They are resistant to polar solvents and chemicals and can be melt-processed. Due to
their high carbon/hydrogen ratio, these polymers have a high refractive index (1.53
for an ethene/norbornene copolymer at 50 mol% incorporation). Their stability
against hydrolysis and chemical degradation, in combination with their stiffness,
makes them interesting materials for optical applications, for example in compact
discs, lenses, optical fibers, or films [105]. Meanwhile, ethene/norbornene COC
material is commercially available under the name of TOPAS (Ticona, Celanese).
Details of the polymerization of cyclic olefins with single-site catalysts can be found
in the contribution by Boggioni and Tritto [106].
3.3 Polyolefin Nanocomposites
In the last few years, a lot of research in academic and industrial laboratories has
focused on polyolefin nanocomposites because of their high potential as materials
with novel properties [96]. The properties of the nanocomposites are not only
influenced by the kind of filler but also by the microstructure of the polyolefins
and the preparation process. Nanofillers commonly used are metal oxides, sulfides,
silica and layered silica as well as fibers such as carbon nanotubes (CNT), carbon
nanofibers (CNF), and polymer fibers [107–111].
In the past, most composites have been prepared by mechanical blending of the
particles or fibers above the melting temperature of the desired matrix. Melt
compounding of polyolefins with nanoparticles often leads to an insufficient filler
dispersion, especially at a high filler content, which leads to aggregation and
intercalation, which in turn causes a deterioration of the mechanical properties.
Such disadvantages can be solved by in-situ polymerization, whereby the cocatalyst
(e.g. MAO) can be adsorbed or anchored on the surface of nanofillers such as
particles, fibers, CNF, or multi-walled carbon nanotubes (MWCNT), thus changing
the surface to a hydrophobic one [112, 113]. The MAO reacts, for example, with the
OH groups of silica or with the carboxy groups of oxidized CNTs or is physically
absorbed at the surface (Fig. 13). The chemical reaction of MAO with polar groups
results in the formation of methane. Excess MAO is washed out.
In a second step, the metallocene is added and forms catalytically active
polymerization sites on the nanosurface. The thickness of the polymer films,
formed by addition of ethene or propene, depends on the polymerization conditions,
especially the polymerization time, the kind of metallocene catalyst, and the
pressure of the monomer. The in-situ polymerization leads to composite materials
in which the particles or fibers are intensively covered with the polymer.
Metallocene/ MAO and other single-site catalysts are soluble in hydrocarbons
and therefore can be perfectly absorbed on the surface of particles and fibers or they
can penetrate the layers of layered silicates and oxides. For a detailed description of
polyolefin nanoparticles and layered silicate nanocomposites see [114–116].
The composite materials show, for example, an improved stiffness with a
negligible loss of impact strength, high gas barrier properties, significant flame
retardant properties, better clarity and gloss, as well as high crystallization rates.
Methylaluminoxane: Key Component for New Polymerization Catalysts
19
their high carbon/hydrogen ratio, these polymers have a high refractive index (1.53
for an ethene/norbornene copolymer at 50 mol% incorporation). Their stability
against hydrolysis and chemical degradation, in combination with their stiffness,
makes them interesting materials for optical applications, for example in compact
discs, lenses, optical fibers, or films [105]. Meanwhile, ethene/norbornene COC
material is commercially available under the name of TOPAS (Ticona, Celanese).
Details of the polymerization of cyclic olefins with single-site catalysts can be found
in the contribution by Boggioni and Tritto [106].
3.3 Polyolefin Nanocomposites
In the last few years, a lot of research in academic and industrial laboratories has
focused on polyolefin nanocomposites because of their high potential as materials
with novel properties [96]. The properties of the nanocomposites are not only
influenced by the kind of filler but also by the microstructure of the polyolefins
and the preparation process. Nanofillers commonly used are metal oxides, sulfides,
silica and layered silica as well as fibers such as carbon nanotubes (CNT), carbon
nanofibers (CNF), and polymer fibers [107–111].
In the past, most composites have been prepared by mechanical blending of the
particles or fibers above the melting temperature of the desired matrix. Melt
compounding of polyolefins with nanoparticles often leads to an insufficient filler
dispersion, especially at a high filler content, which leads to aggregation and
intercalation, which in turn causes a deterioration of the mechanical properties.
Such disadvantages can be solved by in-situ polymerization, whereby the cocatalyst
(e.g. MAO) can be adsorbed or anchored on the surface of nanofillers such as
particles, fibers, CNF, or multi-walled carbon nanotubes (MWCNT), thus changing
the surface to a hydrophobic one [112, 113]. The MAO reacts, for example, with the
OH groups of silica or with the carboxy groups of oxidized CNTs or is physically
absorbed at the surface (Fig. 13). The chemical reaction of MAO with polar groups
results in the formation of methane. Excess MAO is washed out.
In a second step, the metallocene is added and forms catalytically active
polymerization sites on the nanosurface. The thickness of the polymer films,
formed by addition of ethene or propene, depends on the polymerization conditions,
especially the polymerization time, the kind of metallocene catalyst, and the
pressure of the monomer. The in-situ polymerization leads to composite materials
in which the particles or fibers are intensively covered with the polymer.
Metallocene/ MAO and other single-site catalysts are soluble in hydrocarbons
and therefore can be perfectly absorbed on the surface of particles and fibers or they
can penetrate the layers of layered silicates and oxides. For a detailed description of
polyolefin nanoparticles and layered silicate nanocomposites see [114–116].
The composite materials show, for example, an improved stiffness with a
negligible loss of impact strength, high gas barrier properties, significant flame
retardant properties, better clarity and gloss, as well as high crystallization rates.
Methylaluminoxane: Key Component for New Polymerization Catalysts
19