Chemistry Reference
In-Depth Information
In polar solvents, on the other hand, these reactions can go to the other extreme. The propagation
can simply consist of successive additions of the monomers to the growing anions.
In homogeneous anionic polymerizations of simple vinyl monomers steric placement is also
temperature dependent, just as it is in cationic polymerizations. Syndiotactic placement is favored
in polar solvents at low temperature. In non-polar solvents, however, isotactic placement
predominates at the same temperatures. Here too, this results mainly from the degree of association
with the counterion [
181
].
Much of our current knowledge of the propagation reaction is based on studies carried out in
highly solvating ether solvents. Less information is available about homogeneous reactions in non-
polar medium. Generally, though, the rate of propagation increases with solvent polarity and with the
degree of ion pair dissociation [
195
]. Organolithium compounds undergo the greatest degree of
solvation when changed from hydrocarbons to polar ether solvents. Cesium compounds, on the other
hand, are least affected by changes in solvent polarity. In addition, NMR studies of polystyryl
carbanion structures associated with lithium, potassium, and cesium counter cations were studied
in different solvents and at different temperatures [
196
]. The results show an interaction between the
larger radius cations and the phenyl rings of the ultimate monomer units in the chains. The structures
with potassium and cesium counterions, judging from model compounds, were found to be planar
with sp
2
-hybridized
-carbon. It suggests that in the presence of the larger counterions, rotation of the
terminal phenyl ring in styrene polymerization is strongly hindered.
Propagation rates also depend upon the structures of the monomers. For polymerizations initiated
by alkali amides the following order of reactivity was observed [
197
]:
a
N
O
O
-carbon tends to decrease the reaction rate due to the electron
releasing effect of the alkyl group. This tends to destabilize the carbanion and also to cause steric
interference with solvation of the chain end and with the addition of the monomer [
197
].
Also, methyl substitution on the
a
4.4.2.1 Steric Control in Anionic Polymerization
Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes because they
yield some steric control over monomer placement. This is true of both tacticity and geometric
isomerism. As stated earlier, the insertions can be 1,2 or 3,4 or 1,4. Furthermore, the 1,4-placements
can be
cis
trans
. Lithium and organolithium initiators in hydrocarbon solvents can yield polyiso-
prene, for instance, which is 90%
or
cis
-1,4 in structure [
99
]. The same reaction in polar solvents,
however, yields polymers that are mostly 1,2 and 3,4, or
-1,4 in structure. There is still no
mechanism that fully explains steric control in polymerization of dienes. High
trans
-1,4 polyisoprene
forms with lithium or alkyl lithium initiators in non-polar solvents because, propagation takes place
through essentially covalent or intimate ion-pair lithium to carbon bonds [
99
]. An intermediate
pseudo-six-membered ring is believed to form in the process of addition of the diene [
99
]:
cis
Li
CH
2
Li
CH
2
