Chemistry Reference
In-Depth Information
A
L
L
S
S
Fig. 4.1
Steric arrangement.
L
large substituent;
S
small substituent
L(S)
front-side
A
L
S
(L)
S
front-side
back-side
attack
attack
A
L
A
S
A
L
L
S
L
L
L
L
L
S
L
S
S
S
S
S
S
Fig. 4.2
Propagation mechanism
temperatures in homogeneous polymerizations (as stated earlier). This suggests that the fixation of the
conformations of the growing chain ends is very important in enhancing polymer stereoregularity.
In polar solvents the counterions interact only weakly with the growing cations. The steric effects
become major factors in deciding the courses of propagation. In such situations the carbon cations
[
101
] attack the least hindered side (frontal side attacks). These give rise to syndiotactic structures.
The terminal carbon cations probably can rotate freely, so the vinyl monomers should be capable of
approaching from any direction. In non-polar solvents, on the other hand, if the ion pairs are tight
enough, the incoming monomers may approach the cations from the back sides only, giving rise to
isotactic placements. This is illustrated in Fig.
4.2
.
If there is steric hindrance to backside approaches due to the large sizes of the penultimate
substituents, front-side attacks take place. This occurs even in non-polar medium [
101
]. The incom-
ing monomers can, therefore, attack the cation either from the frontside or from the backside. All
depends upon the tightness or the Coulombic interaction of the ion pair and on the difference in the
steric hindrance between the two modes of attack.
In the above reaction mechanism, the possible interactions of the counterions and the monomers
are ignored. This was justified by weak interactions of electron-rich monomers, like
-methyl styrene
and vinyl ethers with weak anions [
101
]. The nature of the counterions as such, however, is not
ignored in this mechanism, because the tightness of the ion pairs is considered.
Later work by Hirokawa et al. [
102
], suggested that the sizes of the R groups of alkenyl ethers play
an important role in determining the steric structures of the resultant polymers. For instance, allyl
vinyl ethers can be polymerized to highly isotactic polymers with the aid of SbCl
5
.
1
H and
13
CNMR
data shows no evidence, however, of steric control, though, it does show a relationship between active
chain ends and incoming monomers. In addition, the amounts of isotactic placement do not differ
significantly at
a
75
C[
103
]. This suggests that isotactic selection is generated by
orienting the substituents in the monomer and in the chain
10
Corat
from each other. A Coulombic
attraction is visualized between the counterion and the positively polarized oxygen of the monomer.
Also, in studies with optically active vinyl ethers it was observed [
104
] that trimethyl vinyl silane,
which is bulky and non-chiral forms highly syndiotactic polymers. Equally bulky, but chiral (
away
)-
menthyl vinyl ether, however, produces isotactic polymers in polar solvents. This suggests that
isotactic propagation is preferred in a polar medium because of helical conformation of the polymer
