• The tendency towards chain transfer to hydrogen seems to be the highest with
metallocenes prone to unimolecular β-H elimination. Metallocenes with high
selectivity towards bimolecular β-H elimination are less reactive towards
hydrogen.
• Isomerization tendency also appears to be the most significant with the catalysts
for which β-H elimination dominates.
3 Long-Chain Branched Polyethylenes Formed In Situ
3.1 LCB Mechanism
Studies on the comonomer response and end group selectivity of bis(indenyl)
zirconium catalysts led to further studies on the tendency of selected single-site
catalyst systems under certain conditions to form long-chain branching.
Long chain branching (LCB) has several benefits relating to the polymer processability because it affects melt viscosity, temperature dependence of viscosity,
melt elasticity, shear thinning, and extension thickening. The effect of LCB on the
melt-state properties of polyethylene depends on the number, length, and distribution of the branches, on molecular weight, and on MWD.
The ability to produce polyethylene with low levels of LCB from ethylene
monomer only is today considered to be reasonably common for single-site and
metallocene catalysts. LCB is thought to occur via an in situ macromer copolymerization mechanism [12, 14, 46, 54, 79–87]. One manifestation of macromer insertion capability is the finding that crosslinked polymer is formed in
copolymerization of ethylene and non-conjugated dienes with metallocene catalysts
[88]. The in situ copolymerization mechanism (Scheme 2) was originally proposed
to explain LCB in polymers with Constrained Geometry Catalyst (CGC) in a
solution process [89–91]. According to this mechanism, the catalyst must first
produce a vinyl-terminated polyethylene chain – a macromonomer – and then
copolymerize it into another growing chain. Even when the fraction of chains
with vinyl ends is relatively high, LCB incorporation would have to compete
with insertion of short chained monomer and comonomer and only very low levels
of LCB are expected to be incorporated. Following the mechanism, low ethylene
concentration should favor LCB incorporation and lead to a more branched polymer. Increasing the amount of comonomer or hydrogen in the polymerization
should result in less LCB due to a decreasing number of vinyl ends. Experimental
data for these dependencies will be shown below.
The polymerization process also plays a role. In solution polymerization at high
temperatures, polymer is dissolved and individual chains are assumed to have free
mobility. The macromer can move from one site to another, and be copolymerized
to a growing chain in a similar manner as other comonomers. However, in the
particle forming process in a slurry or in gas phase, the polymer is not dissolved, but
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J. Seppa ¨la ¨ et al.
metallocenes prone to unimolecular β-H elimination. Metallocenes with high
selectivity towards bimolecular β-H elimination are less reactive towards
hydrogen.
• Isomerization tendency also appears to be the most significant with the catalysts
for which β-H elimination dominates.
3 Long-Chain Branched Polyethylenes Formed In Situ
3.1 LCB Mechanism
Studies on the comonomer response and end group selectivity of bis(indenyl)
zirconium catalysts led to further studies on the tendency of selected single-site
catalyst systems under certain conditions to form long-chain branching.
Long chain branching (LCB) has several benefits relating to the polymer processability because it affects melt viscosity, temperature dependence of viscosity,
melt elasticity, shear thinning, and extension thickening. The effect of LCB on the
melt-state properties of polyethylene depends on the number, length, and distribution of the branches, on molecular weight, and on MWD.
The ability to produce polyethylene with low levels of LCB from ethylene
monomer only is today considered to be reasonably common for single-site and
metallocene catalysts. LCB is thought to occur via an in situ macromer copolymerization mechanism [12, 14, 46, 54, 79–87]. One manifestation of macromer insertion capability is the finding that crosslinked polymer is formed in
copolymerization of ethylene and non-conjugated dienes with metallocene catalysts
[88]. The in situ copolymerization mechanism (Scheme 2) was originally proposed
to explain LCB in polymers with Constrained Geometry Catalyst (CGC) in a
solution process [89–91]. According to this mechanism, the catalyst must first
produce a vinyl-terminated polyethylene chain – a macromonomer – and then
copolymerize it into another growing chain. Even when the fraction of chains
with vinyl ends is relatively high, LCB incorporation would have to compete
with insertion of short chained monomer and comonomer and only very low levels
of LCB are expected to be incorporated. Following the mechanism, low ethylene
concentration should favor LCB incorporation and lead to a more branched polymer. Increasing the amount of comonomer or hydrogen in the polymerization
should result in less LCB due to a decreasing number of vinyl ends. Experimental
data for these dependencies will be shown below.
The polymerization process also plays a role. In solution polymerization at high
temperatures, polymer is dissolved and individual chains are assumed to have free
mobility. The macromer can move from one site to another, and be copolymerized
to a growing chain in a similar manner as other comonomers. However, in the
particle forming process in a slurry or in gas phase, the polymer is not dissolved, but
194
J. Seppa ¨la ¨ et al.