Chemistry Reference
In-Depth Information
4.3.1.4 Metal Alkyls in Initiations of Cationic Polymerizations
Initiations of polymerizations of vinyl and other monomers by metal alkyls generally take place by
anionic mechanisms. This is discussed further in this chapter. There are, however, reports in the literature
[
41
] of cationic polymerizations that are initiated by some metal alkyls. These are polymerizations of
monomers like vinyl ethers,
-methoxystyrene [
42
], and isobutylene [
43
,
44
] that are initiated by
compounds like dialkyl aluminum chloride.
One explanation is as follows [
45
]. These metals are strong electron acceptors. Their valence
shells and their unfilled orbitals can accommodate electrons from donor molecules. As a result, the
organometallic compounds behave like Lewis acids. This was observed in a polymerization of
isobutyl vinyl ether [
46
]. While triethylaluminum is not by itself an initiator for the polymerizations,
the reactions will take place in the presence of typical electron acceptors [
45
]:
o
-and
p
+
Al(C
2
H
3
)
3
ClCH
2
OCH
3
(Al(C
2
H
3
)
3
Cl
CH
2
OCH
3
)
OC
4
H
9
OC
4
H
9
(
Al(C
2
H
3
)
3
Cl
+
CH
2
OCH
3
)
CH
3
OCH
2
Al(C
2
H
3
)
3
Cl
H
The catalyst components must be combined in the presence of the monomer. This is due [
45
]to
high instability of the carbon cation complexes [Al(C
2
H
5
)
3
Cl]
[CH
3
OCH
2
]
+
. It is interesting that the
polymerizations by C
2
H
5
AlCl
2
H
2
O also show rate decreases as the reaction progresses [
47
]. When,
however, additional water is added, rapid polymerizations start again [
47
].
It is possible to modify the catalytic activity of metal alkyls by controlled additions of
modifiers
.
These are water, alcohol [
45
], oxygen [
48
], carbon dioxide [
49
], aldehydes [
50
], organic peroxides
[
45
], and metal oxides, like V
2
O
5
, NiO, and HgO [
45
]. The exact action of these modifiers is not clear
and it is not certain if they should be considered to be co-initiators or initiators. Their addition,
however, can affect catalytic activity, yield, stereospecificity, and molecular weight of the products.
Oxygen, for instance, can act as a modifier for the Grignard reagent, which by itself does not initiate
cationic polymerizations of vinyl ethers. Yet, introduction of oxygen to a vinyl ether-Grignard
reagent system will initiate the polymerization and will yield high molecular weight products [
50
].
It was suggested [
45
] that oxygen may cause transformation of the alkyl magnesium groups into
N alkoxy magnesium groups. This results in greater concentrations of magnesium dihalides that can
induce the polymerizations.
Substances that generate cations can vary widely. They can be molecules that dissociate into ions
or react with other compounds like solvent or monomer to form cations. Iodine is an example of such
a substance. In a system of
n
-butyl vinyl ether-iodine-diethyl ether, the iodine apparently first forms
an inactive
-complex with the solvent. It subsequently dissociates and rearranges into an isomeric
active ion [
12
]:
p
I
2
I
3
I
2
I
+
I
2
OR
RO
RO
Another group of compounds that were recently reported as capable of initiating cationic
polymerizations are metallocene/borate complexes. Such material can, for instance, be generated
from zirconocene dimethyl compounds (Cp
2
ZrMe
2
) and anilinium borate. Thus, [HNMe
2
Ph] + [B
(C
6
H
5
)
4
] will polymerize amine-functionalized
-olefins [
51
] as well as isobutylene homo and
copolymerizations [
52
]. Also, when compounds, like Cp*MMe
3
(M
a
ΒΌ
Ti, Zr, and Hf; where
