LT-PE); the iron (with a smaller discussion of cobalt) pre-catalysts [22–31, 41],
producing highly linear products either by oligomerization of α-olefins or polymerization to afford high-density polyethylenes (HDPEs) or polyethylene waxes; and
the nickel complex pre-catalysts for yielding branched polyethylenes [17–21, 42] as
extremely low-density polyethylenes (LDPEs) or polyethylene elastomers or gasoline depressants.
1 The Characteristic Features of LT-PE
To commercialize LT-PE will require an understanding of how the kind of polyethylene and its microstructure affects its properties and applications. Interestingly,
LT-PE can be divided into two types depending on whether the microstructure is
linear or branched. Highly linear polyethylenes (including oligomers) are
commonly produced by iron and cobalt complex pre-catalysts [22–31, 41], whereas
branched polyethylenes are formed by nickel and palladium complex pre-catalysts
[17–21, 42]. The microstructural differences in LT-PE are caused by their
characteristic mechanistic pathways of polymerization.
The linear products (oligomers and polyethylenes) could be simply explained as
the constantly repeating of the processes of ethylene coordination and insertion
(see Scheme 2: [C] and [I], respectively) but the termination stage will definitely
control the chain length of the products, thereby obtaining oligomers, polyethylene
waxes, or polyethylenes. In addition, in most cases the products were confirmed as
having vinyl groups, therefore the oligomers would be useful α-olefins [43]. The
polyethylene waxes and highly linear polyethylenes have been extensively used,
and new challenges include production of value-adding polyethylenes with narrow
polydispersity or finding efficient catalytic systems, of which the iron and cobalt
complex pre-catalysts are highly potential and promising.
The polyethylenes obtained by nickel or palladium complex pre-catalysts are
usually reported as branched products [17–21, 42], in which the main branches are
methyl groups. Many proposed mechanisms (Scheme 2) are similarly reported in
the literature, in which the polyethylenes with methyl branches are well illustrated
[9–12, 17–21, 32–34, 44]. However, some longer alkyl branches are often observed
within polyethylenes prepared using nickel catalytic systems [45, 46], therefore
intermediates and potential mechanisms have been extensively proposed in order to
complete the understanding of various alkyl-branched polyethylenes. The nickel
and palladium catalytic systems commonly transfer ethylene into various branched
products, including oligomers, polyethylene waxes, and polyethylenes. With the
exception of the nickel-promoted SHOP process for α-olefins [47, 48], most nickel
complex pre-catalysts transform ethylene into branched products and have low
selectivity for vinyl-oligomers. Concerning branched polyethylenes, the current
commercial processes require the co-polymerization of ethylene with an α-olefin.
By employing nickel or palladium complex pre-catalysts, the branched
polyethylenes are obtained by using solely ethylene, which reduces the additional
cost of the co-monomer α-olefin. To enhance the properties and applications of
Novel Polyethylenes via Late Transition Metal Complex Pre-catalysts
165
producing highly linear products either by oligomerization of α-olefins or polymerization to afford high-density polyethylenes (HDPEs) or polyethylene waxes; and
the nickel complex pre-catalysts for yielding branched polyethylenes [17–21, 42] as
extremely low-density polyethylenes (LDPEs) or polyethylene elastomers or gasoline depressants.
1 The Characteristic Features of LT-PE
To commercialize LT-PE will require an understanding of how the kind of polyethylene and its microstructure affects its properties and applications. Interestingly,
LT-PE can be divided into two types depending on whether the microstructure is
linear or branched. Highly linear polyethylenes (including oligomers) are
commonly produced by iron and cobalt complex pre-catalysts [22–31, 41], whereas
branched polyethylenes are formed by nickel and palladium complex pre-catalysts
[17–21, 42]. The microstructural differences in LT-PE are caused by their
characteristic mechanistic pathways of polymerization.
The linear products (oligomers and polyethylenes) could be simply explained as
the constantly repeating of the processes of ethylene coordination and insertion
(see Scheme 2: [C] and [I], respectively) but the termination stage will definitely
control the chain length of the products, thereby obtaining oligomers, polyethylene
waxes, or polyethylenes. In addition, in most cases the products were confirmed as
having vinyl groups, therefore the oligomers would be useful α-olefins [43]. The
polyethylene waxes and highly linear polyethylenes have been extensively used,
and new challenges include production of value-adding polyethylenes with narrow
polydispersity or finding efficient catalytic systems, of which the iron and cobalt
complex pre-catalysts are highly potential and promising.
The polyethylenes obtained by nickel or palladium complex pre-catalysts are
usually reported as branched products [17–21, 42], in which the main branches are
methyl groups. Many proposed mechanisms (Scheme 2) are similarly reported in
the literature, in which the polyethylenes with methyl branches are well illustrated
[9–12, 17–21, 32–34, 44]. However, some longer alkyl branches are often observed
within polyethylenes prepared using nickel catalytic systems [45, 46], therefore
intermediates and potential mechanisms have been extensively proposed in order to
complete the understanding of various alkyl-branched polyethylenes. The nickel
and palladium catalytic systems commonly transfer ethylene into various branched
products, including oligomers, polyethylene waxes, and polyethylenes. With the
exception of the nickel-promoted SHOP process for α-olefins [47, 48], most nickel
complex pre-catalysts transform ethylene into branched products and have low
selectivity for vinyl-oligomers. Concerning branched polyethylenes, the current
commercial processes require the co-polymerization of ethylene with an α-olefin.
By employing nickel or palladium complex pre-catalysts, the branched
polyethylenes are obtained by using solely ethylene, which reduces the additional
cost of the co-monomer α-olefin. To enhance the properties and applications of
Novel Polyethylenes via Late Transition Metal Complex Pre-catalysts
165