Chemistry Reference
In-Depth Information
The six-membered ring is destroyed in the process of propagation:
O
O
O
O
O
O
O
O
O
Li
O
Li
O
O
The transition state formed by a 1,4-dipolar addition to a polarized double bond. Coordination of
the lithium atom to two oxygen atoms determines stereoregulation. Each new incoming monomer
must approach from below the plane because the other side is blocked by an axial methyl group. This
favors isotactic placement. There is doubt, however, whether it is correct to assume a rigid six-
membered cyclic alkoxide structure for a propagating lithium enolate [
209
].
A slightly similar model was suggested by Bawn and Ledwith [
209
]. It is based on the probability
that a growing polymeric alkyl lithium should have some enolic character, with the lithium
coordinating to the carbonyl oxygen of the penultimate unit:
O
O
+
O
O
O
O
O
O
O
O
O
O
Li
Li
The cyclic intermediate forms due to intramolecular solvation of the lithium and due to intramo-
lecular shielding of one side of the lithium ion. The nucleophilic attack by the monomer, therefore,
has to occur from the opposite side. The transition state is similar to an S
N
2
reaction. When the bond
between the lithium and the incoming monomer forms, the oxygen lithium bond ruptures. Simulta-
neously, the charge migrates to the methylene group of the newly added monomer. The resultant new
molecule is stabilized immediately by intramolecular solvation as before. In this manner, the
retention of configuration is assured, if the incoming monomer always assumes the same configura-
tion toward the lithium ion.
NMR spectra of poly (
-butyllithium in both polar and
non-polar solvents show that the penultimate unit does affect monomer placement [
210
]. Also, a
coordination was observed with both heteroatoms [
152
,
153
], the one on the ultimate and the one on
the penultimate unit.
Many refinements were introduced into the various proposed explanations of steric control in
anionic polymerizations [
211
] over the last 30 or more years. Two important features of these
mechanisms are: (1) coordinations of the chain ends with the counterions and (2) counterion
solvation.
Use of complex lanthanide catalysts allows a very high
N
,
N
-dimethylacrylamide) formed with
sec
-1,4 placement of isoprene monomer
and preparation of polymers that are very close to natural rubber [
212
]. Thus, complex neodymium
catalysts can yield polymers that are greater than 98%
cis
-1,4 polyisoprenes. The preparation of
such catalyst, however is difficult. Evans et al. reported, however, that simple TmI
2
,NdI
2
,andDyI
2
will initiate polymerization of isoprene without any additives and can also yield high
cis
cis
-1,4
placement [
213
]:
