Page 130 - 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

478 Catalysis, Homogeneous

FIGURE 50 Stereo-block isotactic PP with an achiral catalyst.

highly isotactic polypropene can be obtained using the are not essential as they are replaced before the polymer-
racemic zirconium analog of the ethylene-bis(indenyl) ization can start by an alkyl group (e.g., CH 3 from methy-
compound. Modification of the cyclopentadienyl ligands lalumoxane) and by a solvent molecule, thus generating
has led to a very rich chemistry and a great variety of mi- the required cationic species.
crostructures and combination thereof has been obtained The two cyclopentadienyl anions are linked together
including isotactic polymer with melting points above by a bridge, here an ethane bridge, and extended with an
◦
160 C, syndiotactic polypropene, block polymers, hemi- organic moiety which renders the molecule its chirality.
isotactic polymers, etc. In the example of Fig. 51 a 1,2-ethane bridged bis(1,1-
tetrahydroindenyl) dianion has been drawn. Two com-
pounds are obtained when this complex is synthesized,
6. Mechanistic Explanations
the rac-isomer mixture and the meso-isomer. Often they
Fundamental studies have led to a detailed insight into the can be separated by crystallization. The sterically less hin-
mechanism of the polymerization and the control of the dered rac-isomers may be formed in excess. The isomers
microstructure through the substituents on the cyclopen- can only interchange by breaking of a cyclopentadienyl-
tadienyl ligands. The nature of the catalyst has been the metal bond and reattaching the metal at the other face
topic of many studies and is now generally accepted to of the cyclopentadienyl ligand. This process is very
+
be a cationic Cp 2 ZrR species. The first requirement for slow compared to the rate of insertion reactions with
obtaining an active catalyst is to ascertain the formation propene.
of cationic species. Secondly, the metallocene (or related The rac-isomers have a twofold axis and therefore C 2 -
ligand) must have the correct steric properties. symmetry.Themeso-isomerhasamirrorplaneasthesym-
TheachiralCp 2 TiPh 2 catalystactivatedwithalumoxane metry element and therefore C s -symmetry. For polymer-
can produce PP with some isotacticity. Hence, the natural ization reactions the racemic mixture can be used since the
tendency of the chain in this 1,2 insertion is to catalyze two chains produced by the two enantiomers are identical
the formation of isotactic PP. How do we explain the im- when begin- and end-groups are not considered. The two
provements made with the bridged, chiral metallocenes? sites (X, in Fig. 51) have the same absolute configuration,
We need a catalysts with a specific preference for coordi- and coordination at both sites will occur with a preference
nation to either the re-or si-face of the incoming propene. for the same face of the propene molecule. A simple way
The metallocene can be equipped with a chiral substituent of looking at it would be that the cyclohexyl rings enforce
and thus we can try if this is indeed effective. By trial a certain position of the methyl group of propene. When
and error and molecular modeling we will find the best
substituent at our Cp-ring. Two sites are needed in our
catalyst: one for the migrating chain and the other for the
propene molecule. We know that insertion takes place in
a migratory manner. The one asymmetric site that we cre-
atedcontrollingthecoordinationofpropeneisinsufficient,
because after this migration the alkyl chain will reside on
the other site. The next propene molecule will coordinate
to the former alkyl site. Thus, the two sites must be the
same and yet chiral. In asymmetric hydrogenation we have
seen how this problem can be solved, viz. with a C 2 sym-
metric site. The first compounds introduced having this
feature are shown in Fig. 51.
Zirconium is tetrahedrally surrounded by two cyclopen-
tadienyl ligands and two chlorides. The latter two ligands FIGURE 51 Structure of 1,2-ethanediyl-tetrahydro-indenyl-ZrX 2 .