of prepolymerization, exotherms of up to 20
C have been measured for individual catalyst particles in the early stages of polymerization [38]. Furthermore,
the catalyst particles have to be mechanically stable to avoid disintegration and
formation of polymer fines; however, they have to be sufficiently fragile to permit
fragmentation into primary particles of submicrometer size by the hydraulic forces
of the growing polymer. The morphology of the starting support is replicated
in the final polymer so that a spherical support in the size range of 50–100 μm
will give a spherical polymer morphology with particle sizes in the range
of 1,000–3,000 μm, depending on the catalyst productivity. In this way, each
polymerizing particle can be considered as a single microreactor with its own
mass and heat balance [39].
For instance, the kinetics of a propylene polymerization promoted by SiO 2 -
supported metallocene catalysts depends on various factors: the applied reaction
engineering (gas-phase, bulk, or slurry-polymerization), the degree of catalyst/
cocatalyst distribution on the support, and the reaction conditions and parameters
chosen.
High polymerization temperature and concentration of active species on the
support lead to an increase in polymerization activity, as does a high monomer
concentration in the reacting solution. A detailed kinetic investigation, however,
is facilitated by choosing especially mild reaction conditions (low temperature, low
catalyst concentration, low monomer concentration). In doing so [40, 41, 44, 45],
it is possible to resolve the individual phases of polymerization and polymer
growth from the start of the reaction. The polymerization rate–time plot (Fig 23a)
shows a course that is characteristic for these systems. The reaction starts with
a short increase in activity, the “prepolymerization period,” followed by a drop
in the polymerization rate to almost zero. The low level is kept for some
minutes; in the case of supported metallocene catalysts, the length of this
“induction period” can vary distinctively. After the induction period, the activity
rises again (“polymer growth”) until a plateau of maximum activity is reached
(“particle expansion”).
These individual kinetic stages of the propylene polymerization can be
interpreted as follows. During the prepolymerization stage, the polymer forms a
regular thin layer around the particle, which partially continues to grow into the
marginal areas of the micro- and mesoporous silica gel (Fig. 23b). The layer of
highly crystalline polypropylene (up to 75%) serves as a diffusion barrier for
following propylene and induces the period of very low activity.
After this diffusion phase, successively active centers in the inner part of
the particles are also provided with monomer and polymer growth from the
outside to the inside continues. Because of the hydraulic forces from the growing
polymer, fragmentation of the SiO 2 support from the surface to the interior occurs
(to consider as “shell-by-shell” or “layer-by-layer” fragmentation). Consequently,
new active centers are released and the overall polymerization rate increases
until the highest possibly activity is reached and the whole support is fragmented
in the polymer.
Contributions to the Ziegler–Natta Catalysis: An Anthology
27
C have been measured for individual catalyst particles in the early stages of polymerization [38]. Furthermore,
the catalyst particles have to be mechanically stable to avoid disintegration and
formation of polymer fines; however, they have to be sufficiently fragile to permit
fragmentation into primary particles of submicrometer size by the hydraulic forces
of the growing polymer. The morphology of the starting support is replicated
in the final polymer so that a spherical support in the size range of 50–100 μm
will give a spherical polymer morphology with particle sizes in the range
of 1,000–3,000 μm, depending on the catalyst productivity. In this way, each
polymerizing particle can be considered as a single microreactor with its own
mass and heat balance [39].
For instance, the kinetics of a propylene polymerization promoted by SiO 2 -
supported metallocene catalysts depends on various factors: the applied reaction
engineering (gas-phase, bulk, or slurry-polymerization), the degree of catalyst/
cocatalyst distribution on the support, and the reaction conditions and parameters
chosen.
High polymerization temperature and concentration of active species on the
support lead to an increase in polymerization activity, as does a high monomer
concentration in the reacting solution. A detailed kinetic investigation, however,
is facilitated by choosing especially mild reaction conditions (low temperature, low
catalyst concentration, low monomer concentration). In doing so [40, 41, 44, 45],
it is possible to resolve the individual phases of polymerization and polymer
growth from the start of the reaction. The polymerization rate–time plot (Fig 23a)
shows a course that is characteristic for these systems. The reaction starts with
a short increase in activity, the “prepolymerization period,” followed by a drop
in the polymerization rate to almost zero. The low level is kept for some
minutes; in the case of supported metallocene catalysts, the length of this
“induction period” can vary distinctively. After the induction period, the activity
rises again (“polymer growth”) until a plateau of maximum activity is reached
(“particle expansion”).
These individual kinetic stages of the propylene polymerization can be
interpreted as follows. During the prepolymerization stage, the polymer forms a
regular thin layer around the particle, which partially continues to grow into the
marginal areas of the micro- and mesoporous silica gel (Fig. 23b). The layer of
highly crystalline polypropylene (up to 75%) serves as a diffusion barrier for
following propylene and induces the period of very low activity.
After this diffusion phase, successively active centers in the inner part of
the particles are also provided with monomer and polymer growth from the
outside to the inside continues. Because of the hydraulic forces from the growing
polymer, fragmentation of the SiO 2 support from the surface to the interior occurs
(to consider as “shell-by-shell” or “layer-by-layer” fragmentation). Consequently,
new active centers are released and the overall polymerization rate increases
until the highest possibly activity is reached and the whole support is fragmented
in the polymer.
Contributions to the Ziegler–Natta Catalysis: An Anthology
27