chain branching [121, 122], and comonomer distribution. Control of morphology is
possible by suspension, cascade, or multizone reactors that improve melt viscosity
and processing.
Late transition metal complexes, which are more stable in water, can be used for
emulsion polymerization [123–125]. Very small and stable polyethylene particles
with diameters of 100 nm are obtained by microemulsion polymerization using
nickel(II) complexes and diphenylphosphine benzene sulfonate as cocatalyst [126].
A lot can be done to tailor the microstructure of copolymers. Today it is only
possible to create statistically ordered or alternating copolymers of polyolefins
[127]. It is not possible to design polyolefins with a required sequence order of
two or more monomers, which will be essential to establish self-organization of the
polymer chain. To design three dimensional crystallizing polyolefins for materials
with special properties, such as cages for catalysts or membranes, the controlled
self-organization will be necessary. Easier synthesis of polyolefins with polar
comonomers is important for this and for polymer blends of polyolefins with
other polymers, such as polyamides or polyesters [128]. Polar monomers
incorporated consist of hydroxyl, carboxyl, ether, ester, siloxy, or amino groups.
Blends containing small amounts of such functional metallocene-based copolymers
are tougher and stiffer than non-compatibilized polymers [120, 129–131].
There has been some initial success in forming block copolymers using singlesite catalysts. These catalysts have to show no or only a slow chain transfer reaction,
like living systems [132]. In a first step, only propylene is polymerized, forming a
hard polypropylene block, then by addition of ethene a soft ethene/propene copolymer or a polyethylene block follows because ethene is inserted much faster.
Another method is described using a chain-shuttling agent to form block
copolymers [133]. Two different single-site catalysts and monomers are used in
one reactor. The cocatalyst is a fluorinated phenylborate. One catalyst is able to
homopolymerize from a mixture of ethene and 1-octene only or mainly ethene,
forming a hard polyethylene segment; the other catalyst copolymerizes ethene and
1-octene to a soft copolymer segment. By the addition of zinc diethyl as a chain
transfer agent, the growing polymer chain shuttles between the two catalysts.
A block copolymer results with soft and hard segments with elastomeric properties.
In the future, it could also be possible to use MAO-activated catalysts for chainshuttling.
Polyolefin nanocomposites open up the approach to new classes of materials
with great property combinations. A soft polyolefin matrix can be combined with
hard inorganic particles, or strong layers of silicates or graphene, or with fibers of
extreme high tensile strength, such as carbon fibers, carbon nanotubes or polymer
fibers. An easy way for the preparation of such polyolefin nanocomposites is in-situ
polymerization using nanoparticles or fibers activated by metallocene/MAO or
other single-site catalysts. Materials with high gas barrier resistance, high thermal
and electric conductivity, and high form stability can be obtained as well as a good
dispersion of the nanofiller in the polymer matrix.
The development and commercialization of metallocene/MAO and other singlesite catalysts have just started and have already expanded the product range of
24
W. Kaminsky and H. Sinn
possible by suspension, cascade, or multizone reactors that improve melt viscosity
and processing.
Late transition metal complexes, which are more stable in water, can be used for
emulsion polymerization [123–125]. Very small and stable polyethylene particles
with diameters of 100 nm are obtained by microemulsion polymerization using
nickel(II) complexes and diphenylphosphine benzene sulfonate as cocatalyst [126].
A lot can be done to tailor the microstructure of copolymers. Today it is only
possible to create statistically ordered or alternating copolymers of polyolefins
[127]. It is not possible to design polyolefins with a required sequence order of
two or more monomers, which will be essential to establish self-organization of the
polymer chain. To design three dimensional crystallizing polyolefins for materials
with special properties, such as cages for catalysts or membranes, the controlled
self-organization will be necessary. Easier synthesis of polyolefins with polar
comonomers is important for this and for polymer blends of polyolefins with
other polymers, such as polyamides or polyesters [128]. Polar monomers
incorporated consist of hydroxyl, carboxyl, ether, ester, siloxy, or amino groups.
Blends containing small amounts of such functional metallocene-based copolymers
are tougher and stiffer than non-compatibilized polymers [120, 129–131].
There has been some initial success in forming block copolymers using singlesite catalysts. These catalysts have to show no or only a slow chain transfer reaction,
like living systems [132]. In a first step, only propylene is polymerized, forming a
hard polypropylene block, then by addition of ethene a soft ethene/propene copolymer or a polyethylene block follows because ethene is inserted much faster.
Another method is described using a chain-shuttling agent to form block
copolymers [133]. Two different single-site catalysts and monomers are used in
one reactor. The cocatalyst is a fluorinated phenylborate. One catalyst is able to
homopolymerize from a mixture of ethene and 1-octene only or mainly ethene,
forming a hard polyethylene segment; the other catalyst copolymerizes ethene and
1-octene to a soft copolymer segment. By the addition of zinc diethyl as a chain
transfer agent, the growing polymer chain shuttles between the two catalysts.
A block copolymer results with soft and hard segments with elastomeric properties.
In the future, it could also be possible to use MAO-activated catalysts for chainshuttling.
Polyolefin nanocomposites open up the approach to new classes of materials
with great property combinations. A soft polyolefin matrix can be combined with
hard inorganic particles, or strong layers of silicates or graphene, or with fibers of
extreme high tensile strength, such as carbon fibers, carbon nanotubes or polymer
fibers. An easy way for the preparation of such polyolefin nanocomposites is in-situ
polymerization using nanoparticles or fibers activated by metallocene/MAO or
other single-site catalysts. Materials with high gas barrier resistance, high thermal
and electric conductivity, and high form stability can be obtained as well as a good
dispersion of the nanofiller in the polymer matrix.
The development and commercialization of metallocene/MAO and other singlesite catalysts have just started and have already expanded the product range of
24
W. Kaminsky and H. Sinn