Chemistry Reference
In-Depth Information
chain and is forced by a bulky chiral substituent. Kunitake and Takarabe [
105
], therefore, modified
the original Kunitake and Aso mechanism. The growing chain ends are crowded by bulky
substituents. This may result in steric interference between a bulky side group and the counterions.
The interactions of the propagating ion pairs decrease when the sizes of the counterions increase.
A frontal attack and syndiotactic placement of the monomers results. When, however, the monomer
side groups are less bulky, steric repulsion becomes insignificant. Larger counterion becomes
responsible for retarding the frontside attack and gives more isotactic placement.
Studies of model reactions for cationic polymerization of alkyl propenyl ethers showed that the
mode of double bond opening is independent of the geometric structure of the ether. Mainly a
threo
opening takes place, but the mode of monomer addition is dependent on the geometric structure of the
monomer or on the bulkiness of the substituent [
106
].
Finally, in still another investigation of model systems, UV and visible spectroscopy were used
together with conductivity measurements. Results showed that charge-transfer complexes do form
between the counterions and
-acceptors, which can be Lewis acids or acceptor solvents [
196
]. This
led Heublein to suggest that interactions with monomers lead to alterations of the solvation spheres of
the ionpairs in the direction of the counterions. The temporary dissymmetry of the sphere of solvation
affects stereoregularities of the structures of the polymers that form. As a result, the propagation
reactions are seen by Hueblein as competing interaction between the chain carriers and the
monomers, the counterions, and the solvents [
106
].
p
Control in Heterogeneous Polymerizations
Several reaction mechanisms were also proposed to explain stereospecific placement with insoluble
catalysts. Furukawa [
46
] suggested that here the mechanism for cationic polymerization of vinyl
ethers depends upon multicentered coordinations. He felt that coordinations of the polymeric chains
and monomers with the catalysts are possible if the complexed counteranions have electrically
positive centers. This can take place in the case of aluminum alkyl and boron fluoride:
þ½
R
3
Al
þ
BF
3
!½
R
2
Al
BF
3
R
Further coordination of aluminum alkyl to the anions is possible if the coordination number of the
central atom is sufficiently large [
46
]:
½
BF
3
R
þ
AR
3
!½
R
3
Al
!
BF
3
R
The products are complex counterions that enable multicentered coordination polymerization.
Thus, the mechanism of vinyl ether polymerization proposed by Furukawa [
46
] is as follows. Two
neighboring ether oxygens that are linked to the polymer chain close to the terminal cation become
coordinated to the metal center of the complexed counterion. The molecules of the monomer can then
approach the growing chain only from the opposite side and isotactic placement results:
OR
B
H
H
O
O
Z
R
R
Me
