Chemistry Reference
In-Depth Information
4.4.1.1
Initiation by Addition of an Anion to an Olefin
When organolithium compounds are dissolved in non-polar solvents there is a strong tendency of the
solute molecules to associate into aggregates. For instance, butyllithium is hexameric in hexane
solution. This is true of ethyllithium [
144
,
145
] as well. Addition of Lewis bases to these solutions
causes formation of strong complexes between the bases and the organometallic compounds
[
146
,
147
]. This causes the clusters to breakup through a succession of equilibrium with tetrameric
and dimeric intermediates, all becoming complexes with the Lewis bases [
146
]. Particularly effective
bases are those that allow a close approach of the lithium ion to the heteroatom. Often, the
carbon-lithium bond, that normally only ranges between 20 and 40% in ionic character, becomes
much more ionic [
148
,
149
].
The breakdown of the aggregates was shown to dramatically enhance the reactivity of the
organometallic compounds [
150
,
151
]. For instance, polymerizations of styrene in benzene with
butyllithium are slow reactions. When, however, these polymerizations are carried out in tetrahydro-
furan they are extremely rapid. Tetrahydrofuran is, of course, a Lewis base. Nevertheless, the
breakdown of the aggregates even in such Lewis bases as tetrahydrofuran or diethyl ether are not
complete, though the clusters are smaller and more solvated. Differences in reactivity, however, can
be observed even in different non-polar solvents [
152
].
The reaction rates depend to a great extent on the nature of the organometallic compounds, such as
polarity of the bonds and the degree of solvation. In polar solvents, where free solvated ion-pairs
predominate, the mechanism of initiation may simply consist of a direct addition of the anion
to the monomer. If the solvents are non-polar, on the other hand, the initiation is more complex.
In these solvents, the metal cation coordinates with the monomer first. This is followed by a
rearrangement [
153
]:
X
+
R
R
M
M
R
M
X
X
-electron cloud of the olefin overlaps with the outer bond orbital of
the metal cation. This causes stretching and eventual rupture of the R-M (metal) bond. An intramo-
lecular rearrangement follows with the migration of the carbanion (R
) to the most electron-deficient
carbon atom of the double bond. A new covalent bond and a new carbanion are formed simultaneously:
In the initial coordination, the
p
H
H
H
H
C
C
R
C
C
M
H
H
X
M
X
R
where M represents the metal.
The basicity of the anion portion of the initiating species is also important. For instance,
fluorenelithium initiates polymerizations of methyl methacrylate but fails to initiate polymerizations
of styrene. A more electronegative butyl anion from butyl lithium, on the other hand, initiates
polymerizations of both monomers. Yet, it was also shown that the order of reactivity is often
contrary to the inherent basicity.
103-146
This may be due to the size of the aggregates that the
particular organometallic compound forms. In addition, higher reactivity may also be due to favor-
able energy requirements for breaking down the aggregates [
146
].
