 wt% of Ti to
values less than 0.1 wt% of Ti. In this case, it is possible to expect that the number
of the active centers increases to 30–40 mol% of the total content of titanium [194].
The number of active centers at polymerization on ZN catalysts essentially
depends on the conditions of polymerization, in particular on the composition
of the organoaluminum cocatalyst and its concentration, the temperature of polymerization, and the presence of additional components added for regulation of
activity and the molecular weight of polymer.
Data on the effect of hydrogen on the number of the active centers and
propagation rate constants for polymerization of propylene and ethylene on ZN
catalysts of various compositions will be presented and discussed in the next section.
2.4 Number of the Active Centers and Propagation Rate
Constants in Polymerization in the Presence of Hydrogen
Hydrogen is the most effective chain transfer agent for olefin polymerization
on ZN catalysts and is widely used for polyolefin production in industry and
in laboratory research for control of molecular weight of polymers. The presence
of hydrogen during the polymerization influences the polymerization kinetics. In
the case of ethylene polymerization, addition of hydrogen into polymerization
usually decreases the polymerization rate. In work [195] for the first time it was
shown that addition of hydrogen during propylene polymerization on ZN catalysts
leads to an appreciable increase in polymerization rate. This phenomenon has
been confirmed in a number of other works [22, 89, 196–198]. To reveal the
reasons for the activating effect of hydrogen (the “hydrogen effect”), data on
the influence of hydrogen on the number of active centers and propagation rate
constants can be the useful. In work [199] it was found that in the presence of
hydrogen during propylene polymerization on ТМC the number of active centers
increases considerably (QR
14
СО method). However, according to data presented
in [184], the presence of hydrogen at a quasi-living polymerization for 0.2 s does
122
L.A. Novokshonova and V.A. Zakharov
polymerization on ТМC, obtained by independent methods (SF and QR methods),
show the following:
1. The great increase in activity of ТМC in comparison with traditional ZN
catalysts is defined, mainly, by an increase in the number of active centers
2. The number of active centers is less than 10 mol% of the total titanium
content in the catalysts (usually in limits from 1% to 10%) and depends on the
composition and procedure of TMC preparation; most of the titanium is in the
inactive form and does not participate in polymerization
It has been found [80, 193] that a great increase in activity of ТМC in
ethylene polymerization (from 50–100 kg/g Ti h atm to 400–600 kg/g Ti h atm)
is observed with a decrease in titanium content in the catalyst from 1–4 wt% of Ti to
values less than 0.1 wt% of Ti. In this case, it is possible to expect that the number
of the active centers increases to 30–40 mol% of the total content of titanium [194].
The number of active centers at polymerization on ZN catalysts essentially
depends on the conditions of polymerization, in particular on the composition
of the organoaluminum cocatalyst and its concentration, the temperature of polymerization, and the presence of additional components added for regulation of
activity and the molecular weight of polymer.
Data on the effect of hydrogen on the number of the active centers and
propagation rate constants for polymerization of propylene and ethylene on ZN
catalysts of various compositions will be presented and discussed in the next section.
2.4 Number of the Active Centers and Propagation Rate
Constants in Polymerization in the Presence of Hydrogen
Hydrogen is the most effective chain transfer agent for olefin polymerization
on ZN catalysts and is widely used for polyolefin production in industry and
in laboratory research for control of molecular weight of polymers. The presence
of hydrogen during the polymerization influences the polymerization kinetics. In
the case of ethylene polymerization, addition of hydrogen into polymerization
usually decreases the polymerization rate. In work [195] for the first time it was
shown that addition of hydrogen during propylene polymerization on ZN catalysts
leads to an appreciable increase in polymerization rate. This phenomenon has
been confirmed in a number of other works [22, 89, 196–198]. To reveal the
reasons for the activating effect of hydrogen (the “hydrogen effect”), data on
the influence of hydrogen on the number of active centers and propagation rate
constants can be the useful. In work [199] it was found that in the presence of
hydrogen during propylene polymerization on ТМC the number of active centers
increases considerably (QR
14
СО method). However, according to data presented
in [184], the presence of hydrogen at a quasi-living polymerization for 0.2 s does
122
L.A. Novokshonova and V.A. Zakharov