Chemistry Reference
In-Depth Information
Another mechanism is derived from the structure of the diethylzinc-water catalyst [
25
] that is
visualized as a dimer:
H
O
C
2
H
5
C
2
H
5
Zn
Zn
O
H
A similar structure pictured can be shown for diethylzinc-alcohol. The asymmetric induction is
suggested to takes place during coordination of the monomer to the catalyst site. This is a result of
indirect regulation that results form interactions between the monomer and the penultimate unit [
25
].
In yet another mechanism the initial coordination and subsequent propagation steps are pictured as
follows [
26
]
R
R
O
O
O
O
R
Zn
Zn
R
O
O
O
+
R
Zn
Zn
R
O
O
O
R
R
R
O
O
R
Zn
Zn
R
O
O
O
R
While the detailed structures of most catalyst sites are still unknown, it was established that
stereoselectivity does not come from the chirality of the growing chain end. Rather it is built into the
catalyst site itself [
27
,
28
]. Normal preparations of the catalysts give equal numbers of (
R
) and (
S
)
chiral catalyst sites. These coordinate selectively with (
R
) and (
S
) monomers respectively in the
process of catalytic-site control [
23
].
5.3.4 Steric Control in Polymerizations of Oxiranes
Cationic polymerizations of oxiranes are much less isospecific and regiospecific than are anionic
polymerizations. In anionic and coordinated anionic polymerizations, only chiral epoxides, like
propylene oxide, yield stereoregular polymers. Both pure enantiomers yield isotactic polymers
when the reaction proceeds in a regiospecific manner with the bond cleavage taking place at the
primary carbon.
In all polymerizations of oxiranes by cationic, anionic, and coordinated anionic mechanisms, the
ring-opening is generally accompanied by an inversion of the configuration at the carbon where the
cleavage takes place. A linear transition state mechanism involving dissociated nucleophilic species
has been proposed [
15
]. Yet, there are some known instances of ring-opening reactions of epoxies that
are stereochemically retentive. For instance, ring opening of 2,3-epoxybutane with AlCl
3
results in
