Chemistry Reference
In-Depth Information
Formation of such intermediates is favorable for lithium because it has a small ionic radius and is
high in the proportion of
-character. Organometallic compounds of the other alkali metals (sodium,
potassium, rubidium, and cesium) are more polar and more dissociated. They react essentially as
solvated ions even in a hydrocarbon medium, yielding high 3,4 placement.
O'Driscoll et al. [
198
] proposed a mechanism for steric control. In isoprene polymerization
the terminal charges are complexed with the metal cations. These cations are close to the active
centers through the occupied
p
-orbitals of the chain ends and the unoccupied p-orbitals of the lithium
ions. In the transition state the monomers are complexed with the cations in the same way [
198
]. The
lithium cations are assumed to be in hybridized tetrahedral sp
3
configurations with four vacant orbitals.
The chain ends are presumed to be allylic and the diene monomers are bidentate [
198
]. During the
propagation steps both the monomers and the chain ends complex with the same counterions:
p
δ
Li
CH
2
δ
In hydrocarbon solvents, the complexes are tight and the rotations of the C
2
-C
3
bonds are
sterically hindered by the methyl groups. This constrains the 1,4-additions to
cis
-configurations. In
polar solvents, however, like tetrahydrofuran, the complexes are loose and thermodynamically
favored
trans
additions take place [
198
].
It was observed, however, that the polymerizations of 2,3-dimethylbutadiene with organolithium
initiators in non-polar solvents result in high
trans
-1,4 structures [
199
]. This appears to contradict the
above-proposed mechanism.
Proton NMR spectra show that solvation shifts the structures of the carbanionic chain ends from
localized 1,4-species to delocalized “
p
-allylic” type structures [
200
]:
CH
2
Li
Li
CH
2
cis
anti
CH
2
Li
Li
CH
2
trans
syn
s
-bonded lithium chain can be expected to predominate. In highly solvating solvents, such as
ethers, the
The
-allyl structure is dominant leading to high 1,2 placements. Because the 2,3-bond is
maintained, the above shown equilibrium should not be expected to lead to
p
isomerization
[
200
]. In fact, such isomerizations do not take place for butadiene or for isoprene when they are
polymerized in hydrocarbon solvents. They do occur, however, in polar solvents at high temperatures.
This suggests that additional equilibrium exist between the
cis-trans
-allylic structures and the covalent 1,2
chain ends [
200
]. Table
4.3
shows the manner in which different polymerization initiators and
solvents affect the microstructures of polyisoprene.
p
