Chemistry Reference
In-Depth Information
H
H
H
O
O
O
H
O
H
(R)
(S)
section of polymer
The polymers are fairly high in molecular weight, approximately 100 times greater than the
products from KOH initiations. Propylene oxide initially reacts with ferric chloride to form an
oligomer, a chloropolyalkoxide. The material contains approximately four or five propylene oxide
repeat units. This forms two different halogen sites. It can be illustrated as follows:
O
Cl
x
O
Cl
Fe
O
Cl
O
y
The above compound may be the catalyst or one closely related to it for forming stereoregular
polymers. Water appears to play a role, because the proportion of crystallinity increases with addition
of water. When water is added in a molar ratio of 1.8:1.0 of water to iron, the proportion of crystalline
to amorphous fraction increases from 0.13 to 0.86. Price and Osgen [
17
] suggested that the polymeri-
zation proceeds in a step-growth mechanism as follows:
possible
catalyst
su
r
fa
c
e
O
H
HH
H
H
surfac
e
Fe
O
Fe
O
O
H
H
H
H
H
The solid surface of the catalyst causes the transition state to be more compressed. Steric
repulsions between the incoming monomer and the ultimate unit are minimized if the incoming
monomer molecule is forced to be
trans
to the methyl group of the previous unit. Such a conforma-
tional approach also results in minimum repulsion between the incoming monomer and the bulky
growing polymer chain [
18
-
20
]. Also, ferric alkoxides are associated in nonpolar solvents. A dimer
may have the following structure:
R
R
O
O
O
R
Fe
Fe
R
O
O
O
R
R
By comparison, intramolecular chelation can be expected to reduce the degree of association of the
catalyst. Addition of water results in increased association after hydrolysis of the ferric alkoxide. This
may explain the effect of promoting stereoregularity by addition of water [
20
]. The ferric alkoxide
catalyst can also be made highly stereospecific by partial hydrolysis and still remain soluble in ether,
the polymerization medium [
21
]. This led to a suggestion [
22
] that the catalyst may contain active Fe-
O-Fe bonds. Such bonds would be formed from condensation of partially hydrolyzed alkoxide
derivative. The monomer insertion between the iron-oxygen bonds can be illustrated as follows:
