longer and the particle formation itself could be a factor. Precipitation has a great
influence on the mobility of the chains and on monomer diffusion rate coefficients.
All above-mentioned phenomena can play a role in the LCB formation. It
appears reasonable to assume that most important are the monomer and the
macromonomer concentrations in the vicinity of the active site. In the particle
forming process, the situation could be described as diffusion-controlled longchain branch formation enabled by vinyl-end formation and the good copolymerization ability of metallocenes. In solution, the decisive factors could be vinyl-end
formation and good copolymerization ability with macromer mobility. The reported
rheological behavior of LCB polyethylenes produced with mono-C P amido
catalysts in slurry [81, 83] differs from that for polymerizations carried out in
solution [80, 91]. The rheologically complex behavior (like that shown in Fig. 8)
and comonomer enhancement effect on LCB has much more often been reported in
particle forming polymerizations [12, 81, 83]. Modeling efforts [119] for the chain
length distribution produced with the in situ mechanism show that the LCB distorts
the MWD and forms a high M w tail.
Adapting the idea of elongational rheological response to reflect species with
longest relaxation times [122–124], we interpret these differences as variations in
long chain branching topology. The continuous processes can be better controlled
for mass and heat transfer properties of the components and system. Therefore, it
seems reasonable that the samples prepared in continuous processes would have
more random branching structure, with branches distributed more evenly among the
molecules. Branches in the polymers produced in a semi-batch system, on the other
hand, may be less homogeneously distributed, forming a multiply branched high
molar mass tail with very long relaxation time.
Despite the big effect on shear sensitivity at low shear rates, low levels of
metallocene LCB do not necessarily yield any improvement at high shear rates
[95, 128] In extensional flows, metallocene-LCB may improve melt strength and
melt stability due to a strain hardening effect, giving improved film blowing
stability and more even film thickness [119].
To summarize long-chain branch formation with metallocenes, as discussed
above:
• The ability to incorporate long-chain branches appears reasonably common
among metallocene catalysts. Experimental results for LCB formation with
both CGC and conventional metallocene-catalyzed polymerizations are in line
with an in situ copolymerization mechanism. For copolymerization of vinylterminated polyethylene molecules to occur, the first requirement is the presence
of termination mechanisms producing vinyl-terminated macromonomers. Secondly, the catalyst must able to incorporate these macromonomers into a growing chain. Macromers probably do not move from one active site to another, but
instead insertion of macromer to another chain takes place at same site where it
was formed in a intramolecular incorporation manner.
• It is the catalyst structure that governs the feasibility of the LCB formation. The
dominating termination reaction and its sensitivity to the presence of chain
transfer agents are determined by the catalyst. Provided that vinyl ends are
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
209
influence on the mobility of the chains and on monomer diffusion rate coefficients.
All above-mentioned phenomena can play a role in the LCB formation. It
appears reasonable to assume that most important are the monomer and the
macromonomer concentrations in the vicinity of the active site. In the particle
forming process, the situation could be described as diffusion-controlled longchain branch formation enabled by vinyl-end formation and the good copolymerization ability of metallocenes. In solution, the decisive factors could be vinyl-end
formation and good copolymerization ability with macromer mobility. The reported
rheological behavior of LCB polyethylenes produced with mono-C P amido
catalysts in slurry [81, 83] differs from that for polymerizations carried out in
solution [80, 91]. The rheologically complex behavior (like that shown in Fig. 8)
and comonomer enhancement effect on LCB has much more often been reported in
particle forming polymerizations [12, 81, 83]. Modeling efforts [119] for the chain
length distribution produced with the in situ mechanism show that the LCB distorts
the MWD and forms a high M w tail.
Adapting the idea of elongational rheological response to reflect species with
longest relaxation times [122–124], we interpret these differences as variations in
long chain branching topology. The continuous processes can be better controlled
for mass and heat transfer properties of the components and system. Therefore, it
seems reasonable that the samples prepared in continuous processes would have
more random branching structure, with branches distributed more evenly among the
molecules. Branches in the polymers produced in a semi-batch system, on the other
hand, may be less homogeneously distributed, forming a multiply branched high
molar mass tail with very long relaxation time.
Despite the big effect on shear sensitivity at low shear rates, low levels of
metallocene LCB do not necessarily yield any improvement at high shear rates
[95, 128] In extensional flows, metallocene-LCB may improve melt strength and
melt stability due to a strain hardening effect, giving improved film blowing
stability and more even film thickness [119].
To summarize long-chain branch formation with metallocenes, as discussed
above:
• The ability to incorporate long-chain branches appears reasonably common
among metallocene catalysts. Experimental results for LCB formation with
both CGC and conventional metallocene-catalyzed polymerizations are in line
with an in situ copolymerization mechanism. For copolymerization of vinylterminated polyethylene molecules to occur, the first requirement is the presence
of termination mechanisms producing vinyl-terminated macromonomers. Secondly, the catalyst must able to incorporate these macromonomers into a growing chain. Macromers probably do not move from one active site to another, but
instead insertion of macromer to another chain takes place at same site where it
was formed in a intramolecular incorporation manner.
• It is the catalyst structure that governs the feasibility of the LCB formation. The
dominating termination reaction and its sensitivity to the presence of chain
transfer agents are determined by the catalyst. Provided that vinyl ends are
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
209