Chemistry Reference
In-Depth Information
form of neighboring group interaction. This interaction (or intramolecular solvation) would be with
oxygen atoms from the penultimate monomer units [
98
]. These are forms of “backside” stabilization of
the growing chains that force reactions to occur at the opposite sides from the locations of the
counterions. The mechanisms are forms of S
N
2
attacks with retention of the configurations. These
configuration are formed between existing and newly formed carbon cations in the transition states:
R
1
RO
H
R
7
O
A
6
5
+
OR
7
O
6
5
A
3
1
3
4
4
RO
OR
2
H
H
CH
2
H
RO
OR
2
6
R
5
RO
O
7
4
3
1
2
RO
OR
A
Solvations of the new cations might even occur before they are completely formed, maintaining
the steric arrangement throughout, provided that the monomers enter as shown above [
98
]. One
weakness of the above mechanism is that it fails to consider the nature of the counterions.
Another mechanism, proposed by Cram and Kopecky [
100
], places emphasis on formation of six-
membered rings. The growing polymeric chains in vinyl ethers occupy equatorial arrangements with
the -OR groups attached to the growing ends by virtue of their size, because they are larger. In
reactions between the monomers and the six-membered ring oxonium ions the relative configurations
of the two asymmetric centers that form determine total chain configurations. If the configurations are
similar, the chains become isotactic, but if they are different they become syndiotactic. Molecular
models suggest that isotactic placement should be more likely [
100
].
OR
OR
RO
RO
RO
HHH
OR
O
O
R
R
RO
RO
RO
RO
HHHH
OR
The Cram and Kopecky mechanism [
43
] fails to explain the influence of the various R groups upon
the stereospecificity of the final product.
In a mechanism proposed by Kunitake and Aso [
101
] two factors were given primary importance.
These are: (1) steric repulsions determine the conformations of the propagating chains with a special
arrangement of the counterions and those of the incoming monomers. (2) The directions of the
monomer attacks are determined by the tightness of the growing ion pairs. It is assumed that the
growing carbon cations are essentially sp
2
hybridized and that the conformations with the least steric
repulsion will, therefore, be [
101
] as shown in Fig.
4.1
.
The position of the counterion is assumed to be at the side of the carbon cation and away from the
penultimate unit. The stability of such conformations should be very dependent on the temperature of
the polymerization and on the size of the substituents. Experimental evidence confirms this. Thus, it is
known that
the stereoregular polymers, whether isotactic or syndiotactic, form only at
low
