Page 131 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

Catalysis, Homogeneous 479

FIGURE 53 “Single site” catalyst for polyethylene.
FIGURE 52 Bridged fluorenyl-cyclopenadienyl complex giving
syndiotactic PP.
The above initial findings were the beginning of a new
generation of catalysts for making polyolefins. It has ini-
we rotate the molecule around the twofold axis we see tiated a lot of research both in industry and academia.
that the site carrying propene is equivalent to the other. Especially interesting is the molecular design of both cat-
In Natta’s words, an isotactic polymer should form (ne- alysts and polymers. For process reasons, however, the
glecting mistakes, and thermodynamic versus kinetic fac- new homogeneous catalysts, for polypropylene at least,
tors). The meso-isomer has no preference for either face should be heterogenized on solid supports.
of propene and gives atactic polypropylene. Only one commercialization of the new metallocene
catalysts will be mentioned here as an example, the so-
called“SingleSiteCatalyst”developedbyDowandshown
7. Site and Chain-End Control Reenforce in Fig. 53. It is not a stereospecific catalyst as it simply
Isotactic Specificity polymerizes ethene and adds higher alkenes. It gives a
narrow MW and a high rate of (re-)insertion of higher
When dealing with cationic titanium and zirconium met-
alkenes. The control of branching and the extent of long-
allocene catalysts, the insertion is a 1,2 mode. As a result,
chain branching leads to a product that can be easily
the two control mechanisms reenforce one another. In-
processed.
deed, the best results in terms of isotacticity are obtained
with these catalysts.
G. Palladium Catalysis
8. Syndiotactic PP by Site Control
Palladium catalysts are nowadays an important ingredient
A catalyst that contains two binding sites with the same of many multistep syntheses both in industry and the lab-
absolute configuration gives isotactic PP. When we can oratory. The versatility of palladium is enormous. Here
make a catalyst that contains two identical sites but with we will present a few typical examples. In several in-
the opposite chirality, each site should coordinate to the stances iso-electronic complexes of nickel and sometimes
opposite face of propene. Following Natta’sdefinition this rhodium can be used or they lead to even better results.
should give syndiotactic polymer. In other words, a cat- The basic principles are the same.
alyst containing a mirror plane—in such a way that the
two coordination sites are mirror images—gives syndio-
1. Allylic Alkylation
tactic polymer, because one site should coordinate to one
face of propene, and vice versa. To this end, Ewen made The palladium-catalyzed allylic alkylation has become a
isopropyl(1-fluorenyl-cyclopentadienyl) ligands and their very important tool for organic chemists. It has been de-
metallocenes; see Fig. 52. This complex has no chirality veloped by Trost, following an initial report by Tsuji. The
(i.e., the dichloride precursor). The catalyst indeed gives reagents used are all very mild and are compatible with
syndiotactic polymer. many functional groups. The method has been applied in

FIGURE 54 Palladium(0)-catalyzed allylic substitution.