the centers C H (surface hydrides of titanium) are deactivated by interaction with
components of the catalytic system the same as and at propylene polymerization.
These reactions lead to a decrease in the number of active centers (С р ) and to a
decrease in ethylene polymerization rate. Thus, the values of the propagation rate
constant are the same for ethylene polymerization with hydrogen and without
hydrogen [201].
So, in the case of propylene polymerization in the presence of hydrogen, the
proportion of the centers participating in the propagation reaction (С р ) is essentially
higher than for polymerization without hydrogen, when some of the centers are
in a “sleeping” condition, C d . Therefore, k p values calculated for experiments
in the presence of hydrogen more precisely correspond to the real reactivity
of active centers in the propagation reaction. These values (see Tаble 7) of
(1.6–2.8)Â10
3 L/mol s at 70
С and (1.2–1.6)Â10
3 L/mol s at 40
С are close to
the k p values found for propylene polymerization on ТМC by the SF method,
(1.0–5.0)Â10
3 L/mol s at 30
С [149, 153, 183–185].
2.5 Heterogeneity of Active Centers of ZN Catalysts, Taking
into Account Data on the Distribution of Active Centers
on Propagation Rate Constants and Their
Stereospecificity
It is known that ZN catalysts are multisite catalytic systems containing active
centers that differ in their k p values and stereospecificity. This is apparent in
the formation of polymers with broad molecular mass distribution (MMD, M w /M n
> 2) and polyolefins containing separate fractions with varying stereoregularity.
Data on the reactivity (k p values) of separate groups of the active centers and their
transformations with the variation in the composition of catalysts and polymerization conditions have an undoubtedly important role in analysis of this phenomenon.
In the literature, various reasons for formation of polymers with broad MMD
on heterogeneous ZN catalysts are discussed. Convincing evidence has been
obtained using the SF method that the reason is heterogeneity of the active
centers on a surface of the catalyst [186]. In conditions of quasi-living polymerization there are no transfer reactions of the growing polymer chain and polymer
is formed on the surface of catalyst in very small quantities. This polymer
cannot cause diffusion restrictions, but nevertheless polymer with broad ММD
(M w /M n ¼ 3.2–4.3) is formed. The further increase in time of polymerization does
not influence the width of ММD (M w /M n ¼ 3.6).
In works [187, 188], using the SF method, the tendency for increased k p values
with increase in stereospecificity of the catalyst has been noted (Table 8,
experiments 1–3). It is supposed that an external donor mainly deactivates
Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler. . .
125
components of the catalytic system the same as and at propylene polymerization.
These reactions lead to a decrease in the number of active centers (С р ) and to a
decrease in ethylene polymerization rate. Thus, the values of the propagation rate
constant are the same for ethylene polymerization with hydrogen and without
hydrogen [201].
So, in the case of propylene polymerization in the presence of hydrogen, the
proportion of the centers participating in the propagation reaction (С р ) is essentially
higher than for polymerization without hydrogen, when some of the centers are
in a “sleeping” condition, C d . Therefore, k p values calculated for experiments
in the presence of hydrogen more precisely correspond to the real reactivity
of active centers in the propagation reaction. These values (see Tаble 7) of
(1.6–2.8)Â10
3 L/mol s at 70
С and (1.2–1.6)Â10
3 L/mol s at 40
С are close to
the k p values found for propylene polymerization on ТМC by the SF method,
(1.0–5.0)Â10
3 L/mol s at 30
С [149, 153, 183–185].
2.5 Heterogeneity of Active Centers of ZN Catalysts, Taking
into Account Data on the Distribution of Active Centers
on Propagation Rate Constants and Their
Stereospecificity
It is known that ZN catalysts are multisite catalytic systems containing active
centers that differ in their k p values and stereospecificity. This is apparent in
the formation of polymers with broad molecular mass distribution (MMD, M w /M n
> 2) and polyolefins containing separate fractions with varying stereoregularity.
Data on the reactivity (k p values) of separate groups of the active centers and their
transformations with the variation in the composition of catalysts and polymerization conditions have an undoubtedly important role in analysis of this phenomenon.
In the literature, various reasons for formation of polymers with broad MMD
on heterogeneous ZN catalysts are discussed. Convincing evidence has been
obtained using the SF method that the reason is heterogeneity of the active
centers on a surface of the catalyst [186]. In conditions of quasi-living polymerization there are no transfer reactions of the growing polymer chain and polymer
is formed on the surface of catalyst in very small quantities. This polymer
cannot cause diffusion restrictions, but nevertheless polymer with broad ММD
(M w /M n ¼ 3.2–4.3) is formed. The further increase in time of polymerization does
not influence the width of ММD (M w /M n ¼ 3.6).
In works [187, 188], using the SF method, the tendency for increased k p values
with increase in stereospecificity of the catalyst has been noted (Table 8,
experiments 1–3). It is supposed that an external donor mainly deactivates
Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler. . .
125