formed, the copolymerization ability appears to be an even more important
aspect. Structural features of the catalyst that influence the copolymerization
ability are interannular bridge and size, and position of substituents in the
ligands. In particular, the Et(Ind) 2 ZrCl 2 -based 3-siloxy-substituted complexes
show high copolymerization ability towards short chain comonomers and a high
tendency for LCB formation.
• Low shear rate rheological characterization in comparison with the molecular
weight and MWD from SEC is a sensitive, yet relatively simple and reliable way
of detecting the LCB. In accordance with the in situ mechanism, the rheological
behavior of conventional metallocene-catalyzed ethylene homopolymers
heavily depends on polymerization conditions such as ethylene pressure, hydrogen concentration, and polymerization time. Therefore, the LCB is not only a
function of the catalyst but also of the polymerization. Moreover, any deviation
from the polymerization rate-controlled polymerization conditions (e.g., due to
the presence of mass and heat transfer limitations) offers a pathway to broadening of polymer structure in terms of MWD, comonomer distribution, and also
the LCB.
• The level of long-chain branching by metallocene catalysts, however, is low and
this complicates the characterization. Metallocene-catalyzed LCB polyethylene
thus consists of a mixture of linear and LCB chains, where the LCB structures
make up only a small fraction of the total number of chains.
• The first structure assumption for LCB is a three-arm star, but complex
branches-on-branches structures may form through copolymerization of more
than one vinyl-ended macromer in the chain. The branched molecules are
located in the highest molecular weight end of the MWD. The LCB mechanism
and the influencing factors, i.e., catalyst copolymerization ability and polymerization conditions, suggest that it is feasible that variations not only in the amount
but also in the distribution of LCB can occur. However, it is postulated that the
catalyst structure also plays a major role as regards distribution/topology of the
long-chain branches along the chains.
4 Polymerization of Functional Comonomers with Olefins
by Using Bridged Bis(indenyl) Catalysts
4.1 Polymerization of Strongly Interacting Comonomers
The coordination polymerization by transition-metal catalysts, such as Ziegler–Natta
catalysts or metallocenes, is the most versatile method for preparing linear polyolefins
under mild and controlled conditions. Unfortunately, attempts at direct incorporation
of functional monomers during polymerization run into the problem of catalyst
poisoning caused by the interaction of organic functionalities with the catalyst
center [129].
210
J. Seppa ¨la ¨ et al.
aspect. Structural features of the catalyst that influence the copolymerization
ability are interannular bridge and size, and position of substituents in the
ligands. In particular, the Et(Ind) 2 ZrCl 2 -based 3-siloxy-substituted complexes
show high copolymerization ability towards short chain comonomers and a high
tendency for LCB formation.
• Low shear rate rheological characterization in comparison with the molecular
weight and MWD from SEC is a sensitive, yet relatively simple and reliable way
of detecting the LCB. In accordance with the in situ mechanism, the rheological
behavior of conventional metallocene-catalyzed ethylene homopolymers
heavily depends on polymerization conditions such as ethylene pressure, hydrogen concentration, and polymerization time. Therefore, the LCB is not only a
function of the catalyst but also of the polymerization. Moreover, any deviation
from the polymerization rate-controlled polymerization conditions (e.g., due to
the presence of mass and heat transfer limitations) offers a pathway to broadening of polymer structure in terms of MWD, comonomer distribution, and also
the LCB.
• The level of long-chain branching by metallocene catalysts, however, is low and
this complicates the characterization. Metallocene-catalyzed LCB polyethylene
thus consists of a mixture of linear and LCB chains, where the LCB structures
make up only a small fraction of the total number of chains.
• The first structure assumption for LCB is a three-arm star, but complex
branches-on-branches structures may form through copolymerization of more
than one vinyl-ended macromer in the chain. The branched molecules are
located in the highest molecular weight end of the MWD. The LCB mechanism
and the influencing factors, i.e., catalyst copolymerization ability and polymerization conditions, suggest that it is feasible that variations not only in the amount
but also in the distribution of LCB can occur. However, it is postulated that the
catalyst structure also plays a major role as regards distribution/topology of the
long-chain branches along the chains.
4 Polymerization of Functional Comonomers with Olefins
by Using Bridged Bis(indenyl) Catalysts
4.1 Polymerization of Strongly Interacting Comonomers
The coordination polymerization by transition-metal catalysts, such as Ziegler–Natta
catalysts or metallocenes, is the most versatile method for preparing linear polyolefins
under mild and controlled conditions. Unfortunately, attempts at direct incorporation
of functional monomers during polymerization run into the problem of catalyst
poisoning caused by the interaction of organic functionalities with the catalyst
center [129].
210
J. Seppa ¨la ¨ et al.