The performance of composites in mechanical, barrier, electrical, thermal, optical,
and tribological applications etc. is dependent on reaching a good dispersion and
adhesion of filler and matrix. Composites of polyolefins show the best mechanical
and rheological properties when a good and lasting dispersion is reached of the filler
or fillers in the matrix [5]. In many cases, the appreciable difference in polarity of
polyolefin matrix and inorganic filler means that there is challenge in obtaining
such a distribution. Several methods have been used for the preparation of
composites. These methods include melt blending in an extruder or kneader,
solution blending, and in situ polymerization or in situ particle formation [6]. The
results of mixing and blending vary in quality depending on the filler, its surface
treatment, and the procedures used (e.g., [7]). Although not established on a large
scale, in situ polymerization has been shown to give finely dispersed fillers in
polyolefins [8, 9]. In this process, the polymer is generated in the presence of the
filler. Prerequisite is the compatibility of the polymerizing catalyst system and
the filler. A lasting dispersion of filler in the matrix is reached in those cases
where the polymer is wrapped around the filler. It may be anticipated that the latter
can be reached if the polymer is formed on or in close vicinity to the surface.
An active polymerization system in the presence of a generic filler (clays/minerals,
glass, carbon nanotubes, graphene) can be reached under the same requirements that
lead to an active supported catalyst [8–16]. It has been reported that certain acid clays
are capable of activating transition metal compounds for polyolefin formation and of
forming an in situ filled polymer [10, 17]. The use of a combination of a transition
metal complex and a polymerization activating agent in the form of aluminum alkyls
or methyl aluminoxane (MAO) and derivatives [18, 19] is much more versatile and
far less dependent on the type of filler and type of catalyst. Good results can be
expected when the agent interacts and/or reacts with the surface of the filler and
remains capable of initiating an olefin polymerization [20]. Many homogeneous
catalysts can be activated for polyolefin formation in combination with aluminum
alkyls or MAO and usually also with a support that has been coated with them
[20]. This is thus a versatile route for generation of polyolefins of diverse microstructure, from high-density polyethylene (PE-HD) to linear low-density polyethylene
(PE-LLD) and also to functionalized polyolefins [21].
The developments in homogeneous ethylene and propylene polymerization
catalysis in the last 10–20 years have widened the scope of polyolefinic products
attainable. Amongst these are: (1) complexes of early transition metals with
non-metallocene ligands [22]; (2) nickel and palladium complexes, with chain
walking leading to highly branched polyolefins (Pd) or chain straightening (Ni) in
the case of 1-olefin polymerization [23]; (3) living olefin polymerization [24]; (4)
catalytic preparation of end-functionalized polyolefins [21]; (5) (ternary) catalytic
chain transfer polymerization [25] and chain shuttling [26]; and (6) facile synthesis
of PE–LLDs and blends from ethylene by using a combination of a polymerization
and/or an oligomerization catalyst (tandem catalysis) [27]. A spin-off of the discovery of further catalysts with easily accessible ligands is the fact that preparation
has become less elaborate and time-consuming and is no longer the field of trained
organometallic chemists working under Schlenk line conditions, thus allowing
Iron Catalyst in the Preparation of Polyolefin Composites
343
and tribological applications etc. is dependent on reaching a good dispersion and
adhesion of filler and matrix. Composites of polyolefins show the best mechanical
and rheological properties when a good and lasting dispersion is reached of the filler
or fillers in the matrix [5]. In many cases, the appreciable difference in polarity of
polyolefin matrix and inorganic filler means that there is challenge in obtaining
such a distribution. Several methods have been used for the preparation of
composites. These methods include melt blending in an extruder or kneader,
solution blending, and in situ polymerization or in situ particle formation [6]. The
results of mixing and blending vary in quality depending on the filler, its surface
treatment, and the procedures used (e.g., [7]). Although not established on a large
scale, in situ polymerization has been shown to give finely dispersed fillers in
polyolefins [8, 9]. In this process, the polymer is generated in the presence of the
filler. Prerequisite is the compatibility of the polymerizing catalyst system and
the filler. A lasting dispersion of filler in the matrix is reached in those cases
where the polymer is wrapped around the filler. It may be anticipated that the latter
can be reached if the polymer is formed on or in close vicinity to the surface.
An active polymerization system in the presence of a generic filler (clays/minerals,
glass, carbon nanotubes, graphene) can be reached under the same requirements that
lead to an active supported catalyst [8–16]. It has been reported that certain acid clays
are capable of activating transition metal compounds for polyolefin formation and of
forming an in situ filled polymer [10, 17]. The use of a combination of a transition
metal complex and a polymerization activating agent in the form of aluminum alkyls
or methyl aluminoxane (MAO) and derivatives [18, 19] is much more versatile and
far less dependent on the type of filler and type of catalyst. Good results can be
expected when the agent interacts and/or reacts with the surface of the filler and
remains capable of initiating an olefin polymerization [20]. Many homogeneous
catalysts can be activated for polyolefin formation in combination with aluminum
alkyls or MAO and usually also with a support that has been coated with them
[20]. This is thus a versatile route for generation of polyolefins of diverse microstructure, from high-density polyethylene (PE-HD) to linear low-density polyethylene
(PE-LLD) and also to functionalized polyolefins [21].
The developments in homogeneous ethylene and propylene polymerization
catalysis in the last 10–20 years have widened the scope of polyolefinic products
attainable. Amongst these are: (1) complexes of early transition metals with
non-metallocene ligands [22]; (2) nickel and palladium complexes, with chain
walking leading to highly branched polyolefins (Pd) or chain straightening (Ni) in
the case of 1-olefin polymerization [23]; (3) living olefin polymerization [24]; (4)
catalytic preparation of end-functionalized polyolefins [21]; (5) (ternary) catalytic
chain transfer polymerization [25] and chain shuttling [26]; and (6) facile synthesis
of PE–LLDs and blends from ethylene by using a combination of a polymerization
and/or an oligomerization catalyst (tandem catalysis) [27]. A spin-off of the discovery of further catalysts with easily accessible ligands is the fact that preparation
has become less elaborate and time-consuming and is no longer the field of trained
organometallic chemists working under Schlenk line conditions, thus allowing
Iron Catalyst in the Preparation of Polyolefin Composites
343