Chemistry Reference
In-Depth Information
Initially, a series of soluble, highly isospecific catalysts were developed for propylene polymeri-
zation [
168
]. These materials are zirconium, titanium, or hafnium based
metallocenes
, such as
racemic 1,1-ethylene-di-
5
-indenylzirconium dichloride. The term metallocene applies to complexes
of transition metals sandwiched between two aromatic rings, usually two cyclopentadienyl. They are
rigid structures, due to ethylene bridges between the two five-membered rings. Syntheses of these
compounds yield racemic mixtures of two enantiomers. Both produce isotactic polypropylene:
Z
Cl
Cl
Cl
Cl
Zr
Zr
Methylaluminoxane co catalyst is required by these metallocenes to become highly isospecific
catalysts capable of very high isotactic placement. They are also very active, yielding very large
quantities of polypropylene per each gram of zirconia.
Catalysts that yield highly syndiotactic polypropylene (86% racemic pentads) were also developed.
One of them is
3
fluorenyl)zirconium dichloride [
291
]. Initial
disclosures of metallocene catalysts were followed by numerous publications in the literature that
described similar materials for the polymerizations of either ethylene or propylene, or both, and for
formation of various copolymers. Thus, for instance, Kaminsky et al. [
292
], reported preparation of a
zirconium dichloride-type catalyst for copolymerization of cyclic olefins with ethylene. These cyclic
olefins are cyclopentene, norbornene, and the hindered cyclopentadiene adducts of norbornene [
292
].
The catalytic system consists of a bridged indene derivative that is combined with methylaluminoxane:
5
-cyclopentadienyl-
i
-propylene(
Z
Z
Z
r
Cl
Cl
It is interesting that the copolymers of some of these cycloolefins with ethylene were reported to be
transparent, amorphous, and oxidatively stable materials. In addition, it is claimed that they have
glass transition temperatures in excess of 200
C, are melt processable, solvent stable, and possess
high mechanical strength [
292
].
Another example is work by Han et al. [
293
], who synthesized
ansa
o
-dichloro[
-phenylenedi-
5
-1-indenyl)]zirconium. Both
methylenebis(
Z
racemic
and
meso
forms were obtained and separated:
Cl
Cl
Zr
Zr
Cl
Cl
racemic - 1
meso - 1
