Traditionally it was considered that ionic catalyst systems produce essentially linear
polymeric architectures. This view changed, however, through analysis of the melt
behavior of metallocene-based polyethylenes, which revealed the presence of longchain branching (LCB) in these materials [9–14]. Interestingly enough, a long-chain
branch structure is different to the structure exhibited by low-density polyethylene
(LDPE) or Cr-based high-density polyethylene (HDPE) materials.
Some metallocenes produce essentially linear polyethylene whereas others are
capable of producing almost crosslinked material at suitable polymerization
conditions. Here, both comonomer response and ability to provide vinyl terminals
have been seen as important in defining the degree of branching.
As is well known, polyethylene properties are dictated by molecular weight,
molecular weight distribution (MWD) and branching. Short-chain branches of less
than 40 carbon atoms influence the crystallinity and crystalline structure, whereas
long-chain branches strongly affect the rheological properties of polyethylene,
giving an additional tool for the modification of processing behavior to complement
that provided by MWD. Thereby, metallocene polyethylenes with narrow MWD
and LCB offer a possibility for multistage processes to produce materials with
specifically tuned polymer property and processability combinations not available
by conventional catalyst technology [15].
Another great challenge in polyolefin technology has been the production of
polyolefins with polar functionalities. The most advanced method for producing
functional polyolefins, in addition to free radical reactions and grafting reactions, is
copolymerization with co-ordination catalysts.
Unfortunately, due to the Lewis acidity of the Ziegler–Natta catalysts, the
copolymerization of polar functional olefins with these catalysts is not straightforward. The σ-donating polar groups have a higher tendency than the π-coordinating
olefins to coordinate with the cationic complexes of these catalysts.
Single-site catalysts enable a broader range of alternative comonomers in ethylene
and propylene copolymerizations through better comonomer response, which allows
the use of much lower polar comonomer concentrations. Dramatically new technical
properties could be achieved in polyolefin materials when the properties of polyolefins
and polar comonomers were combined [16–18].
Functional polyolefins with significant contents of reactive alcohol, acid, ester,
ether, silyl ether, amine, and amide groups have been successfully synthesized
[19–22]. Even though these systems are not commercially feasible, the novel
materials demonstrated gave a boost to the search for ways to fill this gap.
Some vinyl silanes were found to act feasibly as weakly interacting comonomers.
Functional copolymers with ethylene and vinyl-Si(CH 3 ) 3 , allyl-Si(CH 3)3 , 3-butenylSi(CH 3 ) 3 , 4-pentenyl-Si(CH 3 ) 3 , 5-hexenyl-Si(CH 3 ) 3 or 7-octenyl-Si(CH 3 ) 2 Ph as
comonomers were polymerized [23–25]. The short trialkylsilane monomers suffered
from the electronic influence of silicon, which led to poor polymerization performance. In these cases, the chain end of the synthesized polyethylene-co-allyl-Si
(CH 3 ) 3 consisted of reactive allylic silane groups and therefore the functionality
and reactivity of these copolymers was higher than the weakly interacting
trimethylsilane moiety can provide. Also it was found that the phenylene group in
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
183
polymeric architectures. This view changed, however, through analysis of the melt
behavior of metallocene-based polyethylenes, which revealed the presence of longchain branching (LCB) in these materials [9–14]. Interestingly enough, a long-chain
branch structure is different to the structure exhibited by low-density polyethylene
(LDPE) or Cr-based high-density polyethylene (HDPE) materials.
Some metallocenes produce essentially linear polyethylene whereas others are
capable of producing almost crosslinked material at suitable polymerization
conditions. Here, both comonomer response and ability to provide vinyl terminals
have been seen as important in defining the degree of branching.
As is well known, polyethylene properties are dictated by molecular weight,
molecular weight distribution (MWD) and branching. Short-chain branches of less
than 40 carbon atoms influence the crystallinity and crystalline structure, whereas
long-chain branches strongly affect the rheological properties of polyethylene,
giving an additional tool for the modification of processing behavior to complement
that provided by MWD. Thereby, metallocene polyethylenes with narrow MWD
and LCB offer a possibility for multistage processes to produce materials with
specifically tuned polymer property and processability combinations not available
by conventional catalyst technology [15].
Another great challenge in polyolefin technology has been the production of
polyolefins with polar functionalities. The most advanced method for producing
functional polyolefins, in addition to free radical reactions and grafting reactions, is
copolymerization with co-ordination catalysts.
Unfortunately, due to the Lewis acidity of the Ziegler–Natta catalysts, the
copolymerization of polar functional olefins with these catalysts is not straightforward. The σ-donating polar groups have a higher tendency than the π-coordinating
olefins to coordinate with the cationic complexes of these catalysts.
Single-site catalysts enable a broader range of alternative comonomers in ethylene
and propylene copolymerizations through better comonomer response, which allows
the use of much lower polar comonomer concentrations. Dramatically new technical
properties could be achieved in polyolefin materials when the properties of polyolefins
and polar comonomers were combined [16–18].
Functional polyolefins with significant contents of reactive alcohol, acid, ester,
ether, silyl ether, amine, and amide groups have been successfully synthesized
[19–22]. Even though these systems are not commercially feasible, the novel
materials demonstrated gave a boost to the search for ways to fill this gap.
Some vinyl silanes were found to act feasibly as weakly interacting comonomers.
Functional copolymers with ethylene and vinyl-Si(CH 3 ) 3 , allyl-Si(CH 3)3 , 3-butenylSi(CH 3 ) 3 , 4-pentenyl-Si(CH 3 ) 3 , 5-hexenyl-Si(CH 3 ) 3 or 7-octenyl-Si(CH 3 ) 2 Ph as
comonomers were polymerized [23–25]. The short trialkylsilane monomers suffered
from the electronic influence of silicon, which led to poor polymerization performance. In these cases, the chain end of the synthesized polyethylene-co-allyl-Si
(CH 3 ) 3 consisted of reactive allylic silane groups and therefore the functionality
and reactivity of these copolymers was higher than the weakly interacting
trimethylsilane moiety can provide. Also it was found that the phenylene group in
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
183