Chemistry Reference
In-Depth Information
2,2,6,6-tetramethylpiperidine might be used together with different trialkylaluminum compounds as
modifier-cocatalyst systems for the supported catalysts:
TiX
4
AlR
3
Support
N
where X represents a halogen.
Other analogous amines, like 1,2,4-trimethylpiperazine and 2,3,4,5-tetraethylpiperidine, are also
used in preparations of titanium halide catalysts supported on MgCl
2
. The amine remains as a built-in
modifier in the catalyst system [
53
].
Subsequent research efforts concentrated on soluble catalytic systems, like di-
5
-cyclopentadie-
nyldiphenyltitanium and tetrabenzylzirconium complexed with methylaluminoxane, (CH
3
)
2
Al-
[-O-Al(CH
3
)-]
n
-Al(CH
3
)
2
. Such catalysts, however, yield products that contain only about 85%
isotactic polypropylene [
55
-
61
], and only if the reactions are conducted at low temperatures,
Z
45
C
or lower. A major breakthrough occurred when rigid chiral metallocene initiators were developed,
like 1,1-ethylene-di-
5
-indenylzirconium dichloride, complexed with methylaluminoxane. In place
of zirconium, titanium and hafnium analogs can also be used. These catalysts are highly isospecific
[
62
-
64
] when used at low temperatures. The compounds are illustrated in Chap.
4
.
Typical catalysts
consist of aluminum to transition metal ratios of 103 or 104:1. Many of them yield 98-99% isotactic
fractions of the polymer. In addition, these are very active catalysts, yielding large quantities of
polymer per gram of zirconium.
It was also reported that elastomeric polypropylenes can be formed from the monomer with the aid
of some metallocene catalysts [
62
-
64
]. Because rigid, chiral metallocene catalysts produce isotactic
polypropylene, while the achiral ones produce the atactic form, Waymouth and Coates [
62
] prepared
a bridged metallocene catalyst with indenyl ligands that rotate about the metal-ligand bond axis. The
rotation causes the catalyst to isomerize between chivalric and nonchiralic geometries:
Z
isotactic
block
atactic
block
Zr
Zr
Indenyl ligands, however, were found to rotate faster that the polymerization reaction.
This prevents formation of stereoregular polymer blocks [
62
]. To overcome that, phenyl substituents
were added to the ligands to slow down the rotation below the speed of monomer insertion, yet rotate
faster than the time required for formation of the whole polymeric chain. The product, a catalytic
system of bis(2-phenylindenyl)zirconium dichloride plus methylaluminoxane, was found to yield
elastomeric block copolymers of isotactic and atactic polypropylene [
62
].
Vincenzo et al. [
63
] reported that
13
C NMR microstructural analysis of polypropylene samples
produced with two representative “oscillating” metallocene catalysts was found to be largely different
in steric hindrance. The original mechanistic proposal of an “oscillation” between the two enantio-
morphous, a racemic-like (isotactic-selective) and a meso-like (non-stereoselective) conformation,
according to them, cannot explain the observed polymer configuration.
They further feel that isotactic-stereoblock nature of the polymers obtained with this catalyst proves
unambiguously that the active cation “oscillates” between the two enantiomorphous racemic-like
conformations at an average frequency that, even at high propene concentration, is only slightly lower
than that of monomer insertion. The less hindered catalyst gives instead a largely stereoirregular
