Chemistry Reference
In-Depth Information
The olefin is assumed to become coordinated to the transition metal at a
vacant octahedral site through overlap of the olefin
-electrons with the vacant d-
orbitals of the metal
[9,10]
. The bond between the transition metal and the alkyl
group R is weakened by this coordination and propagation takes place by inser-
tion of the complexed olefin between the metal and alkyl group via a four-
membered cyclic transition state. The unsubstituted carbon atom of the olefin
becomes attached to the metal during the opening of the double bond, which is
always a
cis
addition. The alkylated olefin shown attached to Ti in the final prod-
uct of reaction (11-59) is the R group for attack of the next monomer molecule.
π
CH
3
H
R
R
C
C
l
C
l
C
l
Ti
C
l
Ti
C
H
C
l
C
l
H
C
l
C
l
H
(11-59)
+ CH
3
C
CH
2
CH
3
R
H
C
l
C
R
C
l
H
C
l
Ti
Ti
C
l
C
C
C
l
C
l
CH
2
CH
3
C
l
C
l
H
H
= octahedral vacancy
In the polymerization scheme of reaction (11-59), insertion of a monomer
results in an interchange of the polymer substituent and the lattice vacancy. These
are not equivalent positions in the crystal lattice of the catalyst. Under normal
polymerization conditions the macromolecular alkyl appears to shift back to its
original position before the next monomer is added. Isotactic polymers are pro-
duced from olefins and catalysts of this type because the monomer always inserts
by
cis
addition with the unsubstituted carbon of the olefin attached to the transi-
tion metal which always has the same chirality. If the polymer chain and vacant
orbital were to exchange initial positions, however, then the placements of succes-
sive monomers would alternate stereochemically, providing syndiotacticity.
Small changes in the catalyst system or in the choice of monomer can result in sig-
nificant product variations. The stereospecificity and productivity of the polymerization
process can often be improved by procedures such as those described in
Section 11.5.3
.
