Page 129 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: FPP Revised Pages
Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

Catalysis, Homogeneous 477

face or the si-face, or as displayed in Fig. 49, with the
methyl group pointing up or down.
In summary, in the chain-end control mechanism the
last monomer inserted determines how the next molecule
of 1-alkene will insert. Proof for this stems from catalysts
not containing a stereogenic center that give stereoregular
FIGURE 48 Cossee-Arlman site. polymer. Secondly, whatever site-control we try to induce,
the chain that we are making will always contain, by def-
inition, an asymmetric center. These two points would
(see Fig. 48). This simple picture yields an asymmetric
strongly support the chain-end control mechanism. As we
titanium site, but by considering the lattice surface four
have mentioned above, the nature of the solid catalysts
bridging chlorides may also lead to an asymmetric site.
had an enormous influence on the product, and this un-
In Cossee’s view, one way or another, the site has to
derpins the Cossee site-control mechanism. Thus both are
control the way a propene molecule inserts, by doing this
operative and both are important. Occasionally, chain-end
in a very controlled manner one can imagine that a stere-
control only suffices to ensure enantiospecifity. The anal-
oregular polymer will form. This seems obvious, since 13
ysis of the products using high resolution C-NMR has
different catalysts give different stereospecifities (reality
greatly contributed to the mechanistic insight and distinc-
is more complicated as we will see). The asymmetry of
tion between the various catalysts. NMR analysis gives a
the site regulates the mode of coordination of the propene
detailed picture of the relative orientation of the methyl
molecule, in other words it steers the direction to which
groups in the chain, i.e., the regular ones, but more in
the methyl group will point.
particular the mistakes that were made.
4. Site Control Versus Chain-end Control
5. Homogeneous Catalysts
Over the years two mechanisms have been put forward
In the 1970s the first claims appeared concerning the
as being responsible for the stereo-control of the grow-
homogeneous stereospecific polymerization, but they re-
ing polymer chain, first the site-control mechanism and
ceived relatively little attention as during the same years
secondly the chain-end control mechanism. In the site-
the first highly active heterogeneous titanium catalysts,
control mechanism the structure of the catalytic site de-
immobilized on magnesium salts, were reported and the
termines the way the molecule of 1-alkene will insert
industrial interest in homogeneous catalysts diminished.
(enantiomorphic-site control). As we have seen previ-
The development of the new family of homoge-
ously, propene is prochiral and a catalyst may attack either
neous catalysts based on biscyclopentadienyl Group 4
the re-face or the si-face. If the catalyst itself is chiral as
metal complexes for the stereoselective polymerization of
the one in Fig. 48, a diastereomeric complex forms and
alkenesismainlyduetoKaminsky,Ewen,andBrintzinger.
there may be a preference for the formation of a particu-
In 1980 Kaminsky and Sinn reported on an extremely
lar diastereomer. If the catalyst adds to the same face of
fast homogeneous catalyst for the polymerization of
each subsequent propene molecule, we say isotactic PP is
ethene formed from the interaction of Cp 2 Zr(CH 3 ) 2 and
formed (a definition proposed by Natta). Thus, we see that
◦
(CH 3 AlO) n . At 8 bar of ethene and 70 C an average rate
stereoregular polymerization is concerned with asymmet- 7
of insertion of ethene amounting 3 × 10 mole of ethene
ric catalysis and indeed the way the problems are tackled
per mole of Zr per hour was reported. For propene this
these days have much in common with asymmetric hy-
catalyst led to completely atactic polymer. Ewen was the
drogenation and related processes.
first to report the synthesis of stereoregular polymers with
When we look more closely at the intermediate polymer
soluble Group 4 metal complexes and alumoxane as the
chain we see an alternative explanation emerging. After
co-catalyst. He found that Cp 2 TiPh 2 with alumoxane and
thefirstinsertionhastakenplaceastereogeniccenterisob-
propenegivesisotacticpolypropene.Thiscatalystdoesnot
tained at carbon 2; see Fig. 49. Coordination with the next
contain an asymmetric site that would be able to control
propene may take place preferentially with either the re-
the stereoregularity. A stereoblock polymer is obtained,
see Fig. 50. Formation of this sequence of regular blocks
is proof of the chain-end control mechanism.
Using an intrinsically chiral titanium compound (rac
ethylene-bis-indenyl titanium dichloride), first described
by Brintzinger, Ewen obtained polypropene that was in
FIGURE 49 Enantiomorphic chain-end control. part isotactic. Kaminsky and Brintzinger have shown that