in propene polymerization, by hydrogen oxidative addition [94]; indeed, the
concentration of active sites is noticeably increased in the presence of hydrogen
[87, 95] due to hydrogenation of inactive Ti-allyl centers [95].
Thus, the mechanism of the activating action of hydrogen in propene polymerization, as well as the mechanism of reduction of catalyst activity in ethene polymerization in the presence of hydrogen, are still not completely clear and
need further confirmation.
1.7 Heterogeneity of Active Centers
The distinctive property of heterogeneous ZN catalysts is a heterogeneity of
active centers. This influences the kinetics of olefin polymerization and the
characteristics of the polymers obtained. The multicenter nature of heterogeneous
catalysts manifests itself in broadening of molecular weight distribution (MWD),
formation of polymer fractions of different stereoregularity, compositional nonuniformity of copolymers of ethene and propene with α-olefins, and the complicated order of catalyst deactivation reactions. Non-uniformity of active sites is
related, obviously, to the chemical, structural, and energy non-uniformity of the
catalyst surface. The presence on the catalyst surface of active centers of various
types, differing in the magnitude of propagation rate constants, has been reported
in a number of publications [18, 96–98]. This fact has been proposed [99] as a
cause of MWD broadening. Floyd et al. [100] found that the unimodal MWD
curves for polypropylene obtained with heterogeneous ZN catalysts can be
simulated only by assuming the presence of at least three or four types of active
sites, each of which follows Flory’s most probable distribution. The MWD of the
polymer depends on the type of heterogeneous catalyst, nature of the donors in
MgCl 2 -supported catalysts, comonomer presence, and the conditions of polymerization.
For investigation of active site non-uniformity, a method was proposed [101, 102]
based on mass-spectrometric study of temperature-programmed desorption (TPD)
products from the catalyst surface at the most initial stage of olefin polymerization
(up to 10–15 monomer units in chain). The method allows one to obtain information
concerning the energy non-uniformity of active sites in terms of a distribution of
active sites over the activation energy of active Mt–C bond thermal destruction in
active sites. So, it was shown for ethene polymerization with SiO 2 /TiCl 4 /AlEt 2 Cl
and SiO 2 /AlEt 2 Cl/TiCl 4 catalysts that there are at least two groups of active sites
in these catalysts, varying in the activation energy of thermal destruction of active
Ti–C bonds (Fig. 10), and that the distribution depends on the catalyst type.
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L.A. Novokshonova and V.A. Zakharov
concentration of active sites is noticeably increased in the presence of hydrogen
[87, 95] due to hydrogenation of inactive Ti-allyl centers [95].
Thus, the mechanism of the activating action of hydrogen in propene polymerization, as well as the mechanism of reduction of catalyst activity in ethene polymerization in the presence of hydrogen, are still not completely clear and
need further confirmation.
1.7 Heterogeneity of Active Centers
The distinctive property of heterogeneous ZN catalysts is a heterogeneity of
active centers. This influences the kinetics of olefin polymerization and the
characteristics of the polymers obtained. The multicenter nature of heterogeneous
catalysts manifests itself in broadening of molecular weight distribution (MWD),
formation of polymer fractions of different stereoregularity, compositional nonuniformity of copolymers of ethene and propene with α-olefins, and the complicated order of catalyst deactivation reactions. Non-uniformity of active sites is
related, obviously, to the chemical, structural, and energy non-uniformity of the
catalyst surface. The presence on the catalyst surface of active centers of various
types, differing in the magnitude of propagation rate constants, has been reported
in a number of publications [18, 96–98]. This fact has been proposed [99] as a
cause of MWD broadening. Floyd et al. [100] found that the unimodal MWD
curves for polypropylene obtained with heterogeneous ZN catalysts can be
simulated only by assuming the presence of at least three or four types of active
sites, each of which follows Flory’s most probable distribution. The MWD of the
polymer depends on the type of heterogeneous catalyst, nature of the donors in
MgCl 2 -supported catalysts, comonomer presence, and the conditions of polymerization.
For investigation of active site non-uniformity, a method was proposed [101, 102]
based on mass-spectrometric study of temperature-programmed desorption (TPD)
products from the catalyst surface at the most initial stage of olefin polymerization
(up to 10–15 monomer units in chain). The method allows one to obtain information
concerning the energy non-uniformity of active sites in terms of a distribution of
active sites over the activation energy of active Mt–C bond thermal destruction in
active sites. So, it was shown for ethene polymerization with SiO 2 /TiCl 4 /AlEt 2 Cl
and SiO 2 /AlEt 2 Cl/TiCl 4 catalysts that there are at least two groups of active sites
in these catalysts, varying in the activation energy of thermal destruction of active
Ti–C bonds (Fig. 10), and that the distribution depends on the catalyst type.
112
L.A. Novokshonova and V.A. Zakharov