active centers depends on the composition and conditions of preparation of the
catalysts and on the conditions of polymerization and is usually a small fraction
of the total number of surface transition metal compounds in the catalyst. Thus,
the rate of polymerization, which is equal to the rate of propagation of the olefin
polymer chain, is defined by a simple equation (1).
The problem of determination of the number of active centers as the number
of titanium–polymer bonds was formulated for the first time in the pioneering
works of Natta [15, 146] for propylene polymerization on catalyst TiCl 3 þ AlEt 3 .
He used for this purpose cocatalyst AlEt 3 labeled by a radioactive isotope
14
С in
the ethyl group. Active centers of type Cl x Ti–
14
CH 2 CH 3 are formed in this case
and polymer with a radioactive label obtained on these centers. The number of
active centers can be calculated from these data.
In this review, data on the number of active centers and values of propagation
rate constants (k р ) for olefin polymerization on traditional ZN catalysts (mainly
TiCl 3 /AlEt x Cl y ) and highly active supported ZN catalysts containing titanium
chlorides on activated magnesium dichloride will be presented.
Different methods for determination of the number of active centers during
catalytic olefin polymerization are proposed. There are two basic groups of
method applied to determine the kinetic characteristics of propagation reactions
and transfer reactions of polymer chains (values of C p , k p , and K tr ) for catalytic
olefin polymerization.
1. Methods based on the analysis of dependences of polymerization rate and
molecular weight on the time and conditions of polymerization. We will name
them conditionally “kinetic methods.”
2. Methods based on introduction of a radioactive label in the growing polymer
molecule.
2.1 Kinetic Methods
The method of calculation of rate constants of the separate reactions proceeding
in the course of polymerization and based on the analysis of dependences of
polymerization rate (R p ) and polymerization degree (P n ) on the concentration
of reagents was used for the first time in the works of Natta [15, 146] and Chirkov
[28, 29]. This method allows calculation of values for the products of rate constants
of separate reactions and the number of active centers, but does not allow calculation of separate values for rate constants and the number of active centers.
Another kinetic method has been proposed [17]. This method allows one to
determine the k p value and the number of active sites using the dependence on
monomer concentration of the stationary polymerization rate and of the polymerization rate during the acceleration period of the kinetic curve, according to Eqs. (2)
and (6). The method has been applied for determination of these kinetic parameters
in the polymerization of propylene and ethylene with VCl 3 /Al(i-Bu) 3 catalyst.
Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler. . .
117
catalysts and on the conditions of polymerization and is usually a small fraction
of the total number of surface transition metal compounds in the catalyst. Thus,
the rate of polymerization, which is equal to the rate of propagation of the olefin
polymer chain, is defined by a simple equation (1).
The problem of determination of the number of active centers as the number
of titanium–polymer bonds was formulated for the first time in the pioneering
works of Natta [15, 146] for propylene polymerization on catalyst TiCl 3 þ AlEt 3 .
He used for this purpose cocatalyst AlEt 3 labeled by a radioactive isotope
14
С in
the ethyl group. Active centers of type Cl x Ti–
14
CH 2 CH 3 are formed in this case
and polymer with a radioactive label obtained on these centers. The number of
active centers can be calculated from these data.
In this review, data on the number of active centers and values of propagation
rate constants (k р ) for olefin polymerization on traditional ZN catalysts (mainly
TiCl 3 /AlEt x Cl y ) and highly active supported ZN catalysts containing titanium
chlorides on activated magnesium dichloride will be presented.
Different methods for determination of the number of active centers during
catalytic olefin polymerization are proposed. There are two basic groups of
method applied to determine the kinetic characteristics of propagation reactions
and transfer reactions of polymer chains (values of C p , k p , and K tr ) for catalytic
olefin polymerization.
1. Methods based on the analysis of dependences of polymerization rate and
molecular weight on the time and conditions of polymerization. We will name
them conditionally “kinetic methods.”
2. Methods based on introduction of a radioactive label in the growing polymer
molecule.
2.1 Kinetic Methods
The method of calculation of rate constants of the separate reactions proceeding
in the course of polymerization and based on the analysis of dependences of
polymerization rate (R p ) and polymerization degree (P n ) on the concentration
of reagents was used for the first time in the works of Natta [15, 146] and Chirkov
[28, 29]. This method allows calculation of values for the products of rate constants
of separate reactions and the number of active centers, but does not allow calculation of separate values for rate constants and the number of active centers.
Another kinetic method has been proposed [17]. This method allows one to
determine the k p value and the number of active sites using the dependence on
monomer concentration of the stationary polymerization rate and of the polymerization rate during the acceleration period of the kinetic curve, according to Eqs. (2)
and (6). The method has been applied for determination of these kinetic parameters
in the polymerization of propylene and ethylene with VCl 3 /Al(i-Bu) 3 catalyst.
Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler. . .
117