Chemistry Reference
In-Depth Information
high-
cis
-content linear polybutadienes are more elastomeric and perform better on
rubber processing equipment. They are favored for tire applications.
11.5.5
Metallocene Catalysts
Homogeneous catalysts capable of producing polyethylene
[12]
and isotactic
polypropylene
[13]
employ metallocenes. These are organometallic coordination
compounds comprising two cyclopentadienyl rings bonded to a metal atom, as in
11-22.
R
R
R
R
Y
Y
B
M
S
R
R
R
11-22
The cyclopentadienyl ring may be part of an indenyl (11-16) structure.
Catalysts vary in the number of rings, the nature of their substituents (R), the tran-
sition metal (M), and its substituents (Y) as well as the type of bridge (B). Their
essential feature is that the transition metal is constrained between two rings, pro-
viding a sterically hindered catalytic site. They differ also from the heterogeneous
Ziegler
Natta catalysts described above in being homogeneous, i.e., soluble in
hydrocarbons and, most notably, in producing polyolefins with different charac-
teristics. This is because the metallocene catalysts are single-site systems. They
produce more uniform ethylene 1-olefin copolymers than can be obtained with
Ziegler
Natta catalysis. In conventional LLDPE the lower molecular weight spe-
cies are richest in the olefin comonomer, resulting in a fraction of a few weight
percent that is highly branched and not crystallizable (for reasons that are elabo-
rated in Chapter 4). This is a disadvantage in certain food packaging applications
since the amorphous low-molecular-weight material is extractable by greases and
oils. Metallocene-based catalytic systems permit incorporation of the higher olefin
comonomer uniformly or preferentially in the higher molecular weight species of
the copolymer, thus reducing the extractable content.
The generally even comonomer distribution with respect to molecular weight
that is characteristic of metallocene-derived polyolefins is not always advantageous
in polyethylene processing operations. However, use of a combination of metallo-
cenes makes it possible to prepare multisite catalysts to produce polymers with
multimodal compositions that are designed for particular applications. Appropriate
